Time of Sampling:

Collect soil samples when corn plants are 6 to 12 inches tall (measure from the ground surface to the center of the whorl). The test is called the "late-spring" soil test to emphasize that the time of sampling and sample depth make this test different than other soil nitrate tests.

This time of sampling is late enough to reflect the effects of spring weather conditions and early enough that more N can be added if needed. Soil nitrate concentrations may substantially increase or decrease during the spring due to nitrogen release characteristics of the soil.

The increases are caused by nitrification of N from fertilizers, plant residues, animal manures, and soil organic matter. The decreases are caused by leaching, de-nitrification, or immobilization of N into organic forms that are not available to plants. Soil nitrate concentrations decrease with plant uptake after corn is 12 inches tall. Results from samples taken before or after corn is 6 to 12 inches tall should be used with caution because critical concentrations of soil nitrate change with time of sampling.

Depth Of Sampling

Samples collected for the late-spring soil test must be representative of the top foot of soil. Samples representing depths other than 12 inches can result in incorrect assessments of N availability.

Concentrations of soil nitrate in late spring usually decrease with increasing depth within the normal rooting zone of corn. Therefore, optimal concentrations of soil nitrate vary with thickness of the layer sampled.

Concentrations of nitrate in the surface foot of soils in late spring tend to be proportional to the amounts of nitrate in the rooting zone of corn. This means the soil test tends to reflect nitrate below the depth of sampling.

Sampling deeper than one foot may be advisable on sands, but there are relatively few sandy soils in Iowa.

Recommendations are based only on 12-inch sample depths for the side-dress test.

Interpretation:

A chart on how much nitrogen to apply is mailed with the results. A test over 25 ppm generally does not need nitrogen.