Principles of Soil and Plant Water Relations

By M.B. Kirkham

Principles of Soil and Plant Water Relations, Third Edition describes the fundamental principles of soil and water relationships in relation to water storage in soil and water uptake by plants. The book explains why it is important to know about soil-plant-water relations, with subsequent chapters providing the definition of all physical units and the SI system and dealing with the structure of water and its special properties. Final sections explain the structure of plants and the mechanisms behind their interrelationships, especially the mechanism of water uptake and water flow within plants and how to assess parameters.

All chapters begin with a brief paragraph about why the topic is important and include all formulas necessary to calculate respective parameters. This third edition includes a new chapter on water relations of plants and soils in space as well as textbook problems and answers.

• Covers plant anatomy, an essential component to understanding soil and plant water relations
• includes problems and answers to help students apply key concepts
• Provides the biography of the scientist whose principles are discussed in the chapter

• Language: English
• Publisher: Academic Press
• Release date: Jul 13, 2023
• ISBN: 9780323956925

M.B. Kirkham

M. B. Kirkham is a Professor in the Department of Agronomy at Kansas State University. Her research involves two areas: soil-plant-water relations and uptake of heavy metals by crops grown on polluted soil (called “phytoremediation”). Dr. Kirkham is currently collaborating with colleagues at the Kansas State University Northwest Research-Extension Center in Colby, Kansas to study yield and water relations of sorghum grown under the semi-arid conditions of far western Kansas. Dr. Kirkham serves on several editorial boards: Soil Science; Journal of Crop Improvement; International Agrophysics; Crop Science; Australian Journal of Soil Research; Agriculture, Ecosystems and Environment; Agricultural Water Management; Pakistan Journal of Agricultural Research; Agricultural, Food and Analytical Bacteriology; and Journal of the American Society for Horticultural Science. In addition, Dr. Kirkham has received the CSSA Crop Science Research Award and the 2010-11 Iman Outstanding Faculty Award for Research.

Book preview

Preface to the First Edition

This textbook is developed from lectures for a graduate class in soil–plant–water relations taught at Kansas State University. Students in the class are from various departments, including agronomy, biology, horticulture, forestry and recreational resources, biochemistry, and biological and agricultural engineering. This book can be used as a text for graduate- or upper-level undergraduate courses or as a self-study guide for interested scientists. This book follows water as it moves through the soil–plant–atmosphere continuum. The text deals with principles and is not a review of recent literature. The principles covered in the book, such as Ohm's law and Poiseuille's law, are ageless. The book has equations, but no knowledge of calculus is required. Because plant anatomy is often no longer taught at universities, chapters review root, stem, leaf, and stomatal anatomy. Instrumentation to measure status of water in the soil and plant is also covered. Many instruments could have been described, but the ones chosen focus on traditional methods such as tensiometry and psychrometry and newer methods that are being widely applied such as tension infiltrometry and time domain reflectometry. Because the humanistic side of science is usually overlooked in textbooks, each chapter ends with biographies that tell about the people who developed the concepts discussed in that chapter.

Although a textbook on water relations might logically include developments in molecular biology, this topic is not covered. Rather, the text focuses on water in the soil and whole plant and combines knowledge of soil physics, plant physiology, and microclimatology. Chapter 1 reviews population and growth curves and provides a rationale for studying water in the soil–plant–atmosphere continuum. Chapter 2, which defines physical units, at first, may appear elementary, but many students have not had a class in physics. The definitions in this chapter lay the foundation for understanding future chapters. Chapter 3 goes over the unique structure and properties of water, which makes life possible. Chapter 4, on tensiometry, is the first instrumentation lecture. Other instrumentation lectures include Chapter 9 on penetrometer measurements; Chapter 10 on measurement of the oxygen diffusion rate in the soil; part of Chapter 11 on applications of tension infiltrometry to determine soil hydraulic conductivity, sorptivity, repellency, and solute mobility; Chapter 13 on time domain reflectometry; Chapter 16 on psychrometry; Chapter 17 on pressure chambers; Chapter 22, which includes ways to measure stomatal opening and resistance; and Chapter 24 on infrared thermometers. Chapters 4–13 focus on water in the soil; Chapters 14–22, on water in the plant; and Chapters 23–27, on water as it leaves the plant and moves into the atmosphere.

Within any one chapter, the notation is consistent and abbreviations are defined when first introduced. When the same letter stands for different parameters, such as A for ampere or area and g for acceleration due to gravity or grams, these differences are pointed out.

I express my appreciation to the following people who have helped make this book possible: my sister, Victoria E. Kirkham, Professor of Romance Languages at the University of Pennsylvania, who first suggested on March 12, 1999, that I write this book; Dr. Kimberly A. Williams, Associate Professor of Horticulture, who audited my class in 2000 and then nominated me for the College of Agriculture Graduate Faculty Teaching Award, of which I was the inaugural recipient in 2001; Mr. Martin Volkmann in the laboratory of Prof. Dr. Rienk van der Ploeg of the University of Hannover in Germany, for converting the first drafts of the electronic files of the chapters, which I had typed in MS-DOS using WordPerfect 5.1 (my favorite word-processing software), to Microsoft Word documents, to ensure useable backup copies in case my 1995 Compaq computer broke (it did not); my students, who have enthusiastically supported the development of this book; anonymous reviewers who had helpful suggestions for revisions and supported publication of the book; publishers and authors who allowed me to use material for the figures; and Mr. Eldon J. Hardy, my longtime professional draftsman at Oklahoma State University, now retired. His drafting ability is unparalleled. He has redrawn the figures from the original and ensured that they are uniform, clear, and precisely done.

I am grateful to the publishers, Elsevier, including Michael J. Sugarman, Director, for accepting my book for publication, and Kelly D. Sonnack, Editorial Coordinator, for her help during the production of this book. Through my late father, Don Kirkham, Former Professor of Soils and Physics at Iowa State University, I have known of the venerable scientific publications of Elsevier since I was a child, and truly am non solus with a book.

M.B. Kirkham Manhattan, Kansas.

April 6, 2004.

