Organic Gardener's Composting Part 14

Louise Howard, Sir Albert's second wife, made a very similar judgment in her book, _Sir Albert Howard in India._

"A fertile soil, that is, a soil teeming with healthy life in the shape of abundant microflora and microfauna, will bear healthy plants, and these, when consumed by animals and man, will confer health on animals and man. But an infertile soil, that is, one lacking in sufficient microbial, fungous, and other life, will pa.s.s on some form of deficiency to the plants, and such plant, in turn, who pa.s.s on some form of deficiency to animal and man."

Although the two quotes substantively agree, Krasilnikov had a broader understanding. The early writers of the organic movement focused intently on mycorrhizal a.s.sociations between soil fungi and plant roots as _the_ hidden secret of plant health. Krasilnikov, whose later writings benefited from ma.s.sive Soviet research did not deny the significance of mycorrhizal a.s.sociations but stressed plant-bacterial a.s.sociations. Both views contain much truth.

Krasilnikov may well have been the greatest soil microbiologist of his era, and Russians in general seem far ahead of us in this field.

It is worth taking a moment to ask why that is so. American agricultural science is motivated by agribusiness, either by direct subsidy or indirectly through government because our government is often strongly influenced by major economic interests. American agricultural research also exists in a relatively free market where at this moment in history, large quant.i.ties of manufactured materials are reliably and cheaply available. Western agricultural science thus tends to seek solutions involving manufactured inputs.

After all, what good is a problem if you can't solve it by profitably selling something.

But any Soviet agricultural researcher who solved problems by using factory products would be dooming their farmers to failure because the U.S.S.R.'s economic system was incapable of regularly supplying such items. So logically, Soviet agronomy focused on more holistic, low-tech approaches such as manipulating the soil microecology. For example, Americans scientifically increase soil nitrogen by spreading industrial chemicals; the Russians found low-tech ways to brew bacterial soups that inoculated a field with slightly more efficient nitrogen-fixing microorgamsms.

Soil microbiology is also a relatively inexpensive line of research that rewards mental cleverness over ma.s.sive investment. Multimillion dollar laboratories with high-tech equipment did not yield big answers when the study was new. Perhaps in this biotech era, recombinant genetics will find high-tech ways to tailor make improved microorganisms and we'll surpa.s.s the Russians.

Soil microorganism populations are incredibly high. In productive soils there may be billions to the gram. (One gram of fluffy soil might fill 1/2 teaspoon.) Krasilnikov found great variations in bacterial counts. Light-colored nonproductive earths of the North growing skimpy conifer trees or poor crops don't contain very many microorganisms. The rich, black, grain-producing soils of the Ukraine (like our midwestern corn belt) carry very large microbial populations.

One must be clever to study soil microbes and fungi. Their life processes and ecological interactions can't be easily observed directly in the soil with a microscope. Usually, scientists study microorganisms by finding an artificial medium on which they grow well and observe the activities of a large colony or pure culture--a very restricted view. There probably are more species of microorganisms than all other living things combined, yet we often can't identify one species from another similar one by their appearance. We can generally cla.s.sify bacteria by shape: round ones, rod-shaped ones, spiral ones, etc. We differentiate them by which antibiotic kills them and by which variety of artificial material they prefer to grow on. Pathogens are recognized by their prey.

Still, most microbial activities remain a great mystery.

Krasilnikov's great contribution to science was discovering how soil microorganisms a.s.sist the growth of higher plants. Bacteria are very fussy about the substrate they'll grow on. In the laboratory, one species grows on protein gel, another on seaweed. One thrives on beet pulp while another only grows on a certain cereal extract.

Plants "understand" this and manipulate their soil environment to enhance the reproduction of certain bacteria they find desirable while suppressing others. This is accomplished by root exudates.

For every 100 grams of above-ground bioma.s.s, a plant will excrete about 25 grams of root exudates, creating a chemically different zone (rhizosphere) close to the root that functions much like the culture medium in a laboratory. Certain bacteria find this region highly favorable and multiply prolifically, others are suppressed.

