Potassium management in crops

Overview

Potassium (K) is a macronutrient required by all plant and animal life. While K is not a commonly limiting soil nutrient in crop production in Alberta, about 15% of Alberta soils used for annual crop production are estimated to have slight to moderate K deficiency.

Adequate soil potassium results in:

• superior whole-plant quality due to improved photosynthesis efficiency
• increased crop resistance to some diseases
• greater water use efficiency
• a normal balance between carbohydrates and proteins
• stronger straw and improved seed filling for cereal crops

Potassium deficiency in cereal crops results in:

• reduced growth
• delayed maturity
• lodging caused by weaker straw
• lower bushel weight

Potassium deficiencies are most common on well drained, coarse-textured soils. These deficiencies can be corrected with potassium fertilizer (K₂O) such as potash.

Soil potassium

The majority of soils in Alberta contain sufficient plant-available potassium to satisfy crop growth.

The total amount of K in soil often exceeds 40,000 kg/ha (36,000 pounds per acre – lbs/ac) in the top 15 cm (6 in) of soil. However, only 1 to 2% of the total K in soil is in a form available to plants.

The parent geologic material on which Alberta soils developed generally contains considerable K-bearing clay minerals. Potassium becomes available to crops through weathering of these soil minerals.

Forms of K in soil

There are 3 forms or pools of inorganic K in soil:

1.    Unavailable K

About 90 to 95% of the total soil K is locked within the structure of the layered clay mineral sheets and is not available to plants.

2.    Slowly available K

About 5 to 10% of the total soil K is slowly becoming available to plants. Weathering of the clay minerals occurs on the surface of the minerals and results in a very slow release of K from the unavailable K pool locked within the clay minerals. The weathering of clay minerals gradually releases K from the minerals to recharge the K removed from the available soil K pool.

3.    Available and exchangeable K

Available and exchangeable K represents about 1 to 2% of the total soil K. The K in soil available to plants is dissolved in soil water while exchangeable K is loosely held on the exchange sites on the surface of clay particles. Typically, this K pool or fraction represents about 1 to 2% of the total soil K. A portion of this pool is plant-available K dissolved in the soil water.

The exchangeable K, which is positively charged (K+), is loosely held on the negatively charged exchange sites on the surface of clay minerals and is referred to as exchangeable K. As the available K dissolved in the soil water is taken up by plant roots, exchangeable K is released into the soil solution to maintain an equilibrium between the 2 forms. Soil tests attempt to measure the available and exchangeable K in soil to determine the K-supplying power for the soil K for crop production.

Available and exchangeable levels of K generally range between 300 and 1,000 kg/ha (270 to 900 lbs/ac) in Alberta soils in the top 15 cm (6 in) of soil. A very small percentage of Alberta soils have as little as 100 kg/ha (90 lbs/ac) of available K. A minimum of 200 kg K/ha (180 lbs/ac) in the top 15 cm (6 inches) of soil is generally required for adequate growth of most crops grown in Alberta.

K adsorption

Potassium only occurs in soils in inorganic form and does not make up part of the soil organic matter. Potassium in soil solution and in exchangeable form occurs as a positively charged ion, K+.

Soil colloids and the surface of soil clay minerals are negatively charged. Therefore, K will tend to be adsorbed onto the surface of negatively charged soil particles. As a result, soil K tends to be fairly immobile in soil and is not subject to leaching or downward movement in soil.

Soils that have the greatest potential for K 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 in Alberta. Irrigated coarse textured soils that have intensive crop rotations that include potatoes, sugar beets and/or alfalfa in the rotation may require K. A soil test is often very useful to determine whether K fertilizer application is warranted.

Plant K uptake

It is important to note that plants take up K in ionic form as K+. Potassium fertilizer is sold as K₂O. The most commonly sold K fertilizer in Alberta is K chloride (KCl) (0-0-60), which contains 60% K₂O.

To convert from lb of K₂O to lb of K, divide by 1.6. Soil tests normally report available and exchangeable K as K and provide K fertilizer recommendations in lb of K₂O.

