Soil Minerals and Soil Testing for Organic Gardeners
26 August 2018
The Errors of Regenerative Agriculture
Error 1: Increasing the amount of carbon and carbonaceous organic matter does not and will not increase the mineral fertility of a soil.
Error 2: It is impossible to increase stable organic matter, or sequester carbon in the soil, without increasing the soil's reserves of phosphorus, sulfur, and certain trace minerals. If any essential elements are deficient or absent, additional organic matter added will simply decompose and return to the atmosphere as CO2, carbon dioxide, and NH3, ammonia.
Let's take these one at a time. What makes a soil fertile? Mineral nutrients, not carbon content. Higher plants require at least seventeen mineral elements to complete their life cycle, to grow and reproduce and form viable seed for the next generation. The higher animals and humans need at least twenty-two essential minerals, and possibly fifty or more.
The proven essential minerals for plants:
The macronutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sulfur (S), magnesium (Mg), carbon (C), oxygen(O), hydrogen (H)
The micronutrients (or trace minerals): iron (Fe), boron (B), chlorine (Cl), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni)
In the absence of any one of these essential minerals the growth and reproduction cycle will fail. The failure may not be as obvious as a fruit tree not blooming or forming fruit. A plum or peach tree may bloom and form loads of beautiful, sweet fruit, but the stone or pit of the fruit will be empty or contain a shriveled infertile embryo that cannot grow. I'm sure many readers have seen this, a plum or peach with an empty, hollow seed. That is caused by the lack of sufficient manganese, either because there is little or no manganese in the soil, or because the plant was unable to absorb and utilize manganese due to a deficiency of another essential mineral such as iron.
Of these seventeen known essential elements, four come primarily from the atmosphere: carbon, hydrogen, oxygen, and nitrogen. These elements form carbohydrates such as sugars and starches, hydrocarbons such as oils and waxes, and amino acids and proteins where nitrogen is involved. If the soil is downwind from a coal-fired power plant or a volcano, the plants may be able to obtain sulfur from the atmosphere, so potentially five elements can be sourced from the air; the other twelve must come from the mineral reserves of the soil.
Do soils have an unlimited supply of these other essential elements? Of course not, no more than a gold or copper mine has an unlimited amount of gold or copper. The planet has thousands of abandoned gold and copper mines that simply ran out of ore. We also have thousands of abandoned farms that simply ran out of essential mineral nutrients. In addition, many soils never did contain sufficient quantities, or any amount, of some of the essential minerals. When the prairies of northern Florida were first used for grazing cattle, the cattle did very poorly, and often wasted away and died. It turned out that the soil of these prairies was lacking in cobalt, necessary for bacteria in the cattle's digestive tract to synthesize vitamin B12. Notice that cobalt, Co, is not on the above list of essential elements for plants, but it is necessary for a bacteria that produces a vitamin needed for the growth and reproduction of higher animals.
In reality, very few soils contain large reserves of all of the essential minerals. A soil that formed from the decomposition of limestone may well have an endless supply of calcium, but it is unlikely to have much iron. As a matter of fact, a limestone soil may be unable to grow food crops at all without iron amendments. On the other hand, a soil that formed from decomposing sandstone is likely to be severely deficient in calcium, though it will probably have plenty of iron.
In wild nature, plants that are well-adapted to thrive in various soil types will colonize any soil in which they can obtain sufficient minerals for growth and reproduction. Over time, these soils will foster plant communities that absorb minerals from the soil, and as the plants die and decompose, those minerals will stay where they originated, increasing the available fertility of the topsoil. In addition, grazing animals and predators will bring in minerals from outside the area and leave them as manure or as their bodies decompose after death.
Modern agriculture works in an entirely different way. Modern agriculture is extractive. Plants are grown and animals are fed on the minerals extracted from the soil, but rather than being returned to the soil, the crops and animals are harvested and sold out the farm gate. Each animal, vegetable, fruit or grain exported from the farm depletes the mineral reserves of the soil. No matter how rich in minerals the soil started off, eventually the soil will run out of one or more essential minerals and crops can no longer be grown profitably, if at all. Suppose a crop of potatoes removes fifty pounds of calcium per acre per year. A calcium-rich soil may contain enough calcium to continue this extraction for hundreds of years. That same soil is unlikely to contain enough zinc, copper, or potassium to be extracted and exported for more than a few decades.
It will not matter how much organic matter is grown as a cover crop, or what vegetation from the field is composted and returned to the soil; eventually the soil will be depleted of one or more essential nutrients and the crops will fail.
The spokespersons for regenerative agriculture tell us that we can overcome this limitation by increasing the biological activity in the soil; they tell us that microbes and fungi can access hard to find minerals and supply them to the crops, but this is very short-sighted. The total amount of mineral reserves in any soil is limited. Microbes and fungi cannot create a mineral where it doesn't exist. At best they can scavenge the last remaining traces of a scarce mineral and supply that to the crop. This is at the same time the best and worst case scenario, because when the last traces of a scarce essential mineral has been extracted and sold out the farm gate, the soil will truly be depleted. The remaining soil may have a wonderful texture and be high in organic matter, but it will no longer be able to grow crops that can feed a person or animal.
That is the real reason that our food supply has become more and more deficient in minerals and vitamins: because the minerals have been extracted and exported and not replaced. No amount of additional organic matter or microbial activity will overcome the law of diminishing returns. When the mineral reserves are exhausted the mine will be abandoned.
