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~5 U 158 Ludwick The amount of nutrient absorbed by the plants is then compared with that extracted 6y the chemical solutions. The soil extract that compares (correlates) best with plant uptake of the nutrient is selected. The second step in development of a soil test is called "cali- bration." This involves determining the appropriate rate of fertili- zation for a given soil test level. This work must be done under actual production conditions to reflect those same conditions experienced by growing crops. This involves a series of experiments in which fertilizer rates for the nutrient in question are applied from zero [o more than adequate for maximum yield. By conducting these experiments on a wide range of soils over a period of time it is possible to relate the laboratory soil test values to actual field yields and rates of fertilizer necessary [o achieve maximum yields (or other desired guals). Since various crops have different growth habits and nutrient requirements, the same procedure must be Followed for each of [hose important in the testing area. ACTUAL PRACTICE i ~a {{ i~ i !'~ 11 The most serious problem of the CSU Soil Testing Laboratory in its daily operation is obtaining a reliable sample for analysis. This requires both careful sampling of the soil and handling between the Field and the laboratory. Fields can be very heterogeneous in plant available nutrients and selecting reasonably unifor~ areas is diffi- cult. Usually areas for a single composite sample l' are grouped based on field history and soil characteristics (color, slope, texture, drainage, and degree of erosion). Other sampling decisions besides determining the area per composite sample include sampling procedure (i. e., random vs. systematic samplings within the selected area), sampling depth, time of year to sample, and handling between the field and the laboratory. Careful attention to each of these is necessary if the soil sample is to be truly representative of the field area. In order [hat personnel at [he testing laboratory can formulate a proper fertilizer recommendation, they must have specific information on the crop to be grown (whether it be irrigated corn or a grass mix- ture to revegeta[e a mine spoil) and its growing conditions. impro- per information in this regard can lead to a bad recommendation just as can happen when poor technique is used to obtain the sample in the first place. 1~A._composite soil sample is the soil mixture representing a ~r' single field area composed of a number of individual samples (subsamples). ~._: i ;}f ;'I . ~. ,"~,