Soil Testing

Regularly checking soil nutrient status by sampling and analysis should be part of one’s crop production plans regardless of whether manure use is an option. However, soil testing is especially important with manure application to insure that excessive amounts of nutrients are not accumulating in the soil.

Soil samples should be collected to accurately reflect the nutrient status of the field. Guidelines for Soil Sampling provides details on how to collect samples. It’s important to collect samples for non-mobile nutrients, such as phosphorus, potassium, and pH, to a depth of 8 inches or less, and to collect deeper samples for nitrate-N to a depth of at least 24 inches. The most critical nutrients to monitor with regular soil sampling for manure management are nitrogen and phosphorus. Nitrate-nitrogen should be sampled before manure application and the residual nitrogen should be credited to next year’s crop nitrogen requirement.

Another soil nitrogen test of value to livestock producers is the pre-sidedress nitrate test (PSNT). This test is based on a soil sample collected to a depth of one foot in early June, around the V6- to V8-growth stage of corn. Nitrate analysis of samples collected at this time of year accounts for nitrate which has been mineralized from soil organic matter and manure during the spring. A PSNT may better predict crop nitrogen needs for the remainder of the growing season than preplant nitrate sampling, particularly in situations where manure has been applied. Research from Iowa and other states has shown that a PSNT sample with 25 ppm nitrate-N or greater reflects an adequate nitrogen supply to meet crop needs of corn during the growing season.

Since nitrogen in soil is very dynamic, it is important in any cropping situation to closely monitor the nitrogen status of the crop. This is especially important if most or all of the required nitrogen is anticipated to come from manure. If nitrogen availability from soil and manure is less than expected (due to cool weather, excessive rain, etc.), nitrogen deficiencies may develop (yellowing of lower leaf tips and down the midrib). A regular scouting program for the field is encouraged. Assessment tools such as a chlorophyll meter or remote sensing may help detect developing nitrogen deficiency before yield potential is reduced.

Residual nitrate-N or PSNT sampling is only necessary if the crop requires significant nitrogen inputs. It is not necessary to sample for nitrogen if the crop to be planted is soybean or alfalfa.

Part I of Crop Planning | Part II - Soil Testing | Part III - Manure Nutrient Analysis and Application | Complete the Quiz