There is a common belief that humus is the result of the breakdown of organic materials in the soil. While this is true it is less than true because the organic materials do need to break down into simple organic compounds—and from there they need to be built back up again into large, complex carbon molecules by soil organisms whose role is to store nutrients for rainy days. These organisms, primarily actinomycetes and mycorrhizae, work in tandem with plants, storing humic acids in an easy to access form. Humic acids are too large for most organisms, such as bacteria, to absorb. Yet they are accessible to the actinomycetes and mycorrhizae and thus are insoluble but available nutrients. And that’s how we want nutrients in the soil—insoluble so they are not easily lost when it rains, but available.
The NPK theory that all soil nutrients must be soluble all at once is rather like feeding a pig six months’ worth of slop in one meal—initially it is too much. Try though the pig will, he can’t handle it all. As time goes on the banquet sours and the pig is left lacking a balanced diet while flies, yeasts, moulds and various pests move in. This is modern agriculture, and it’s not a pretty picture—you wouldn’t feed your kids that way. Surely, plants are more resilient than pigs, but as living organisms they aren’t that different.
Basically we do not want most of our nutrients to be soluble. Rather, we want them to be insoluble but available. A plant can only consume a small amount of its needs every day. Having more soluble than the daily optimum in the near vicinity of uptake roots invites unwonted guests to the table, and this creates unnecessary problems for crops. Nature, left to her own devices, provides insoluble but microbially available nutrients in the humus flywheel. Crop-symbiotic micro-organisms mop up loose nutrients and store them in the humus reserve in large, carbon complexes. Acting like bees storing honey, they maintain this nutrient reserve. Photosynthesis and root exudation feed the microbes that stock this storehouse when conditions are good, and when conditions are poor these microbial plant partners—along with protozoa—draw energy and nourishment from the humus reserves to feed the crop.
The Humus Flywheel
This reveals humus as the soil’s flywheel to keep plant growth going by feeding the digestive activity around plant roots. Humus sustains this microbial activity by providing uptake of a steady stream of quality amino acids and mineral complexes—like mother’s milk—that makes it easy for crops to assemble their proteins and grow, photosynthesize, and make nectars that are shared with the soil as root exudates—like honey. These root exudates provide energy for soil microbes that unlock minerals, fix nitrogen and feed the soil’s digestive activity—which in turn provides a milky, mineral amino acid rich feed for growth. Observation of this millennia old interplay in nature is honoured in Mosaic Scripture and elsewhere as a land flowing in milk and honey. Humus is the flywheel whose momentum fosters and sustains the milk and honey flow through thick and thin—the better the storage of insoluble but available nutrients, then the more momentum the system has.
Soluble nutrients, such as the salts of nitrogen, phosphorous and potassium, must be extremely dilute or they interfere with the sensitive micro-life of the humus flywheel. Like urine, these salts are the wastes of microbes that fix nitrogen, solubilize phosphorous and release potassium. In the soil these salts shut down the microbes that otherwise might make them available when they are awash in their own waste. If these salts are applied at rates sufficient for a couple months’ supply, they kill off soil microbes and release nutrients—which results in a flush of crop growth; but it also leads to leaching of key minerals such as sulphur, boron, silicon, calcium, copper, zinc and manganese. Chlorides tend to sterilize the soil, while phosphates and sulphates, though useful to soil microbes, can still cause harm in excess. Nitrates are especially notable for causing a flush of available nutrients and a lush response that looks good, but it’s like the long haul trucker using ‘speed’, keeping double log books and driving 5 day runs in 48 hours. The result is problematic, and there is a price.
Humic vs Fulvic
Both humic and fulvic acids are so complex and varied they are only distinguished by the size of their molecules. Fulvic acids are of low enough molecular weight they can pass through bacterial cell walls as bacterial food. Humic acid molecules are larger and can only be consumed by microbes that can ingest them, like protozoa, or by silica oriented microbes like fungi and actinomycetes (aka actinobacteria) that can take the carbon skeleton apart. Since fungi and actinomycetes often live in close partnership with plant roots, especially our food crop roots, they provide access to the humic complexes in the soil, stripping out the silicon and carbon frameworks of the clay/humus colloids, thereby releasing all the other nutrients held on these structures. However, like bees drinking nectar and concentrating it into honey, these microbes also can mop up root exudates and loose nutrients in the soil solution and combine them for storage in clay/humus complexes so bacteria and leaching do not let them go to waste.
Many bacteria and protozoa are consumers that thrive in a nutrient rich broth and break things down. When soluble nutrient levels are high in the soil, the bacteria that fix nitrogen, solubilize phosphorous and release potassium can’t function because they are awash in their own waste. This is why tilling in a green manure crop requires a waiting period of 3 or 4 weeks, over which rampant bacterial breakdown subsides, before humus formation resumes and the excesses are stored in insoluble but available complexes. Only then can crops be planted and a stable plant/microbe partnership established.
Justus von Liebig, the great 19th century chemist who introduced chemical agriculture, acknowledged toward the end of his life his mistake in assuming productive soils required the nutrients to be soluble. By then, however, the chemical industries had seen great prospects for sales. Liebig, in his retirement, was ignored, and today the error of thinking solubility is good still continues.
