A fertile and healthy soil is the basis for healthy plants, animals, and humans. Soil organic matter is the very foundation for healthy and productive soils. Understanding the role of organic matter in maintaining healthy soil is essential for developing ecologically sound agricultural practices. But how can organic matter, which only makes up a small percentage of most soils, be so important that we devote so much of the book to discussing it? The reason is that organic matter positively influences or modifies the effect of, essentially all soil properties, and it is what makes the soil fertile. That is the reason it’s so important to our understanding of soil health and how to manage soils better. Organic matter is essentially the heart of the story, but, as we will discuss later, certainly not the only part. In addition to functioning in a large number of key roles that promote soil processes and crop growth, soil organic matter is a critical part of several global and regional cycles.
In this section of the book, we give you a lot of terms and concepts. It is probably the first time you are learning this vocabulary and coming across these soil principles. We will fully explain more concepts as we work our way through the book. We will start with an introduction through the lens of organic matter, exploring cation exchange and soil microbiology.
You can indeed grow plants on soils with little organic matter. You don’t need to have any soil at all. Although gravel and sand hydroponic systems, and even aeroponics (where a nutrient solution is sprayed directly on plant roots) without soil, can grow excellent crops, large-scale systems of this type may have ecological problems and make sense economically only for a limited number of high-value crops grown close to their markets. It’s also true that there are other important issues aside from organic matter when considering the health of a soil. However, as soil organic matter decreases, it becomes increasingly difficult to grow plants, because problems with fertility, water availability, compaction, erosion, parasites, diseases, and insects become more common. Ever higher levels of inputs—fertilizers, irrigation water, pesticides, and machinery—are required to maintain yields in the face of organic matter depletion. But if attention is paid to proper organic matter management, the soil can support a good crop with less need for expensive fixes.
The organic matter content of agricultural topsoil is usually in the range of 1–6%. A study of soils in Michigan demonstrated potential crop yield increases of about 12% for every 1% increase in organic matter. In a Maryland experiment, researchers saw an increase of approximately 80 bushels of corn per acre when organic matter increased from 0.8% to 2%. The enormous influence of organic matter on so many of the soil’s properties—biological, chemical, and physical—makes it of critical importance to healthy soils. Part of the explanation for this influence is the small particle size of the well-decomposed portion of organic matter, the humus. Its large surface area–volume ratio means that the humus is in contact with a considerable portion of the soil. The intimate contact of humus with the rest of the soil allows many reactions, such as the release of available nutrients into the soil water, to occur rapidly. However, the many roles of living organisms make soil life an essential part of the organic matter story.
On Full Circle Organic Farm, which practices managed grazing, soil organic matter values are about 3.5%. Other regional farms that have cropped corn and soybeans annually for 25 years have lost organic matter and now have levels of only 1.5%. Corn yields on the organic farm match yields on the conventional farms while using only 1/3 of the nitrogen fertilizer. This is due to the extra boost in soil health and available nitrogen in the extra 2% organic matter in the soil.
Plant Nutrition
Plants need 17 chemical elements for their growth (these will be discussed in later chapters). For now, understand that plants obtain carbon as carbon dioxide (CO2) from the atmosphere with some of that diffusing up from the soil underneath as organisms decompose organic substances. Oxygen is also mostly taken from the air as oxygen gas (O2). The remaining essential elements are obtained mainly from the soil. The availability of these nutrients is influenced either directly or indirectly by the presence of organic matter.
Nutrients from Decomposing Organic Matter
Most of the nutrients in soil organic matter cannot be used by plants as long as those nutrients exist as part of large organic molecules. As soil organisms decompose organic matter, nutrients are converted into simpler, inorganic (mineral) forms that plants can easily use. This process, called mineralization, provides much of the nitrogen that plants need by converting it from organic forms. For example, proteins are converted to ammonium (NH4+) and then to nitrate (NO3–). Most plants will take up the majority of their nitrogen from soils in the form of nitrate.
The mineralization of organic matter is also an important mechanism for supplying plants with such nutrients as phosphorus and sulfur, as well as most of the micronutrients. This release of nutrients from organic matter by mineralization is part of a larger agricultural nutrient cycle. Mineralization is a very important concept as we discuss the form nutrients must be in so that they can be taken into the plants via root hairs.
