By Hugh Lovel
What many farmers probably don’t know about soil testing is that most soil tests only tell us what is soluble in the soil. They do not tell us what is actually there in the soil, no matter what fertilizer salesmen might like to imply. To find out what is actually there requires a total acid digest similar to what is used for plant tissue analysis. Mining labs run these total acid digests on ore samples which are crushed, ground and extracted with concentrated nitric and hydrochloric acid solutions, but a mining assay does not determine total carbon, nitrogen and sulfur as a plant tissue analysis would. These elements need a separate procedure essential for evaluating soil humic reserves.
Most soil tests measure total carbon, which then is multiplied by 1.72 to calculate soil organic matter. This assumes that most of the carbon in the soil is humus of one form or another. While this may or may not be true, determining the carbon to nitrogen, nitrogen to sulfur, and nitrogen to phosphorus ratios is a good guide for evaluating organic matter, and this requires testing total nitrogen, sulfur and phosphorus as well as carbon.
While carbon in almost any form is a benefit to the soil, it helps enormously if it is accompanied by the right ratios of nitrogen, sulfur and phosphorus. Though these ratios are not set in stone, a target for carbon to nitrogen is 10:1, for nitrogen to sulfur is 5.5:1 and for nitrogen to phosphorus is 4:1. This works out to an ideal carbon to sulfur ratio of 55:1, and a carbon to phosphorus ratio of 40:1. Because soil biology is very adjustable these targets are not exact, but achieving them in soil total tests is a good indication of humus reserves that will supply the required amounts of amino acids, sulfates and phosphates whenever the soil food web draws on them.
Humus as Vague Science
Humus formation and utilization is a fuzzy subject that has long been poorly understood. Humification may result from long-term geological processes as with the formation of peat, brown coal and leonardite. But humification can also result from humus-forming activity by mycorrhizal fungi, actinomycetes or any microbial species that can add to or withdraw, somewhat like bees storing honey in the hive from the soil’s storehouse of humic acids. The precise carbon structures of humic acids are enormously difficult to characterize, which means carbon structures end up classified as humic acids whenever they are too large to pass through bacterial cell walls. This pretty much limits humic acids to consumption by fungi, actinomycetes or protozoa. This vague but useful rule draws the dividing line between humic and fulvic acids at somewhere around 2,000 atomic weight units — above is humic acid, and below is fulvic.
It is not much easier to determine the precise structures of fulvic acids. Though fulvic acids can also be extracted from peat, brown coal or leonardite, generally fulvic acids are low molecular weight residues from the breakdown of plant and animal wastes. However, much of the carbon chemistry that plants give off around their roots as root exudates could be classified as fulvic acids based on molecular weight. This low molecular weight fulvic chemistry is very versatile and may be taken up by plants, consumed by soil bacteria, or used by humus building microorganisms to assemble stable, high molecular weight humic acids.
Many of these humus-forming microbes form symbiotic relationships with crop roots and capitalize on the fact that virtually all plants that are growing well also give off some of their sap as an energy-rich bonanza of root exudates. When photosynthesis is abundant these microbes convert surplus root exudates into humic acids and store them in the soil as clay/humus complexes. Then when there is rain or photosynthetic conditions are not ideal they tap into these stores, much like bees do in the hive. This evens out plant and soil food web interactions and keeps things going on a fairly even keel.
Where we really see the benefits of this plant/microbe/humus interaction is where we see root exudate overlap, which will be dealt with later. The important bit here is the organisms that consume humic acids also store them as clay/humus complexes. This is a good reason to use 10 percent soil in making compost to ensure adequate soil surfaces for humus complexes to form. The large molecular weight carbon compounds in the resulting clay/humus complexes will incorporate amino acids, sulfates and phosphates along with silicates and various cations. Only a small portion of these materials show up on soluble soil tests even though they are available to the mycorrhizae, actinomycetes and/or protozoa.
Charcoal & Fossil Humates
Carbon is the basis of life, and in almost any form carbon benefits the soil by attracting life. Biochar is a very beneficial carbon source. But just because something is a carbon source does not mean it has sufficient other elements associated with it. The process of making biochar pretty much guarantees that most of the nitrogen, sulfur and phosphorus are driven off; and since these elements are anions, the char that results — while bio-active — will have a high pH because it will still contain most of its original calcium, magnesium, potassium and silicon.