Preface to the Second Edition

The second edition results from an invitation that I received on April 22, 2013, from Elsevier/Academic Press to revise the first edition. The major changes in the new edition include the addition of three chapters: one on dual thermal probes to monitor soil–water content (Chapter 9); one on sapflow (Chapter 21); and one on interception of direct-beam solar radiation by leaves of plants (Chapter 30). The comments made in the Preface to the first edition hold for the second edition. These new chapters result from lectures that I give in my graduate class on soil–plant–water relations. They deal with principles and are not a review of the literature. As for the original chapters, I have ended the new chapters with biographies about the people who developed the concepts in the chapters.

In revising the book, I have interviewed experts on the topics—i.e., Loyd R. Stone about tensiometers (Chapter 5); Gerard J. Kluitenberg about the dual thermal probe technique (Chapter 9); and Kevin P. Price about drones (Chapter 27). I thank each of them for sharing their knowledge with me.

Based on a suggestion of one of the reviewers, I have kept all of the original chapters (27 of them). Changes made in the original chapters, based on comments from the reviewers, are the following: a reorganization of the order of the chapters; a shortening of some of the titles; and an addition of the history of the concept of the soil–plant–atmosphere continuum (added to Chapter 22). Other additions include the following: a problem showing how to convert grams to slugs (Chapter 2); Helmert's equation for calculation of the acceleration due to gravity (Chapter 4); a problem showing how to determine soil–water content with time domain reflectometry probes (Chapter 8); a discussion of the least limiting water range (Chapter 10); a biography of Katherine Esau (Chapter 15); notation of the continuing controversy about the cohesion theory (Chapter 20); a discussion of the normalized difference vegetation index (Chapter 27); and an analysis of the effect of elevated carbon dioxide on water-use efficiency (Chapter 29).

I express my appreciation to the following people who have helped make this new edition possible: Candice Janco, Senior Acquiring Editor of Elsevier/Academic Press, for inviting me to revise the first edition; Sean R. Coombs, Editorial Project Manager, Elsevier, Inc. Science and Technology Books, for his prompt and kind help throughout the revision; Brent E. Clothier and Steven R. Green, both of the Systems Modelling Group, the New Zealand Institute for Plant and Food Research, Ltd., Palmerston North, New Zealand, for figures provided in the chapter on sapflow (Chapter 21); Soil Measurement Systems, Tucson, AZ, for providing figures of tensiometers (Chapter 5); Eldon J. Hardy, my longtime professional draftsman, for redoing the population figure (Chapter 1); and Marsha K. Landis, Graphic Designer and Web Manager, Kansas State University, for taking pictures of equipment shown in the new edition. I also want to give special thanks to the three reviewers of the draft proposal, who supported publication of the revised edition: Bingru Huang, Professor II, Department of Plant Biology and Pathology, Rutgers University, New Brunswick, NJ; William L. Kingery, Professor and Graduate Coordinator, Department of Plant and Soil Sciences, Mississippi State University; Mississippi State, MS; and Anvar R. Kacimov, Professor and Dean, College of Agriculture and Marine Sciences and Head of the Department of Soils, Water and Agricultural Engineering, Sultan Qaboos University, Al-Khob, Sultanate of Oman.

As for the first edition, I dedicate the second edition to my family whose support made the book possible.

M.B. Kirkham Manhattan, Kansas.

January 1, 2014.

Preface to the Third Edition

As noted in the preface to the first edition, this textbook results from lectures that I give in my graduate class at Kansas State University dealing with soil-plant-water relations. Each chapter is meant to be a lecture or two.

On April 9, 2020, I received an e-mail from Candice Janco, Senior Acquiring Editor of Elsevier/Academic Press, inviting me to revise the second edition, which I agreed to do. She then got reviews for the proposed third edition. On June 28, 2021, I received the reviews, which were from five anonymous reviewers in Australia, Canada, Croatia, Germany, and the United States. I thank these reviewers for their comments.

There are four main changes in the third edition. The first one relates to the COVID-19 pandemic, which was sweeping the world when Ms. Janco invited me to do a third edition. On March 12, 2020, Kansas State University suddenly shut down due to the pandemic. All classes had to be taught remotely. I had never taught my class in a digitized form, and suddenly I was forced to put all my lectures in PowerPoint and show the slides by Zoom. I had always used the blackboard to present information in class. I find PowerPoint a boring way to give a lecture. In order to make the PowerPoint presentations more interesting, I spent time searching out information of scientific and humanistic interest, which I added to the Zoom lectures. I have put this information in the third edition of the book. Thus, I call it the plague edition of my textbook. My sister, Victoria Kirkham, Professor Emerita of Romance Languages at the University of Pennsylvania, especially encouraged me to include in the book the new information that I gave the students in the Zoom lectures during the pandemic.

The second change is the addition of problems at the end of each chapter. These are the problems that I hand out to my class when I teach the course. I have put the answers after each problem.

The third change relates to an idea of the reviewer from Croatia, who suggested that I add photographs of the people whose biographies I have at the end of each chapter. I was able to add 15 pictures. They are of the more recent scientists. Images of well-known scientists of the past, like Newton, Faraday, Kepler, and Galileo, are easy to find on the Web. I thank the scientists and their families who allowed me to put these pictures in the third edition.

The fourth change is the final chapter of the book, which is a new chapter dealing with microgravity. With the increased interest of space travel, soil and plants under reduced gravity cannot be ignored. I included information in this chapter from notes about gravity that I took when I visited with my father, Don Kirkham, while he was alive. He was professor in the Departments of Physics and Agronomy at Iowa State University from 1946 until his death in 1998.

As for the first and second editions, I made no attempt to review the literature. The book presents principles, and principles do not change. Also, I made no attempt to review recent instrumentation. I present principles for operation of important instruments used in soil-plant-water relations, like the tensiometer and the infrared thermometer. Instrumentation is constantly changing, and the reader is referred to review articles for recent information on instrumentation. Also, I do not consider models. Today's students have many opportunities to learn about modelling. This book presents the principles that students need to know when they do their modelling.

