By Neal Kinsey
Soybean production factors out as follows: a 50-bushel crop takes 280 pounds of nitrogen. Some texts tell you that a 60-bushel crop requires 295 to 300 pounds of nitrogen. The same general rule governs both soybeans and alfalfa. For ease of computation, I like to think in terms of 300 pounds of nitrogen and 60 bushels of beans.
In a normal growing season, you are going to get 50% efficiency from the nodulation on your beans. If it takes 300 pounds of nitrogen to grow a 60-bushel soybean crop, and you are getting 50% efficiency, this means another 150 pounds of nitrogen must be accounted for. What if you have 2.3% organic matter? That factors in another 66 pounds of nitrogen. Subtract 66 pounds from 150. This leaves another 84 pounds of nitrogen to be accounted for. Without that extra nitrogen, you will not grow 60 bushels of soybeans per acre.
The first clue for a decent production level is calcium in the 60% range. If the soil colloid does not have a proper calcium level, nitrogen will not deliver a top yield. Once you have your basics in place—60 to 70% calcium, 10 to 20% magnesium—then and only then do you start looking at nitrogen as a factor in growing high yield beans. There are very few farmers on my program who raise 60 bushels of beans without obtaining some extra nitrogen above the amount supplied by the soil and nodulation.
A typical farmer question runs approximately as follows. If a grower wants to raise a 60-bushel bean crop and he is at a 60% calcium level, how many pounds of extra nitrogen would he have to add, the organic matter level being 2.5%? At 2.5% organic matter, such a farmer will have 70 pounds of nitrogen supplied by the soil. He will need 150 pounds of additional nitrogen in a normal year, after accounting for the 50% supplied by nodules. Again, if you have 70% efficiency from your beans, that goes up. We are considering a 50% efficiency factor right now because it is easy—and expected in a normal year. In other words, 150 pounds of nitrogen generated by bacteria goes into production. We have another 150 pounds to go. Take that 70 pounds from 2.5% humus off the equation and we have to come up with another 80 pounds of nitrogen from some other source. All this assumes that we have a good fertile soil and that there is a proper phosphate and potassium level.
MINERAL DEFICIENCY SYMPTOMS IN SOYBEANS
(from Modern Soybean Production, 1983, p. 171-73)
- Nitrogen. Pale green or yellowish leaves. Seldom a problem if root nodule bacteria are present. Can be due to a molybdenum deficiency.
- Phosphorus. Plants stunted; leaves blue-green and sometimes cupped.
- Potassium. Irregular yellow border around leaves.
- Calcium. Few nitrogen-fixing root nodules, causing nitrogen deficiency symptoms.
- Magnesium. Leaves turning yellow or brown between veins; leaf tip curled down.
- Sulfur. Slow growth; leaves becoming yellowish.
- Iron. Slow growth; new leaves yellow or brown between veins.
- Manganese. Leaves light green to white between veins.
- Molybdenum. Reduced growth; leaves with nitrogen deficiency symptoms.
- Zinc. Plants stunted; lower leaves turning yellow to brown to gray and dropping off; young plants with pale green leaves. Few flowers and pods; pods mature slowly.
It is not discussed generally, but there is a vicious nitrogen cycle that you can get into just by pushing nitrogen. And there are other things that can contribute to crop production for which nitrogen gets the credit. All things considered and all things in perspective should be a norm. We say legumes can get nitrogen out of the air, but we have to add that they don’t get it all from the air. At least 30% has to come from another source, and that 30% either has to be supplied by microbial activity in the soil or by applying some type of N source. The farmer must decide, but when the choice is made, he or she has to deal with the consequences.
Source: Hands-On Agronomy
Neal Kinsey will be making an appearance at the 2019 Acres U.S.A. Eco-Ag Conference & Trade Show this Dec. 9-12 in Minneapolis, Minnesota. On Dec. 9, he will be hosting an Eco-Ag U workshop, Foundations for Soil Health. Then, on the 11th, Kinsey will host the seminar The Critical Role of Magnesium in Our Soil during the Eco-Ag conference.