Potassium deficiency

Potassium deficiency symptoms on cereal grains appear as a burning or scorching of the older, lower leaves. The burning begins at the leaf tip and continues down the leaf margin. Lower leaves are affected first because K in these older leaves is translocated to the new upper leaves to meet growth requirements.

In alfalfa, K deficiency appears as white or yellow spots on the lower leaves. Many other factors can affect the appearance of lower leaves, so a diagnosis based on visual deficiency symptoms must be confirmed with soil tests.

Potassium deficiency in Alberta is less common than nitrogen, phosphorus or sulphur deficiency for 3 reasons:

1. The parent geologic material on which the soils developed generally contains considerable K-bearing clay minerals.
2. Alberta soils are young with minimal weathering and leaching.
3. Crop removal has been relatively minor where cereal, oilseed and pulse crops have been the dominant crops, especially where straw was left on the field or manure returned. About 80% of the K taken up by crops remains in the straw and is returned to the soil.

There are an estimated 3,000,000 acres of K-deficient soils in Alberta (see Figure 1). Of this total:

• 2,500,000 are moderately deficient
• 500,000 are very deficient

K-deficient soils tend to be sandy (light to medium textured), alkaline, carbonated and/or imperfectly to poorly drained in their natural state. Organic soils are also frequently deficient in K.

Forage, silage, potato and sugar beet crops have a high K requirement; they take up and remove relatively large amounts of K from soils. When these crops are included in an intensive crop rotation on sandy soils, there is an increased risk of K deficiency.

Potassium is a relatively inexpensive nutrient, and deficiencies can often be corrected with moderate rates of application.

Correcting a K deficiency can result in excellent economic returns. Potassium chloride (KCl) (0-0-60), also called muriate of potash, is the most commonly-used K fertilizer, which contains 60% K₂O.

Figure 1. Soil areas of Alberta more susceptible to having K-deficient soils.

Table 1. Approximate nutrient uptake of crops for specific yields

Crop K requirements

The potassium content of the above-ground portion of commonly grown crops is shown in Table 1 above. Note the distribution of K between grain and straw as compared to nitrogen and phosphorus. Potassium removal from the soil is relatively low when only the grain of cereals is removed from the field. Potassium removal is much higher when forage, silage crops or potatoes are grown. If the forage is fed to livestock on the farm, the manure can be used to return much of the K removed by the forage crops.

While an awareness of the amount of K removed by various crops is useful, it is not in itself a basis for determining fertilizer requirements. The majority of soils in the prairie region contain high levels of plant-available K and can supply adequate amounts, even for high-use crops, for many years. On soils marginal to deficient in K, high-use crops will require higher rates of K fertilizer to maintain adequate nutrition.

Determining potassium fertilization need

The easiest way to determine the need for K fertilization is through a soil test. In Alberta, response to K fertilizer has been related to the amount of K extracted from the soil with ammonium acetate.

The most common soil test method used in Alberta to extract K from the soil is the modified Kelowna method.

In studies, large increases in barley yield were usually obtained when K fertilizer was applied to soils with less than 151 lbs/ac of extractable K. Sites with a soil test K level of less than 150 lbs/ac all responded to added K fertilizer. On soils with 200 to 250 lbs/ac of extractable K, moderate fertilization of 15 to 30 lbs/ac of K₂O may result in a profitable response.

The probability of response above a soil test level of 250 lbs/ac was low. However, at soil test K levels between 250 and 300 lbs/ac, producers should consider a maintenance application of 15 to 20 lb K₂O/acre to help replace K taken up by crops. This application could also potentially improve crop yield in areas of a field that may have slightly deficient soil K levels.

A response to K fertilization is sometimes obtained on soils not considered deficient in K. Research, principally in Oregon, Washington and South Dakota, has shown that the presence of chloride in K chloride fertilizer can result in increased yield through the suppression of plant diseases such as take-all and common root rot in wheat and barley. Such responses have not been observed in Alberta research with wheat and barley; therefore, the use of K chloride on non K-deficient soil for the suppression of disease must be on a trial basis.

