Effect of Potassium on Wheat Plant Growth

Wheat requires potassium for optimal growth and development. Adequate potassium results in superior quality of the whole plant due to improved efficiency of photosynthesis, increased resistance to some diseases, and greater water use efficiency. It helps maintain a normal balance between carbohydrates and proteins. Sufficient potassium results in stronger wheat straw and assists in grain filling.

Wheat Plant Potassium Content and Requirement

Wheat requires significant amounts of potassium for optimum growth (Table 1). Of the total potassium required by a 2690kg/ha (40 bu/ac) wheat crop, 18 kg/ha (16 lb/ac) of potassium is contained in the seed and 54 kg/ha (48 lb/ac) fertilizer potassium in the straw. Unlike phosphorus and nitrogen, potassium is not incorporated into organic compounds in the plant tissue but remains in ionic forms within cells and tissue. After wheat straw is incorporated into the soil, a large quantity of potassium is (75%) released back into the soil for subsequent crop use.

Table 1. Potassium requirements of wheat.

Adapted from Simplot, 1986
  Potassium (K)
Yield Crop Part Kg/ha (lb/ac)
Wheat Seed 18 16
2690 Kg/ha Straw 54 48
(40 Bu/ac) Total 72 64

Potassium Deficiency Symptoms

In wheat, potassium deficiency may result in shorter internodes, weaker straw, increased lodging and slower growth. A bronzing or yellowing of the leaf tip followed by marginal yellowing and dying of the tip may be noted. Lower leaves are affected first because potassium in the plant is translocated to the new upper leaves. Deficiency symptoms can be easily confused with symptoms of nitrogen or phosphorus deficiency. Standard tissue tests can be useful to identify and confirm potassium deficiencies.

Potassium Supply

The majority of soils in Alberta contain sufficient potassium to satisfy crop growth. Total amount often exceeds 40,000kg/ha (36,000 lb/ac) in the top 15 cm (6 in). However, only 1 to 2% of the total K is in a form that is potentially available to plants.

Geologic material on which soils developed generally contained considerable potassium bearing minerals. Potassium becomes available to crops through weathering of these minerals. Available and exchangeable levels of potassium generally range between 300 and 1,000 kg/ha (270-900 lb/ac). However, some soils have as little as 100 kg/ha (90 lb/ac) of available potassium, and a minimum of 200 kg/ha (180 lb/ac) of available potassium is required for optimum growth of wheat. Soils that have the greatest potential for potassium deficiencies are the coarse (sandy) and medium (loam) textured soils in the black, gray-black and gray soil zones, as well as the organic soils. Irrigated coarse textured soils that are intensively cropped and include potatoes, sugar beets or alfalfa in the rotation, would probably require potassium. A soil test is recommended to determine whether a potassium application is warranted in any specific situation.

Potassium Fertilizer Recommendations

General potassium fertilizer recommendations are provided for wheat in Table 2.

Table 2. Potassium fertilizer recommendations for wheat

*D=Dry, M=Moist, W=Wet
Seedbed soil moisture conditions at seeding D=25%; M=50%; W=75% of field capacity.
Soil test K
(lb/ac)
Brown Dark
Brown
Thin
Black
Black Gray
Wooded
Irrigated
  D M W D M W D M W D M W D M W  
K20
lb/ac
0 100 105 110 105 110 115 105 110 115 110 115 120 110 115 120 120
25 90 95 100 95 100 115 95 100 105 100 105 110 100 105 110 110
50 80 85 90 85 90 105 85 90 95 90 95 100 90 95 100 100
75 70 70 80 75 75 95 75 75 85 80 80 90 80 80 90 90
100 60 60 70 65 65 85 65 65 75 70 70 80 70 70 80 80
125 50 50 55 55 55 75 55 55 60 60 60 65 60 60 65 70
150 35 40 45 40 45 50 40 45 50 45 50 55 45 50 55 55
175 25 30 35 30 35 40 30 35 40 35 40 45 35 40 45 45
200 15 20 25 20 25 30 20 25 30 25 30 35 25 30 35 35
225 15 15 15 15 15 20 15 15 20 15 20 25 15 20 25 25
250 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15
>250 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Potassium Fertilizer Placement and Time of Application

In Alberta, potassium fertilizers have been used much less than nitrogen and phosphorus fertilizers and therefore much less research work has been done concerning their placement. However, placement of potassium fertilizer with the seed has appeared to be the most effective method of application provided the rate of application is not greater than the seed can tolerate. With average soil moisture conditions and for medium soil textures, the total amount of seed-placed fertilizer materials should not exceed 196 kg/ha (175 lb/ac) and the amount of nitrogen 45 kg/ha plus potassium should not exceed 45 kg/ha (40 lb/ac). These recommendations are based on the use of a hoe or similar drill which places the seed and fertilizer in a relatively narrow band. If the opener spreads the seed over a wider band, higher rates of fertilizer can be safely placed with the seed.

Side banding is an efficient means of applying potassium. In this placement the fertilizer is in a band to the side and below the seed. Banding prior to seeding has not received a great deal of research attention, however, side banding should give similar results. Broadcasting potassium prior to seeding is less efficient than applying potassium in a band with or near the seed.

Potassium fertilizers are not subject to leaching or volatilization; therefore, can be applied in the fall or spring. However, seed-placed or side banded potassium is normally best, versus broadcast or a random band application.