Bacterial counts adjacent to roots will be in hundreds of millions to billions per gram of soil. A fraction of an inch away beyond the influence of the exudates, the count drops greatly.

Why do plants expend energy culturing bacteria? Because there is an exchange, a _quid pro quo._ These same bacteria a.s.sist the plant in numerous ways. Certain types of microbes are predators. Instead of consuming dead organic matter they attack living plants. However, other species, especially actinomycetes, give off antibiotics that suppress pathogens. The multiplication of actinomycetes can be enhanced by root exudates.

Perhaps the most important benefit plants receive from soil bacteria are what Krasilnikov dubbed "phytamins," a word play on vitamins plus _phyta_ or "plant" in Greek. Helpful bacteria exude complex water-soluble organic molecules that plants uptake through their roots and use much like humans need certain vitamins. When plants are deprived of phytamins they are less than optimally healthy, have lowered disease resistance, and may not grow as large because some phytamins act as growth hormones.

Keep in mind that beneficial microorganisms cl.u.s.tering around plant roots do not primarily eat root exudates; exudates merely optimize environmental conditions to encourage certain species. The main food of these soil organisms is decaying organic matter and humus.

Deficiencies in organic matter or soil pH outside a comfortable range of 5.75-7.5 greatly inhibit beneficial microorganisms.

For a long time it has been standard "chemical" ag science to deride the notion that plant roots can absorb anything larger than simple, inorganic molecules in water solution. This insupportable view is no longer politically correct even among adherents of chemical usage.

However, if you should ever encounter an "expert" still trying to intimidate others with these old arguments merely ask them, since plant roots cannot a.s.similate large organic molecules, why do people succeed using systemic chemical pesticides? Systemics are large, complex poisonous organic molecules that plants uptake through their roots and that then make the above-ground plant material toxic to predators. Ornamentals, like roses, are frequently protected by systemic chemical pesticides mixed into chemical fertilizer and fed through the soil.

Root exudates have numerous functions beyond affecting microorganisms. One is to suppress or encourage the growth of surrounding plants Gardeners experience this as plant companions and antagonists. Walnut tree root exudates are very antagonistic to many other species. And members of the onion family prevent beans from growing well if their root systems are intermixed.

Many crop rotational schemes exist because the effects of root exudates seem to persist for one or even two years after the original plant grew That's why onions grow very well when they are planted where potatoes grew the year before. And why farmers grow a three year rotation of hay, potatoes and onions. That is also why onions don't grow nearly as well following cabbage or squash.

Farmers have a much easier time managing successions. They can grow 40 acres of one crop followed by 40 acres of another. But squash from 100 square feet may overwhelm the kitchen while carrots from the same 100 square feet the next year may not be enough. Unless you keep detailed records, it is hard to remember exactly where everything grew as long as two years ago in a vegetable garden and to correlate that data with this year's results. But when I see half a planting on a raised bed grow well and the adjacent half grow poorly, I a.s.sume the difficulty was caused by exudate remains from whatever grew there one, or even, two years ago.

In 1990, half of crop "F" grew well, half poorly. this was due to the presence of crop "D" in 1989. The gardener might remember that "D" was there last year. But in 1991, half of crop "G" grew well, half poorly. This was also due to the presence of crop "D" two years ago. Few can make this a.s.sociation.

These effects were one reason that Sir Albert Howard thought it was very foolish to grow a vegetable garden in one spot for too many years. He recommended growing "healing gra.s.s" for about five years following several years of vegetable gardening to erase all the exudate effects and restore the soil ecology to normal.

Mycorrhizal a.s.sociation is another beneficial relations.h.i.+p that should exist between soil organisms and many higher plants. This symbiotic relations.h.i.+p involves fungi and plant roots. Fungi can be pathogenic, consuming living plants. But most of them are harmless and eat only dead, decaying organic matter. Most fungi are soil dwellers though some eat downed or even standing trees.

