HOW DO plants eat? I am pretty sure this is an age-old question. It probably came up 10,000 years ago after some early gardener noticed that rotting fish did wonders for plants. The observation that one’s urine had a beneficial impact on plants could not be missed, either. These and other natural fertilizers not mentioned in public helped trigger the Neolithic Revolution, the transition from hunter-gatherer to farmer-gardener. Even in ancient times, feeding an ever-growing population required horticultural advances. The Aztec and Mayan civilizations, for example, were all about growing food to support burgeoning populations. They offered their gods sacrificial blood to ensure a good harvest. Perhaps this practice arose from their observation that soil bloodied from butchering an animal or as a result of some mortal blow during a heated battle grew better plants.
I come from a long line of natural fertilizer users. My grandfather and dad taught me to bury the uneatable, bony fish we used to catch every summer under roses and tomatoes. We had a horse for a while, too, and chickens, geese, ducks, and rabbits. We knew about the wonders of manure. I won’t go into my use of urine as a fertilizer, but with three boys growing up on eight acres, you can bet it was applied to all manner of plants liberally, with varying impacts on the plants. Today, gardeners use homemade and commercial fertilizers, composts, and mulches. Many simply follow the directions with little or no thought about what those powders and liquids really do. We’re just glad they do it.
After more than 50 years of gardening, I realized that I didn’t know much about fertilizers other than what I picked up from my family and my own observation over the years. When I started to ask my gardening friends what they could tell me about fertilizers, I discovered a startling fact: I couldn’t find one gardener who could tell me how they work. It seems that today’s gardeners are just as clueless about how fertilizers work as were our early ancestors. We rely on the same principle, observation, which, sad to say, includes advertising.
Still, how plants eat has been the subject of discussion probably ever since the early days. Ancient Greeks, for example, engaged in arguments about what should be used as fertilizers, and these sound like modern organic versus inorganic arguments. The seventeenth-century scientist Jean Baptiste van Helmont proved that plants actually didn’t need fertilizers, only water. A century later, the Englishman Jethro Tull and others promoted the notion that the carbon in plants came from organic particles in the soil that only entered through the roots. It is from here that Tull came up with the idea of pulverizing soil particles to make them more edible to the roots, and the practice of rototilling was born.
In the mid-1800s, the German chemist Justus Von Liebig showed that plants actually get carbon from the carbon dioxide in the atmosphere. He did so, incidentally, by demonstrating that after plants die and decay, they leave soil richer in carbon. He also examined the ashes of burned plants and saw they contained minerals that obviously didn’t come from organic humus or soil particles. From these studies came Von Liebig’s Mineral Theory of Plant Nutrition, which states that plants need certain minerals and that these minerals can be put back into the soil in inorganic forms to enable plants to grow in soils depleted of these minerals. This was pretty heady stuff in its day—and it was controversial.
Von Liebig experimented in a field in the English countryside from 1845 to 1849, growing crops using artificial manures, the first synthetic fertilizers. He made one mistake, however. Von Liebig thought plants got their nitrogen from the atmosphere, so he didn’t add it to his soils. His plants didn’t do well. Quickly proven wrong on where nitrogen came from, he and others prepared inorganic fertilizer formulations that contained nitrogen and worked well. In fact, they worked as well as manures. After only a few years of testing, the results were so impressive that they caused Von Liebig to predict, “A time will come, when fields will be manured with a solution of glass (silicate of potash), with the ashes of burnt straw, and with the salts of phosphoric acid, prepared in chemical manufactories, exactly as at present medicines are given for fever and goitre.”
Von Liebig’s experiments and the efforts of competing contemporaries led to the birth and growth of the chemical fertilizer industry. The first fertilizers of this kind were sold in 1845, and thus began a huge shift in how society lived. For the first time in history, farmers did not have to be dependent on animals for manures. They could spread inorganic chemicals on their fields instead, chemicals that could be made in factories or mined from the earth.
