As observation is a key to learning, closely comparing a handful of rich garden soil with one from a worn out field can teach us a lot. The garden soil, with its additions of organic matter and minerals, will be dark and crumbly, while the worn out soil will be lighter in color and compact. The difference is that the former has humus.
Lichens and mosses are the first plants to grow on newly weathered rocks. As they grow, acids leaching out from their roots further decay the rocks, slowly creating and improving the soil. Higher forms of plants can then grow and the process of improving soil slowly continues.
Animals eventually enter the picture, eating plants and excreting. Their waste products speed up the soil improvement process. Acids in these wastes continue dissolving the minerals in the rocks which then become nutrients for further plant growth.
Worn out soil can be made into good garden soil, but only plants can do it, and they need the help of animals. Another difference is one we can’t see, and that is the number of living organisms that can only be seen with a microscope. A spoon full of garden soil can have billions of these microbes, where the worn out soil has just a few million, or 1,000 times less.
Farmers rest their worn out fields by sowing them back into grass and clover. The immense network of the grass roots subdivide the cloddy soil into smaller crumbs, and the clover roots dive deeper down and bring air into the soil. You may have noticed that good soil has air pockets in it, while worn out soil does not.
The life in the soil, just like you, needs air to breathe. Air is made of the elements nitrogen, oxygen, and carbon dioxide. Nitrogen is usually the limiting factor in plant growth and other chemical reactions, and oxygen is necessary for combining with the mineral elements so they can be of use. Both plants and animals cannot live without these important elements. As the sod grows, it opens up the soil so air can enter.
When the soil is open, water can enter instead of running off. Water is a combination of two elements, hydrogen and oxygen. Careful tillage opens the soil, but too much will damage the precious soil life.
Plants incorporate the element carbon into the soil through photosynthesis, which is the process of converting the carbon dioxide in the air into the carbohydrates in the plant. The water and air in the vicinity of a plant’s leaves and roots are the source of the four free elements; nitrogen, oxygen, hydrogen, and carbon. Farmers do not have to buy these when they routinely recover their cropland with a grass and clover sod.
Another great asset of grass and clover is that cows, sheep and goats love to eat them. Their stomaches contain different microbes which end up in the soil and help it get better. These animals are called ruminants, and are capable of improving soil even while living off the plants grown there.
Many other smaller animals help as well. Ants bring to the surface the finest sand. Earthworms make channels and take organic matter from the surface deeper into the soil, pulverizing it as they go. Birds, reptiles and mammals are all poking around the soil, moving minerals and organic matter around, and leaving valuable soil-building wastes behind.
The acids in the plant roots and animals wastes continually help decay rocks and release mineral nutrients. The major mineral nutrients are the elements silica, calcium, potassium and phosphorus. Other minor but vital elements are sulfur, magnesium, iron, aluminum, sodium, copper, zinc, manganese, molybdenum, cobalt, chlorine and boron. These are known as the trace elements. Most soils contain enough of the major and trace elements, in an unavailable, insoluble form, to grow abundant crops for thousands of years, with the possible exception of phosphorous. If a soil is deficient in a major or minor element, farmers need to add them.
By spreading calcium-like substances, such as lime and wood ashes, a chemical reaction occurs. These bases react with the acids to speed up the soil improvement process and release of nutrients. They are necessary for the growth of legumes, which is the family of plants clover belongs to.
But this brings a problem, as all solutions inherently bring other problems to a greater or lesser degree. Once the nutrients are released from their parent materials, the rocks, they can be leached out and washed away when it rains. We need microbes to incorporate loose nutrients into their bodies to prevent leaching, and now we see why the life in the soil is so important.
Farmers do many different things to make the land fertile and in good tilth. These things don’t work so well by themselves. Besides growing grass and clover, raising livestock and spreading wood ashes and rock dusts, farmers take great care of the life in the soil so they can grow healthy crops.