Crop Planning - Soil Testing

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Crop Planning - Soil Testing

Crop Planning - Soil Testing

Soil Testing
Soil samples should be collected to accurately reflect the nutrient status of the field.
Soil samples should be collected to accurately reflect the nutrient status of the field.

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.

Effluent Needed to Meet Crop Nitrogen Needs
Application Availabilities: 7.9 lbs/ac-in × 35% = 2.8 lbs organic-N
44.5 lbs/ac-in × 50% = 22.3 lbs ammonium-N
Total N Applied: 2.8 + 22.3 = 25.1 lbs N/ac-in
Crop Needs: PSNT = 60 lbs N/acre
60 lbs N/acre ÷ 25 lbs/ac-in = 2.5 in
Therefore, the producer should apply 2.5 inches/acre of effluent to meet crop nitrogen needs.
Example of a swine producer who has a lagoon with a center pivot irrigation system

The PSNT calls for 60 lbs/acre of nitrogen to meet the corn crop’s need at sidedress time. Samples of the effluent are sent to a testing lab and the results indicate that 7.9 lbs/ac-in of organic nitrogen and 44.5 lbs/ac-in of ammonium nitrogen are available. Application losses from sprinkler irrigation must now be taken into account (reference to availability factors). About 35 percent of the organic-N and 50 percent of the ammonium- N are available with sprinkler irrigation. The result is 2.8 lbs/ac-in of organic-N and 22.3 lb/ac-in of ammonium-N for a total of 25.1 lbs/ac-in. The producer can meet the crop nitrogen needs by applying about 2.5 inches of effluent per acre.

Drag the boxes below into the appropriate blank cells of the table to the right

soil probe



grab sampling

grid sampling


separate different areas

nutrient analysis


fall and spring

zig-zag pattern

representative samples

Related to Soil SamplingRelated to Manure SamplingRelated to BOTH Manure and Soil Sampling

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

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