May 1994 SCSB# 380
Chapter 4 Critical Level: Definition and Usage in Interpretation F. R. Cox In a soil testing program, the concentrations of nutrients
removed with a given extractant are determined. For each nutrient,
field experiments are conducted on specific crops to determine
the extractable nutrient concentration below which there will
be a response to application of that nutrient. The concentration
that indicates the division between responsive and non-responsive
conditions is termed the critical level.
The M3K critical levels were 0.12 and 0.09 cmol L-1 (Fig. 2), which average 0.105 cmol L-1. This value would be 32 mg K kg-1 with an assumed sample density of 1.3 g cm-3. As Mehlich-3 and Mehlich-1 remove similar amounts of K, the M1K critical level range could be 24 to 40 or 16 to 48 mg kg-1 depending on the degree of inclusion of expected observations desired.
These examples suggest creation of a range in critical levels by differences in annual climatic conditions. There are also differences in soil and management factors that would decrease the precision of the critical level range if they are not taken into account. For instance, the range in P critical level decreases with increasing clay content (Cox and Lins 1984). This may not be an important factor when growing peanuts as the crop is ordinarily grown on sandy, low-clay sites. The nutrient content of the subsoil also affects the amount of that nutrient required from the topsoil to meet plant needs. Woodruff and Parks (1980) found this especially true for K. If K fertilization is routinely greater than K removal, the subsoil would have a substantial reserve of available K and the critical level in the surface soil could still be rather low. Disregarding soil and management factors should expand the critical level ranges in soil test interpretation. Similar results can be achieved by combining data from numerous sites, in which case the data are often transformed to relative yield. This approach is used frequently in soil test interpretation studies. The range would be the same, however, whether using actual or relative yields. When a critical level range has been established for a crop, points within that range vary in probability of getting a response to fertilization. At the low end of the range, a yield response is highly probable and should occur almost 100% of the time. On the other hand, at the high end of the range, responses would seldom occur. This range is represented by the medium class in many soil test evaluation schemes. It covers the variable response range, whereas in the low class responses are always expected and in the high class responses are never expected. The examples cited above are based upon interpreting the critical level range with the linear plateau method. When the quadratic plateau technique was applied to the peanut data, critical level ranges were 25 to 30% greater. When an exponential function at 95% of maximum yield was compared to the linear plateau with the three-crop data, the M3P critical level range was 67% greater with the former. The method of interpretation, therefore, may markedly affect the critical level range determined and should be made known. ReferencesCOX, F. R. 1992. Range in soil phosphorus critical levels with time. Soil Sci. Soc. Am. J. 56:1504-1509. COX, F. R., AND I. D. G. LINS. 1984. A phosphorus soil test interpretation for corn grown on acid soils varyng in crystalline clay content. Commun. Soil Sci. Plant Anal. 15:1481-1491. WOODRUFF, J. R., AND C. L. PARKS. 1980. Topsoil and subsoil potassium calibration with leaf potassium for fertility rating. Agron. J. 72:392-396. Document Prepared by: Leigh H. Stribling, lstribli@acesag.auburn.edu Alabama Agricultural Experiment Station Auburn University |
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