April's Ag Advice by April Borders Crops will soon be harvested and even though we would like to be able to take a break from our farming activities, we still have work to do. As we finish with one crop, we need to start thinking about next year's crops and what we need to do to ensure good future crop production. Soil testing is one of the most important crop production practices that we can do. By doing soil tests, producers might be able to save on fertilizer costs for next year.
Predicting nitrogen carryover on fields is difficult. Because each field will be different, 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 the next year. Soil sampling at SDSU has found that fields, even on the same farm, will vary in carryover nitrogen depending on factors such as rainfall, crop growth, nitrogen fertilizer applied and many other variables.
Careful soil sampling is essential for accurate fertilizer recommendations. A sample must reflect the overall or average fertility of a field so analyses, interpretations and recommendations accurately represent the nutrient or mineral status of the soil. An accurate evaluation will result in more efficient fertilizer use, which can increase yields, reduce costs and potentially reduce environmental pollution.
Consider each of the following before obtaining a soil sample: field area, sampling procedure, sampling depth, when to sample, sampling tools, sample handling, information form and handling and mailing.
A composite soil sample should represent your field area. Fifteen to 20 sub-samples should be taken for each area sampled. Stay out of any obviously poor areas of the field as you will want to sample these areas separately. The samples taken should represent a good average of the area you want to sample.
Use a systematic sampling scheme. In your mind's eye, create a grid plan and sample once within each grid you created. As soil cores are collected, the entire core for the desired depth should be placed in a plastic pail for mixing. Separate pails are needed for surface cores (top 6 inches of soil) and for subsurface samples (6-24 inches). Soil cores in each pail can be mixed thoroughly and at least one pint should be saved for analysis. This pint mixture is the composite soil sample (one for the top 6 inches and one for 6-24 inches).
Soil sampling depth is very important to ensure accurate test results. Depth of sampling is critical because of tillage and nutrient mobility in the soil can greatly influence nutrient levels in different soil zones. Sampling depth depends on the crop, cultural practices, tillage depth, and the nutrients to be analyzed.
Soil tests for nutrients that do not move readily with water such as phosphorous, potassium, and most of the micronutrients are tested in samples taken to a 6-inch sampling depth. Nutrients such a nitrogen, sulfur, and chloride, that leach down into soil readily with water, need to be sampled down to the two-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 of soil and the other bag needs to contain the other 18 inches of soil. The top 6-inch depth is tested for all nutrients, nitrogen, phosphorus, potassium and micronutrients, 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 inaccurate fertilizer recommendation.
Freeze or air-dry soil samples within 12 hours of sampling and before sending to the lab. Air drying samples prevents microbes from mineralizing soil organic matter that can cause less accurate nitrogen fertilizer recommendations.
Fill out the information form completely. Interpretations depend on a series of field and crop factors. Such items as past crop, manuring, crop to be grown and its yield goal, all enter into the final fertilizer recommendation. Without this information, a fertilizer recommendation cannot be tailored to the specific situation.
Once samples have been taken, they can be mailed directly to the laboratory at SDSU. Soil samples bags and information forms are available at the Clay County Cooperative Extension Office at 515 High St. or by calling 677-7111.