This is the time of year when farmers and landowners start thinking about next year's crops. Crop prices are down significantly from last year's peaks and prices for inputs are still very high. Determining fertilizer requirements for this year's crop will have to take in to account these economic factors. Additionally, we must remember to look at the agronomic factors such as crop needs and current soil fertility levels. Soil tests are essential in accurately determining fertilizer needs.

Most of the chemical elements used to build crop plants come from air and water, carbon, hydrogen and oxygen. The rest comes from the soil. In the process of harvesting we are removing some of those chemical elements from the area. A 200 bushel corn yield is removing is removing 4.76 tons of dry matter per acre. This removal overtime will quickly draw down soil fertility levels if the removed nutrients are not replaced. Plant available nitrogen is not held static in the soil. Every year plants depend on several sources to get the needed nitrogen, for soil microorganisms to release it, for mineralization of organic matter and/or the application of manure or commercial fertilizer. Over a 2 year period in a corn-soybean rotation with a 200 bushel corn yield and a 60 bushel soybean yield about 135 pounds each of phosphorus and potassium will be removed.

In addition to crop removal some fields may have very low levels of phosphorus and potassium that may hold back crop yields. The best way to determine if such a condition exists is to have your soil tested. The basic soil test should include pH, phosphorus (Bray P1) and potassium. Farm samples will cost about $5.00 per sample for this basic package. The price for home and garden samples is generally little higher, expect to pay around $7.00 to $15.00 per sample. There more paperwork per sample for these type samples.

The more samples you take per field the greater the probability that you will accurately map the variability of the field. Many agronomists feel the best compromise between cost and accuracy is one sample for every 2 ½ acres. Most commercial samplers also use GPS to evenly sample a field, this also allows them to make fertility maps of the field. These maps, in addition to helping you visualize nutrient variability, also allow for the variable rate application, VRA. VRA can allow you to concentrate nutrients on areas of the field that most need it. Each submitted sample is a composite of 5 cores or subsamples taken within a 10 ft radius of each sampling point. Our recommendation system is based on a 7 inch sampling depth.

Most Illinois soil labs report the phosphorus and potassium values in pounds per acre, however, some labs will report in parts per million, ppm. It is easy to convert if you know that pounds per acre is equivalent to parts per two million, so just multiply ppm by 2.

In East-Central Illinois, if soil test phosphorus levels are below 45 lbs/A fertilizer should be applied in addition to crop removal needs. Test results between 45 and 65 indicate you need only to match crop removal needs and if you have levels over 65 you can skip two years and soil test again. For potassium, if soil test potassium levels are below 300 lbs/A fertilizer should be applied in addition to crop removal needs. Test results between 300 and 400 indicate you need only to match crop removal needs and if you have levels over 400 you can skip two years and soil test again, unless you are growing corn silage or alfalfa.

More detailed explanations of soil testing and fertilizer recommendations can be found in the Illinois Agronomy Handbook or online at http://iah.ipm.uiuc.edu.