Part II: Increasing Soil Organic Matter and Stable Humus
For any given soil and climate the limiting factors for soil organic matter (OM) are the minerals needed to form stable humus. The primary elements needed are carbonaceous matter, nitrogen, phosphorus, and sulfur. In the absence of any one of the above, stable humus will not form. In most agricultural soils the limiting factors are phosphorus and sulfur, primarily sulfur.
Nitrogen is the active component of amino acids and proteins. Phosphorus is a critical component of DNA and RNA. Phosphorus is also a critical component of the Krebs cycle:
Citric acid cycle
The citric acid cycle – also known as the tricarboxylic acid cycle or the Krebs cycle (named after Hans Krebs) – is a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA (derived from carbohydrates, fats, and proteins) into carbon dioxide and chemical energy in the form of adenosine triphosphate. [emphasis added] More at Wikipedia
An essential amino acid is one that cannot be synthesized by the organism requiring it; it must be supplied as part of the diet. For humans, there are nine essential amino acids. All of them contain nitrogen, but one of them, methionine, also contains sulfur. As methionine Is needed for the synthesis of other sulfur containing amino acids such as cysteine and taurine, a deficiency of sulfur in the soil leading to a deficiency of amino acids could well explain why it is so difficult to increase stable humus in the soil. Because the soil's decomposition microbes are not able to synthesize the full range of amino acids, the microbes cannot complete the conversion of organic matter into humus. In the absence of essential minerals, as the organic matter breaks down it releases the stored nitrogen to the air as ammonia, while the carbonaceous components of organic matter off-gas as CO2. The percentage of stable soil organic matter remains unchanged, no matter how much organic matter is added.
Not that long ago, as recently as the 1950s and ‘60s, sulfur was a common component of many fertilizers. Ammonium sulfate was used as a primary N fertilizer and single super-phosphate, made by treating ground-up phosphate rock with sulfuric acid, was a primary P fertilizer. The development of commercial urea fertilizers in which ammonia is reacted with CO2 took sulfur out of the lineup in nitrogen amendments. The widespread use of Mono- and Di-ammonium phosphate fertilizers, replacing single and triple super-phosphate, eliminated sulfur from P amendments (along with many trace elements and calcium). Placing pollution controls on coal-fired powerplants removed that source of atmospheric S. All we are left with to supply sulfur are volcanoes, and there are not enough of them going off at any given time to supply much. Most agricultural soils are seriously deficient in sulfur. At least 95% of all soil tests this writer looks at show a severe sulfur deficiency.
The image above results from a survey of thousands of soil tests submitted to A&L Great Lakes Laboratories between 1996 and 2017. Note the steady decline in Sulfur levels over 20 years, from an average of 17ppm S to 9ppm S. Even the higher levels twenty years ago were not sufficient to maintain a steady level of soil organic matter, much less an increase in stable humus, no matter how much carbonaceous material was added.
If the promoters of regenerative agriculture wish to increase the amount of carbon sequestered in the soil, they would be well advised to study the role of soil chemistry and soil minerals in humus formation. An in-depth treatment of the subject can be found in the book Cycles of Soil: C, N, P, S, and Micronutrients by F J Stevenson (John Wiley and Sons, 1986). Also a paper published in 2014 that is worth checking out: Long-term S-fertilization increases carbon sequestration in a sulfur-deficient soil. http://www.bioone.org/doi/full/10.1139/CJSS2013-022
Due to a deficiency of sulfur and other essential minerals, growing cover crops and composting and recycling agricultural waste, though it provides short-term benefits in moisture retention and soil texture, is not sequestering atmospheric CO2. It is simply spending a lot of time and effort recycling carbon right back to the air where from which it came. If the goal is to increase long-term stable humus, the soils need to be supplied with enough sulfur to balance the nitrogen content of the soil. This would be a bare minimum of 50 parts per million sulfur (100 lbs S per acre), and much more in soils with a high exchange capacity or a high pH.
Our food is deficient in nutrients because our soils have been depleted of minerals by extractive agriculture. We have been selling the crops and animals out the farm gate without replacing the exported minerals. No soil is a bottomless well of nutrients that can be exploited forever; just like a gold or copper mine, eventually the ore is exhausted. Some modern gold mines are seeding the leftover low-grade tailings piles with specialized bacteria that can extract the minute traces of gold that still remain, but when that has been done, there won't be any gold left to extract. Employing specialized bacteria and fungi in agriculture to extract the last crumbs of scarce minerals from depleted soil is no different; it is the same process. Anyone believing and teaching that soil biota can create minerals where they don't exist, or that we can continue to extract and export scarce minerals endlessly from the same soil, is deluding themselves and others. If we wish to return our food supply to the nutrient content it had in the past, when crops were grown on highly mineralized virgin soils, we are going to have to measure the mineral content of our soils and supply the missing elements.
If we wish to pull carbon out of the atmosphere and sequester it in the soil, we will need to supply the soil with the elements necessary to create and maintain high levels of stable humus.
Both problems/goals have the same solution: Determine which mineral elements are needed and supply them to the soil. Doing this will provide a further and greater benefit: When we provide all of the mineral elements needed to grow truly nutrient dense food there will no longer be a need for more farmland. It's not the volume of food we eat that makes us healthy and satisfied, it is the amount of nutrients in the food, When the same amount of land provides two or three times as many nutrients, we will need less land to feed people and animals, and both will be much healthier.
If you found this essay interesting you may also like the Interview with Agricola
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