I found it challenging to get permissions from publishers to reproduce copyrighted material for the new figures that I have added to this book. When I wrote my previous editions, I could get permissions without too much trouble. Many figures were in journals published by societies. I could often get them for free from the society. Now many societies have turned over publication of their journals to commercial publishers. And getting permissions from commercial publishers to reproduce figures is costly. In addition, many book publishers have failed and been taken over by large publishing conglomerates. All permissions must be obtained electronically and trying to get permission to publish a figure in a book by a defunct publisher, whose books are now owned by a large publisher, is difficult. One large publisher denied me permission to reproduce a figure. The publisher gave no reason, but said it just was not giving permissions. I see this as a type of censorship. When a publisher sequesters information in books, it cannot be shared and knowledge cannot be advanced. I was always glad to find a figure or table in a publication of the U.S. government, because information in governmental publications cannot be copyrighted. It can be freely reproduced.

I have many people to thank who made this third edition possible. I thank Dr. Arthur W. Warrick, Professor Emeritus of Soil Physics at the University of Arizona, for help with some of the problems. I thank three graduate students in the Department of Agronomy at Kansas State University: Haidong Zhao, who put equations in the proper format; Lina Zhang, who put my drawings in Word format; and Gustavo Roa, who put tables copied from books in Word format. I also thank Jahvelle Rhone, Media Coordinator, and his student helpers at the Media Studio at Kansas State University for scanning the new figures in this book. The student helpers were Jacqué Jones, Emmett Lockridge, Thane Ray Meadows, and Wilhelm A.S. Wiedow. I also thank Arthur Selman, Computer Systems Specialist in Throckmorton Plant Sciences Center at Kansas State University, for help in scanning the figures.

As for the first and second editions, I dedicate the third edition to my family whose support made the book possible.

M.B. Kirkham

Manhattan, Kansas.

October 26, 2022.

Note added after reading the proofs: Due to computerized editing, many errors were introduced into the text that did not appear in the second edition, which was published before editing was turned over to computer software. I have done my best to remove these errors.

M.B. Kirkham

January 10, 2023

Chapter 1: Introduction

Abstract

This chapter tells why it is important to study soil-plant-water relations. Water is the most important substance necessary for food production. People depend upon plants for food, so the challenge of feeding a growing population is discussed. The human population growth curve, a logarithmic one, is presented. Rules of logarithms are then given. A calculation is done to show that the human population is limited by the productivity of the land. The calculation shows that it requires two square yards (16,700 cm²) to feed one person. The sigmoid plant growth curve is presented followed by a mathematical analysis of Blackman's compound interest law for plant growth. Finally, data from corn and soybean are analyzed to show that their growth rate is exponential. A biography of Napier, the inventor of logarithms, is given in the appendix.

Keywords

Blackman growth curve; John Napier; Logarithms; Plagues; Population curve; Sigmoid growth curve; Soil-plant-water relations

1.1. Why study soil-plant-water relations?

1.1.1. Population

Of the four soil physical factors that affect plant growth (mechanical impedance, water, aeration, and temperature) (Shaw, 1952; Kirkham, 1973), water is the most important. A classic analysis was done by Boyer (1982) to determine the reasons for crop losses over a four-decade period in the United States. He found that drought causes 40.8% of crop losses and excess water causes 16.4%. Insects and diseases amount to 7.2% of the losses. Thus, soils that are too dry or too wet are the major reasons for lost productivity.

People depend upon plants for food. Because water is the major environmental factor limiting plant growth, we need to study soil-plant-water relations to provide food for a growing population. What is our challenge?

The earth's population is growing exponentially. The universe is now considered to be 13,000,000,000 (13 thousand millions or 13 billion) years old (Zimmer, 2001). The earth is thought to be 4.45 billion years old (Allègre and Schneider, 1994). The earth's oldest rock is 4.03 billion years old (Zimmer, 2001). Primitive life existed on earth 3.7 billion years ago, according to scientists studying ancient rock formations harboring living cells (Simpson, 2003). Human-like animals have existed on earth only in the last few (less than 8) million years. In Chad, Central Africa, six hominid specimens, including a nearly complete cranium and fragmentary lower jaws, have been found that are 6–7 million years old (Brunet et al., 2002; Wood, 2002). The earliest fossil in Europe, which belongs to the genus Homo, was found in Spain and has been dated to 1.1–1.2 million years ago, which suggests that the first settlement of Western Europe was related to demographic expansion out of Africa (Carbonell et al., 2008).

In 8000 BCE, at the dawn of agriculture, the world's population was 5 million (Wilford, 1982). At the birth of Christ in 1 A.D., it was 200 million. In 1000, the population was 250 million (National Geographic, 1998) (Fig. 1.1). By 1300, it had grown larger (Wilford, 1982). But by 1400, the population had dropped dramatically due to the Black Death, also called the bubonic plague (McEvedy, 1988), which started in Asia and moved to Europe and Africa (Wright, 2020).

Figure 1.1  The human population growth curve. Drawn by author from data found in literature.

Three forms of the plague occur that vary depending upon location of the infection: pneumonic plague, an infection in the lungs; bubonic plague, an infection in the lymph nodes; and septicemic plague, an infection in the blood. All forms of the plague are caused by a bacterium, Yersinia pestis. It is spread by fleas on rats. The bubonic plague was called the Black Death because the lymph nodes that it infected became blackened and swollen after the bacteria entered through the skin. The swollen lymph nodes, which occur in the groin and armpits, are known as buboes (Wright, 2020). The Black Death raged in Europe between 1347 and 1351 and killed at least half of its population. It caused the depopulation or total disappearance of about 1000 villages. Starting in coastal areas, where rats were on ships, and spreading inland, it was the greatest disaster in western European history (Renouard, 1971). People fled to the country to avoid the rampant spread of the disease in cities. The great piece of literature, The Decameron, published in Italian in 1353 and written by Giovanni Boccaccio (1313–1375), tells of 10 people who in 1348 went to a castle outside of Florence, Italy, to escape the plague. To pass time, they each told a tale a day for 10 days (Bernardo, 1982). The Decameron means 10 days (from the Greek deka, meaning 10 and hēmera, meaning day"). Geoffrey Chaucer (1340–1400) in England knew of The Decameron and used several plots from it in his Canterbury Tales (1387–1400). During the Black Death, people not only isolated themselves to avoid it but they also used masks. The masks had a long, bird-like beak, which perhaps was a method to keep people apart in crowds (Victoria Kirkham, personal communication, December 22, 2021).

