The idea that nutrients are depleted from soil when crops are harvested is not correct. Nutrients are removed when plants are harvested, but this doesn’t mean the soil has no nutrients left. Consider that every second of every day nutrients are replenished from rocks, pebbles, gravel, bedrock and parent material. Organic farming practices nurture the organisms that perform the service of replenishing plant-available nutrients, while chemical-intensive practices destroy these very important organisms.
As defined by the “Father of Soil Science,” Hans Jenny, soil is made up of (1) mineral components, (2) organic matter, and (3) organisms. With the proper sets of organisms in the soil, mineral nutrients are cycled from plant-unavailable forms in rocks, sand, silt, clay, etc. into plant-available forms every second of every day. But, the only way to make these nutrients available from the rocks, pebbles, gravel, sand, silt and/or clay is to have life in the soil.
There is no soil on this planet that lacks nutrients to grow plants. Nutrients abound in rocks, pebbles, sand grains, silt particles and clay colloids, as well as any organic matter present in the soil. Until the day your soil runs out of sand, silt, clay or rocks, there is no reason to add nutrient amendments to soil.
So, why do fields “need” inorganic fertilizers? Because something happened to destroy the food web of organisms that normally do the work to pull the nutrients from the rocks, sand, silt, clay and organic matter. The diversity of life is no longer present to make those nutrients available to a plant’s root system. When these organisms are lost, soil structure cannot be built, leading to inability to hold water or nutrients. Erosion will occur once these services are lost. Without the correct soil life to support the plants we desire, we get weeds, diseases and pests.
What causes plant-beneficial organisms to disappear from soil? Why does our soil turn into dirt? Simply put, the organisms are killed or put soundly to sleep. Any of the following insults will severely reduce, if not completely destroy, the beneficial organisms in soil.
• Tilling too much and in a highly destructive fashion slices, dices and crushes the organisms.
Till only the strips where crops will be put in, or better yet, use zero-tillage. Both of these management practices require active, functioning sets of beneficial organisms in the soil in order to work.
• Stop the "nuke-it" approach. No pesticides should be used. All pesticides kill far more than just the target organism(s). Avoid copper, sulfur, or rotenone applications, even though they may be allowed by organic regulations.
• Avoid high levels of inorganic fertilizers. On average, application of more than 100 pounds per acre per year will result in loss of critically important beneficial soil organisms. As explained above, applying inorganic fertilizers, lime, gypsum, etc. is just adding more of what is already present in the soil. Why waste money, time, and equipment?
• Avoid driving equipment over the soil, as heavy objects cannot help but compact the soil. Compaction reduces movement of oxygen into the soil, and when anaerobic organisms begin to grow, soluble nutrients will be lost through volatilization and leaching. Soil pH will drop, soil structure will be lost, and highly toxic compounds will be produced. Roots of most plants die in anaerobic zones, with the exception of wetland or riparian plants.
With the loss of aerobic organisms, soluble nutrients will leach from the soil easily, polluting surface and ground waters. Anaerobic conditions promote the growth of organisms which volatilize N, P, and S, contributing to greenhouse gas emissions. Because of all these losses, conventional chemical systems require massive inputs of soluble inorganic N, P, S, Ca, Mg, Fe, B, etc. to be “added back” to the soil each cropping cycle in hopes that some of those nutrients will be captured by contact with root systems. But in dirt, most nutrients are lost, destroying water quality for everything downstream.
Plant roots release exudates, or in more everyday language, “cakes and cookies,” which select for the growth of the thousand or so bacterial and fungal species the plant might need on any particular day (depending on the temperature, moisture, etc.). If a plant is limited for cobalt, or iron, or zinc, the plant releases the specific exudates (i.e., cakes and cookies) that feed the specific bacteria and/or fungi which make the specific enzymes to convert the non-plant-available mineral nutrients in the rocks, sand, silt, clay, or organic matter into bacterial or fungal biomass. The bacteria and fungi have to then be eaten by one of their predators—protozoa, nematodes, microarthropods, earthworms, etc.—in order to release, right at the plant root surface, the soluble nutrients the plant requires. The plant controls this process, and what better controller of nutrient cycling for the plant than the plant?
When soil organisms are lacking in soil—which should then properly be called dirt—nutrients that are in mineral forms (e.g., rocks, sand, silt, clay) cannot be processed into plant-available forms. Scientists raised in the “Green Revolution” era, whose research was funded by the inorganic fertilizer and pesticide industries, were taught nothing about soil life. They were encouraged to ignore any evidence that soil life could be important, and in fact were taught to ridicule anyone suggesting that soil organisms served important functions. The only way to grow plants, therefore, was to pour on more and more soluble, inorganic fertilizers.
Consider however that inorganic fertilizers do not supply the full balance of nutrients that plants require. Nitrates or phosphates or calcium is applied in extremely high concentrations because the fertilizer industry knows that most of it will leach or volatilize out of the dirt where it is applied. But plants need a proper balance of all nutrients, not just the standard N-P-K. The nutrient cycling performed by soil life supplies the full balance of all the soluble nutrients the plant needs.
Would you rather eat products from plants grown with the full balance of all the nutrients—the exact balances controlled by the plant—or products from plants where the plant has been overloaded with just N, or P, or Ca at different times of its life, but never allowed to have the full balance of everything?
Benefits of Compost
Compost and water-extracts of compost, properly made and aerobic, will replenish the complex, highly diverse sets of organisms needed to support a healthy plant. Compost also replenishes organic matter which feeds the organisms the plant needs. Although compost contains plenty of nutrients, its real value is that it replenishes living microorganisms.
