Apri's Ag Advice by April Borders Soil testing is one of the most important crop production practices that we can do. It is especially important after a drought year. By doing deep nitrate-N soil testing a producer will be able to tell what the level of carryover nitrogen is after a failed crop year. Crop growth that was limited during the drought can result in the applied N and mineralized N not being fully utilized. This can lead to larger than normal carryover N levels that would be available for use by next years crop.
By doing soil testing, producers might be able to save on fertilizer costs for next year, especially when looking at nitrogen applications. Predicting nitrogen carryover on fields is difficult and each field will be different, so don't risk guessing or using an average for your fields. Also don't make the mistake of assuming that you don't need fertilizer for next year. Recent sampling at SDSU has found that fields, even on the same farm, will vary considerably in carryover nitrogen depending on factors such as rainfall, crop growth, nitrogen fertilizer applied and many other variables.
Because many fields will have higher that normal carryover N levels, there are some sampling basics that should be kept in mind. Fifteen to twenty sub samples should be taken for each area sampled. The samples should be air dried or frozen within 12 hours of sampling and before sending to the lab. Stay out of any obviously poor areas of the field unless you want to sample these areas separately. The samples should represent a good average of the area you want sample.
Soil sampling depth is very important to ensure accurate test results. Soil tests for nutrients that do not move readily with water such as phosphorus, potassium, and most micronutrients are tested in samples taken to a 6-inch sampling depth. Nutrients such as nitrogen, sulfur, and chloride that leach down into soil readily with water, need to be sampled from 6 inches down to the 2-foot depth. A soil sample that will result in accurate fertilizer recommendations for all nutrients needs to be sent in two bags. One bag needs to have the top 6 inches and the other bag needs to have the other eighteen inches.
The top 6-inch depth is tested for all nutrients and the 6 to 24-inch depth is tested only for nitrogen and sulfur or chloride if requested. Many soil-testing labs bill these "two bag" samples as one sample. Mixing subsoil and topsoil will result in an incorrect recommendation.
The only proven method to determine carryover N is with a soil test. These tests can save significant input dollars next spring.