The bubonic plague came in three pandemics. The one that came from Asia and hit Italy in 1347 was the second pandemic. Subsidiary outbreaks of it continued to appear in Europe for 300 years. The first pandemic, known as the Plague of Justinian, lasted from the 6th century until the 8th. The third one, the Great Plague of London, arrived in 1665. The plague then mysteriously faded away (Wright, 2020).

By 1500, the world's population was about 250,000,000 again. In 1650, it was 470,000,000; in 1750, it was 694,000,000; and in 1850, it was 1,091,000,000. At the beginning of the nuclear age in 1945, it was 2.3 billion. In 1950, it was 2,501,000,000; in 1970, 3,677,837,000; and in 1980, 4,469,934,000. In 1985, it was 4.9 billion, and in 1987, it was 5.0 billion (New York Times, 1987). In 1999, the world's population reached 6 billion (National Geographic, 1999). On October 31, 2011, the United Nations Population Fund estimated that the seventh billion person was born in the world (McGurn, 2011).

Note that it took more than six million years for humans to reach the first billion; 120 years to reach the second billion; 32 years to reach the third billion; and 15 years to reach the fourth billion (New York Times, 1980). It took 12 years to add the fifth to sixth billion (1987–99), and 12 years to add the next billion (sixth to seventh billion; 1999–2011). The population is projected to reach 9.3 billion by 2050 (Chin et al., 2011).

However, the population may fall due to plagues (Weiss, 2002). The plague that devastated Europe in the 14th century was not unique. Current potential plagues may result from acquired immune deficiency syndrome (AIDS), caused by the human immunodeficiency virus (HIV), but now under control with medications; influenza, another viral infection—for example, there may be a recurrence of the 1918 pandemic (Gladwell, 1997); sudden acute respiratory syndrome (SARS), a deadly infectious disease caused by a coronavirus (Lemonick and Park, 2003); hospital infections, which cannot be treated with any known antibiotics; and mad-cow disease, which is formally called bovine spongiform encephalopathy (BSE). BSE is called Creutzfeldt–Jakob (also spelled as Creutzfeldt–Jacob) disease, when it occurs in humans (Hueston and Voss, 2000). It is thought to be caused by prions, which were discovered by Stanley Prusiner (born in 1942), director of the Institute for Neurodegenerative Diseases at the University of California, San Francisco (the discovery won Prusiner both the Wolf Prize (1996) and the Nobel Prize (1997) in medicine). Prions are a new class of protein, which, in an altered state, can be pathogenic and cause important neurodegenerative diseases by inducing changes in protein structure. Prions are designed to protect the brain from the oxidizing properties of chemicals activated by dangerous agents such as ultraviolet light. Humans get the disease by eating brains of infected cattle, and there was an outbreak in 2000. It has a long incubation period (11–12 years), is fatal, and there is no cure.

Other recent plagues are the Middle East respiratory syndrome (MERS), Ebola, and Zika. Middle East respiratory syndrome is caused by a coronavirus and was identified in Saudi Arabia in 2012, but it was contained. There is no vaccine for MERS. Ebola is caused by a group of viruses in the genus Ebolavirus, mainly located in sub-Saharan Africa. It was identified in 1976 and named after a river in Africa. An outbreak occurred in West Africa in 2014–16, but it was stopped by the development of the Ebola vaccine. Zika is caused by the Zika virus, which is a virus in the genus Flavivirus. It is transmitted to humans through the bite of infected mosquitos. Zika is passed from a pregnant woman to her fetus causing microcephaly (a small brain, which is a serious birth defect). The virus was first isolated in 1947 from a monkey in the Zika forest in Uganda. In 2016, there was an outbreak in South America, but since then it has been contained. There is no vaccine or medicine for Zika.

Polio is a plague of the past, which was eradicated in the United States by the development of a vaccine. Polio, or poliomyelitis, is caused by a poliovirus and is spread when the stool of an infected person is introduced into the mouth of another person through contaminated water or food. It is especially virulent during the summer. It is also called infantile paralysis because it mainly affects children. In the past, a child with the disease had to use an iron lung to breathe because of the loss of muscle control of the diaphragm. Jonas Salk (1914–95) developed the first polio vaccine while a professor at the School of Medicine at the University of Pittsburgh. Between 1948 and 1955, he worked on developing the vaccine, and on April 12, 1955, he announced that he had found one. Immunization began immediately. Parents were eager to get their children vaccinated, and children again could play freely in the summer without their parents fearing they would get polio. Since 1979, no cases of polio have originated in the United States. Vaccinations have eradicated it. However, polio still affects children in other countries. Smallpox is another example of a disease that has been eradicated by vaccines. It is an acute, infectious, viral disease characterized by fever, vomiting, and pustular eruptions that often leave pitted scars, or pockmarks, when healed. The last, naturally occurring case of smallpox was reported in 1977.

Now, we are in the midst of another pandemic caused by a coronavirus. It is called SARS-CoV-2, named COVID-19, by the World Health Organization. The acronym is derived from coronavirus disease 2019 because it was first identified in late 2019. As of March 2022, over one million people in the United States have died from COVID-19, and, worldwide, over six million people have died of it. Why was not the United States prepared to prevent the deaths from COVID-19? It had no better techniques for protection than those used during the Black Death in 1347–1351—i.e., social distancing, as was done by the 10 people in The Decameron, and the wearing of masks. No one heeded warnings of a pandemic. The January 15, 2018, issue of Time magazine (Gibbs, 2018) published an interview with Bill Gates (born 1955; software developer and philanthropist). He said, Of all the major bad things that could happen—a nuclear war, an asteroid, a gigantic earthquake—the one that is the most scary is a big epidemic, like the flu epidemic sweeping the world as it did in 1918. Vaccines for COVID-19 were developed in record time (less than a year), and they are effective in controlling the disease. Nevertheless, the US population grew at a slower rate in 2021 than in any other year on record due to COVID-19. The year 2021 was the first time since 1937 that the US population grew by fewer than 1 million people, reflecting the lowest numeric growth since at least 1900, when the Census Bureau began annual population estimates.

