From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Winter 1996, No. 1


Healthy soil contains many living organisms, which help to recycle the nutrients essential to plants. Fungi account for about half of the soil's biological biomass (the total weight of the living organisms in soil). Another one-third includes bacteria and actinomycetes. The remainder consists of yeast, algae, protozoa, nematodes and earthworms. Larger organisms begin the breakdown of crop residues, but microorganisms usually limit the release of nutrients.

From the crop producer's point of view, some aspects of soil organisms are beneficial while others are detrimental. Even while they break down organic matter and release nutrients, they also incorporate these nutrients into their living tissues. Organisms contribute to building soil organic matter with the by-products of the crop residues they break down, but they also decompose organic matter. High biological activity is not necessarily beneficial, as it can mean excessive decomposition of soil organic matter.

Plants can grow well without soil organisms. Experience with hydroponic gardening or commercial greenhouse production shows that many species of plants can grow vigorously on a sterile mixture of essential elements. Hydroponic management is very demanding, however. It is more economical to grow most crop plants within a healthy soil ecology where the cycling of nutrients becomes important to ensure continuity of supply, matching the demands of plant growth.

Fertilizers impact the soil's biological biomass. Soil organisms generally require all the nutrients that are essential to crop plants, plus more. The most important nutrient for most soil organisms is fixed carbon as an energy source. Green plants fix far more carbon than any other soil organism. The reason is that plants are highly adapted to capturing energy from sunlight and converting it to the many forms of fixed carbon: carbohydrates, fibers, oils, proteins, etc. Crop plants are harvested to remove the useful portion of their fixed carbon, but even the most efficient crops return a substantial amount to the soil. The return of fixed carbon is in the form of root exudates, roots, stover and leaf litter.

Plants produce the most important nutrient for the soil's biology. It follows, therefore, that any nutrient which enhances the growth of crop plants will also enhance the soil's biological health. For example, most forms of nitrogen fertilizer contain ammonium, which can easily reach toxic concentrations in the soil near the applied fertilizer. Yet, research has shown that crops fertilized with nitrogen can build up soil organic matter, and sustain a higher level of soil microbial activity than without fertilizer. How is this possible? Applied at normal rates, fertilizers kill organisms only in small zones in the soil. These zones are quickly recolonized, and the net effect is to speed up the recycling of nutrients contained in the soil's biomass.

A recent study in Quebec compared microbial biomass nitrogen in soils under two long-term fertilizer rates. Corn was grown at a normal recommendation of 150-90-150 and at a high rate of 400-270-360 pounds per acre of N-P2O5-K2O. In soil with the higher fertilizer rates, microbial biomass nitrogen was enhanced during the early growing season. Over a nine-year period, soil organic carbon increased 20 to 25 percent. With continuous corn in short growing seasons, microbial activity and the accumulation of soil organic matter can be limited by available nitrogen.

Keep your soils healthy! Nourish your crops, and they will nourish your soil.

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