Wheat Nutrition and Fertilizer Requirements: Potassium

Adapted from the Canada Grains Council's Complete Guide to Wheat Management

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 2690 kg/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.

		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

Adapted from Simplot, 1986

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 40,000 kg/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

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

*D=Dry, M=Moist, W=Wet
Seedbed soil moisture conditions at seeding D=25%; M=50%; W=75% of field capacity.

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.

Wheat Nutrition and Fertilizer Requirements: Potassium

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