In 2020, due to the pandemic, most colleges and universities across the United States closed and moved to online-only instruction. Students were sent home. The University of Padua in Italy was closed in 1528 due to the bubonic plague. The closure was chronicled by the Paduan jurist, Marco Mantova Benavides (died 1582, aged 93) (Kirkham, 2011). Here is what he said, as translated from the Italian by Victoria Kirkham (information in brackets is her commentary): Now it befell in those calamitous and infelicitous times during which not only our region, as you know, but all of Italy universally was suffering both from the deathly pestilence and from the scarcity of food [there was famine] … the greater part of the students went back home [they came from countries all over Europe and, once at the university, established clubs of co-nationals; universities were organized by nations of students] … and the year that left memory of itself for many centuries that will come after us was the year of the salvation-giving incarnation of our lord, one thousand five hundred twenty eight. The year 2020 will leave a lasting memory for all students in the United States.

1.1.2. The two-square-yard rule

The population is limited by the productivity of the land. There is a space limitation that our population is up against. Many of us already have heard of this limitation, which is a space of two square yards per person. The sun's energy that falls on two square yards is the minimum required to provide enough energy for a human being's daily ration. Ultimately, our food and our life come from the sun's energy. The falling of the sun's energy on soil and plants is basic. We want to make as many plants grow on those two square yards per person as possible, to make sure we have enough to eat.

Let us do a simple calculation to determine how much food can be produced from two square yards, using the following steps:

1. Two square yards is 3 ft by 6 ft or 91 cm by 183 cm.

    91 × 183 cm = 16,653 cm² or, rounding, 16,700 cm².

2. The solar constant is 2.00 cal/cm²/min, or, because one langley = 1 cal/cm², it is 2.00 langleys/min. The langley is named after Samuel Pierpoint Langley (1834–1906), who was a US astronomer and physicist who studied the sun. He was a pioneer in aviation. Langley Air Force Base, located in Hampton, Virginia, is named after him and was established after the entry of the United States into World War I in April 1917. The solar constant is defined as the rate at which energy is received upon a unit surface, perpendicular to the sun's direction in free space at the earth's mean distance from the sun (latitude is not important) (Johnson, 1954). The brightness of the sun varies during the 11-year solar cycle but typically by less than 0.1% (Lockwood et al., 1992). However, the solar cycle or activity is important in determining climate change. Solar activity is usually associated with the variation (number and magnitude) of sunspots over an 11-year solar cycle. When sunspots are few, the climate cools (Kutílek and Nielsen, 2010, pp. 83–94).

3. 16,700 cm² × 2.00 cal/cm²/min = 33,400 cal/min.

4. 33,400 cal/min × 60 min/h × 12 h/day = 24,048,000 cal/day, or, rounding, 24,000,000 cal/day. We multiply by 12 h/day, because we assume that the sun shines 12 h/day. Of course, the length the sun shines each day depends on the day of the year, cloudiness, and location.

5. There is 6% conversion of absorbed solar energy into chemical energy in plants (Kok, 1967). This 6% is for the best crop yields achieved; 20% (Kok, 1967) to 30% (Kok, 1976) conversion is thought possible, but it has not been achieved; 2% is the conversion for normal yields; and under natural conditions, ≤1% is converted (Kok, 1976). The solar energy reaching the earth's surface that plants do not capture to support life is wasted as heat (Kok, 1976). Let us assume a 6% conversion:

    24,000,000 cal/day × 0.06 = 1,440,000 cal/day

6. The food calories we see listed in calorie charts are in kilocalories. So, dividing 1,440,000 cal/day by 1000, we get 1440 kcal/day, which is not very much.

We recognize that the earlier calculation of productivity from two square yards is simplified, and more complex and thorough calculations of productivity, which consider geographic location, sky conditions, leaf display, and other factors, have been carried out (e.g., De Wit, 1967). Nevertheless, the 1440 kcal/day is a useful number to know. It would be a starvation diet. One could live on it, but the calories probably would not provide enough for active physical work, creative intellectual activity, and reproduction. Women below a minimum weight cannot reproduce (Frisch, 1988). Civilization would advance slowly with this daily ration. People begin to die of starvation when they lose roughly a third of their normal body weight. When the loss reaches 40%, death is almost inevitable.

Triage is a system developed during World War I. It is the medical practice of dividing the wounded into survival categories to concentrate medical resources on those who could truly benefit from them and to ignore those who would die, even with treatment, or survive even without it. This practice has been advocated to allocate scarce food supplies. If we can grow more food, then this system does not need to be put into effect. Unfortunately, triage was being practiced in hospitals in the United States in 2020 and 2021 because they could not handle the huge number of COVID-19 patients. People died because there were not enough medical supplies and personnel. We still have enough land to grow food for our population, unlike during the plague year in Padua, Italy, in 1528, so population is not yet limited by the two-square-yard rule.

However, population still depends upon many factors, as follows: (1) Migration. The population of a country increases when people move into it or decreases when people move out of it. (2) Life span. The longer people live, the higher the population is. (3) Time of first birth. The longer a couple waits for the first birth, the smaller the population. (4) Education of women. Women who are educated often delay starting families, thereby reducing the rate of population growth. (5) Social security. Social security has reduced the need for parents to have many children to support them in old age. (6) Plagues and malnutrition. We have discussed how plagues reduce population.

In this book, we seek a better understanding of movement of water through the soil-plant-atmosphere continuum, or SPAC (Philip, 1966), because of the prime importance of water in plant growth. We focus on principles rather than review the literature. Many references are given, but no attempt is made to cite the most recent papers. Articles explaining the principles are cited. They often are in the older literature, but we need to know them to learn the principles. No knowledge of calculus is required to understand the equations presented.

In the book, we divide the movement of water through the SPAC into three parts: (1) water movement in the soil and to the plant root; (2) water movement through the plant, from the root to the stem to the leaf; and (3) water movement from the plant into the atmosphere. We then combine all parts when we look at electrical analogues for water movement through the soil-plant-atmosphere continuum (Chapter 22). However, before we turn to principles of water movement in the SPAC, let us first consider plant growth curves.

