Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Increasing global demand for nitrogen fertilizers with limited increase in production capacity has resulted in much higher prices and limited availability of some nitrogen fertilizer materials. Also, safety, security and insurance concerns have resulted in very limited available of ammonium nitrate, which over the years has been a favorite sidedress and topdress material in vegetable production. Urea, urea-ammonium-nitrate, calcium nitrate and ammonium sulfate are the nitrogen materials most likely to be available for use this year. Growers may also want to consider the use of one of the slow release nitrogen materials. The following are a few comments about each of these materials.
Urea (46 percent N) will be one of the primary materials available. It is a synthetic organic nitrogen material (contains carbon as part of the base structure) that as it is broken down forms ammonia. If the urea is incorporated into the soil, the ammonia reacts readily with water to form the ammonium (NH4+) form of N that is held on the exchange sites of the soil. However, when urea is applied on the soil surface some of the N in urea may be lost by volatilization of the ammonia before it moves into the soil. The percent of N lost in this way from surface applied urea is more of a concern on soils with a pH near or above 7.0. If incorporated within two days after application loss, in most situations, will be less than 2 percent. A rain or irrigation providing over 0.5 inch water will move the urea into the soil sufficiently to minimize loss. Lost does increase with temperature. In a pH 6.5 loam soil at 45oF, N lost during the first four days after application will be less than 3 percent and at 75oF lost during this time period may range from 5 to 10 percent Potential for loss is also much greater in sandy soils than in loam and clay loam soils. Urea broadcast over plants usually results in minimal injury to the leaves. For crops, such as lettuces, where any damage to the leaves is a quality issue, nitrogen materials should be sidedressed along side the row.
Urea-ammonium nitrate (UAN or 28 percent N) is a liquid material that is approximately a 50:50 blend of urea and ammonium nitrate. This material is a good material for sidedressing N. It can also be sprayed or dribbled in a band on the soil surface between rows of plants, but care needs to be taken to minimize the amount of material that gets on the foliage of plants. Spraying more than 3.5 gallons per acre directly over the foliage will cause some burn. Since half of the N is in the urea form, applying this material to the soil surface has the same risk of volatile N loss as with granular urea.
Calcium nitrate (15.5 percent N, 20 percent Ca) is a good material to use when additional calcium is need. The N is all in the nitrate form which is readily available for plant uptake, but is also more at risk for loss by leaching or denitrification with heavy rainfall events.
Potassium nitrate (13 percent N, 42 percent K2O) is a good material for supplying some additional potassium.
Ammonium sulfate (21 percent N, 23 percent S) is becoming more available in the market place. It is a good N source, especially where additional sulfur is needed. There is no concern for volatile N loss when this material is broadcast on the soil surface.
Slow-release N materials tend to release N over a 75 to 90 day period depending on the properties. With these materials, all of the nitrogen can be applied just prior to planting, and the N will gradually be released over the growing period of the crop, eliminating the need for sidedressing. Since the N is gradually released over time, the potential for N loss is reduced. Cost of the various nitrogen materials vary considerably. Be sure to compare the costs of materials on the cost per pound of N, not per ton of material. The way to do this is as follows:
Cost per lb N = (Price per ton material) divided by (lbs of N in a ton of material).
For example, one ton of urea costs $598 and contains 920 lbs N (2000 x .46)
Therefore cost per lb N = ($598) ÷ 920 lbs N = $0.65/ lb N.