Michigan State University Extension
Soils & Soil Management - Fertilizer - 06109705
07/10/97
Potato Fertilizer Recommendations Extension Bulletin E-2220, April 1990
Maurice L. Vitosh Extension Soil Specialist Department of Crop and Soil Sciences
Potassium (K) is very similar to N in that it can greatly affect potato tuber quality. High rates of K fertilizers will lower specific gravity (percent solids). Complicating the issue is the fact that potatoes have a high requirement for K Five hundred hundredweight (cwt) of potatoes will remove approximately 300 lb of K20 per acre. Because most potatoes are grown on sandy soils in Michigan and sandy soils do not hold large amounts of K, growers must pay a great deal of attention to K fertilization. Soil testing is the key to determining the amount of K fertilizer to apply.
Potassium Fertilizer Rates
Potassium fertilizer recommendations based on K soil test level and yield goal are given in Table 4. MSU recommends more K than is removed at very low soil test levels, maintenance amounts at low to medium soil test levels, and less than is removed at medium to high soil test levels. Recommendations also vary with yield goal. Higher yields require higher rates of K fertilizer than lower yields because of greater removal.
Potassium Fertilizer Placement
Potassium fertilizer may be applied broadcast before
planting or banded at planting time. Do not apply
more than 100 lb of K20 per acre banded at planting
time. Large amounts of K salts in this location can
cause salt injury to young plants. If more than 100
lb of K20 is needed based on the soil test, it
should be applied before planting Potash (0-0-60)
may be applied in the fall of the year on fine- and
medium-textured soils. Fall applications will reduce
the amount of chloride available during the growing
season, which should have a positive effect on tuber
quality. Fall applications of potash on loamy sand
or sand soils, however, is not recommended because
some can be potentially lost over winter by
leaching. Topdress applications of potash
after planting are not recommended.
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Table 4. Potassium (K2O) recommendations for
potatoes grown on mineral soils.
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Yield goal, cwt/acre
--------------------
Soil test 300 400 500
--------- --------------------
lb K/acre lb K20/acre
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sandy loams and loamy sands
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50 280 330 380
100 230 280 330
200 130 180 230
300 30 80 130
400 0 0 30
500 0 0 0
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loams, clay loams and clays
-------------------------------
50 280 350 410
100 220 280 350
200 100 160 220
300 0 30 100
400 0 0 0
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Potassium Fertilizer Sources
Potassium chloride, also known as muriate of potash (0-0-60), is the cheapest and most common source of K fertilizer. It is used in most bulk blend starter fertilizers and for direct application. Some research has shown that potassium sulfate and potassium nitrate will have less effect on tuber specific gravity than potassium chloride, although all these materials will reduce specific gravity at high rates of application. Under good irrigation management and moderate rates, very little difference in tuber quality has been observed in the field. Because of the higher cost of potassium sulfate and potassium nitrate and the lack of consistent improvement in tuber solids, these sources of K are not recommended over potassium chloride.