By Brian Hefty

Plant tissue analysis is great for knowing what is in the plant today for nutrients.  If you want to know how much nitrogen you need to sidedress for corn or how much to streambar in wheat this year, you need a different test.

Our suggestion is to soil sample for nitrogen levels a week or two before you want to apply your in-season N.  Pulling a simple nitrogen test is less expensive than running a complete soil analysis.  The big question is, how deep will you pull soil cores?  Many people like to sample down to two feet, as deep nitrogen can move up through capillary action, not to mention the fact that many fields have decent root growth down to two feet.  However, I think more farmers sample down to 12 inches only when they are pulling a lot of nitrogen samples in the spring or summer.  Personally, I’d rather have a deeper test, but either way is better than no test at all.

In-season nitrate tests will tell you how much nitrogen is sitting there in the soil, ready to go.  The two unanswered questions are:

  • How much nitrogen will be lost before it can be used by your crop?
  • How much nitrogen will become available through organic matter mineralization?

We usually figure 20 to 30 pounds of N for every one percent of organic matter, but that nitrogen comes available all throughout the growing season, from May through October on our farm.  How much will come available for your crop by the time it needs it?  That’s always a guess.

Very little we do on the farm is an exact science.  However, by having information like in-season soil nitrate tests, you can better play the odds.  For example, on our farm last year we were prepared to sidedress our corn, but our pre-sidedress nitrate tests showed we had far more nitrogen than we thought.  We saved our money, spending nothing on extra nitrogen in many fields.  The program worked because we had our best corn yields ever.

Before you invest in nitrogen in your fields, do at least a little bit of sampling to find out if you even need nitrogen; and if you do, how much do you need?  Even though nitrogen prices have come down, there’s no point in wasting money if you can help it; but on the flip side, you want to make sure you apply enough nitrogen to maximize yield and profit.