Having a good amount of topsoil is important. But what gives topsoil its beneficial characteristics? Is it because it’s on TOP? If we bring in a bulldozer and scrape off one foot of soil, will the exposed subsoil now be topsoil because it’s on the surface? Of course, everyone knows that there’s more to topsoil than its location on the soil surface. Most of the properties we associate with topsoil—good nutrient supply, tilth, drainage, aeration, water storage, etc.—are there because topsoil is rich in organic matter and contains a huge diversity of life. These characteristics diminish the farther down you dig, making topsoil a unique and indispensable part of the soil profile.
Addition of Nitrogen
Bacteria living in nodules on legume roots convert nitrogen from atmospheric gas (N2) to forms that the plant can use directly. A number of free-living bacteria also fix nitrogen.
Storage of Nutrients on Soil Organic Matter
Decomposing organic matter can feed plants directly, but it also can indirectly benefit the nutrition of the plant. A number of essential nutrients occur in soils as positively charged molecules called cations (pronounced cat-eye-ons). The ability of organic matter to hold on to cations in a way that keeps them available to plants is known as cation exchange capacity (CEC). Humus has many negative charges, and because opposite charges attract, it is able to hold on to positively charged nutrients, such as calcium (Ca++), potassium (K+), and magnesium (Mg++). This keeps them from leaching (washing through the soil) deep into the lower soil. Nutrients held in this way can be gradually released into the soil solution and made available to plants throughout the growing season. However, keep in mind that not all plant nutrients occur as cations. For example, the nitrate form of nitrogen is negatively charged (NO3–) and is actually repelled by the negatively charged CEC. Therefore, nitrate leaches easily as water moves down through the soil and beyond the root zone.
Clay particles also have negative charges on their surfaces, but organic matter may be the major source of negative charges for coarse and medium-textured soils. Some types of clays, such as those found in the southeastern United States and in the tropics, tend to have low amounts of negative charge. When those clays are present, organic matter is even more critical as it is the main source of negative charges that bind nutrients.
Protection of Nutrients by Chelation
Organic molecules in the soil may also hold on to and protect certain nutrients. These particles, called chelates (pronounced key-lates) are byproducts of the active decomposition of organic materials or are secreted from plant roots. In general, elements are held more strongly by chelates than by binding of positive and negative charges. Chelates work well because they bind the nutrients at more than one location on the organic molecule. In some soils, trace elements, such as iron, zinc and manganese, would be converted to unavailable forms if they were not bound by chelates. It is not uncommon to find low-organic-matter soils or exposed sub-soils deficient in these micronutrients.
Other Ways of Maintaining Available Nutrients
There is some evidence that organic matter in the soil can inhibit the conversion of available phosphorus to forms that are unavailable to plants. One explanation is that organic matter coats the surfaces of minerals that can bond tightly to phosphorus. Once these surfaces are covered, available forms of phosphorus are less likely to react with them. In addition, some organic molecules may form chelates with aluminum and iron, both of which can react with phosphorus in the soil solution. When they are held as chelates, these metals are unable to form an insoluble mineral with phosphorus.
Beneficial Effects of Soil Organisms
Soil organisms are essential for keeping plants well supplied with nutrients because they break down organic matter, including other dead organisms. These organisms make nutrients available by freeing them from organic molecules. Some bacteria fix nitrogen gas from the atmosphere, making it available to plants. Other organisms dissolve minerals and make phosphorus more available. Without sufficient food sources, soil organisms aren’t plentiful and active, and consequently, more fertilizers will be needed to supply plant nutrients.
Directly
- As organic matter is decomposed, nutrients are converted into forms that plants can use directly.
- CEC is produced during the decomposition process, increasing the soil’s ability to retain calcium, potassium, magnesium, and ammonium.
- Organic molecules are produced that hold and protect several micronutrients, such as zinc and iron.
- Some organisms make mineral forms of phosphorus more soluble while others fix nitrogen, which converts it into forms that other organisms or plants may use.
Indirectly
- Substances produced by microorganisms promote better root growth and healthier roots. With a larger and healthier root system, plants are able to take up nutrients more easily.
