AGRI-BRIEFS   AGRONOMIC NEWS ITEMS

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

Spring 1998, No. 7

Of all the nutrients taken up by roots, phosphorus is the most dependent on diffusion for movement to the root. Its concentration in the soil solution is very low, so the mass flow of water into the plant does not carry enough phosphorus. Diffusion is active over short distances only; no farther than a quarter of an inch. In order for diffusion to supply the crop's need, the root density needs to be as high as possible, to bring more of the soil's phosphorus within that quarter inch range of the root.

In the field, many crops have an association with fungi that helps to expand the absorbing surface of the root system. These fungi are called mycorrhizae, and their long filaments are called hyphae. Mycorrhizal hyphae have much smaller diameters than roots. They can penetrate finer pores, and get close access to parts of the soil where roots are excluded. A mycorrhizal plant may have hyphae 100 to 1000 times longer than its roots. Mycorrhizae can extend the phosphorus uptake range by four times or more.

Mycorrhizal fungi are more active in no-till systems. Part of the reason is that the extensive networks of hyphae remain intact from year to year in no-till systems. Particularly with continuous corn, mycorrhizal effectiveness in supplying phosphorus to the plant can be much higher when the soil is not disturbed by tillage. The benefit of an intact mycorrhizal network is seen mainly in the first few weeks of the growing season, when phosphorus is critical.

Mycorrhizae do not supply nutrients to the plant without cost. The vast network of hyphae requires carbohydrate energy from the plant for its support. Indeed, in calcareous soils, mycorrhizae are thought to dissolve insoluble calcium phosphates by releasing large amounts of carbon dioxide from their active metabolism. Under certain conditions, mycorrhizae may build up to excessive levels. One hypothesis explaining why continuous corn yields are lower than those in rotation is that continuous corn eventually builds up the mycorrhizal population to levels that are detrimental. Alternating between corn and soybeans may maintain mycorrhizae at more beneficial levels.

Phosphorus application can reduce the activity of mycorrhizae, but at normal rates the organisms still play a vital role in nutrient uptake. The fungi can benefit the plant in absorption of micronutrients as well. Excessive applications of phosphorus can eliminate mycorrhizae altogether, but, at extremely low levels of soil phosphorus, mycorrhizal activity can actually increase in response to fertilizer phosphorus additions.

Phosphorus availability cannot be taken for granted, no matter what the tillage management. Less tillage imposes physical and chemical constraints on phosphorus availability, but opens up biological opportunities. Nevertheless, in all tillage systems, biological activity and economic productivity depend on an adequate supply of phosphorus.