Most people do not realize that plant roots adsorb water and water-soluble nutrients only through the tiny hairs and actively growing tips near the very end of the root. The ability for any new root to absorb nutrition only lasts a short time, then the hairs slough off and the root develops a sort of hard bark. If root system growth slows or stops, the plant's ability to obtain nourishment is greatly reduced. Roots cannot make oxygen out of carbon dioxide as do the leaves. That's why it is so important to maintain a good supply of soil air and for the soil to remain loose enough to allow rapid root expansion.

When roots are cramped, top growth slows or ceases, health and disease resistance drops, and plants may become stressed despite applications of nutrients or watering. Other plants that do not seem to be competing for light above ground may have ramified (filled with roots) far wider expanses soil than a person might think. Once soil is saturated with the roots and the exudates from one plant, the same s.p.a.ce may be closed off to the roots of another. Gardeners who use close plantings and intensive raised beds often unknowingly b.u.mp up against this limiting factor and are disappointed at the small size of their vegetables despite heavy fertilization, despite loosening the earth two feet deep with double digging, and despite regular watering. Thought about in this way, it should be obvious why double digging improves growth on crowded beds by increasing the depth to which plants can root.

The roots of plants have no way to aggressively breakdown rock particles or organic matter, nor to sort out one nutrient from another. They uptake everything that is in solution, no more, no less while replacing water evaporated from their leaves. However, soil fungi are able to aggressively attack organic matter and even mineral rock particles and extract the nutrition they want. Fungi live in soil as long, complexly interconnected hair-like threads usually only one cell thick. The threads are called "hyphae." Food circulates throughout the hyphae much like blood in a human body.

Sometimes, individual fungi can grow to enormous sizes; there are mushroom circles hundreds of feet in diameter that essentially are one single very old organism. The mushrooms we think of when we think "fungus" are actually not the organism, but the transitory fruit of a large, below ground network.

Certain types of fungi are able to form a symbiosis with specific plant species. They insert a hyphae into the gap between individual plant cells in a root hair or just behind the growing root tip. Then the hyphae "drinks" from the vascular system of the plant, robbing it of a bit of its life's blood. However, this is not harmful predation because as the root grows, a bark develops around the hyphae. The bark pinches off the hyphae and it rapidly decays inside the plant, making a contribution of nutrients that the plant couldn't otherwise obtain. Hyphae breakdown products may be in the form of complex organic molecules that function as phytamins for the plant.

Not all plants are capable of forming mycorrhizal a.s.sociations.

Members of the cabbage family, for example, do not. However, if the species can benefit from such an a.s.sociation and does not have one, then despite fertilization the plant will not be as healthy as it could be, nor grow as well. This phenomenon is commonly seen in conifer tree nurseries where seedling beds are first completely sterilized with harsh chemicals and then tree seeds sown. Although thoroughly fertilized, the tiny trees grow slowly for a year or so.

Then, as spores of mycorrhizal fungi begin falling on the bed and their hyphae become established, scattered trees begin to develop the necessary symbiosis and their growth takes off. On a bed of two-year-old seedlings, many individual trees are head and shoulders above the others. This is not due to superior genetics or erratic soil fertility. These are the individuals with a mycorrhizal a.s.sociation.

Like other beneficial microorganisms, micorrhizal fungi do not primarily eat plant vascular fluid, their food is decaying organic matter. Here's yet another reason to contend that soil productivity can be measured by humus content.

CHAPTER EIGHT

Maintaining Soil Humus

Organic matter benefits soil productivity not because it is present, but because all forms of organic matter in the soil, including its most stable form--humus--are disappearing. Mycorrhizal fungi and beneficial bacterial colonies around plant roots can exist only by consuming soil organic matter. The slimes and gums that cement soil particles into relatively stable aggregates are formed by microorganisms as they consume soil organic matter. Scats and casts that _are_ soil crumbs form only because organic matter is being consumed. If humus declines, the entire soil ecology runs down and with it, soil tilth and the health and productivity of plants.

If you want to manage your garden soil wisely, keep foremost in mind that the rate of humus loss is far more important than the amount of humus present. However, natural processes remove humus without our aid or attention while the gardener's task is to add organic matter.

So there is a very understandable tendency to focus on addition, not subtraction. But, can we add too much? And if so, what happens when we do?

