As was mentioned in other links (Soil parts and functions, Improving soil), that small portion of the soil called organic matter is extremely important because it is a reservoir of nutrients for plants and beneficial soil organisms, and especially because it helps improve the soil’s structure and texture.  The most important type of organic matter for improving soil structure is the decomposed substance called humus.

 Decomposition

 Fresh (or raw) organic matter, such as manure and dead plants and animals, is made up of a wide variety of complex substances, which can be grouped into major categories: carbohydrates, fats and proteins, along with minerals (as in bones and teeth) and a few others.  Most of those cannot be used by plants, but instead can be decomposed (used as food) by certain animals (vultures, worms, insects) and especially by microbes (bacteria, fungi).

 During decomposition, the complex materials are broken down into simpler substances, some of which are used by plants, including carbon dioxide, nitrogen compounds, potassium, phosphorus, sulfur, calcium, magnesium, and several trace minerals.  The most common carbohydrate components of plants are cellulose, hemicellulose, lignin, sugars and starches.  They are called high-carbon materials, since they contain mostly carbon, oxygen and hydrogen.  In contrast, proteins and other such materials (enzymes, nucleic acids) contain nitrogen, phosphorus, sulfur and the other elements listed above.  These nitrogen-containing materials are very important because plants need nitrogen to manufacture their own proteins, enzymes and nucleic acids; and nitrogen tends to be an uncommon soil nutrient.  So, when fresh organic matter is decomposed, the ideal situation is to have a mixture of high-carbon and nitrogen-containing materials.

 Composting

 In nature, organic matter is recycled as the decomposing species in an ecosystem break down whatever dead bodies and wastes become available [see the link on The agricultural ecosystem].  This sustainable recycling may only allow for a moderate amount of plant growth, as in dry or cold climates.  In human-directed agricultural systems, where large amounts of food are desired, the farmer or gardener usually has to add additional crop nutrients beyond what nature can provide – in other words, some type of fertilization.  One way to do this is by adding extra organic matter.  Supplying plant nutrients is covered in more detail in the links on Crop growth and nutrition and Fertilizers, but here we are covering primarily organic soil additives, since they also can greatly improve soil structure.

 When the decomposition of organic matter and its addition to soil is controlled by humans, it is often called composting, or the decomposed material is compost.

 There are two approaches to composting.  First, the farmer or gardener can add the fresh organic matter directly to the soil (either on the surface, or better yet in the upper several inches) and thus allow decomposition to occur in the soil. This is sometimes called sheet composting.  Or else the farmer or gardener can collect the fresh organic matter into a pile or a windrow (long ridge) and let it decompose there before applying it to the soil.

 Either method has advantages and disadvantages.  It takes time for organic matter to decompose, and the best way for this to happen is in an aerated environment, since the microbes that require oxygen produce substances useful to plants, while those that grow in low-oxygen places can cause diseases or can release toxic by-products.  Thus, if too much raw organic matter is worked into soil that is dense, as in sheet composting, it can cause problems.  Composting in a pile or windrow requires extra attention and labor, but can produce better quality compost, sometimes more rapidly.

 As you accumulate fresh organic matter to compost, you should include both high-carbon and nitrogen-containing materials, with at least 5 pounds of nitrogen-containing materials per 100 pounds of high-carbon materials, although in well-aerated piles or windrows, you might use up to 10 or 20 pounds of nitrogen materials.  High cellulose materials such as sawdust, straw or wood chips should make up less than one-third of the total, and animal manure can make up as much as 10 to 20 percent (horse, sheep and poultry manures are “stronger” than cattle manure, so less should be used).  The exact amounts aren’t that important, and different experts have different recommendations.  One general formula is one-third nitrogen-containing material to two-thirds high-carbon material.

 Some examples of high-carbon materials include crop residues, straw, old hay, leaves, grass clippings, green manures (freshly-cut plants), sawdust, wood chips, weeds, pine needles, tree bark and newspapers (coarse materials should be chopped or shredded).

 Types of nitrogen-containing materials include animal manures (but not from carnivores such as mink, dogs or cats, which may contain parasite eggs), legume hay (clover, alfalfa, vetch), sewage sludge (if treated to eliminate diseases, parasites and toxins), garbage (avoid items that could attract scavenging animals), fish meal, blood meal, bone meal, cottonseed meal and soybean meal.

 If you do not have enough nitrogen-containing materials to promote rapid decomposition, you can add a small amount (about one or two pounds per 100 pounds total) of a commercial fertilizer, such as urea or ammonium sulfate.  Sometimes adding a couple pounds per hundred of lime, granite dust or rock phosphate will stimulate microbe growth and correct too-acid conditions.  Do not add much wood ashes because they may cause too-alkaline conditions and slow decomposition. Other items to avoid adding to a compost pile include high-fat materials, coal ashes or charcoal, diseased plants, toxic chemicals such as pesticides, and manure from animals treated with antibiotics.

 To increase microbial activity in a compost pile, you can add some topsoil or previously-made compost.  Some people recommend adding a special inoculant product containing beneficial microbes, but adding soil or compost usually works just as well.  Soaking compost in water overnight also provides a good liquid “starter.”

