Michigan State University Extension
Soils & Soil Management - Fertilizer - 06029703
07/10/97
Fertilizer Recommendations for Field Crops in Michigan Extension Bulletin E-550A, March 1992
D. R. Christenson, D. D. Warncke, M. L. Vitosh, L. W. Jacobs and J. G. Dahl, Department of Crop and Soil Sciences
A soil test must be made from a representative soil sample to obtain reliable soil results and fertilizer recommendations. For most field crops, sample and test soils at least once every three years. For high-value field crops grown under intensive management, sample and test soil annually.
Mineral Soils. Soil samples may be taken at any time during the year when soil temperature and moisture conditions permit. Before sampling a field, check for differences in soil characteristics. A soil survey map helps. Consider the productivity, topography, texture, drainage, color of topsoil and past management of the area to be sampled. If these features are uniform throughout the field, each composite sample of the topsoil should represent no more than 20 acres. Avoid sampling close to gravel roads, dead furrows, previous locations of brush, lime and manure piles, burned muck or any unusual areas. Areas of the field which differ from the general characteristics should be sampled and fertilized separately if sufficiently large to manage. For example, a sandy ridge could be sampled separately and fertilized with additional potash if required. Avoid sampling small areas which are not typical of the larger field. Fertilize these areas the same as the rest of the field.
Each composite sample should consist of at least 20 subsamples taken at random over the field. Mix the subsamples breaking the cores or chunks. Place a pint of the soil in the sample container for mailing to the laboratory. Soil sample boxes are available from your county Cooperative Extension Service office.
Organic Soils. For newly reclaimed organic soils or soils that have not been heavily fertilized, time of sampling is not important. On organic soils that have been fertilized for more than two or three years, the time of sampling is important. Considerable amounts of potassium may leach over winter. The potash recommendations are given for organic soils collected in the fall and assume the potassium test level decreases 25 percent because of leaching. For samples collected between March and June, decrease these recommendations for potash by 25 percent.