Improving Soil Sampling Strategies

Issue: Soil sampling is generally not practiced often enough by Texas producers, and the usual sampling strategies have not been re-evaluated in many years. Improper soil sampling can result in incorrect fertilizer applications, loss of production and profits, as well as contamination of the environment.

What has been done/discovered: Depth to soil sampling is the first concern. Historically in Texas, soil sampling for nitrate and nitrogen fertilizer recommendations has been done to 6 inches. Our project has shown that this practice can greatly over-estimate fertilizer recommendations by ignoring the often high levels of residual nitrate in clayey subsoils of the 6 to 24 inch layer. Where to take soil samples in the field is the second major concern. Grid soil sampling from 0.5 acre to 2.5 acre-grid has been used by researchers and 2.5 acre-grid is used by commercial fertilizer applicators to produce soil test maps. Variable-rate fertilizer application is a service that is now being offered by commercial applicators in the Southern High Plains of Texas. However, grid-soil sampling has received criticism as a practice that a producer could not be able to do profitably. Therefore, there has been interest in “management zone”– based soil sampling. Results from our research show that “directed” soil sampling from management zones based on soil type, landscape position, and/or zones in yield maps can be as effective as grid soil sampling. Specifically, four to six soil samples can be taken from each zone and composited, so that one soil sample from each zone can be sent to a state or private soil testing laboratory for analysis. With this approach, producers can apply different rates of fertilizer to a small number of zones. This would especially be doable, even without variable-rate equipment, if soil tests in certain zones recommend no fertilizer application.

Impact: The management zone approach to soil sampling and deep soil sampling are easy to understand and to implement. These improved strategies can result in substantial reductions in farmers’ fertilizer application rates and associated costs. Additionally, these approaches will mitigate excessive nitrate buildup in the subsoils of the High Plains. The amount of subsoil nitrate that leaches deep enough to reach the groundwater (> 100 feet deep) is not clear, but reducing residual soil nitrate clearly minimizes that possibility. As of the 2001 growing season, we know of at least two prominent area crop/soil consultants who are presently doing deep soil sampling and zone soil sampling in Castro, Hale, and Lamb counties. We are promoting these improved soil sampling strategies so that their adoption widen.

Funding Sources: Texas Legislature Special Initiative on Precision Agriculture/Phosphate and Potash Institute/Foundation for Agronomic Research

Contact:
Kevin Bronson
Soil Fertility & Nutrient Management
Research & Extension Center - Lubbock
Texas A&M Agriculture Program
Phone: 806-746-4013 Fax: 806-746-6528
k-bronson@tamu.edu
http://precisionagriculture.tamu.edu/Annualrpt/report00/bronson00.htm