Fossil humates, such as are mined or extracted from brown coal or leonardite, also tend to be deficient in nitrogen, sulfur and phosphorus. Even composts, which tend to be better balanced, may be deficient in certain elements. Chars, fossil humates and composts will increase soil life, but will that soil life scavenge the soil for such things as nitrogen, sulfur and phosphorus and tie them up so they aren’t soluble? We only need small amounts to be soluble on a steady basis.
If we want to achieve the best results we should test and adjust our ratios of carbon to nitrogen, nitrogen to sulfur and nitrogen to phosphorus, not only in our soils but also in the chars, humates or composts we apply — and this requires total testing. The significance of these ratios is huge in developing a long-range plan for thriving, robust growth, efficient photosynthesis and biological nitrogen fixation without resort to nitrogen fertilizers.
Just suppose the ratio of C to N in the soil reserve is 15:1 or even 20:1 and there’s not enough amino acid nitrogen in the soil’s humus reserve. In cloudy weather when photosynthesis is reduced, root exudation and nitrogen fixation are low and the microbial symbiosis with crop roots mines the humus flywheel — then it comes up short in amino acids.
Or suppose the N:S or N:P ratios don’t deliver enough S or P. Will there be enough free in the soil or will the plant come up short? Deficiencies may also include silicon or boron, or any macro- or micronutrients that might be stored in the soil’s clay/ humus complexes. What can the soil’s humus flywheel deliver? Total tests are our best clue.
Keep in mind that we do not want more than a steady trickle of soluble nutrients. For the most part we want our nutrients to be insoluble but available. We should also keep in mind Liebig’s law of the minimum. The great 19th century chemist, Justus von Liebig, pointed out that plants can only grow to the extent of their most deficient element, and it won’t matter how much other stuff they have. This implies that whenever there is a shortage of something in the soil’s humus flywheel, the plant may have to slow down and limp along.
Building N, S & P
Truly amino acids are of first importance for protein development, but as long as nitrogen fixation supplies a steady stream of amino acids from the microbial symbiosis around crop roots there is no other element closer to hand in greater abundance than nitrogen.
A more urgent deficiency to remedy is sulfur. Sulfur works at surfaces and boundaries making things accessible. As such it is the catalyst for most of plant and soil chemistry. For example, sulfur is what peels the sticky, miserly magnesium loose from its bonding sites in the soil. Without sufficient sulfur the plant may not take up enough magnesium even if it is abundant in the soil. This deprives the plant of sufficient chlorophyll to make efficient use of sunshine, and then there is a shortage of sugary root exudates to feed nitrogen fixation — which requires 10 units of sugar to produce one amino acid. Considering how common magnesium deficiency is in plants growing on magnesium-rich soils, we shouldn’t ignore sulfur deficiencies in the soil reserves. Many soils are abundant with magnesium, but without the 55:1 carbon to sulfur ratio needed for optimum growth, plants can easily be magnesium deficient, poor in photosynthesis — and when they don’t make enough sugar they won’t have good nitrogen fixation.
One can amend sulfur in the soil in various ways. With chars or raw humates, both of which are deficient in nitrogen and sulfur, small amounts of ammonium sulfate (30 to 80 pounds per acre depending on the case) can be helpful. But keep in mind this is a soluble chemical and only so much can be absorbed by the soil’s carbon complexes and the microbial life they support.
Potassium sulfate might also be of use, but total soil testing often indicates an abundance of total potassium and more in soluble form interferes with magnesium uptake, which usually is counterproductive. Gypsum (calcium sulfate) is most commonly used for corrections, though only about 50 ppm of sulfur (0.4 to 0.6 tons per acre) can be absorbed by the soil in one application.
The problem here is sulfate tends to leach if there’s too much. That might be good if all it carried with it was magnesium as most soils are high in magnesium. But, what if the sulfate carries copper, zinc, manganese or even potassium along with it? Can we afford such losses?
