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Soil Fertility Basics: Organic Matter

by Steve Peters, Seeds of Change Agricultural Planner

The key to growing healthy, nutritious plants is maintaining fertile soil. Much more than dirt, garden soil is a richly complex ecosystem that is as dynamic and diverse as a tropical rainforest. Most soil activity, however, is invisible to the human eye so we tend to think of soil as merely a medium for holding up plants. Nothing could be further from the truth. A single gram of soil can host billions of living organisms!

The most important component for sustaining a healthy, living soil is organic matter (OM). OM comprises all organic substances in varying stages of decay including plant roots, stems, and leaves, microorganisms, and added organic materials such as compost, manure, and straw. When organic matter reaches its most mature and stable form it is called humus, a dark and fragrant, nutrient rich substance essential for long term soil fertility.

OM, and in particular humus, serves many vital purposes. It attracts and holds essential plant nutrients. It acts much like a sponge by retaining moisture. It produces vitamins, hormones, and other growth substances important for plant health. It is also critical for maintaining good soil structure by binding the mineral portion of the soil into loose aggregates. This permits air to reach the roots, allows for capillary movement of water, and enables roots to penetrate the soil.

Finally, and perhaps most significantly, organic matter is the food source for soil based organisms, including bacteria, fungi, actinomycetes, yeasts, soil-dwelling insects and earthworms. Without these living organisms plants would cease to exist. They perform a variety of critical processes including:

• Converting organic nitrogen, which is unusable to plants, into inorganic nitrogen compounds (ammonia and nitrates) which are available to plants.
  
  
• Fixing atmospheric nitrogen and making it available to plants.
  
  
• Producing organic acids that make mineral nutrients soluble and available to plants.
  
  
• Living symbiotically on plant roots and aiding the plant in the uptake of many important nutrients and growth substances.
  
  
• Preying on pathogenic organisms that would otherwise be detrimental.
  
  
• Contributing to soil aggregation by producing gum-like polysaccharides.
  
  
• Producing carbon dioxide (CO2) which is released into the atmosphere and is essential for photosynthesis.

As a home gardener, the best way to increase the organic matter content of your soil is to add mature, high quality compost. Composting is that seemingly magical process that transforms raw organic residues into a stable, sweet-smelling earthy substance that contains a balanced blend of all the essential plant nutrients. Compost can be applied to the soil at almost any rate without fear of toxicity or loss of nutrients, although more frequent, smaller applications are preferred. Compost provides a slow and steady release of nutrients throughout the growing season, and can cure a variety of problems including high or low pH or a nutrient imbalance. Mature compost is particularly important in sandy soils which lack the ability to retain much water or nutrients.

Cover Crops
Another important source of organic matter are cover crops, which are grown specifically for soil improvement and are incorporated into the soil prior to growing vegetables and other garden plants. Cover crops can be grown any time of the year.

Spring Annuals: Oats and Triticale are planted as soon as the soil can be worked in early spring.

Summer Annuals: Buckwheat, Cowpea, and Sorghum are grown only during frost-free months.

Winter Annuals: Austrian Winter Pea, Hairy Vetch and Winter Rye are planted in late summer/early fall, over winter, and resume growing the following spring.

Biennials: Yellow Blossom Sweetclover and perennials Red Clover can be grown for longer term soil-building.

Cover crops are not as effective as compost for building humus, but they stimulate a greater amount of biological activity. In heavy clay soils, fresh cover crop residues may be preferred over compost because they help open up the soil and provide drainage.

Animal Manures
Raw animal manures are not generally recommended for direct application to the garden because much of the nutrients are lost by leaching. They can also introduce unwanted weed seeds and pathogens, and can cause over-stimulated leafy growth because of excess available nitrogen. We recommend composting raw manures before putting them in the garden.

Carbon and Nitrogen
Many organic residues such as straw, leaves, and sawdust, are very high in carbon (C) relative to nitrogen (N). When incorporated into the soil, these materials will stimulate biological activity. The problem is, however, that because of a lack of N, the soil microorganisms tie up most of the available soil N, causing a nitrogen deficiency in plants. Eventually these microorganisms will die and release their N, but this could take weeks or months. Therefore, it is best not to incorporate these high-carbon residues into the soil but rather to use them in pathways, spread them as surface mulches, or to add them to your compost piles.

Soil Tests
There are soil tests available through many cooperative extension services and private soil testing laboratories, which can give you an idea of the organic matter content of your soil. However, this is often not cost effective for the home gardener. A simple way to measure the biological activity in your soil is to measure CO2 respiration, which is associated with the breakdown of organic matter by soil microorganisms. Higher levels of biological activity in the soil reflect healthier soils, which provide essential plant nutrients like nitrogen and phosphorous. The Woods End Institute, a leading soil and compost quality laboratory, has developed a simple method for the home gardener to test for CO2 respiration in soil and compost called the Solvita Garden Care Test.

For more on soil tests, click here