The importance of this change can’t be missed by anyone who has had just a dog or cat. Animals require costly feeding and lots of time-consuming care. They take up valuable land, and consume a lot of nutrients. The invention of artificial manures meant that one could farm without having any animals (though I would hope the dog, at least, could stay).
Before the 1850s, when a farmer exhausted his land he either had to resort to using expensive, animal-based manures or, as American farmers did, find fresh land. The American West was settled in part because early American farmers didn’t have enough animals to produce enough manure to replenish their soils. The American slogan that expansion of the country was “manifest destiny” had more to do with farming than we’re taught in school. When the nutrients in the soil ran out, it was cheaper and easier to just move west and start over.
Few farmers really cared, much less understood, why these inorganic chemicals worked, just as long as they did so. What else was there to know? However, the new science of agriculture was burgeoning. Justus Von Liebig, considered by most as the father of modern agricultural science, again contributed to the field in 1863 when he proposed the Law of the Minimum. It states, “A manure containing several ingredients acts in this wise: The effect of all of them in the soil accommodates itself to that one among them which, in comparison to the wants of the plant, is present in the smallest quantity.” In other words, plant growth is limited by the least abundant mineral, no matter how abundant the other minerals happen to be. This idea affects how fertilizers are used even today. (Incidentally, Von Liebig may have appropriated the Law of the Minimum from Carl Sprinkle, who at least deserves mention here.)
The application of the Law of the Minimum revealed that the elements nitrogen (N), phosphorus (P), and potassium (K) were used by plants in the greatest quantities and were the nutrients plants most needed if they were to thrive in depleted soils. This resulted in the era of measuring the amount of nitrogen, phosphorus, and potassium (and other essential plant nutrient elements) and amending deficiencies with inorganic fertilizers. Today, any fertilizer sold around the world carries three numbers on the package representing the percentage by weight of each of those elements: the N–P–K ratio.
Consequently, many gardeners think they only need to look for these three numbers to know which fertilizer to use. In fact, fertilizer manufacturers are making it even easier. These days all a gardener needs to do is look at the representative pictures on fertilizer packages. Spot the tomatoes, annuals, perennials, houseplants, or orchids you want to feed and buy that package. Of course, there is all manner of advertising dollars spent to convince you that you need to know even less: just buy a particular brand, and your plants will do fine.
Fortunately, like Neanderthals, those early Neolithic gardeners, most of us are able to make a bit of sense of things as a result of our own observations. Gardeners today know that if you put lots of nitrogen on the lawn, it grows like crazy and has to be mowed more often. So nitrogen has something to do with leaves and growth. Yellow leaves with green veins turn back to green leaves with green veins when iron is added to the soil. So iron must have something to do with the plant being green.
Von Liebig’s Law of the Minimum states that if one nutrient doesn’t reach the minimum required by a plant, then it doesn’t matter how much more of the others you apply. In short, the barrel will only hold up to its lowest stave.
Still, many gardeners simply believe in the power of fertilizers because they are exposed to advertising. Phosphate, we have been told, helps grow healthy roots. Unless you dig up roots, who can really tell? But I am a believer. Boron is needed for pollination. O.K. And calcium or magnesium has something to do with preventing spots on tomatoes.
Although gardening is all about growing plants, many gardeners don’t understand how nutrients actually get inside plants so they can grow in the first place, or how nutrients contribute to growth (and better flowers, tastier vegetables, healthier trees) once inside. By understanding how fertilizers work, how they get into plants, and what they do thereafter, you won’t have to rely on someone else who is only guessing what your plants need. You will know something about how fertilizers work and whether what you are paying for is worth it. Information is power.
Knowing how fertilizers work should also make you a more sustainable gardener and help the planet. Take nitrogen, for example. The production of nitrogen-based synthetic fertilizers is a very energy intensive manufacturing process. More than 5 percent of the world’s natural gas production is used to make these fertilizers. Less than 100 years ago, all of the plant available nitrogen in the world, except for a tiny bit fixed during electrical storms, was produced by microbes. All of the nitrogen in your body was naturally produced, whereas today half of your nitrogen comes from synthetic sources. In addition, the excessive use of nitrogen fertilizers causes severe pollution. Gardeners use three times the nitrogen per acre that farmers use. This results in excess nutrients that are washed into waterways and harm aquatic ecosystems.