How Much Humus is Soil Supposed to Have?

If you measured the organic matter contents of various soils around the United States there would be wide differences. Some variations on crop land are due to great losses that have been caused by mismanagement. But even if you could measure virgin soils never used by humans there still would be great differences. Hans Jenny, a soil scientist at the University of Missouri during the 1940s, noticed patterns in soil humus levels and explained how and why this occurs in a wonderfully readable book, _Factors in Soil Formation._ These days, academic agricultural scientists conceal the basic simplicity of their knowledge by unnecessarily expressing their data with exotic verbiage and higher mathematics. In Jenny's time it was not considered demeaning if an intelligent layman could read and understand the writings of a scientist or scholar. Any serious gardener who wants to understand the wide differences in soil should become familiar with _Factors in Soil Formation._ About organic matter in virgin soils, Jenny said:

"Within regions of similar moisture conditions, the organic matter content of soil ... decreases from north to south. For each fall of 10 degree C (18 degree F) in annual temperature the average organic matter content of soil increases two or three times, provided that [soil moisture] is kept constant."

Moist soil during the growing season encourages plant growth and thus organic matter production. Where the soil becomes dry during the growing season, plant growth slows or stops. So, all things being equal, wet soils contain more organic matter than dry ones.

All organic matter eventually rots, even in soil too dry to grow plants. The higher the soil temperature the faster the decomposition. But chilly (not frozen) soils can still grow a lot of bioma.s.s. So, all things being equal, hot soils have less humus in them than cold ones. Cool, wet soils will have the highest levels; hot, dry soils will be lowest in humus.

This model checks out in practice. If we were to measure organic matter in soils along the Mississippi River where soil moisture conditions remain pretty similar from south to north, we might find 2 percent in sultry Arkansas, 3 percent in Missouri and over 4 percent in Wisconsin, where soil temperatures are much lower. In Arizona, unirrigated desert soils have virtually no organic matter.

In central and southern California where skimpy and undependable winter rains peter out by March, it is hard to find an unirrigated soil containing as much as 1 percent organic matter while in the cool Maritime northwest, reliable winter rains keep the soil damp into June and the more fertile farm pastures or natural prairies may develop as much as 5 percent organic matter.

Other factors, like the basic mineral content of the soil or its texture, also influence the amount of organic matter a spot will create and will somewhat increase or decrease the humus content compared to neighboring locations experiencing the same climate. But the most powerfully controlling influences are moisture and temperature.

On all virgin soils the organic matter content naturally sustains itself at the highest possible level. And, average annual additions exactly match the average annual amount of decomposition. Think about that for a moment. Imagine that we start out with a plot of finely-ground rock particles containing no life and no organic matter. As the rock dust is colonized by life forms that gradually build in numbers it becomes soil. The organic matter created there increases nutrient availability and accelerates the breakdown of rock particles, further increasing the creation of organic matter.

Soil humus steadily increases. Eventually a climax is sustained where there is as much humus in the soil as there can be.

The peak plant and soil ecology that naturally lives on any site is usually very healthy and is inevitably just as abundant as there is moisture and soil minerals to support it. To me this suggests how much organic matter it takes to grow a great vegetable garden. My theory is that in terms of soil organic matter, vegetables grow quite well at the humus level that would peak naturally on a virgin site. In semi-arid areas I'd modify the theory to include an increase as a result of necessary irrigation. Expressed as a rough rule of thumb, a mere 2 percent organic matter in hot climates increasing to 5 percent in cool ones will supply sufficient biological soil activities to grow healthy vegetables if _the mineral nutrient levels are high enough too._

Recall my a.s.sertion that what is most important about organic matter is not how much is present, but how much is lost each year through decomposition. For only by decomposing does organic matter release the nutrients it contains so plants can uptake them; only by being consumed does humus support the microecology that so markedly contributes phytamins to plant nutrition, aggressively breaks down rock particles and releases the plant nutrients they contain; only by being eaten does soil organic matter support bacteria and earthworms that improve productivity and create better tilth.