Fall is upon us, and this is the time of year to consider soil tests and the fertility of your cropland.

To maximize yield of corn and soybeans, your soil has to have the proper pH level and adequate levels of P and K. For corn and soybean growth to reach optimum levels, agronomists recommend a soil pH between 6.0 and 6.5. Soil pH declines when basic compounds are removed by crops, leaching, and acid residues left from nitrogen fertilizing. Low pH can lead to reduced availability of calcium, magnesium, and molybdenum in the soil, and can increase the risk that imidazolinone-based herbicides will carry over between seasons. Low pH also reduces the rate of nitrogen fixation. Fortunately, low pH is relatively easy to fix – just add lime.

If your soil pH is too high, there’s not really much you can do to lower it in an economical fashion, but knowing the level allows you to manage the problems created by high pH. High pH reduces bioavailability of iron, manganese, copper, boron, and zinc, and increases the risk of herbicide carryover for sulfonylurea-based herbicides.

P and K are removed from the soil each year by your crop. Each bushel per acre of corn harvested removes about 0.43 pounds of P2O5, and 0.28 pounds of K2O. It’s important to track your yield across fields so that you know how much maintenance fertilizer each field will need to replace this intrinsic loss. Remember that the loss will be much higher on acreage that was cut for silage – the above figures assume that the crop residue is staying on the field for reabsorption!

Agronomists recommend soil tests at least every four years – and more often if there is a concern about fertility. Sample fields at the same time of year each year, so that seasonal variations don’t throw off your results. The best time to test is after harvest but before the ground freezes. Soil cores should be taken in the top six or seven inches of soil. If you are using no-till or reduced tillage, be aware that nutrients can become stratified, and run a separate test on the top two inches of your soil cores.

Remember that almost any soil deficit can be remedied – but you have to know about the problem to be able to implement a solution. Don’t be afraid to test – it’s a lot better to find out about a soil issue before you plant than after you harvest.