Using plant-essential nutrients the proper way will help head off some serious environmental disasters already in the making. Manufacturing nitrogen so that gardeners can waste it in prodigious amounts is not our only problem. The world is about to reach the peak of its ability to produce phosphorus. The precipitous decline in availability of this key nutrient has led some to predict that there is less than 50 years’ worth remaining. Gardeners had better use sustainable practices, or we will really test the Law of the Minimum with regard to essential plant nutrients. This is important stuff.
Researchers are studying the uptake and use of nutrients by plants because it influences many important technologies that affect us all. The genetic modification of plants and the creation of organic and inorganic pesticides, herbicides, and biocides all have to do with how plants eat. For example, many herbicides work by interfering with a plant’s nutrient system, preventing some nutrient from getting into the plant and causing it to die. Many genetically modified plants are engineered to resist glyphosate (available to gardeners as Roundup). As more genetically modified crops are being grown, an increasing amount of glyphosate is making its way into the natural environment. If you don’t understand how the plant nutrient system works on a chemical level, you can’t understand the current GMO-glyphosate debate. Once you understand how plants eat, you’ll realize where the debate is going to end, and you can make some decisions about whether to use glyphosate in your yard.
Plants are what they eat. If you want the best possible plant, you need to feed it properly. It helps to know what each essential element really does and the best sources for them. Along the way, I hope you become infected by the knowledge of the wondrous universe inside plants: cells, organelles, secret pathways, manufacturing on a scale of unimaginable proportions—life at its most basic, simplest form. Seeing the beauty in this system will surely enhance what you already feel about plants and gardening. I found myself comparing cellular systems to human systems to planetary systems and ended up confronting the meaning of life, all while zeroing in on the plant cell, its structure, and wondering (in the truest sense of that word) about the amazing workings. Plant cells build everything that plants are, which results in much of what we are. It’s truly awesome.
While this is a book about how plants eat and includes some suggestions about what kinds of fertilizers to use on plants, let me assure my loyal readers that I am not abandoning the principles of the soil food web outlined in my previous book, Teaming with Microbes: The Organic Gardener’s Guide to the Soil Food Web. I now have a better understanding of the importance of mycorrhizal and actinorhizal relationships with plants. It is important to establish and maintain these and other soil organism so that they, in turn, can feed your plants. The practices recommended in this book are organic and are not chemically oriented. All of the nutrients plants require can be easily supplied without having to resort to any of Von Liebig’s artificial manures.
Von Liebig eventually recognized the negative impacts of artificial fertilizers on life in the soil of his own vegetable garden. Thereafter, he preferred the use of organic matter to inorganic fertilizers. In fact, he spent a good part his later life arguing that the Britons should use their sewage as fertilizer, this despite having created the inorganic fertilizer industry.
Describing the process of how plants take in and use nutrients necessarily involves chemistry and biology. Don’t worry. Each chapter builds on the previous one, so by the time you get to the punch line, you will have the science under your belt and get it. Let’s take some of the mystery out of it right now with a short summary.
In chapter 1, I discuss the various parts of a typical plant cell, because this is where the action takes place. The outer cell wall and the plasma membrane up against it act as barriers and regulators of what can enter and leave a cell. Special membrane proteins assist water and nutrients in entering the cell, while keeping unwanted materials out. The cytoplasm holds structures and organelles that perform special jobs related to taking up and using fertilizers. They provide power to the cell and serve as sites for photosynthesis. The nucleus is the command center where the DNA is housed. Cells have transportation and communication infrastructure, protein construction areas, and even tunnels that connect every single cell in a plant.
