Organic Matter and Soil Fertility in the Garden - August 26, 1998
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

Most crop plants grown today have been genetically altered through plant breeding to produce a larger, more succulent crop. These crops place higher demands on soils by requiring greater inputs of plant nutrients, especially nitrogen. Corn is an excellent example. The corn we today is the result of many hybridization experiments conducted by ancient Meso-American cultures. Primitive maize varieties had much lower yields, and therefore, lower nitrogen requirements. In modern production agriculture, these nitrogen requirements are usually met by using industrially produced fertilizers because of market demands, low costs, and availability.

How did humans manage soil fertility before the industrial age? One way was by incorporating animal manure back into the soil. In visualizing societies that used animals as the sole means of transportation, we come to a realization that there must have been no shortage of manure. Manure contains all the essential plant nutrients, including a high percentage of nitrogen, and indigestible plant fiber (cellulose). Most animals cannot digest cellulose, but many soil microbes (bacteria and fungi) can. These soil microbes also break down dead plant roots as well as stems and leaves that come in contact with the soil. This is how soil organic matter is constantly replenished.

Soil organic matter consists of partially decomposed plant (and animal) tissue and humus. The partially decomposed material contain mineral nutrients, improves soil condition (porosity, structure and density) and increases water holding capacity. Humus imparts a darker color to soil, is very resistant to decomposition, increases water holding capacity, and acidifies (lowers pH) of soil. In addition, humus exists as microscopic particles that have the ability to attract and maintain a pool of plant nutrients (calcium, magnesium, potassium, etc.) on their negatively charged surfaces.

Getting back to nitrogen, it is the soil nutrient that most often limits plant growth and crop yields worldwide. The process that allows atmospheric nitrogen to become available to plants is called nitrogen fixation. It is a long, involved process and volumes of scientific literature are devoted to it. For the sake of simplicity (and to prevent readers from dozing), I'll streamline the story: specialized soil bacteria form a partnership with leguminous plant roots (the bean/pea family) resulting in the conversion of nitrogen in the air to nitrogen in the soil. This truly amazing process also occurs in many of our local native plants including mesquite, palo verde and catclaw.

Industrially produced nitrogen fertilizers such as ammonium sulfate are an easy way add nitrogen to the soil, but do not increase organic matter. One way to increase both nitrogen and organic matter is through composting. Another way is through green manuring. Green manure is live plant material grown in place for the sole purpose of being incorporated into the soil. Plants used for this purpose are called cover crops.

Cover crops are usually grown during fall or spring then tilled into the soil three to four weeks before crops are planted. Grasses and legumes are commonly used as cover crops. Grasses are great for biomass production and have a fine-textured, fibrous root system that is easily decomposed. Some common grass cover crops are sudangrass, brome, rye, oats, barley and wheat. As previously stated, legumes are desirable because they fix nitrogen. Some legumes that are commonly used for cover crops are alfalfa, clover, vetch, peas and beans. If you are planting legumes, use seeds inoculated with Rhizobacteria. This is the soil organism responsible for nitrogen fixation. Inoculants can be purchased separately or some catalogues sell pre-inoculated seeds. Make sure that the inoculants you use are fresh and the correct strain of Rhizobacteria for the legume you are planting.

In your vegetable garden, think of the soil as a bank account where you must maintain a balance of nitrogen and organic matter. As the account grows, so do the benefits to your garden.

The University of Arizona Cooperative Extension has publications and information on composting and cover crops. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at and be sure to include your address and phone number.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: March 15, 2001
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