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F •° <br />~4 <br />PRINCIPLES OF SOIL TESTING FOR <br />FERTILIZER RECOMPfENDATIONS <br />A. E. Ludwick <br />Department of Agronomy <br />Colorado State University <br />C .'.: <br /> <br />Soil testing for fertilizer recommendations is most frequently <br />used in intensive crop production systems. In this capacity con- <br />siderable research has been conducted over the years on testing metho- <br />dology and relationships to fertilizer recommendations. The potential <br />of soil testing as a useful tool for other plant-soil systems, in- <br />cluding high altitude revegetation, certainly exists. In fact, soil <br />testing is being used in this capacity today, but considerable re- <br />search is needed to fully develop its potential. To do this, basic <br />testing principles must be clearly understood. <br />BASIC CONCEPT <br />Nutrients exist in soil in diverse chemical forms of differing <br />availabilities to plants. Plant roots can absorb only certain of <br />these forms which are usually only a small amount of the total. <br />Therefore, tests are developed to extract only a "portion" of the . <br />nutrients from soil--that portion related to plant availability; <br />~: Such tests frequently are called "availability indexes." Total <br />nutrient content is, therefore, of little concern [o a laboratory <br />analyzing soils for the purpose of fertilizer recommendations. <br />DEVELOPMENT <br />Development of a soil test involves two steps. The first is <br />the selection (or creation) of a soil extract that will remove an <br />amount of a nutrient from the soil that. is proportional [o what a <br />plant extracts. This is usually done b.y conducting experiments•in <br />the greenhouse where plants are grown in pots on soils typical of <br />'~ those to be analyzed for fertilizer recowoendations. Following the <br />desired growth period(s), the plants are harvested and analyzed. <br /> <br />157 <br />