 In order to compost well at a rapid rate, a compost pile or windrow should be at least 3 to 5 feet in diameter (farm-scale windrows are typically 4 to 5 feet high and 9 to 12 feet wide at the bottom, tapering to a point [ridge] at the top).  It is best to build a pile on a base of waterproof material (packed clay or concrete), or else spread a layer of absorbent sawdust, wood chips, corn cobs, etc., to collect moisture.  Garden-scale piles can be enclosed by a bin or woven-wire wall to keep loose material in place.  Even a large plastic trash bag can work, if ventilated.

 The organic matter in the pile will compost better if the various materials are mixed together.  If this is not done at the beginning, it will happen later when the pile is aerated (turned).  The materials should be kept moist, but not soaking wet, in order for the microbes to do their job.  Thrust your hand deep into the pile to judge moisture.  If the pile is rather dry, sprinkle it with water (if rainfall doesn’t suffice).  Avoid letting the pile get too wet in the first place, by putting it in a well-drained location and if necessary covering it during rainy periods.  If you squeeze a handful of the material and water drips out, it is too wet.  Wet material can be dried somewhat by turning it or by adding some absorbent organic matter (straw, ground corn cobs, shredded newspaper, etc.).

 Turning a compost pile or windrow means somehow lifting and mixing it, so as to admit air (and thus keep proper decomposition going, especially for the first eight weeks.  Farm-scale windrows can be turned with a special mechanical compost turner.).  Turning can also be done to dry a wet pile or to lower the temperature of a too-hot pile, since as the microbes become active, they generate a surprising amount of heat.  The temperatures in the center of an active pile should be between 100° to 150° F for best composting (it may even appear to be “steaming”).  If it gets above 160° F, the pile should be turned to cool it down.  The material should remain at above 100° F for two or three weeks, in order to kill weed seeds, parasites and disease microbes.

When the composting process is approximately complete, the temperature will drop and the material inside the pile will have an “earthy” smell.  Its volume will be only 20 to 60 percent of the original pile.  Finished compost should not have many recognizable structures left, such as plant stems or leaves; rather, it should be a crumbly, moist, humus-like material.  Finished compost can be turned a couple more times several weeks later (keep it moist), and it should be protected from heavy rain (cover with a tarp).

 Finished compost can be added to the topsoil at any time and in almost any amount without harm.  Anywhere from one-half to ten tons per acre have given good results.

 Well-decomposed compost is essentially humus, and a humus content of 5 to 10 percent greatly improves a hard, compact soil, in several ways:

  1. It improves soil texture, making it loose and crumbly, and thus well-aerated, able to absorb precipitation, and easy to till.  Soil high in organic matter resists erosion and holds more water in dry conditions than hard soil.
  2. It is a storehouse of many plant nutrients, especially nitrogen.  The nutrients in humus become available to plants slowly over the growing season, as plants need them, rather than as a sudden dose of strong nutrients that commercial fertilizers give.
  3. It helps to break down nutrients locked in soil mineral particles and put them into forms more easily used by plants (called chelates).
  4. It helps buffer (reduce severity) extreme soil pH (acidity or alkalinity).
  5. It contains millions of beneficial organisms, some of which can help nourish plants, as well as stimulate plant growth.  Soil microbes can temporarily hold nutrients inside their cells, but when they die, the nutrients become available to plants (in this way, the nutrients are kept from leaching away).

 Other sustainable methods

 In addition to composting organic matter, there are several other methods that can improve soil and help nourish plants.

  1. Compost extract.  By soaking compost in non-chlorinated water overnight or for a day, you get a brownish extract often called “compost tea.”  You can either dump some compost into a bucket or tub of water and later strain out the solids (if necessary, especially if you want to run it through a sprayer), or else put the compost in a burlap or cloth bag to keep the solids separated.  The “tea” will be swarming with beneficial microbes, along with soluble plant nutrients and soluble components of humus which help loosen dense soil.  Compost extract is an excellent plant growth stimulant and can be either sprayed on leaves or watered into the soil.
  2. Crop rotation.  This time-honored practice helps reduce soil depletion from growing the same crop year after year.  Typically, two or more crops that have completely different nutrient needs and different root systems are grown one after another.  Some deep-rooted plants draw up minerals from the subsoil, and some (the legumes, such as clovers, alfalfa, vetch, beans and peas) add extra nitrogen to the soil that is useable by the next crop.  When the crop is harvested, the root systems die and add organic matter, along with any top parts that are tilled in.  Also, growing a different crop each year confuses insect pests, which often will only eat certain types of plants.
  3. Cover crops, green manures.  Soil can be improved and protected from erosion by planting some other crop besides the main one during the “off season,” after harvesting (usually the fall and winter).  Cover crops are intended to cover bare soil and thus reduce erosion (and sometimes to shade out weeds), while green manure crops are intended to be tilled into the soil to stimulate soil microbes and add to soil humus (also called sheet composting).  One crop can serve both purposes.  Frequently used cover crops and green manures include small grains (wheat, oats, barley, rye), ryegrass, buckwheat, legumes (clovers, alfalfa, vetch), turnip, radish, kale, canola and sunflower.  You can also plant a mixture, such as oats and rye, rye and vetch, or clover and ryegrass.  Even a “crop” of weeds can improve soil, as long as you mow them before they go to seed.