Soil Basics Part V: Topdressing and Sidedressing Nitrogen

In the last fact sheet, we discussed soil testing and nutrient application. Routine soil tests are not good predictors of nitrogen (N) availability, but we discussed how we can estimate N availability based on sources such as soil organic matter, cover crops, manure and compost. Generally, it is best to apply only a small amount of N at planting and supply more as plant demand increases during the growing season. Decomposition of organic materials in the soil will supply some or all of the required N and sidedress or topdress applications should be adjusted accordingly. A pre-sidedress soil nitrate test (PSNT) is helpful in determining if an application of additional N is needed. This fact sheet will address using the PSNT and methods of topdressing and sidedressing N, including application using trickle irrigation.

Introduction

Regardless of its source, most soil N is converted to nitrate-N, which is highly soluble and easily leached. It is not desirable to have high levels of soil nitrate when crop demand is low. When we broadcast large amounts of N prior to planting, most of it will be converted to the nitrate form several weeks before crops can utilize much of it. In the mean time, the N is subject to leaching. The degree to which leaching actually occurs is dependent on the amount of rainfall and irrigation and soil characteristics. Leaching risk from pre-plant N applications can be reduced somewhat by using a slow release N fertilizer such as sulfur-coated urea. Soil organic matter, compost and residues from previous crops are generally considered to be slow-release sources of N, but certain organic sources of N are readily available. Up to half the N in dairy manure and 75% of the N in poultry manure is readily converted to nitrate-N. A large part of the N in legumes is converted to nitrate shortly after plow down. Pre-plant incorporation of manures and legumes can result in leaching risks similar to fertilizer N.

Plastic mulch protects N from leaching from the portion of the field that is covered by the mulch. However, the bare soil between the plastic strips is not protected, and in fact, may be more susceptible to leaching because, during a rain storm, it receives water being shed from the plastic in addition to that which falls directly on the soil. Many growers are applying N in wide bands and covering them with plastic.

Managing N

Good N management involves supplying the right amount at the right time for crop needs. Lack of sufficient N can reduce yields, but any N in excess of crop needs is subject to leaching. Studies in New York have shown that even a small amount of over-fertilization with N increased nitrate levels above drinking water standards in ground water. In one study, when N was applied to corn at 30 lbs/A above the optimum rate, 40% of the excess was lost to leaching. The study was done on a sandy soil, typical of many agricultural soils. Leaching becomes less likely as soil texture becomes finer.

In some crops, such as corn, application of excess N is simply a waste of money, costing a grower as much as $30 to $60 per acre (depending on the N source) without any benefit. With other crops, over-applying N can also suppress yields or quality. In studies in New York and Massachusetts, pumpkin and butternut squash yields were reduced by applying more N than required by the crop. High levels of N can increase the incidence of blossom-end rot in tomatoes and delay maturity of onions and potatoes.

For most crops, it is appropriate to apply 20 to 40 lb of N per acre as a band at planting, or if banding is not practical, up to 50 lb per acre as a pre-plant broadcast. This practice should be adequate because crops use small amounts of N during the first few weeks after germination or transplanting. As the plants grow, their need for N increases, and additional N can be applied as a sidedress or topdress. When the soil is warm and if soil conditions are appropriate (see Soil Basics IV), N will be released or mineralized as microbes break down soil organic matter. This N can supply some, and sometimes all, of the crop's remaining N requirement, reducing or eliminating the need for additional N.

Pre-sidedress Soil Nitrate Test (PSNT)

The PSNT was developed originally for use in field corn on dairy farms where manure is routinely applied and legumes are often included in the cropping system. It is now well established that if the nitrate-N level in the soil is above a threshold level of 25 ppm when the corn is six to twelve inches tall, additional N from fertilizer will not increase yield. Most vegetable growers do not include manure or legumes in their cropping systems. However, several years of research and experience on vegetable farms shows that decomposing soil organic matter can release substantial amounts of N which can be measured by the PSNT.

Samples for the PSNT should consist of a well-mixed composite of 10 to 20 cores or slices of soil to a depth of 12 inches. 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 the soil thinly to air dry. A fan will 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. Fields should be sampled for the PSNT about a week before the time when side or topdressing is normally done. This should allow adequate time for drying, shipping, and testing (turn around time in the lab is about 24 hours) and for you to plan your program. See Soil Basics IV for mailing information and prices for soil testing.

As with field corn, sweet corn does not respond to N application if the soil nitrate-N level exceeds 25 ppm as measured by the PSNT. Based on research and experience, a threshold of 35 to 40 ppm seems appropriate for peppers, tomatoes, butternut squash or pumpkins. This suggestion may appear contradictory since the N requirement for peppers and tomatoes is about the same as sweet corn and it is lower for pumpkins and squash. However, sweet corn has a deeper and more extensive root system than most other vegetables and is better able to extract N from the soil. Vegetables with shallower root systems would logically require a higher concentration of N in their root zones to supply their needs.

