Chapter 7. Organic Matter in Soils

From the time plants first ventured out of the protecting waters of the sea, there has existed a kinship of soil, organic matter and plant life which has continued down through the ages. Even before the first feeble formation of land began, the processes of soil devel­opment were well under way. For millions of years, mineral elements had been accumulating: sand and gravel flaked away from rocks high in silicates; wind, water and frost disintegrated various oxides into clay, and sedimentary layers lifted above the seabed and gave up their lime.

These accumulations of rock debris were not, however, true soils. Plants, microorganisms and organic matter—life elements that had not yet appeared on dry land—were necessary for the formation of true soils.

No fossils remain today to give us a picture of the first "plants" that crept slowly out of the waves and onto the rock. Soft, without skeletal structure or fiber, these plant forms left behind no clue of how they were able to escape their dependence upon the sea. Eon after eon must have passed before one form less fragile than the rest was at last able to leave the protecting waters and establish itself permanently on dry land.

Footprints on The Sands of Time

From here on, the record is easier to read, since the descendants of these primordial bits of vegetation still cling to the rocks at the ocean's edge—the lichens that grow much as their ancestors did millions of years ago. Primitive in structure and function, they ask little of their environment. Their life-cycle helps to form soil today just as it did then. Since lichens have no roots, they have no need for soil. Instead, their hold-fasts cleave to the naked rock, emitting an acid that dissolves the stone beneath them, releasing solutions of minerals on which they live. Some of the rock resists dissolution, leaving harder grains that tumble down the slope to collect in hol­lows at the base. The lichens themselves die, crumble to dust and join the mineral build-up at the base. Fungi and bacteria invade to feed upon this pile of mineral and plant matter, and thus does the world's stock of soil increase.

In the course of checking various reference works and subject-index-files, I noted nearly 850 different phases of organic matter and their relation to soils and plants. Many of these phases would easily occupy a full chapter, while others could not be adequately treated in anything less than a full-length book.

For example, there is humus, an important end product of organic matter in soil. In 1935, Dr. Selman A. Waksman, of Rutgers Univer­sity, the noted microbiologist who developed some of our most effec­tive antibiotics, wrote a book entirely on humus and its relation to soil. It is one of the most exhaustive studies of a soil fraction in existence, yet in the end it leaves unanswered as many questions as it answers. (Anyone who thinks soil organic matter is a simple sub­ject should read Dr. Waksman's book, published by Williams and Wilkins Co., Baltimore, Md.)

The recycling of organic matter, using and reusing basic elements over and over to keep unbroken the chain of existence on earth, is vital to all life. Were it not for death and decay, all the carbon and nitrogen in the world would soon be locked up in permanent form in bodies of dead plants, animals and men.

Typical Pattern

Let us follow a plant in the garden as it dies and falls to the ground in late fall. In spring, the gardener digs it into the soil; the dead plant (what's left of it) then begins the process of changing to organic matter and humus. The words "organic matter" are used here in a special sense—the dead remains of plants and animals. Tissues of the plant that has just been plowed under are largely water—between 75 and 85 per cent in most garden vegetables. Of minerals other than water, about 10 per cent will be carbon and another 10 per cent oxygen (in compounds other than water), plus about 2 per cent hydrogen and 2 per cent ash.

What is particularly striking about the mineral makeup of organic matter is that only 2 per cent of the total of the fresh plant—the ash —is derived from soil. The rest of the plant is made up of elements from air and water. Even the nitrogen, a vital part of the total, small as it is percentagewise, came originally from the air. This empha­sizes the importance of air and water relationships (see Chapter Eleven).

By the time our dead plant is plowed under, it has lost much of its water but because winter temperatures are low, bacteria and fungi have not yet been able to attack the solid matter it contains. As soon as soil temperatures go above 60 degrees F. for several days, bacteria and fungi will become active. Their first attacks will be on the starches and sugars in the plant tissues—energy foods they need for continued activity. They will also go to work on proteins, which they need for cell growth.

Fast and Slow

Cellulose (the fibrous plant substance that forms the cotton and flax of commerce, for example) decays more readily than do waxes, fats and lignin, but cellulose is more resistant than protein, starches and sugars. The more readily decomposed substances break down quickly into alcohols, aldehydes, amides, amino acids and similar products.

The starches and sugars are almost immediately absorbed by soil bacteria and fungi and used as energy foods. Since these plant forms do not produce chlorophyll, they must get these energy foods from an outside source.

At this stage of breakdown, some humus is formed by an interest­ing process. Protein is attacked at the same time as starches and sugars but because a more elaborate breakdown process is required for it, there will be some free protein in the soil solution along with lignin. These two substances have a strong attraction for each other and will combine to form humus long before it would otherwise be produced in the end stages of organic breakdown. Any protein which does not combine at this stage will go to feed bacteria and fungi, except for the portion that rotifers, protozoa and other animal forms manage to steal from them.

Cellulose next breaks down, releasing hydrogen, carbon dioxide and methane as by-products. Only two species of bacteria are known to decompose cellulose, both of which are slowed up by acid (low pH) soil. One reason why adding lime to compost (raising the pH) speeds up decomposition is that the cellulose bacteria are stimulated. However, too high pH is just as bad as too low.