Consider that most crop seeds contain a food supply so they can give off nourishment for beneficial microbes—thereby attracting and multiplying their microbial partners as their roots emerge. On the other hand, most weeds have tiny seeds which rely on soluble nutrients rather than microbial partnerships. They soak up loose nutrients by design, sprouting and growing vigorously when cover crops or raw manures are tilled in. They do not rely on the humus flywheel or feed its microbes. If crops are planted immediately after mixing in fresh vegetation or manures they do not grow well. It doesn’t take much experience to see the difference between application of raw manures and the application of humified compost—the former feeds weeds and the latter feeds crops.
Likewise if we apply large doses of highly soluble fertilisers—anhydrous ammonia, superphosphate and muriate of potash—our crops then have to compete with weeds that love soluble salts like potassium nitrates. It is only when we apply humified compost that we feed the crop/microbe interactions that feed our crops with a mix of amino acids and minerals akin to milk.
Most soil tests use mild acids that do not reveal what is stored in the humus flywheel. The concept behind these tests is that several months’ worth of nutrients, especially the nitrogen, must be present in soluble form. But in reality, feeding a plant is more like feeding your kids. Plants only need a little bit of soluble food on a steady basis, rather than having it all on the table at once. To reveal what could be available from the humus reserve on a daily basis requires a testing method more like what is used for tissue analysis—a total acid digest.
Many organic growers take it on faith that if they build organic matter they will have good crops and their problems will go away. However, this is rarely the case. The clay/humus complexes in the soil are like a storehouse, and unless this storehouse has everything it needs, growth is limited to whatever is in short supply.
Since sulphur is the bio-catalyst that acts as the key in the ignition, when it is deficient both soil and plant life suffer. When boron—which leaches unless held in clay/humus complexes—is deficient, nutrient uptake lags because boron’s interaction with silicon is what draws fluids through the plant’s capillary system. And silicon, which lines the capillaries themselves, must also be sufficient, along with boron, to transport calcium and other nutrients. And, if calcium—which is essential for nitrogen chemistry and cell division—is deficient, then growth suffers. Moreover, if too much soluble potassium gets in the way of calcium and magnesium uptake, photosynthesis suffers. And even if everything else is working, without sufficient phosphorous and its trace element co-factors, chlorophyll burns up because its energy can’t be transferred into making sugar. So all these things need to be stored in the right proportions, which means we need to get the mix of major and minor nutrients right in the humus flywheel.
Understanding the Mix
In some of the world’s premier soils, such as the Ukraine, Western Missouri or Australia’s Liverpool Plains, nature’s virgin conditions provided black, crumbly clays with cation exchange capacities of nearly 80, and the first couple plantings of wheat and other cereals produced crops beyond anyone’s previous experience without any fertilisers. However, with insufficient understanding and poor management these soils went straight downhill and their enormous momentum was lost. Nevertheless, measurements of the carbon to nitrogen ratios in unexploited remnants still in their virgin state are between 9 and 10 to 1, carbon to nitrogen. Interestingly, it takes roughly 10 units of sugary carbon to fix one unit of amino acid nitrogen, so this does not seem mere coincidence. Even making industrial ammonia takes ten units of methane to make one unit of ammonia.
Comparing hundreds of total acid digest tests to field responses also revealed that a six-to-one nitrogen to sulphur ratio is desirable. When these two ratios are achieved and major and minor nutrient targets are approached so that microbial partnerships interact efficiently with the humus flywheel, then the only limit to nitrogen fixation is the energy provided by root exudation.
Since grasses make more sugars and can get them to their roots a lot faster than legumes, they can feed several times more nitrogen fixation than legumes. However, because legumes unlock minerals better with their acidic root exudates, they can feed nitrogen fixation in nodules on their roots and kick off nitrogen fixation in an otherwise mineral deficient soil. Because legumes unlock far more minerals than they use in nitrogen fixation, and because they leave these minerals behind for plants that follow, they have a reputation for getting nitrogen fixation going under tough conditions. Besides, it is easy to measure their nodules and estimate how much nitrogen was fixed, though it may be a mistake to credit their follow-on effects solely to the nitrogen fixed in their nodules. After legumes have made sufficient minerals available, grasses can easily supply the energy needed for further fixation.
Soil test information is useful in blending the right amounts of major and minor nutrients into composts or fossil humate fertilisers to ensure that both grasses and legumes have what they need. Composts and raw humates can be combined in humus based fertiliser programs, and as such they are food for life and are appropriate for growing quality crops.
Manure composts are richer in minerals and nitrogen than fossil humates, but either or both are an excellent way to add deficient nutrients in a humate complexed form. Even at only a quarter ton per acre composts and mined humates fortified with deficient nutrients can deliver significant adjustments, although imbalances and deficiencies usually require many small corrections. Fossil humates, which are more notable for nitrogen and sulphur deficiencies, generally need ammonium sulphate added along with whatever else is needed as rock phosphate, gypsum, borax, copper, zinc, manganese and sea minerals.
The total test ratios of carbon to nitrogen and sulphur can be used for nitrogen and sulphur targets while calcium, magnesium and potassium targets are derived from their percentage of base saturation. Other targets vary depending on the test used, and achieving these targets is likely to require many partial adjustments. Exact formulas for restoring optimum balance in soils is the job of a professional consultant, but in general never add more than 10 kg/ha borax, 15 kg/ha copper sulphate, 25 kg/ha zinc or manganese sulphate or 1 kg/ha sodium molybdate, cobalt sulphate of sodium selenate. In sum, blending these mineral supplements in with humified compost and/or raw humates before spreading turns an expense into a capital investment.