1.2. Plant growth curves

1.2.1. The importance of measuring plant growth and exponential growth

The world population growth curve (Fig. 1.1) is an exponential curve. An exponent is a small symbol placed above and to the right of another symbol to show how many times the latter is to be multiplied by itself, for example, b ³ = b × b × b. What do plant growth curves look like? Because water is the most important soil physical factor affecting plant growth, it is important to quantify plant growth to determine effects of water stress. In any experiment dealing with plant–water relations, some measure of plant growth (e.g., height, leaf area, biomass) should be obtained. Plant growth curves also exemplify quantitative relationships that we seek to understand basic principles of plant–water relations. If we can develop equations to show relationships, then we can predict what is going to happen. Equations describing plant–growth curves demonstrate how we can quantify, and thus predict, plant growth.

We first consider the growth of the bacterium Escherichia coli. In the early 19th century, when plants and animals were being classified, the bacteria were arbitrarily included in the plant kingdom, and botanists first studied them (Stanier et al., 1963, pp. 55–56). Even though bacteria are not plants or animals, we can follow their growth to understand plant growth curves.

Under ideal conditions, a cell of E. coli divides into two cells approximately every 20 min; for the sake of simplicity, we assume that it is exactly 20 min. Let us consider the propagation of a single cell. Our purpose is to find a relation between the number N of cells at some moment in the future and the time t that has elapsed. At the start of our observations, at the time 0 min, there is one cell. When 20 min have elapsed, there are two cells. When 40 min have elapsed, there are 2 × 2 = 2² cells. When 60 min have elapsed there are 2 × 2² = 2³ cells, that is, when three time intervals of 20 min each have passed, there are 2³ cells. We observe a pattern developing: when m time intervals each of 20 min have passed, at the time t = 20 m min, there are 2 m = 2 t/20 cells. Thus, if N denotes the number of cells present at the moment when t minutes have elapsed, then the relation we seek is given by the equation

(1.1)

Because the time t appears in the exponent of the expression 2 t/20, this equation is said to describe exponential growth of the number N of cells (De Sapio, 1978, pp. 21–23).

A famous book called On Growth and Form by D'Arcy Wentworth Thompson contains the following statement (Thompson, 1959; vol. 1, p. 144): Linnaeus shewed that an annual plant would have a million offspring in 20 years, if only two seeds grew up to maturity in a year. Linnaeus is, of course, Carolus Linnaeus (born Karl von Linné) (1707–78), the great Swedish botanist. We can show that what Linnaeus said is true by adapting the preceding equation, as follows:

(1.2)

where X is the number of offspring from the plant in 20 years.

To solve this equation, we will use logarithms. John Napier (1550–1617), a distinguished Scottish mathematician, was the inventor of logarithms. (See the Appendix, Section 1.4, for his biography). This equation can be solved on computers without knowing logarithms. But it is important to know the rules of logarithms, to know how it is solved. A logarithm is defined as follows: In mathematics, the exponent of the power to which a fixed number (the base) must be raised in order to produce a given number (the antilogarithm). The logarithm of a positive number N to a given base b (written as log b  N) is the exponent of the power to which b must be raised to produce N (Ayres, 1958, p. 83). For example, since 1000 = 10³, log10 1000 = 3. Since 0.01 = 10 −², log10 0.01 = −2. Since 10 = 10¹, log10 10 = 1. Since 9 = 3², log3 9 = 2. Since 1 = 10°, log10 1 = 0. Anything raised to the 0 power is 1. Here are the fundamental laws of logarithms (Ayres, 1958, p. 83):

1. The logarithm of the product of two or more positive numbers is equal to the sum of the logarithms of the several numbers. For example,

(1.3)

2. The logarithm of the quotient of two positive numbers is equal to the logarithm of the dividend minus the logarithm of the divisor. For example,

(1.4)

3. The logarithm of a power of a positive number is equal to the logarithm of the number, multiplied by the exponent of the power. For example,

(1.5)

4. The logarithm of a root of a positive number is equal to the logarithm of the number, divided by the index of the root. For example,

(1.6)

    In calculus, the most useful system of logarithms is the natural system in which the base is a certain irrational number e = 2.71828, approximately (Ayres, 1958, p. 86). The natural logarithm of N, ln N, and the common logarithm of N, log N, are related by the formula

(1.7)

    To solve our equation, we take the logarithm of each side:

    Using logarithm Rule No. 3, we get

    Solving (and reading out all the digits on our hand calculator):

    Linnaeus was right.

1.2.2. Sigmoid growth curve

The S-shaped, or sigmoid, curve is typical of the growth pattern of individual organs, or a whole plant, and of populations of plants (Fig. 1.2). It can be shown to consist of at least five distinct phases: (1) an initial lag period during which internal changes occur that are preparatory to growth; (2) a phase of ever-increasing rate of growth; (because the logarithm of growth rate, when plotted against time, gives a straight line during this period, this phase is frequently referred to as the log period of growth or the grand period of growth); (3) a phase in which growth rate gradually diminishes; and (4) a point at which the organism reaches maturity and growth ceases. If the curve is prolonged further, a time will arrive when (5) senescence and death of the organism set in, giving rise to another component of the growth curve (Mitchell, 1970, p. 95).

1.2.3. Blackman growth curve

Since about 1900, people have used growth curves to analyze growth. Significant relationships of a mathematical nature, however, are difficult to apply to such a complex thing as growth (Hammond and Kirkham, 1949). One well-known theory of plant growth is the compound interest law of Blackman (1919). He related plant growth to money in a bank. When money accumulates at compound interest, the final amount reached depends on:

1. The capital originally used;

2. The rate of interest; and

3. The time during which the money accumulates.

Comparing these factors to plants

1 = the weight of the seed;

2 = the rate at which the seed material is used to produce new material;

3 = the time during which the plant increases in weight.

Blackman related the three factors into one exponential equation,

(1.8)

where

Figure 1.2  Five phases in the sigmoid growth curve. From Mitchell (1970, p. 95). Reprinted by permission of Roger L. Mitchell.

W1 = the final weight

W0 = the initial weight

r = the rate of interest

t = time

e = the base of natural logarithms (2.718 …).

The Blackman equation works best for early phases of growth (the log phase of growth in the sigmoid growth curve). In the later growth stages, the decreasing relative growth rate has appeared to make impossible the application of this theory to the entire growth curve. Blackman attempted to do this, nevertheless, by using the average of all the different relative growth rate values as the r term in Eq. (1.8). He called this term the efficiency index of plant growth.