Variable Methane

The mention of methane in garden soils may seem strange, since it is a gas usually associated with marshy lands and swampy areas. Actually, while methane is called marsh gas, it is released by many forms of microorganisms that work on carbohydrates, organic acids and proteins. However, in normal soils, it is used as a source of energy by both bacteria and fungi and so does not reach the atmos­phere. In swamps, lack of air in the soil favors anaerobic forms of microorganisms that work in the absence of oxygen and do not utilize methane for energy, allowing it to escape into the atmos­phere unchanged.

By this time, many series of chemical compounds are in the soil from the tissues of our plant. In addition to those already mentioned, there are sulfates, phosphates, calcium compounds and others. With all of these being released, and processes by the dozens proceeding simultaneously, a seething ferment of infinite complexity results and tremendous amounts of energy are being used. Someone has called organic matter the fuel for bacterial fires in the soil—certainly an apt metaphor.

Organic material containing a great deal of lignin, such as saw­dust or wood shavings, presents a problem because usually its starch content is either limited or unavailable, and if nitrogen is supplied in large amounts, the lignin-protein conversion to humus is heavy. For this reason, sawdust and other woody fibers do not become available readily and should be considered as long-time amendments to the soil. Eventually the humus does break down, of course, but most gardeners are too impatient to wait for this effect.

Oils for Humus

Our plant originally plowed under has now released most of its elements to the soil. However, except for the lignin-protein, true humus formation has yet to take place. A major source of humus, surprisingly enough, is a portion of plant materials—fats, waxes, oils and resins—so often mentioned in British gardening literature as likely to ruin a compost pile. I was once told by a farmer that he wouldn't dare spread spoiled shell corn on his fields to rot because corn grain was so oily it would ruin his soil.

Like so many traditional but false ideas in agriculture and garden­ing, this notion was accepted without question. I believed it up to about twenty years ago, until one day I was lunching with Dr. Emil Truog, the very able head of the soils department at the University of Wisconsin. I mentioned I had thrown away some oil-soaked saw­dust rather than add it to my compost pile. His only remark was "Why?" In the discussion that followed, I learned that here we are dealing with a half-truth—an observation not carried to a logical conclusion. It is true that as we watch dissolution of organic matter in a compost pile we do find that waxes, fats and resins are still intact at the end of the first year. It is, however, this very resistance to decay which makes them ideal elements in humus formation.

This does not mean the perfect compost pile is one made up of candles, rancid lard and spent crank-case oil. A pile composed en­tirely of fatty materials would never get started on decomposition. It does mean that we should not avoid including some oil, fatty or waxy wastes in making compost piles.

A point worth restating here is that all soil processes involve living organisms. When we attempt to study them by chemical analyses, the bacteria and fungi (even if not killed) are no longer in their normal environment. Thus tests are inaccurate. It is not surprising, therefore, to find the careful soil scientist using words such as "I feel that …," "possibly," or "probably," far more often than positive, dogmatic words about the action and nature of organic matter.

SOIL CONDITIONING

During the first weeks following the introduction of chemical soil conditioners, I often made a statement to the effect that "Five dollars spent on organic matter will do your soil far more good than $50.00 spent on chemical conditioners." At that time, these chem­icals were being touted as "permanent amendments and condi­tioners." Today, we know that they have a life of about three years in soils that are worked annually. If I were making that statement today, I would change the latter figure to $100.

In contrast to the three-year life for chemicals, barnyard manures have shown both soil conditioning and fertilizing effects for as long as 50 to 60 years following application.

Organic matter is a soil conditioner which can be matched by no other material. When it can be had practically free, as it can from the home compost pile, it is sheer folly to let it go to waste. Humus is a storehouse for plant "foods" and (along with absorption by bacteria and fungi) prevents the loss of nutrients if they are not used by plants immediately. The extent of these nutrient reserves can be enormous, as in the case of rich prairie soils of the Middle West, many of which continued to produce crops for half a century after they were broken by the plow, often without the addition of any outside source of fertility. While such soil depletion is to be con­demned, the case mentioned does show the extent to which soil rich in humus can build up stores of plant nutrients.

In addition to this function, humus also makes a soil more porous so that air and water move freely. Humus-rich soil turns easily under the plow or spade and does not pack readily. There is a special "feel" to such gardener's loam which the true gardener learns to know.

One of the most valuable functions served by organic matter is in providing favorable living conditions for bacteria, fungi and other microorganisms. When soil is examined under the microscope, the bacteria will be found clustered around nodes of decaying vegetation or clinging to crumbs of humus. Here they find the food, moisture and air they must have to maintain life and carry on their soil-improving functions.

One of the first steps in improving problem soils such as sands and heavy clays is to mix in plenty of organic matter and thus build up the population of microorganisms. Whether this applied material is compost, rotted manure, spent mushroom manure or dried sheep manure bought in bags from the garden center, it will help "seed" the soil with essential bacteria at the same time that it creates an environment in which they can thrive.

Chapter Digest

Organic matter is the "intermediary" that keeps all soil functions in harmony. It is made up of dead plant and animal tissues (plus minerals), and provides, in turn, the essential environment for the growth of plants that give food and pleasure to man. The ideal soil, "Gardener's Loam," is basically one that is high in organic content. The processes involved in the production of humus are complex, but the gardener's chief concern is with how to use the humus.