Topdressing and Sidedressing Nitrogen
Topdressing is defined as a fertilizer application to a crop any time after planting. In popular usage, topdressing sometimes refers to a broadcast application of fertilizer made after planting. Alternatively, fertilizer can be sidedressed as a band along the side of the row of a growing crop. Sidedressing is commonly done immediately before or during cultivation. When urea-containing fertilizers are used, cultivation helps reduce volatilization losses.
Sidedressing of relatively soluble N fertilizer is an important component of efficient nitrogen management. The N accumulation pattern for annual crops is very similar to biomass accumulation (Figure 2). Early in the season, when crop growth is slow, crop N needs are very small. A starter fertilizer is generally sufficient to satisfy those needs. Any soil nitrate in excess of crop N needs during this period is prone to leaching and/or denitrification losses. The next phase of crop development is characterized by rapid vegetative growth. The N demand during this phase is the highest of the growing season. As much as 85% of the total N uptake occurs during this period. Efficient recovery of the fertilizer portion of N can be achieved by sidedressing fertilizer N immediately before this phase. Delaying application of a large portion of N fertilizer until sidedress also allows growers to use the Pre-sidedress Soil Nitrate Test (PSNT) to help determine N needs.
Pre-sidedress Soil Nitrate Test (PSNT)
The dynamic nature of the N cycle and its sensitivity to weather limits the value of routine, pre-season soil testing for predicting N availability during the season in our humid environment. However, under certain circumstances, in-season soil testing has proven useful. The PSNT, developed by Dr. Fred Magdoff at the University of Vermont in the early 1980’s, was originally intended to help estimate the amount of available N for field corn in fields where manure had been applied and/or forage legumes were grown in rotation. Over the last thirty years, research conducted in the Northeast has found the PSNT useful for improving N management of several vegetable crops including sweet corn, peppers, pumpkin, winter squash, and cabbage. The PSNT is most suitable for use with annual crops, which accumulate N rapidly within a single growing season.
The PSNT is especially useful where large amounts of N from mineralization are expected, and the test works best when pre-plant and starter fertilizer N rates are less than about 50 lbs N per acre. PSNT samples are collected about a week before the rapid growth phase (see Figure 2), to provide an indication of how much N has been made available from mineralization. During wet springs with heavy leaching rains, or in sandy soils with rapid losses, the PSNT will also provide some indication of how much N remains in the root zone.
As with all soil testing, information from a PSNT should be used along with the grower’s experience and knowledge of the field. Interpretation of the PSNT is also crop-specific. Research in the Northeast has shown that when the soil nitrate N level is above 20-25 ppm there is rarely an economic response to the application of sidedress fertilizer N for sweet corn. Based on research and experience in New England, New Jersey, and New York, a threshold of 25-30 ppm seems appropriate for peppers, tomatoes, butternut squash, cabbage, pumpkin, and probably other long-season vegetable crops. When PSNT values are below threshold levels, the appropriate rate of sidedress N should be determined based on the level of nitrate N reported, previous N application, realistic yield expectation, the field’s management history, and growing season conditions. See Table 2 for recommendations on timing of sampling and making sidedressing applications of N based on PSNT for many vegetable crops.
Table 2. Timing of PSNT and sidedress nitrogen needs of crops1
| CROP | SOIL SAMPLING TIME FOR PSNT | SIDEDRESS lbs n per ACRE2 |
|---|---|---|
| Beets | After thinning (2-4 leaves) | 30 |
| Cabbage, Brussels sprouts, broccoli | 2 weeks after transplanting | 60 |
| Cauliflower | 2 weeks after transplanting | 30 |
| Celery | 2 weeks after transplanting, again 3-4 weeks later | 40 twice, 3-4 weeks apart |
| Cucumber, muskmelon | Before vines are 6" long | 40 |
| Eggplant | 3-4 weeks after planting, again 3-4 weeks later | 30-50 |
| Lettuce, endive, escarole | 2 weeks after transplanting or after thinning (2-4 leaves) | 30-50 |
| Pepper | 3-4 weeks after planting, again 3-4 weeks later | 50, and 40 later at fruit set |
| Spinach | 2-4 leaves, again after first cutting | 30 |
| Sweet corn | when plants are 6-10" tall | 60-90 |
| Tomato | 3-4 weeks after planting, again 3-4 weeks later | 30 twice, 3-4 weeks apart |
1 Adapted from: Rutgers Cooperative Extension Bulletin by J. Heckman, “Soil Nitrate Testing as a Guide to Nitrogen Management for Vegetable Crops”
2 If soils have 0-30 ppm nitrate, apply the full sidedress amount recommended. For sweet corn, the threshold is 25 ppm nitrate. Above 30 ppm no additional N is needed and could hurt yields.
Samples for the PSNT should consist of a well-mixed composite of 10-20 cores or slices of soil to a depth of 12 inches. This is a deeper sample than what is recommended for routine soil sampling. A deeper sample is required for nitrate testing to accurately reflect the concentration in the effective root zone due to its mobility in soil. Avoid sampling fertilizer bands or areas that may have received extra N. About one cup of the composite should be dried to stabilize the nitrate. A good method is to spread a thin layer of the soil on a cookie sheet or aluminum foil to air dry. Use a fan to reduce drying time. Do not place damp samples on absorbent material because it can absorb some of the nitrate. You can skip the drying step if you can deliver the samples to the soil testing lab in less than 24 hours; however, samples should be kept cool. Fields should be sampled for the PSNT about a week before the time when sidedressing is normally done. This should allow adequate time for drying, shipping, and testing (lab results are generally available within 24 to 48 hours) and for you to plan your fertilizer program.