Update to Iowa P, K and lime recommendations
click image to zoom Table 2 shows the new crop P and K concentrations and the suggested default yield levels that laboratories could use when they do not get yield information with the soil sample submitted for analysis. Some default crop yield levels were increased, especially corn yield, to reflect increasing yield over time. We must emphasize that the prevailing yield level should be provided to the laboratory for a more accurate maintenance recommendation, but actual yields will provide a better estimate of removal.
The suggested concentrations were changed because (1) lower grain nutrient concentrations in samples from research studies have been observed in Iowa and other states, (2) we have better information of nutrient concentrations for many crops and harvested plant parts, and (3) previous publication versions had concentrations expressed on a dry matter basis that resulted in too high removal estimates when they were directly multiplied by crop yield expressed on standard moisture content basis. Therefore, the suggested nutrient concentrations better reflect the observed concentrations and have been adjusted so that they can be directly multiplied by yield expressed using common moisture content standards.
New suggested P and K application rates
Recent research has shown a need to change the P and K amounts recommended to maintain the optimum soil-test category for several crops, due to adjustments in nutrient concentrations of harvested plant parts or default yield levels. Also, adjustments were made to P and K amounts recommended for corn silage and some forages when soil-test results are in the very low or low categories. The new application rates cannot be possibly shown in this short publication. Also, the updated publication includes equations to calculate P and K rates that are useful for variable-rate application for corn, soybean and the two-year rotation.
Lime recommended for soil association areas with calcareous subsoil
Previous recommendations indicated that soil pH 6.5 was sufficient for corn and soybean in soil association areas with low subsoil pH, but that 6.0 was sufficient in areas with high (calcareous) subsoil pH. However, a target pH of 6.5 was suggested for all areas when calculating the amount of lime to apply. This apparent incongruence could be justified, but it created confusion. Therefore, the recommend target pH is 6.0 for areas classified as having calcareous subsoil. The classification of Iowa soil association areas according to subsoil pH was not changed. The updated publication includes the equations based on buffer pH test results used to determine the amount of lime needed to raise soil pH to desireable values that are useful for variable rate lime application.