Michigan State University Extension
Soils & Soil Management - Fertilizer - 06109703
07/10/97
Potato Fertilizer Recommendations Extension Bulletin E-2220, April 1990
Maurice L. Vitosh Extension Soil Specialist Department of Crop and Soil Sciences
Nitrogen (N) is essential for all living plants. It is the basic element of amino acids and proteins. Nitrogen is the most critical nutrient -it has the greatest effect on yield and tuber quality. Inadequate N can drastically reduce yields. Excess N can reduce tuber quality by lowering specific gravity, and it is a potential contaminant of groundwater. Nitrogen is the most mobile nutrient in soil and requires skillful management, particularly on sandy soils, to provide adequate available N to the plant throughout the growing season while avoiding contamination of groundwater.
Nitrogen Fertilizer Rates
MSU does not have a soil test for determining N fertilizer needs. Research is currently being done on a soil nitrate test that can be used just before planting so that growers can adjust their standard N fertilizer recommendation. At present, N fertilizer recommendations are based on expected yield goal and then adjusted for varietal differences and N fixed by previous legume crops (Table 2). Russet Burbank is the only variety for which MSU nitrogen recommendations differ. Because of its long growing season and demonstrated higher N requirement, Russet Burbank should receive an additional 40 lb of N per acre above the standard recommendation. ==================================================== Table 2. Nitrogen fertilizer recommendations for potatoes.
Yield goal N recommendation cwt/acre lb/acre ---------------------------- 300 150 350 170 400 180 450 200 500 210
For Russet Burbank, add 40 lb of N to the recommendations. ==================================================== Nitrogen Credits
Nitrogen credit for legumes such as alfalfa and clovers should be based on the kind of legume and the percent stand when plowed under. Use the following formula for determining the amount of N credit for a previous alfalfa crop.
N credit (lb/A) = 40 + .6 x percent stand
For legumes other than alfalfa, use half the rate determined by the formula.
Credit should also be given for high levels of organic matter. Reduce the standard N recommendation by 20 to 40 lb per acre for soils with 2 to 4 percent organic matter, 40 to 80 lb of N per acre for soils with 4 to 8 percent organic matter, and 80 to 100 lb for soils with greater than 8 percent organic matter.
Growing potatoes on soils that have been manured is not recommended because of problems associated with potato scab disease and the difficulty in assessing the N availability from manure.
Nitrogen Fertilizer Efficiency
Nitrogen fertilizer efficiency can be improved by supplying N fertilizer several times throughout the growing season. Fertilizing to achieve maximum efficiency usually requires applying some N at planting or before emergence, some at hilling time and some through the in irrigation system. Apply approximately one-third of the recommended N at planting, one-third at hilling, and one-third through the in irrigation system. The amount applied through the in irrigation system should be applied no later than July 15. Later N applications are inefficiently utilized and may contaminate groundwater.
Apply no more than 60 lb of N or 800 lb of starter fertilizer in bands 2 inches to the side and 2 inches below the seed pieces at planting. Excess N and K at planting can cause salt injury to young seedlings, particularly if dry-soil conditions persist.
Foliar applications of N. P and K are not recommended. Soil applications are preferred because they allow for season-long uptake of these nutrients and because the amount of N. P or K that can be absorbed through the leaf tissue is very limited. Potatoes have large requirements for these nutrients; therefore it is impractical to try to feed them through the leaves. Applying nutrients through the in irrigation system is not considered a foliate application because the amount of water applied washes nearly all of the nutrients off the plant leaves.
Nitrogen Fertilizer Sources
Most N fertilizer sources perform equally well as long as they are applied properly. Ammonium sources such as urea (46-0-0), ammonium sulfate (21-0-0-18S), diammonium phosphate (18-6-0), monoammonium phosphate (11-48-0) and ammonium polyphosphate= (10-3-0) are preferred sources at planting time because all of the N is in the ammonium (NH4+) form. Excess rain one to two days after these materials are applied will not cause the ammonium N to be leached from the soil profile because it is held on the clay and organic soil particles. As the soil warms, the ammonium will be converted to nitrate, which is subject to leaching. Anhydrous ammonia, another ammonium source, has also been shown to be equally effective but very little ammonia is used today on potatoes. Applying ammonia to dry, sandy soil may be undesirable because the cation exchange capacity of these soils is low which may allow the ammonia to escape from the soil before it is absorbed.
The use of ammonium nitrate (33-0-0), N solution (28-0-0) or calcium nitrate (18-0-0) at planting Time is less desirable because the nitrate (NO3) portion of these materials is subject to leaching loss immediately after application.
All of the above sources of N, except the phosphate materials, may be used at hilling time. Phosphorus is most efficiently utilized if applied in a band at planting time. Urea and N solutions should be incorporated into the soil by cultivation or irrigation within 24 hours to minimize any loss of N by ammonia volatilization. Do not broadcast urea when the plant foliage is wet, because it will burn the leaves.
N solution (28 percent) is the preferred source for application through the irrigation system because of its convenience. Dry sources of N such as urea, ammonium sulfate and ammonium nitrate can be dissolved in water and applied through the irrigation system. Anhydrous ammonia should not be applied through the irrigation system because of ammonia losses to the atmosphere.
Ammonium sulfate (21 percent), when banded at planting time and again at sidedress time, has been shown to reduce, but not control, potato scab disease. Good irrigation management at tuber initiation and use of scab-tolerant varieties has the greatest potential for educing potato scab. The use of ammonium sulfate in addition to good irrigation management and tolerant varieties may further reduce scab in badly infested fields.