By Brian Hefty
Nitrogen is expensive. Too much nitrogen can excessively lower your soil pH and contaminate groundwater. Too little nitrogen means lower yields. Of all soil nutrients, I believe nitrogen is the most difficult to manage. That said, proper management can be extremely rewarding and profitable. Here are some of the key things I look at when making nitrogen recommendations for farmers, including on our own farm.
- Cation Exchange Capacity – This is a measurement of holding capacity in your soil. As a rough guideline, we will tell you to multiply 10 times your CEC, and that’s the maximum we want in your soil at any one time. Is it possible your soil could hold more or less? Absolutely, but this is a great starting point, and it’s simple and easy to calculate. Here’s an example. Let’s say you have a CEC of 12, and your soil test shows 30 pounds of N already in the soil. 12 X 10 = 120 total pounds minus 30 already there = 90. The most we would want you to apply now is 90 pounds, and if it was me, I’d err on the cautious side and go with 70 or 80 pounds. The lower your CEC, the more likely you need to split-apply your N. By the way, if you decide to research this topic, you’ll find many articles supporting and many articles contradicting our guideline of 10 times CEC. The problem is, no one has developed a simple, easy to use formula other than this. 10 times CEC will at least get you close. The key is to not overapply early in the season, so if you ever have doubts, keep your initial rate low and apply more once the crop is growing.
- Carryover Nitrogen and the Soybean Credit Myth – For years you have likely heard about the “soybean credit”, meaning after a soybean crop you can figure 30 to 50 pounds of nitrogen will be there. That is simply a guess. I’d rather test the soil and see how much nitrogen is left. When we’ve done that on our farm, sometimes we’ll have 10 pounds left, and sometimes we’ll have 100. That’s a big difference! If you do have lots of nitrogen left, I would absolutely lower my rate going into the next year and then test my soil again mid-season to see how much sidedress nitrogen I need before making my final trip across the field.
- Organic Matter Mineralization – One of the most important factors that few farmers know about, is that some of your soil’s organic matter mineralizes each year, releasing free nitrogen! In the Midwest, we figure 20 to 30 pounds for each one percent of organic matter. In 5% O.M. soil, that’s 100 to 150 pounds of nitrogen you get for free between spring thaw and fall freeze-up. If you are raising a full-season corn that you harvest just prior to freeze-up, your corn has the ability to use all that free nitrogen if you don’t overapply another N source. Organic matter mineralization is increased when soil health is great and the weather is warm with adequate rainfall.
- Nitrogen Stabilizers to Protect From:
By using a nitrogen stabilizer, you can help keep nitrogen in the ammonium (non-leachable) form longer. Plants prefer ammonium, because if they bring in nitrate (the leachable form, by the way), they have to convert that back to ammonium inside the plant, using up more of the plant’s energy. Nitrogen stabilizers are great tools to keep nitrogen in your field.
- When Will You Apply? The closer you apply your nitrogen to when the nitrogen is needed, the better chance you have for nitrogen uptake and reduced nitrogen loss.
- How Much Does EACH Acre Need? This is the most important question, in my opinion. Think about the yield variance across your fields. I can tell you on our own farm last year that variance was roughly 50 bushels per acre on the low end to over 300 on the high end in corn. Our average was 224. If I fertilize everything for 224 bushels, I’ll have areas with almost 200 pounds too much nitrogen, meaning I’ll drop my soil pH and have to spend money on lime, as well as potentially contaminate the water and waste a whole bunch of money on N. On the flip side, I’ll be shorting my best areas, limiting my top-end yield and income potential. I know this may be a process for you, especially if you haven’t done it before, but setting up variable rate zones and using variable rate technology for nitrogen is really important.
If you want to do the right thing for your soil, your crop, your water, and your pocketbook, I strongly encourage you to work hard on your nitrogen management program this spring and summer.