Nitrogen source

Nitrogen is available in a number of forms. These include urea, ammonium and nitrate. Common fertilizer sources of N used in New England include urea, ammonium nitrate, diammonium phosphate, monoammonium phosphate, calcium nitrate and potassium nitrate. In the soil, urea is converted by hydrolysis to ammonium, which in turn is converted through nitrification to nitrate. In warm soils these reactions usually happen fairly quickly if soil pH is over 6.0 and soil moisture and aeration are adequate. Nitrate is the predominant form of N taken up by most plants, but any of these fertilizers can be used because they will be converted to nitrate. However, high ammonium levels can interfere with calcium uptake and induce calcium related disorders such as blossom-end rot of tomatoes, tip burn of cabbage and greens and cavity spot of carrots. This problem is exacerbated by heat and moisture stress and may not be an issue every year. Many growers use calcium nitrate and sometimes potassium nitrate for topdressing or sidedressing N on crops subject to calcium related disorders. High ammonium levels can also be injurious to soil microbes leading to a prolonged period of high ammonium. Ammonium nitrate provides half the N in the nitrate form and half in the ammonium form. Urea N is converted to ammonium N and then to nitrate N. In effect, applying urea is similar to applying N in the ammonium form. Application of urea and ammonium phosphates are most likely to interfere with calcium uptake whereas calcium nitrate and potassium nitrate are not likely to do this. Ammonium nitrate is intermediate in this regard. When a slow release form of urea is used, only a small amount of ammonium is present at a given time and is unlikely to cause a problem with calcium nutrition, but N may not be available quickly enough to meet the demands of a rapidly growing crop.

Applying additional N

Supplemental N is typically applied during the growing season as a sidedress or topdress. A sidedress application involves placing a band of N into the soil at a desired distance from the row of plants and is done with a fertilizer applicator mounted on a cultivator. The machine should be adjusted so that the fertilizer band is far enough from the crop row to avoid root damage by the equipment. For a topdress application, fertilizer is broadcast over the entire field, usually with a spin-type spreader. Most of the fertilizer granules bounce off the plant, but a few remain on the leaves. The adhering granules can burn the leaves, leaving small dead spots. Other than leaf crops, this injury is not much of a problem, but if the leaves are moist during application, more fertilizer sticks to the foliage and serious leaf burn can occur. Many growers cultivate the crop after topdressing to incorporate the fertilizer, and others try to apply it just prior to rain or irrigation, which can move soluble N into the soil. Some growers are using liquid solutions of N in water. Typically, these are sidedressed. There are no significant differences between liquid or dry materials from a horticultural standpoint. The main considerations in deciding which of these to use are equipment and convenience.

Trickle irrigation

Trickle, or drip irrigation has become increasingly popular with vegetable growers and is frequently used under plastic mulch. By using a fertilizer injector, trickle irrigation can be used effectively to apply N during to growing crops. (NOTE: Certain precautions must be followed when injecting fertilizers. See recent articles in Vegetable Notes by Anne Carter on components of a trickle irrigation system.) The need for supplemental N can be determined using the PSNT as it is with other application methods. Samples for the PSNT should be take from under the plastic, if used. The best way is to use a soil sampler which will punch a small hole in the plastic and remove a core of soil. Be sure to avoid cutting the irrigation tape when sampling under plastic.

When topdressing or sidedressing, it is common to apply all the N in one or two applications. Smaller, but more frequent applications are desirable from a N management standpoint, but are time consuming, and may be impossible when plants grow large. With trickle irrigation, it is convenient to apply small amounts of N weekly or even daily. For example, if you want to apply about 50 lb N per acre, you can inject a little over seven lb N per acre per week for seven weeks, or about one lb per day if you prefer. Small weekly applications provide for more efficient crop use of N than one or two larger applications. Daily application offers little advantage over weekly application, but may be necessary if the injector can not inject a week's worth of N during the appropriate irrigation run time. To prevent leaching, the irrigation system should not be run longer than necessary to effectively wet the root zone of the crop. If there is not enough time to inject all the fertilizer needed for the week in one injection, then smaller, daily injections are preferable. Before injecting fertilizer, the entire system should be filled with water and at full operating pressure. When all the fertilizer has been injected, the system should be run long enough to flush all fertilizer from the lines. If fertilizer is left in the lines, clogging may occur due to chemical precipitates or growth of bacterial slimes.

Water problems

There is a potential for certain fertilizer materials to react with chemicals in irrigation water. If the water pH is below 7.0, there is little potential for problems, but at pH 8.0 and above, the risk is high. At levels above 40 to 50 ppm, calcium and magnesium are likely to react with phosphorus, if present in the fertilizer, causing precipitation of phosphates. If fertilizer containing calcium is added to water with concentrations of bicarbonates above 2 meq/liter, calcium carbonate may precipitate. Sulfates in fertilizers can react with calcium in the water resulting in the precipitation of gypsum. These precipitates can clog emitters.

Phosphorus- and sulfate-containing fertilizers, if needed should be applied before planting because we are not concerned about these leaching. Nitrogen is the element that is most appropriate for injection into trickle irrigation water. Calcium nitrate has the potential to cause clogging if the water pH and bicarbonate levels are high as noted above. If calcium nitrate causes clogging, potassium nitrate or urea can be used as an alternative N source.

Water testing labs can analyze water for pH, calcium, magnesium and bicarbonates. You can also perform a simple test: Mix fertilizer into a container of irrigation water at the same concentration it will be after injection into the trickle system. Cover the mixture to exclude dust and let it sit for at least the length of time it will be in the system before it reaches the soil. If the water becomes cloudy or a precipitate collects on the bottom of the container, you can expect this to happen in the irrigation system with the likelihood of clogging. If it is necessary to lower the water pH, acid can be injected into the irrigation water. This requires special handling precautions and special injection equipment. Be sure to carefully follow directions to avoid personal injury or damage to crops or equipment.

Summary

Nitrogen is easily leached from the soil. If this happens, money is wasted and ground water may be contaminated. Nitrogen applications should be timed to meet crop demands. Large pre-plant broadcast N applications should be avoided. A PSNT should be used to determine the need, if any, for additional N during the growing season. If needed, additional N can be applied by topdressing, sidedressing or injection into a trickle irrigation system.

This is the last of a series of fact sheets on soil and nutrient management for vegetable growers. Special thanks are extended to Dr. Allen Barker for reviewing these fact sheet and making helpful suggestions.

John Howell, Department of Plant and Soil Sciences