- Organic matter improves soil structure, which results in increased water infiltration following rains and increased water-holding capacity of the soil; it also enhances root growth into more permeable soil. This results in better plant health and allows more movement of mobile nutrients (such as nitrates) to the root.
A varied community of organisms is your best protection against major pest outbreaks and soil fertility problems. A soil rich in organic matter and continually supplied with different types of fresh residues, through the use of cover crops, complex rotations, and applied organic materials such as compost or animal manure, is home to a much more diverse group of organisms than soil depleted of organic matter. The residues provide sufficient food sources to maintain high populations of soil organisms.
There are two aspects to biological diversity, both aboveground and belowground:
- the range of different organisms present
- their relative populations (referred to as evenness).
It is good to have diverse species of organisms, but it is a richer environment when there are also similar population sizes. For example, if there is a moderate population of disease organisms, we do not just want a small population of beneficial organisms present; the soil is biologically richer if there is also a moderate population of beneficial organisms. Good populations of diverse organisms help ensure that fewer potentially harmful organisms will be able to develop sufficient numbers to reduce crop yields.
Since erosion tends to remove the most fertile part of the soil, it can cause a significant reduction in crop yields. In some soils, the loss of just a few inches of topsoil may result in a yield reduction of 50%. The surface of some soils low in organic matter may seal over, or crust, as rainfall breaks down aggregates and as pores near the surface fill with solids. When this happens, water that cannot infiltrate into the soil runs off the field, carrying away valuable topsoil.
Protection of the Soil Against Rapid Changes in Acidity
Acids and bases are released as minerals dissolve and organisms go about their normal functions of decomposing organic materials or fixing nitrogen. Acids or bases are excreted by the roots of plants, and acids form in the soil from the use of nitrogen fertilizers. It is best for plants if the soil acidity status, referred to as pH, does not swing too wildly during the season. The pH scale is a way of expressing the amount of free hydrogen (H+) in the soil water, but in soils, it is strongly related to the availability of plant nutrients and toxicity of certain elements like aluminum. It is a log scale, so soil at pH 4 is very acidic and its solution is 10 times more acidic than soil at pH 5. A soil at pH 7 is neutral: there is just as much base in the water as there is acid. Most crops do best when the soil is slightly acidic and the pH is around 6 to 7, although there are acid-loving crops like blueberries. Essential nutrients are more available to plants in this pH range than when soils are either more acidic or more basic. Soil organic matter is able to slow down, or buffer changes in pH by taking free hydrogen out of solution as acids are produced or by giving off hydrogen as bases are produced.
Stimulation of Root Development
Humic substances in soil may stimulate root growth and development by both increasing the availability of micronutrients and by changing the expression of a number of genes. Microorganisms in soils produce numerous substances that stimulate plant growth. These include a variety of plant hormones and chelating agents. The stimulation by chelating substances (siderophores) is mainly due to making micronutrients more available to plants, which causes roots to grow longer and to have more branches. In addition, free-living nitrogen-fixing bacteria provide the plant with additional sources of that essential nutrient while some bacteria help dissolve phosphorus from minerals, which makes it more available to plants.
Darkening of the Soil
Organic matter tends to darken soils. You can easily see this in coarse-textured sandy soils containing light-colored quartz minerals. Under well-drained conditions, a darker soil surface allows the soil to warm up a little faster in the spring. This provides a slight advantage for seed germination and the early stages of seedling development, which is often beneficial in cold regions.
Protection Against Harmful Chemicals
Some naturally occurring chemicals in soils can harm plants. For example, aluminum is an important part of many soil minerals and, as such, poses no threat to plants. As soils become more acidic, especially at pH levels below 5.5, aluminum becomes soluble. Some soluble forms of aluminum, if present in the soil solution, are toxic to plant roots. However, in the presence of significant quantities of soil organic matter, the aluminum is bound tightly and will not do as much damage.
Organic matter is the single most important soil property that reduces pesticide leaching. It holds tightly onto a number of pesticides. This prevents or reduces the leaching of these chemicals into groundwater and allows time for detoxification by microbes. Microorganisms can change the chemical structure of some pesticides, industrial oils, many petroleum products (gas and oils), and other potentially toxic chemicals, rendering them harmless.