Knowledge of crops to be planted in the future rotation is important for planning sampling time. For example, in cropping systems including peanuts, better results are obtained from applying potassium to the crop in the preceding year than on the pean ut crop. Also, in double cropping of small grains followed by soybeans, it is often best to add enough fertilizer to the small grain to provide for both crops. It is best to sample soils in the Fall for Spring planted crops and in the Summer for Fall pl anted crops. It is important to sample annually in the Coastal Plain and Sandhill regions where many nutrients are easily leached through the soil and the pH value can rapidly decrease. This is particularly important when high levels of nitrogen have be en added due to its ability to significantly acidify the soil.
For fields where surface broadcast fertilizer has been applied in the previous year, sample to a depth of 6 inches and take 15 to 20 cores from each sampling area. An area can be any consistently managed field where the soil properties such as clay co ntent, organic matter content, etc. are fairly uniform.
If phosphorus or potassium has been banded in the previous years, the number of cores collected from random locations throughout the field should be increased from 20 to 40. As in the broadcast situation, sample to a depth of 6 inches.
An alternate sampling approach for phosphorus or potassium banded fields is to make a composite sample consisting of one sample in the band for every 20 samples taken between the bands for wide (30 inch) band spacing. For narrow (12 inch) band spacing , take one sample in the band for every 8 samples between the bands. Sample to a depth of 6 inches.
In no-till situations where the soil is not mixed and the fertilizer and lime applications are all broadcast on the surface, the soil pH tends to be lower near the surface and then increase with depth. Surface broadcast applications of nitrogen do not concentrate at the surface due to its transformations and mobility in the soil but usually it will cause a decrease pH values in surface soil layers. To account for pH differences with soil depth, it may be beneficial periodically to separate samples in to 0 to 3 inch and 3 to 6 inch depth increments. Even though a 0 to 6 inch sampling may indicate an ideal soil pH value, a 0 to 3 inch sampling may show a concentration of acidity near the surface which would warrant a lime application to provide better growth conditions in that rooting zone.