If we try to keep soluble sulfur topped up at 50 ppm (Morgan test) by using gypsum mixed with compost or raw humates, gypsum will probably work beautifully and not acidify the soil. It may take a few years to build sulfur levels into the soil totals, but patience is a virtue. However, when the soil pH is already 7.0 or above, elemental sulfur becomes the input of choice. Elemental sulfur pulls oxygen out of the atmosphere as it oxidizes to sulfate and this lowers pH — which for alkaline soils is desirable. Again, try to keep the soluble sulfur level around 50 ppm and gradually build this element into the soil reserves as humic reactions or interactions progress.
Sometimes we can see a field that had water standing in a streak, puddle or blanket for a day or two, which leached some of the sulfur and left a meandering, light-color streak or area where the water was. Often such events are repeated, which can make the area of leaching stand out rather clearly. This is sulfur deficiency, which leads to magnesium deficiency in plant growth on what is probably a high mag soil — which would explain not draining fast enough in the first place. Usually on such soils the calcium leaches leaving the magnesium behind. Fixing such problems takes careful applications to the deficient area rather than just making a simple recommendation for an entire field. It may be possible to remedy such a deficiency by eye by following the lighter colored area with one or more sulfur applications — most likely gypsum — along with compost, fossil humates or biochar.
Phosphorus may also be deficient, though sometimes total phosphate is surprisingly high without sufficient phosphorus availability. If a total test shows the N:P ratio is too high, add enough rock phosphate to compensate for the deficiency and apply this with compost, raw humates or char inputs. As with sulfur, calculate the amounts once the inputs are spread and don’t go overboard. Adding too much can be like having a soup with too much salt in it.
Keep in mind it is not rare for total tests to show 10 to 100 times as much total P as shows up on soluble tests. Although sulfur deficiency limits phosphorus availability, the key deficiency that often must be remedied to make phosphorus available from soil totals is copper. Phosphorus is useless without copper. Though 2 ppm soluble copper is generally considered adequate, 5 ppm gives more margin and 10 is not harmful unless the soil is extremely light with poor humus reserves.
Zinc deficiency can also keep phosphorus tied up, and a 10:1 phosphorus to zinc ratio is a desirable target in total tests. Total tests of rock phosphates generally show the desired amount of zinc. Usually trace mineral deficiencies such as copper and zinc show up most clearly in winter where these elements work 1/100th less efficiently at 30 or 40°F as they do at 70 or 80°F. The signs of these deficiencies are quite obvious in winter, and if the deficiencies are remedied, growth in cool periods of spring or autumn will be much better.
Silicon & Boron
Even though silicon is secondary in importance to sulfur, silicon accounts for all transport in plants. It is the basis of capillary action. As a co-factor, boron works with silicon to provide sap pressure and often is found in appropriate amounts in siliceous rock formations. Boron has an affinity for silicon in the capillary linings where borate molecules take the place of silicate molecules. However, boron forms three electron bonds where silicon forms four. Boron’s inability to form the fourth bond creates a hunger in the surrounding silicate molecules, which causes them to draw water and electrolytes from the roots through the capillary system to the transpiration sites in the canopy. Without sufficient boron, plants with high boron requirements like legumes, crucifers, vines, etc., will have too little sap pressure to feed their canopy. Then they may wilt at mid-day or not have enough root exudation at night. Where plants have high brix in the early morning, boron is deficient.
Lest we forget, however, the key role of sulfur is in the soil biology around plant roots where sulfates and sulfur-containing amino acids interact with the surfaces of soil particles, most of which are siliceous. Actinomycetes and mycorrhizal fungi in particular need sulfur to peel silicon and boron away from the surfaces of clay and sand particles in the soil. This is a gradual process because it only works at surfaces. It is the nitrogen to sulfur ratio in soil total tests that lets us know whether the soil food web can do an adequate job of silicon and boron access — and this makes a huge difference with how well alfalfa, tomatoes, grapes, wheat or other crops can transport things.
Most importantly, since photosynthesis is hugely dependent upon the efficiency of transport, silicon and boron are essential for efficient photosynthesis. Energy has to travel in chemical form from the chloroplasts, which capture sunlight, to where sugars are made. Also any newly made sugars have to get out of the way of the next sugars being made, and so forth. Anything that slows down transport slows down photosynthesis and will ultimately slow down the nitrogen fixation that chlorophyll formation depends on.
