Understanding Your Soil Sample Results

When you are sitting down with your FSA’s and going through your soil sample results, you may be wondering: What do all these numbers mean and how does my FSA sort it all out?  So in this month’s article I will try to do the best I can on making sense of your soil sample results.  We will start with some basic indicators to keep in mind and then move on to the nutrients to watch and what ranges we would like to see the values come back in.  Finally, we will examine what to do when those values do not match the optimum ranges for top end production.

By Mike Zwingman

Next we want to inspect the Nitrogen, Phosphorus and Potassium values, which are the building blocks to any fertility program.  I am not going to spend time on Nitrogen this month, as that will be the focus of my January article.  For the most part, we have significant variation in P levels across our territory; which has to do with things like manure history, depth of top soil and silting of lower ground due to erosion.  Phosphorus levels are, for the most part, on the low side in our area.  I would like to see our P levels in the 25-35ppm range to maximize efficiency of the plant.  This is where we will most likely talk to you about strategies like Build, Maintenance, and Nutrient removal applications.  I would always urge a grower to be on the Build side of P applications when good economics (like now) allow it, and I would never do anything less than a removal rate application.We generally have good soil test levels of Potassium (K), but we have horrible soil release and plant uptake of this nutrient.  I want you to start thinking of Potassium in two ways:

  1. How should we use K to increase Nitrogen Use efficiency?
  2. How can we use K for Water Use efficiency?

I laid out the water use part in a Reachout article earlier in November. Ask your FSA for a copy if you haven’t picked one up at one of our locations already.  I believe that on your highest potential ground we should be applying potassium at anywhere between 30-50 percent of your total N application.  This will improve conversion of N in the plant and enhance the plant’s ability to use N.

Of the micronutrients, Zinc and Sulfur are most important. Zinc is essential due to its impact on a lot of the enzyme processes in the plant. It also helps drive Phosphorus into the plant and is critical to early root development.  We would like to see Zinc soil test levels anywhere above .75 ppm.  Corn has a huge response to Zinc applications and is critical to proper grain development, so we do not want to end up deficient in this nutrient.  An ideal range for Sulfur soil test levels is 8-15ppm.  This is another nutrient that is key to a plant’s ability to use Nitrogen, so I always use a balanced approach:  For every 5 lbs of Nitrogen applied, I would apply 1 lb of sulfur regardless of what soil samples tell me.  This balance in the soil helps us maintain balance in the plant so we can make amino acids at the right rate, not to mention maintaining good photosynthetic ability and helping the plant fend off diseases.

At the end of the day this is only a start to understanding your soil test levels. The next step is to understand what to do when you see them.  Remember this: Our best fertility plan begins with knowing a few things like realistic yield goals, means and timing of application, and what fits your budget in a given year.  With a little math and some thoughtful discussion, you and your FSA can write a successful fertility program in any given year.