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Composting > Humus and Soil Productivity

Humus and Soil Productivity

Books about hydroponics sound plausible. That is, until you actually see the results. Plants grown in chemical nutrient solutions may be huge but look a little "off." Sickly and weak somehow. Without a living soil, plants can not be totally healthy or grow quite as well as they might.

By focusing on increasing and maximizing soil life instead of adding chemical fertility, organic farmers are able to grow excellent cereals and fodder. On richer soils they can even do this for generations, perhaps even for millennia without bringing in plant nutrients from elsewhere. If little or no product is sent away from the farm, this subsistence approach may be a permanent agricultural system. But even with a healthy ecology few soils are fertile enough by themselves to permit continuous export of their mineral resources by selling crops at market.

Take one step further. Cereals are mostly derived from hardy grasses while other field crops have similar abilities to thrive while being offered relatively low levels of nutrients. With good management, fertile soils are able to present these lower nutritional levels to growing plants without amendment or fortification with potent, concentrated nutrient sources. But most vegetables demand far higher levels of support. Few soils, even fertile soils that have never been farmed, will grow vegetables without improvement. Farmers and gardeners must increase fertility significantly if they want to grow great vegetables. The choices they make while doing this can have a strong effect, not only on their immediate success or failure, but on the actual nutritional quality of the food that they produce.

How Humus Benefits Soil

The roots of plants, soil animals, and most soil microorganisms need to breathe oxygen. Like other oxygen burners, they expel carbon dioxide. For all of them to grow well and be healthy, the earth must remain open, allowing air to enter and leave freely. Otherwise, carbon dioxide builds up to toxic levels. Imagine yourself being suffocated by a plastic bag tied around your neck. It would be about the same thing to a root trying to live in compacted soil.

A soil consisting only of rock particles tends to be airless. A scientist would say it had a high bulk density or lacked pore space. Only coarse sandy soil remains light and open without organic matter. Few soils are formed only of coarse sand, most are mixtures of sand, silt and clay. Sands are sharp-sided, relatively large rock particles similar to table salt or refined white sugar. Irregular edges keep sand particles separated, and allow the free movement of air and moisture.

Silt is formed from sand that has weathered to much smaller sizes, similar to powdered sugar or talcum powder. Through a magnifying lens, the edges of silt particles appear rounded because weak soil acids have actually dissolved them away. A significant amount of the nutrient content of these decomposed rock particles has become plant food or clay. Silt particles can compact tightly, leaving little space for air.

As soil acids break down silts, the less-soluble portions recombine into clay crystals. Clay particles are much smaller than silt grains. It takes an electron microscope to see the flat, layered structures of clay molecules. Shales and slates are rocks formed by heating and compressing clay. Their layered fracture planes mimic the molecules from which they were made. Pure clay is heavy, airless and a very poor medium for plant growth.

Humusless soils that are mixtures of sand, silt, and clay can become extremely compacted and airless because the smaller silt and clay particles sift between the larger sand bits and densely fill all the pore spaces. These soils can also form very hard crusts that resist the infiltration of air, rain, or irrigation water and prevent the emergence of seedlings. Surface crusts form exactly the same way that concrete is finished.

Have you ever seen a finisher screed a concrete slab? First, smooth boards and then, large trowels are run back and forth over liquid concrete. The motion separates the tiny bits of fine sand and cement from denser bits of gravel. The "fines" rise to the surface where they are trowelled into a thin smooth skin. The same thing happens when humusless soil is rained on or irrigated with sprinklers emitting a coarse, heavy spray. The droplets beat on the soil, mechanically separating the lighter "fines" (in this case silt and clay) from larger, denser particles. The sand particles sink, the fines rise and dry into a hard, impenetrable crust.

Organic matter decomposing in soil opens and loosens soil and makes the earth far more welcoming to plant growth. Its benefits are both direct and indirect. Decomposing organic matter mechanically acts like springy sponges that reduce compaction. However, rotting is rapid and soon this material and its effect is virtually gone. You can easily create this type of temporary result by tilling a thick dusting of peat moss into some poor soil.

A more significant and longer-lasting soil improvement is created by microorganisms and earthworms, whose activities makes particles of sand, silt, and clay cling strongly together and form large, irregularly-shaped grains called "aggregates" or "crumbs" that resist breaking apart. A well-developed crumb structure gives soil a set of qualities farmers and gardeners delightfully refer to as "good tilth." The difference between good and poor tilth is like night and day to someone working the land. For example, if you rotary till unaggregated soil into a fluffy seedbed, the first time it is irrigated, rained on, or stepped on it slumps back down into an airless mass and probably develops a hard crust as well. However, a soil with good tilth will permit multiple irrigations and a fair amount of foot traffic without compacting or crusting.

Crumbs develop as a result of two similar, interrelated processes. Earthworms and other soil animals make stable humus crumbs as soil, clay and decomposing organic matter pass through their digestive systems. The casts or scats that emerge are crumbs. Free-living soil microorganisms also form crumbs. As they eat organic matter they secrete slimes and gums that firmly cement fine soil particles together into long lasting aggregates.

I sadly observe what happens when farmers allow soil organic matter to run down every time I drive in the country. Soil color that should be dark changes to light because mineral particles themselves are usually light colored or reddish; the rich black or chestnut tone soil can get is organic matter. Puddles form when it rains hard on perfectly flat humusless fields and may stand for hours or days, driving out all soil air, drowning earthworms, and suffocating crop roots. On sloping fields the water runs off rather than percolating in. Evidence of this can be seen in muddy streams and in more severe cases, by little rills or mini-gullies across the field caused by fast moving water sweeping up soil particles from the crusted surface as it leaves the field.

Later, the farmers will complain of drought or infertility and seek to support their crops with irrigation and chemicals. Actually, if all the water that had fallen on the field had percolated into the earth, the crops probably would not have suffered at all even from extended spells without rain. These same humusless fields lose a lot more soil in the form of blowing dust clouds when tilled in a dryish state.

The greatest part of farm soil erosion is caused by failing to maintain necessary levels of humus. As a nation, America is losing its best cropland at a nonsustainable rate. No civilization in history has yet survived the loss of its prime farmland. Before industrial technology placed thousands of times more force into the hands of the farmer, humans still managed to make an impoverished semi-desert out of every civilized region within 1,000-1,500 years. This sad story is told in Carter and Dale's fascinating, but disturbing, book called _Topsoil and Civilization _that I believe should be read by every thoughtful person. Unless we significantly alter our "improved" farming methods we will probably do the same to America in another century or two.

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