Hammond and Kirkham found that the growth curves (dry weight versus time) of soybeans [Glycine max (L.) Merr.] and corn (Zea mays L.) were characterized by a series of exponential segments, which were related to the growth stages of the plants. The exponential equation for all segments had the form:

(1.9)

where

w = weight of the plant at time t

wo = weight of the plant at an arbitrary time to

r = relative growth rate

e = base of natural logarithms (2.718 …).

Taking the natural logarithm of each side, we get

Converting to common logarithms by dividing each term by 2.303, we get

Now let

We get y = a + bx, which is the equation of a straight line.

The differential form of Eq. (1.9), is

(1.10)

where r, the relative growth rate, is the increase in weight per unit weight per unit time. It is obtained by multiplying the slope, b, of the line by 2.303. We shall not be using differential equations in this book. In mathematics, a differential is an infinitesimal difference between two consecutive values of a variable quantity, e.g., weight or time in Eq. (1.10). Gerard J. Kluitenberg, theoretical soil physicist in the Department of Agronomy at Kansas State University, showed how Eq. (1.10) is derived from Eq. (1.9). The document is entitled Solution of the Ordinary Differential Equation for Exponential Growth. An interested reader can contact the author for this solution.

Hammond and Kirkham (1949) plotted the common logarithm of dry weight versus time and found that soybeans have three growth stages, I, II, and III. The analysis showed that the plants produce dry matter at the greatest relative rate during period I; at a smaller rate during period II; and at a still smaller rate during period III. That is, the slopes declined with age (slope = r/2.303). They saw that the dates of change in the growth curves from period I to period II were also the dates when the plants began to bloom. The dates of the second change in the growth curve, or the change from period II to period III, were the dates when the plants reached maximum height. The soybeans grew on two different soils, Clarion loam and Webster silt loam. The soybean plants in the Clarion soil bloomed and reached maximum height about a week earlier than the soybeans on the Webster silt loam soil. The growth curves clearly showed this difference (Fig. 1.3). Growth curves, therefore, can be used to see the effect of the soil environment on plant growth. Hammond and Kirkham (1949) did not give a reason for the difference in rate of growth on the Webster and Clarion soils, but it probably was related to one of the four soil physical factors that affect plant growth: water, temperature, aeration, or mechanical impedance.

For corn, they found four periods of growth (Fig. 1.4). The additional period in corn apparently was related to the difference in time of appearance of male and female flowers in corn. The physiological changes associated with the breaks in the curves were connected with tasseling, silking, and cessation of vegetative growth. The last break occurred after the corn plants had reached maximum height. In sum, the data for soybeans and corn showed that a quantitative analysis of the complete growth curve can be accomplished if the overall growth is partitioned into segments based on the growth stages of the plants.

The equations for plant growth show that we can develop significant mathematical relationships for a quantitative analysis of plant growth. This is probably because plant growth is governed by basic chemical and physical laws. From these relationships, we can predict plant growth.

Figure 1.3  Logarithmic dry matter accumulation curves of soybeans grown in the field in Iowa on Webster and Clarion soils. From Hammond and Kirkham (1949). American Society of Agronomy, Madison, Wisconsin. Reprinted by permission of the American Society of Agronomy.

1.3. Problem

The problem comes from Putka (1988) but updated to the current time.

Each plant of a certain variety yields 50 seeds in the autumn and then dies. Only 40% of the seeds produce plants the following summer, and the other seeds never produce plants. At this rate, a single plant yielding 50 seeds in the autumn of 2018 will produce how many plants as descendants 3 years later (2021)?

Answer.

Use Eq. (1.1) in the following format:

N = (0.40 × 50)t/y

where

N = number of individuals

t = years

y = generation time

t = 3

y = 1

N = (0.40 × 50)³/¹ = 20³ = 8000 plants

Figure 1.4  Logarithmic dry matter accumulation curves of Iowa 939 corn in 1938 and 1939. From Hammond and Kirkham (1949). American Society of Agronomy, Madison, Wisconsin. Reprinted by permission of the American Society of Agronomy.

The problem also can be answered using words, which Putka (1988) did, as follows: The 50 seeds of 2018 yield 20 plants in 2019 (50 plants × 0.40 = 20 plants). They produce (20 × 50 = ) 1000 seeds, which yield 400 plants in 2020 (1000 plants × 0.40 = 400 plants). They produce (400 × 50 = ) 20,000 seeds, which yield 8000 plants in 2021 (20,000 plants × 0.40 = 8000 plants).

1.4. Appendix: biography of John Napier

John Napier (1550–1617), a distinguished Scottish mathematician, was the inventor of logarithms. The son of Scottish nobility, Napier's life was spent amid bitter religious dissensions. He was a passionate Protestant. His great work, A Plaine Disco u ery of the Whole Re u elation of Saint John (1594), has a prominent place in Scottish ecclesiastical history as the earliest Scottish work on the interpretation of the scriptures. He then occupied himself by inventing instruments of war, including two kinds of burning mirrors, a piece of artillery, and a metal chariot from which a shot could be discharged through small holes. Napier devoted most of his leisure to the study of mathematics, particularly to developing methods of facilitating computation. His name is associated with his greatest method, logarithms. His contributions to this mathematical invention are contained in two treatises: Mirifici logarithmorum canonis descriptio (1614; translated into English in 1857) and Mirifici logarithmorum canonis constructio, which was published 2 years after his death (1619) and translated into English in 1889. Although Napier's invention of logarithms overshadows all his other mathematical work, he has other mathematical contributions to his credit. In 1617, he published his Rabdologiae, seu numerationis per virgulas libri duo (English translation, 1667). In this work, he describes ingenious methods of performing the fundamental operations of multiplication and division with small rods (Napier's bones). He also made important contributions to spherical trigonometry (Scott, 1971).

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Chapter 2: Definitions of physical units and the international system

Abstract

This chapter defines seven different physical concepts: force, weight, work, energy, power, pressure, and heat. Units for each are given. The definitions and their units form the foundation for understanding the principles of soil and plant water relations, and they are used throughout the remainder of the book. In defining force, Newton's laws of motion are given. The three systems for presenting units are defined: the mks (meter-kilogram-second) system; the cgs (centimeter-gram-second) system; and the English system. The international system (SI) of units is reviewed. Notes about presenting appropriate units and making graphs are given. The chapter ends with an example in which the units of work and pressure are applied to a plant root. A biography of Newton is given in the appendix.