Be aware that compost, extracts and teas do not normally contain adequate soluble, inorganic nutrients to allow them to be sold as chemical-industry defined “fertilizers.” Most composts are labeled “soil amendments” because the chemical industry has co-opted the term “fertilizer” to mean only the soluble inorganic forms. Check EPA and USDA regulations to find out more about this interesting conundrum. But in fact, compost and water extracts of compost (extracts and teas) do not—and should not—contain high levels of in-solution, ionic or inorganic forms of nutrients. Nutrients in compost should be present as organisms and organic matter. With the right biology, these nutrients will rapidly meet the demands of the plants growing in that aerobic, properly made compost.
Issues with Soil Tests
When you send a sample into a soil chemistry lab, what information appears on that soil chemistry report? What’s the difference between soluble nutrients and exchangeable nutrients? How important is the extracting agent used to assess soluble nutrients or exchangeable nutrients? Why is it that each and every extracting agent gives different numbers? Each extracting agent extracts something different, so it is important to know which agent was used, or you will think your nutrients levels are increasing or decreasing when in fact they may not be changing at all. Which extracting agent tells you how much nutrient the plant is going to take up, or how much of that pool the plant could potentially take up?
Newsflash: none of them. There is no good correlation. Across hundreds or thousands or tens of thousands of samples, we can’t predict how much of the “soluble pool” extracted by any extracting agent will actually end up in the plant.
Let’s back up for a second and define these “pools of nutrients” in soil. Soluble nutrients are those in the form of salts. Salts dissolve in water because they have positive “ends” and negative “ends” which allows water to “pull them apart” so they dissolve. Exchangeable nutrients are nutrients bound to surfaces of rocks, pebbles, sand, silt, clay, organic matter, etc. Again, they have to have positive or negative parts of the molecules so they will interact with positive or negative sites on these surfaces. Thus, these nutrients on these surfaces can “exchange” with soluble nutrients dissolved in water.
Back to the conundrum with that soil chemistry report. There are hundreds of extracting agents touted as “the best” in order to measure exchangeable nutrients, and perhaps as many as a thousand different extracting agents that have been used to extract soluble nutrients from soil. Which extracting agent gives the highest value? But is highest actually the best? Which agent actually gives you information on what the plant will take up? Isn’t that what we want to know? Just because you use an extremely strong acid or base to extract nutrients does not mean anything relative to what the plant will obtain. So in terms of telling us how much inorganic fertilizer needs to be added, these measurements are misleading at best and a complete waste of money at worst.
To predict which nutrients will be available to plants requires knowledge of how much food is available for the bacteria and fungi to do their jobs of making enzymes to solubilize nutrients from the rocks, from sand, silt clay, from humus and organic matter. We want to know the populations of the predators of the bacteria and fungi that are actively consuming bacteria and fungi, and thus releasing soluble nutrients, right next to the root.
Consider the following information based on extractions of nutrients from a set of soil samples. The original soil contained on average only 1.8 µg of phosphate per gram (ppm) of soil (average of five replicate soil samples). Rock phosphate and compost were added to achieve a value of 75 ppm phosphate, on average, through the entire field. Two weeks later, five separate samples of soil were taken from the same field using the same methods, but now phosphate measured 200 ppm. Bacteria had decreased slightly, and fungi had increased by nearly twofold compared to the soil at the beginning, before the fertilizer addition. Nothing else was added to the field during those two weeks—no fertilizer, no pesticides, no tillage, no seed—just grass growing in the field. Where did the “extra” phosphate come from?
Did the chemistry lab mess up? Let’s think it through. Plants were growing, making exudates, feeding bacteria and fungi in the root system. Phosphate was solubilized from the TOTAL mineral not detected with either exchangeable or soluble extracting agents. Predators eat the bacteria and fungi, and nutrients were released in soluble forms, which can be detected by both soluble and exchangeable extracting agents. No error—just an explanation that doesn’t enter into the lexicon of conventional agriculture.
The lesson here is that the plant itself provides the best test of its potential uptake. So let the plant take care of itself. Let it be the determinant of what bacteria and fungi are going to be working, and what nutrients will be taken from the soil. Let the predators do their jobs. And consider that every second of every day, the parent material underlying your soil is being broken down into rocks, pebbles, sand, silt and clay, such that until the bedrock of the planet “runs out,” or you run out of organic matter, you should never have to worry about a lack of minerals in your soil.
The problem in agriculture has not been a lack of nutrients, but a lack of the proper biology to make those nutrients available to plants. Total nutrients are present in natural soil in great excess. But in agriculture, the soil has been abused with excess tillage and excess toxic chemical use, and the organisms which supply nutrients to plants have been destroyed.
Rather than applying nutrients and minerals, we must concentrate on repopulating the soil biology so that the organisms can break down the nutrients to make them available to plants. Organic farming practices, such as the application of properly made aerobic compost, repopulate the bacteria and fungi which solubilize nutrients from rocks, sand, silt and clay as well as organic matter, and the protozoa, nematodes and microarthropods which will release nutrients from the bacteria and fungi in balanced, plant-available forms. That’s how organic farming practices work with the natural soil system rather than against it.
Dr. Elaine Ingham is a world-renowned soil microbiologist and founder of Soil Foodweb, Inc., an organization that helps farmers all over the world grow more resilient crops by understanding and improving soil life.
Dr. Ingham will present “Soil Biology and the Soil Food Web” at the 2015 Organic University on Feb. 26, prior to the 2015 MOSES Conference.
This article reprinted from the Organic Broadcaster with permission from the Midwest Organic & Sustainable Education Service (MOSES)