Michigan State University Extension
Soils & Soil Management - Fertilizer - 06109704
07/10/97
Potato Fertilizer Recommendations Extension Bulletin E-2220, April 1990
Maurice L. Vitosh Extension Soil Specialist Department of Crop and Soil Sciences
Potatoes are rather inefficient at making use of soil phosphorus (P). As a result, potato responses to P fertilizers have frequently occurred at relatively high soil test levels (300 to 500 lb P per acre). For instance, yield responses have been observed at the Montcalm Research Farm, where the soil test showed 481 lb per acre. Limited studies have been conducted on soils testing higher than 500 lb P per acre. On-farm P demonstrations in 1988 on soils testing 900 to 1,000 lb P per acre showed no noticeable growth or yield differences due to P fertilizer. Phosphorus contamination of Michigan lakes has recently become a public concern. Non- point source P contamination from agricultural land is often blamed for much of the P addition to these lakes. Because of the concerns about additional P contamination of Michigan lakes and the lack of good response data on higher testing soils, MSU does not recommend P fertilizer on soils testing above 600 lb P per acre.
Phosphorus Fertilizer Rates
MSU P fertilizer recommendations based on P soil test level and yield goal are given in Table 3. One hundred hundredweight (cwt) of potatoes remove approximately 13 lb of P2O5. per acre. MSU recommends more P than is removed at low soil test levels, maintenance amounts at medium soil test levels, and less than is removed at high soil test levels.
Recommendations vary, depending on the yield goal. Higher yields require higher rates of P fertilizer than lower yields because of greater removal. Studies conducted by MSU show that P fertilizer has no significant effect on tuber solids. ==================================================== Table 3. Phosphorus (P2O5) recommendations for potatoes grown on mineral soils.
Yield goal, cwt/acre
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Soil test 300 400 500
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lb P/acre Phosphorus recommendation
lb P2O5/acre
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100 80 85 90
200 50 50 55
300 30 30 35
400 20 20 20
500 0 20 20
600 0 0 20
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Phosphorus Fertilizer Placement
Most of the P fertilizer added to soil is precipitated out in an unavailable form-only small amounts remain in soil solution. For this reason, band applications alongside the seed pieces are more efficient than broadcast applications. Any P fertilizer that is applied to potatoes should be applied in bands 2 inches to the side and 2 inches below the seed pieces at planting time.
Soil pH and texture greatly affect phosphorus availability. The availability of P is greatest in sandy soils at pH 6.5. Most potatoes are grown on acid soils where soluble P fertilizer is readily precipitated as iron and aluminum phosphate. The P soil test level in sandy soils can easily be raised by P fertilizer additions, but fine-textured soils require moderate additions of P fertilizer to raise the P soil test level.
Phosphorus Sources
The most common P fertilizers are diammonium phosphate (18-46-0), a dry solid, and ammonium polyphosphate (10-34-0), a liquid. They are the common ingredients in bulk blended and liquid mixed starter fertilizers. Monoammonium phosphate (11-48-0), another solid material, is sometimes used in bulk blends, but because of its added expense, it is used less frequently. All sources are rated equal if applied in the same manner. Liquid 10-34-0 has the advantage of being easy to handle, and it can be easily mixed with 28 percent N solutions to obtain a lower P analysis starter, which is needed on many potato fields. Liquids that can be pumped also have the advantage of allowing lower application rates than solid fertilizer. The primary disadvantage of using liquid fertilizers is their incompatibility with certain inorganic micronutrient sources, such as manganese sulfate.