Keywords

Energy; Force; Heat; Isaac Newton; Power; Pressure; SI units; Weight; Work

2.1. Definitions

In soil-plant-water relations, we will be using units based on physical definitions. Therefore, we need to review the definitions. We will define the following: force, weight, work, energy, power, pressure, and heat. The definitions come from Schaum (1961), but they can be found in any physics textbook.

2.1.1. Force

Force is a push or pull exerted on a body. If an unbalanced force acts on a body, the body accelerates in the direction of the force. Conversely, if a body is accelerating, there must be an unbalanced force acting on it in the direction of the acceleration. The unbalanced force acting on a body is proportional to the product of the mass and of the acceleration produced by the unbalanced force.

2.1.1.1. Newton's laws of motion

For completeness, we now review these three laws, even though the second law is the one we are interested in for the definition of force (See the Appendix, Section 2.5, for a biography of Newton).

1. A body will maintain its state of rest or of uniform motion (at constant speed) along a straight line unless compelled by some unbalanced force to change that state. In other words, a body accelerates only if an unbalanced force acts on it.

2. An unbalanced force F acting on a body produces in it an acceleration a, which is in the direction of the force and directly proportional to the force, and inversely proportional to the mass m of the body. In mathematical terms, this law states that ka = F/m or F = kma, where k is a proportionality constant. If suitable units are chosen so that k = 1, then F = ma.

3. To every action, or force, there is an equal and opposite reaction or force. In other words, if a body exerts a force on a second body, then the second body exerts a numerically equal and oppositely directed force on the first body. These two forces, although equal and oppositely directed, do not balance each other because both are not exerted on the same body.

2.1.1.2. Units of force

In equation F = ma, it is desirable to make k = 1; that is, to have units of mass, acceleration, and force such that F = ma. To do this, we specify two fundamental units and derive the third unit from these two.

1. In the meter-kilogram-second or mks absolute system, the fundamental mass unit chosen is the kilogram and the acceleration unit is m/s². The corresponding derived force unit, called the newton (nt or N), is the unbalanced force that will produce an acceleration of 1 m/s² in a mass of 1 kg.

2. In the centimeter-gram-second or cgs absolute system, the fundamental mass unit is the gram, and the acceleration unit is cm/s². The corresponding derived force unit, called the dyne, is that unbalanced force that will produce an acceleration of 1 cm/s² in a mass of 1 g.

3. In the English gravitational system, the fundamental force unit is the pound and the acceleration unit is the ft/s². The corresponding derived mass unit, called the slug, is the mass that when acted on by a 1 lb force acquires an acceleration of 1 ft/s².

Thus, the following indicate three consistent sets of units that may be used with equation F = ma (F = kma with k = 1):

• mks system:F (newtons) = m (kilograms) × a (m/s²)

• cgs system:F (dynes) = m (grams) × a (cm/s²)

• English system:F (pounds) = m (slugs) × a (ft/s²)

2.1.2. Mass and weight

The mass m of a body refers to its inertia, and the weight w of a body is the pull or force due to gravity acting on the body, which varies with location (Inertia is the tendency of matter to remain at rest if at rest, or, if moving, to keep moving in the same direction, unless affected by some outside force). Weight w is a force with a direction approximately toward the center of the earth.

If a body of mass m is allowed to fall freely, the resultant force acting on it is its weight, w, and its acceleration is that due to gravity, g. Then, in any consistent system of units, equation F = ma becomes

Thus

w (newtons) = m (kilograms) × g (m/s²)

w (dynes) = m (grams) × g (cm/s²)

w (pounds) = m (slugs) × g (ft/s²).

It follows that m = w/g. For example, if a body weighs 64 lb at a place where g = 32 ft/s², its mass is m = w/g = 64 lb/(32 ft/s²) = 2 slugs. If a body weighs 49 N at a place where g = 9.8 m/s², its mass m = w/g = 49 N/(9.8 m/s²) = 5 kg.

Problem: How many grams are there in one slug?

We remember that one pound = 454 g. We abbreviate pound as lb and gram as g.

One slug of mass = 32 lb of mass = 32 × 454 g of mass = 14,528 g of mass. The conversion was made by Don Kirkham (personal communication, February 15, 1994).

2.1.3. Work

A force does work on a body when it acts against a resisting force to produce motion in the body. Consider that a constant external force F acts on a body at an angle θ with the direction of motion and causes it to be displaced a distance d (Fig. 2.1). Then, the work W done by the force F on the body is the product of the displacement d and the component of F in the direction of d. Thus

If d and F are in the same direction, cos θ = cos 0° = 1 and W = Fd.

2.1.3.1. Units of work

Any unit of work equals a unit of force × a unit of length.

• One foot-pound (ft-lb) of work is done when a constant force of 1 lb moves a body a distance of 1 ft in the direction of the force.

• One newton-meter (N-m), called 1 J (J), is the work done when a constant force of 1 N moves a body a distance of 1 m in the direction of the force. Because 1 N = 0.2248 lb and 1 m = 3.281 ft,

Figure 2.1  Illustration for definition of work. Adapted from Schaum (1961, p. 49). This material is reproduced with permission of The McGraw-Hill Companies.

One dyne-cm, called 1 erg, is the work done when a constant force of 1 dyne moves a body a distance of 1 cm in the direction of the force. Since 1 N = 10⁵ dyn and 1 m = 10² cm,

2.1.4. Energy

The energy of a body is its ability to do work. Because the energy of a body is measured in terms of the work it can do, it has the same units as work.

The potential energy (PE) of a body is its ability to do work because of its position or state. The potential energy of a mass m lifted a vertical distance h, where g is the acceleration due to gravity, is

In the mks system: PE (joules) = m (kg) × g (m/s²) × h (m). In the cgs system: PE (ergs) = m (grams) × g (cm/s²) × h (cm).

Because mg = w, we may also write: PE = mgh = wh.

2.1.5. Power

Power is the time rate