Michigan State University Extension
Soils & Soil Management - Fertilizer - 06109721
07/10/97
Nutrient Management to Protect Water Quality Extension Bulletin WQ-25, January 1996
Maurice L. Vitosh and Lee W. Jacobs Extension Soil Specialists Department of Crop and Soil Sciences Michigan State University
When crops are irrigated, conscientious management of irrigation water is necessary for good N management. Because nitrate N is readily soluble in water, excess water from irrigation or precipitation can cause nitrates to move below the root zone. Precise scheduling of irrigation water during the growing season can minimize these percolation losses.
Several methods are available to schedule irrigation water accurately. All methods require knowledge of the soil's water-holding capacity and ability to determine or estimate the available soil moisture at any time during the growing season.
The soil moisture procedure requires an estimate of soil moisture. Irrigation begins when 50 percent of the available soil moisture is depleted. Tensiometers, which measure the tension with which soil holds water, are excellent tools for use on sandy soils. Their use on fine-textured soils, however, is usually unsatisfactory. Soil moisture blocks, which measure water conductivity, are better instruments to use.
The water balance approach requires an estimate of crop water use (i.e., evapotranspiration). Computerized programs do an excellent job of estimating crop water use and keeping track of soil moisture. Additional information on irrigation scheduling can be found in the Michigan Irrigation Guide, which is available from county MSU Extension offices or the Department of Agricultural Engineering at MSU.