by Tim Mundorf, CCA
National Training & Precision Ag Specialist

With grain prices a little lower than they have been in past years we get questions at the laboratory concerning where growers can cut back their fertilizer recommendations. Balancing crop revenue vs fertilizer cost for any given year and a particular grower’s financial situation as well as renting vs land ownership is really outside the role of the laboratory. I like to just point growers back to the basics.

Here is what I usually talk about:

  1. Soil fertility and pH levels vary across a field. The better you can address this variation and apply the right rate of fertilizer for that acre the better you can maximize your return. That is our primary role at the lab.
  2. There is a priority list that should be considered when it comes to pH and soil fertility. Here is what that list looks like for me when growing corn.
    • Nitrogen: Sometimes timing is more important than rate. Early season loss can cause what would normally be a sufficient rate to leave the crop a little short. Splitting nitrogen application to time some of the application to when crop uptake of N is reaching its maximum makes sense. The appropriate rate varies by yield goal, previous crop, nitrogen credits and efficiency in managing loss.
    • Soil pH: Soil pH often does not get the attention it deserves. I really like to see soil pH levels between 6.3 and 6.8. If you have large areas under 6.0 you need to lime them to keep your applied and natural nutrients available and to maximize soil biological health. Soil pH under 5.5 can really start to cause problems.
    • Phosphorus: I like to see a P1 Bray value of around 25 ppm for raising high yield corn and soybeans. If your level gets below University recommended levels of 15-20 ppm you have a problem.
    • Potassium: I like to see a K value of around 200 ppm. If you have areas under 160-180 ppm you really should address them.
    • Sulfur: We used to get more sulfur from atmospheric pollution. We do get sulfur from mineralized organic matter but its release is weather dependent. I like to see some sulfur added especially in lower OM soils.
    • Zinc: Zinc is the micronutrient with the greatest likelihood of response in corn. I like to see soil test levels at 1.5 ppm or above.
    • Other micronutrients: Copper, Manganese, Boron, and Iron are all essential micronutrients in plant development.   They can limit yield but are much less likely than those given above. If a grower has taken care of other yield limiting factors and is pursuing higher yield these may be a good area to address next. Appropriate soil test levels or plant leaf nutrient concentrations are often disputed.
  1. Soybeans have a very similar priority list as the one for corn. They produce much of their own nitrogen through root nodules so that is much less of a factor. The other issues all apply. Soybeans are often treated like a secondary crop. We should fertilize them very similar to how we handle corn.
  2. Crop removal: Two hundred bushel corn will take out approximately 70 lbs of P2O5 and 55 lbs of K2O with the grain. This is the equivalent of 135 lbs of MAP and 100 lbs of KCl (0-0-60). Sixty bushel soybeans will take out approximately 50 lbs of P2O5 and 80 lbs of K2. This is equal to nearly 100 lbs of MAP and 135 lbs of KCl. Applying less than this removal will reduce soil test levels.
  3. Of course, mineral nutrition is only one aspect of maximizing crop yield. Growers will find many opportunities for yield increase when they look at their entire system of water management, residue management, appropriate hybrid choice, weed control, insect control, disease control, reducing soil compaction, and other factors.
  4. There is probably no better practice than regularly walking your fields and using good data management to compare yields to what you are seeing in the field through the year. Remember to dig up some plants to see how the roots are developing and to take some pictures through the year to help you remember what things looked like at different times.
  5. Soil fertility and pH adjustment is not really a single season decision. Many of the decisions you make today will affect yield for years.  Setting up fields for maximizing long-term yield is an important part of fertility management. Fields with good fertility and soil characteristics can generally handle high crop stress years with less loss of yield. They are also able to reach higher yields in the good years.