Care with plant nutrient analysis
Iowa research with whole plant analysis (V5 to V6 growth stage) for P and K has not been successful in corn and soybean, and similar very poor results occurred with analysis of leaves collected at silking in corn or at pod initiation (R2 to R3) in soybean (Figures 1 and 2). In corn with sulfur (S), the S concentration in ear leaf samples has not provided a useful correlation with fertilizer response (Figure 3). As can be seen in these graphs, similar nutrient concentrations can range across widely varying yield response levels, hence the reason for poor relationship to a critical level. Probably the best plant analysis relationship that has been found is in alfalfa for S, where samples of the top six inches of the plant at early bud stage provided a good relationship to fertilizer response, and indication of a critical level at 0.23 percent S (Figure 4). That concentration is quite similar to past research reports from other states.
click image to zoomFigure 4. Relationship between alfalfa yield increase per cut to S fertilization and S concentration of the 6 inch plant top at early bud stage. For nitrogen (N) in corn, Iowa research on plant deficiency and critical levels during the late 1980s and 1990s with small plants showed very poor correlations with yield response to N. Therefore, research during the last few years has focused on use of sensing tools rather than plant analysis, such as the SPAD chlorophyll meter (Figure 5) or active canopy sensors. With those sensing tools, differentiating slight to moderate N deficiency has been difficult.
Available research results indicate that plant sampling and analysis for corn and soybean is of limited value. Its use can aid in determination of nutrient deficiencies in diagnostic situations, where good and poor growth areas in a field can be sampled for comparison; and when is used in conjunction with soil testing to help confirm potential deficiency issues. Unlike soil testing, calibrations and interpretations of plant analysis results for N, P, K, and S for corn and soybean have not been produced in Iowa for reasons given above. For micronutrients, with the only exception of zinc for corn, neither soil tests nor plant analysis have been successfully correlated to fertilizer response or calibrated to nutrient application requirement because of a major reason: if no deficiencies exist and there are no responses to fertilizer application, then critical levels cannot be determined.
click image to zoomFigure 5. Relative SPAD chlorophyll meter (RCM) value versus N rate difference from economic optimum N rate, R1 corn growth stage (from ISU Extension and Outreach publication PM 2026). Ongoing research continues to investigate plant analysis for P, K, and S. New research began this year to study the relationship between plant analysis for micronutrients at different corn and soybean stages to yield response to foliar and soil applied fertilizer at many research farms and farmers’ fields. If yield responses are found in a sufficient number of fields, then this research may be helpful for determining critical plant analysis concentrations. Stay tuned.