Daniel Gill, Koske, Thomas J., Boudreaux, James E., Fontenot, Kathryn
Nitrogen (N) is a very important plant nutrient for all fall vegetable crops. A sufficient and continuous amount must be supplied to all plants to achieve good yields. It also is the nutrient most often lacking in soils since spring and summer crops as well as rains have removed some residual soil N.
All of the fall vegetable crops are fast-growing plants with lush vegetation and a high nitrogen requirement. Because these crops are grown in times when cool temperatures occur, the uptake of N by the plants may be restricted.
These factors, along with the extreme mobility of nitrogen in the soil, make it necessary to split nitrogen applications for fall crops into a preplant application and one or two later sidedressings.
Most fall vegetables should start with a preplanting application of from 40 to 80 pounds of nitrogen per acre. This amount would be about 1/3 to 2/3 lb. N per 100 feet of row. This is usually applied with a complete fertilizer, such as 13-13-13 or 8-24-24 to include adding phosphate and potash as well. The choice of materials should depend on the residual of these two nutrients in the soil -- especially the residual phosphate. Choose fertilizer based on results from your soil analysis. Soil test your vegetable garden at least every three years. The application before planting usually provides a sufficient amount of nitrogen to achieve good, early plant growth.
One or two follow-up sidedressings will provide enough nitrogen for high yields of good-quality crops. Applications of 30 to 50 pounds of nitrogen per acre, dressed at the appropriate stage of plant development, are recommended. They are the equivalent of about ¼ to ½ lb. N per 100 feet of row. These amounts of nitrogen easily can be obtained by applying 75 to 150 pounds of ammonium nitrate per acre or 2/3 to 1 lb. per 100 feet of row. Ammonium nitrate is an excellent source of nitrogen for the vegetable garden. However, it is increasingly becoming more difficult to find. An alternate fertilizer that can be used for side dressing is calcium nitrate. Just remember that calcium nitrate typical has a guaranteed analysis of 15% nitrogen compared to 33% nitrogen in ammonium nitrate. So when using calcium nitrate you'll need to increase the amount of fertilizer you apply to the garden to obtain the 30-50 pounds of nitrogen/ acre required.
Cabbage, broccoli and cauliflower are generally sidedressed when the oldest leaves are 2 to 4 inches wide, or at three to four weeks after transplanting. Additional sidedressings two to three weeks apart may be beneficial in obtaining high yields.
Greens -- mustard, turnips and collards -- usually are sidedressed three to four weeks after planting. A single sidedressing normally is adequate for greens.
Summer crops still hanging on and those sensitive to cool temperatures and frost will not benefit from late-season sidedressing since they are almost finished producing.
To obtain the maximum benefit from a nitrogen sidedressing, moisture is needed within two days after application. Vegetable producers and gardeners should try to time sidedressings with natural rainfall or plan to irrigate soon after the nitrogen is applied.
Some will sidedress with a complete fertilizer, such as 13-13-13 or 8-24-24, believing that the crop receives an additional benefit from the split application of phosphorus and potassium. This may be true in some years and on some lighter soil types. A sufficient amount of this complete fertilizer (with 13 percent N) should be applied with the sidedressing to provide the required amount of nitrogen, but beware of overfertilizing with too much phosphate. Phosphorous can build in the soil to detrimental levels because it is slow to move and leach out.