Sugars & Nitrogen Fixation
Usually sugar is the most limiting factor in nitrogen fixation. This shows up in root exudate overlap. Where garlic, ginger, corn, beans, bananas, etc., double their root density in the soil and have root exudate overlap between plants, they grow more vigorously.
Ever notice where corn is planted too thickly so that five or six seeds sprout in just a few inches? Always the corn sprouts in the middle grow fastest. Later if the corn isn’t thinned there may be competition for nutrients and moisture; but if nutrient and moisture competition was all that was going on the middle corn seedlings wouldn’t be the most robust.
Native Americans used to plant corn — without fertilizer — as a soil-building crop by planting their seeds in triangle shaped groups or hills to maximize root exudation, nitrogen fixation, and amino acid uptake. They grew big, tall, long-season corns that built carbon into their soils. In some cases they bundled the stover for winter fuel, which they burned, sprinkling the ashes back on their fields. They did this for hundreds and even thousands of years without recourse to nitrogen fertilizers. In terms of efficiency, agriculture took some giant steps backward in the 20th century.
If we had corn planters that perfectly singulated seed and we could plant with double drills that alternated seeds from left and right drills with 10-inch spacing in each drill and 5 inches in between drills, the seeds would come up in a zigzag pattern that maximizes root exudate overlap in high population corn plantings. This would minimize the need for nitrogen fertilizers.
Soil Testing: An Eye-Opener
As an agricultural consultant in far northern Queensland, Australia, I grew $2,000-$3,000 of culinary ginger in my garden as well as an aloe vera nursery without nitrogen fertilizers. Both were high-silicon crops. At nearby Mt. Garnet we had a diatomaceous earth mine that sold diatomaceous earth (DE) at $300/ton — somewhat pricey, but an excellent silicon fertilizer. When I sprinkled this DE on my ginger it grew beautifully and was twice as robust wherever I spilled a liberal amount. The same was true for my aloe vera. What was clear was that nitrogen fixation and amino acid uptake by both ginger and aloe was far more abundant with high-silicon availability. On a nearby banana farm using the same diatomaceous earth at a rate of 1 ton per hectare (2.5 acres) there were 1.28 more new leaves per month, a sure sign of quality nitrogen availability and robust growth. This meant silicon was a huge influence in nitrogen fixation.
One of the most common problems is too much soluble nitrogen at any given time. A little nitrogen on a steady basis is good, but it is easy to go overboard. Nitrogen availability is a double-edged sword because too much soluble N leads to the nitrification of amino acids, which strips silicon and boron from the soil while shutting down nitrogen fixation. The result is insufficient transport in following crops. We have to be observant and intelligent in our management of soil nitrogen, as ignorance is hardly bliss.
Grasses usually are the best silicon accumulators, which makes maintaining them in our soil cover along with legumes a good idea. Bare soil is always a dead loss and a sure way to ensure silicon and boron leaching — which easily results from too much cultivation, and this welcomes weeds. Weeds love soluble nutrients, which is one of the reasons we don’t want soluble nutrients. What we want is insoluble but available nutrients, and we want to get all our nitrogen from the air where it is abundant.
My target on pastures is to keep soluble silicon levels above 80 ppm with totals above 1,000 ppm — not so hard without nitrogen fertilizer abuse. For tomatoes I like 100 ppm soluble silicon which is more difficult; and for cherries — a really silicon-sensitive crop — I aim for 120 ppm. This really takes good management though it pays off handsomely. Hopefully American soil laboratories will take total soil testing on board like my Australian lab, Environmental Analysis Laboratories (EAL).
Though growers can send samples to EAL, I’d prefer a quicker, more responsive domestic approach. So far Texas Plant and Soil Lab in Edinburg, Texas, and Midwest Laboratories in Omaha, Nebraska, have indicated interest. I’m not sure how they do with the Mehlich III analysis, my preference, but I’d like to think they can perform adequate total soil testing including totals for C, N and S.
This article was published in the April 2013 issue of Acres U.S.A. magazine.