Soil aeration is a common topic in most pasture management discussions. Quickly, testimonials will surface for both sides of the aeration issue. Since there is very little data to support either side, it creates an interesting topic for discussion. Actually, as with most agricultural practices, "It Depends". No doubt, where the soils are compacted from traffic or a hard pan has developed, impervious zones must be fractured for water and roots to penetrate those zones.
Often, soil that is hard when it is dry is confused with compacted or impervious zones. Hard soils are those that are firm because they are dry — a natural process. When rainfall occurs, firm soils will allow moisture percolation into the profile. This is often confused with a hard pan. Where hard pans exist, plant rooting depth and soil moisture will be limited to the impervious zone.
Soils exist in southern Oklahoma that have water ponded on the surface while there is a dry compacted zone at 18 inches. After a long wet period, it is easy to examine the soil profile and determine the moisture content at each level. Examining the soil moisture distribution will provide insight into a soil's permeability.
When the soil profile is filled with moisture, there is not a physical zone that interferes with the infiltration of water. If there is a zone that does interfere with moisture movement, then it must be fractured with some form of tillage or we have to accept the soil's limitation. Most often, plow and natural hard pans will develop from 6 to 24 inches deep. To be effective in fracturing those zones, some form of sub-soiling is required which is quite different from aeration.
A pasture aeration experiment was conducted by Jim Enis, Area Extension Agronomy Specialist, on the Thomas Ranch at Ada, Oklahoma. The results were published in the January issue of the Southeast Oklahoma Beef and Forage Newsletter. He reported that over the years 1995 and 1996, aeration of a pasture on one specific soil had little to no effect on bermudagrass, tall fescue, and clover forage production. Where nitrogen fertilizer was applied, there was an expected response to the applied nitrogen (one ton/50 pounds nitrogen) but again there was little to no effect from the aeration.
One use of shallow aeration is when overseeding sod. It appears that the small amount of soil disturbance from the aerator stimulates the germination and establishment of overseeded legumes and ryegrass. Spring planted grasses have also developed into good stands with the aerator treatment at seeding.
Aeration will result in increased yields on a limited number of soil types that have hard pans and compacted layers; therefore, other areas of management will usually pay larger dividends. Look for the most limiting factor and act upon that factor before going to the next limitation. From this perspective, aeration will be down the priority list in many pasture management systems.