Chapter 2 covers the necessary basic chemistry needed to understand the journey of nutrients. You don’t have to remember anything from school. I discuss atoms, electrons, and chemical bonds. (Finally, we have a reason to know about covalent, ionic, and hydrogen bonds, which affect the qualities and availability of the various nutrients.) This chemistry results in the four types of molecules that are necessary for life: carbohydrates, proteins, lipids, and nucleic acids. I also describe ATP (adenosine triphosphate), the energy currency in all cells, and enzymes that speed up the millions of chemical reactions that occur within plant cells.
In chapter 3, I discuss the botany that affects nutrient uptake and utilization. Four kinds of plant tissues and their organization into special organs (leaves, stems, and roots) have roles in the uptake of nutrients. Some play surprising and unexpected roles, including aiding in the formation of symbiotic relationships and other biological partnerships important to nutrient uptake.
The seventeen elements essential for the lives of plants are covered in chapter 4, including the macronutrients (nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur) needed by plants in the greatest quantities and the micronutrients (boron, chlorine, copper, iron, manganese, zinc, molybdenum, and nickel), which are needed in only trace amounts. The other three essential nutrients, carbon, hydrogen, and oxygen, are also covered, as are the different degrees of mobility of the various nutrients and its implications.
Water plays the starring role in this story. In chapter 5, the chemistry and botany described in the previous chapters are used to help explain how water moves through the soil to get to and then into the roots of a plant. Once water is inside a plant, there are different ways it can to get to the xylem, where it is carried up to the stems and leaves along with the nutrients dissolved in it. Water is later moved throughout the plant in the phloem, along with the sugars, proteins, enzymes, and hormones produced in plant cells. I describe the interplay between the two tissues in the vascular system when it comes to plants taking up and distributing nutrients.
Chapter 6 covers the movement of nutrients into and then inside a plant, starting with their movement in the soil around plant roots. Once inside the plant, nutrients must be transported across cell membranes so they can be used to make all the compounds the plant needs for growth and maintenance.
In chapter 7, I explain the role of the essential nutrients in the makeup of the four molecules of life. Carbohydrates are produced via photosynthesis. Proteins are constructed from various combinations of the twenty amino acids. Lipids are made up of fatty acids and glycerol. Finally, nucleic acids are the molecules DNA and RNA that carry the genetic code.
In chapter 8, the book gets down to actual gardening and applying some of the knowledge gained in the previous chapters to make our art more of a science. I discuss whether you even need to fertilize, and, if you do, what steps you should take. The use of fertilizers should be based on sound knowledge, which can only be obtained by having your soil tested.
In chapter 9, I discuss the other factors that influence nutrient uptake and the use of nutrients: temperature, soil microbes, moisture, soil compaction, and the chemical reactions that occur within plants and in soil. Many gardening practices come into focus when the science behind them becomes clearer.
Finally, in chapter 10 I offer recommendations of what to feed plants based upon knowledge of how plants take up nutrients and how they use them. I provide fertilizer recipes designed for annuals, vegetables, and lawns and describe the best ways to apply fertilizers, including the timing of application, and other characteristics of commonly obtainable natural fertilizers.
I am the first to admit that to understand how plants take up nutrients requires more botany, chemistry, and cellular biology than the average (and probably above-average) gardener knows. This doesn’t mean you have to learn college-level science before getting it. Also, we are gardeners, not chemists, and it is my book. So, in my book I avoid chemical equations like the plague. I don’t need to explain everything. That’s what the Internet is for. I supply enough information so you can get to the end of the story without being overwhelmed—or at least to know where to start using the Internet.
And for goodness sake, don’t read this book like it’s a textbook. There is a lot here, sure, but if you just read it and don’t get hung up on memorization, you’ll find that pretty soon the lingo flows and the understanding increases with each chapter. It helps to look at the illustrations, which were drawn especially to depict things in three dimensions. Everything builds on what came before, and all will be clear. Just relax and read for fun.
So, let’s go. Hopefully, you will never again take for granted what happens to make your plants grow.