Research in Montana has indicated that K deficiency may occasionally occur on soil with high soil test K because of slow K diffusion in cold, dense soils. These results have been used to promote the need for K fertilization for early seeding of cereal crops in central and northern Alberta.

Research in Alberta on K-deficient soils has shown equal response of barley to K with early and later seeding. If soil K was less available at lower soil temperatures, greater response to K should have been obtained with early versus later seeding. The results do not support a higher K requirement for early seeded cereal crops.

Choosing application rates

Application rates are based on the soil test level and the responsiveness of the crop. On soils marginal to deficient in K, crop removal (Table 1) should also be taken into account when planning an on-going K fertilization program.

Rates above 15 to 30 lb of K₂O/ac for cereals and pulse crops should be banded or broadcast to avoid seedling injury. At low rates of application, placement with the seed is more effective than banding, and banding is more effective than broadcast. See Application methods below.

The majority of K-deficient soils in Alberta are only slightly to moderately deficient. Maximum response to K on these soils is usually obtained with an application of 15 to 30 lbs/ac of K₂O to cereal crops. On soils that are very deficient in K (soil test K less than 150 lbs/ac), high rates of K fertilization are usually required to achieve maximum productivity. On very K-deficient soils, soil K can be built-up with an initial application of 200 to 300 lbs/ac of K₂O (broadcast and incorporated), followed in subsequent years with annual applications of about 30 lbs/ac. Similar results could be achieved by banding 50 to 80 lbs/ac for a few years to build up soil K levels.

Research in Alberta indicates that soil tests do not always adequately reflect residual fertilizer K where high rates have been applied to very K-deficient soils. Following the application of up to 810 lbs/ac of K₂O on very K-deficient soils, soil test K was still in the deficient range, but subsequent crops showed little or no response to additional K fertilizer. These heavily fertilized soils contained residual, crop-available K that was not reflected in the soil test.

Therefore, when high rates of K fertilizer are applied to very K-deficient soils, the subsequent need for additional fertilizer should be determined on the basis of crop response, rather than solely on the basis of a soil test.

For detailed potassium responses of different crops across major Alberta zones, see Alberta Farm Fertilizer Information and Recommendation Manager (AFFIRM).

Types of K fertilizer

The most common form of potassium fertilizer used in Alberta is potassium chloride (KCl), which has the analysis 0-0-60 or 0-0-62 (Table 2). It is mined and refined in Saskatchewan. Potassium chloride can be safely blended with nitrogen and phosphate fertilizers to produce various blends to achieve optimum N-P₂O₅-K₂O application.

Table 2. Potassium fertilizers available in Alberta

Application methods

Annual crops

Since soil potassium is not mobile in soil, placement of K fertilizers with or near the seed is usually the most effective and efficient method of application provided the rate of application is not greater than the seed can tolerate. Potassium chloride (KCl) is a salt. If too much K or a combination of other fertilizers that have a high salt index are placed with the seed, a “salt effect” can delay or reduce seed germination and emergence. The salt effect of fertilizer will interfere with the moisture uptake by the seed and may result in the death of a germinating seed.

The safe level of K that can be applied with the seed depends on the crop. In general, smaller seeded crops such as canola have a lower tolerance than cereal crops. The clay and organic matter content of the soil and the soil moisture content will also have an effect on possible germination problems. Soils with a higher clay content or higher organic matter content can hold more water. This situation can slightly reduce the salt effect when soil moisture levels are good by diluting the amount of K salt dissolved in the soil water.

With average soil moisture conditions and medium- textured soils, the total amount of seed-placed fertilizer materials should not exceed 175 lbs/ac, and the total amount of N plus K₂O should not exceed 40 lbs/ac at a seed-bed utilization (SBU) of 10%. For less tolerant crops such as canola, flax and peas, the application of K with the seed should not exceed 15 lb K₂O/acre, provided other fertilizers are not seed placed.

These recommendations are based on the use of a seed drill with a 10% SBU, which places the seed and fertilizer in a very 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

Side-band placement is an efficient means of applying K and much safer than seed placement, particularly when higher rates of K must be applied. Ideally, with this placement, the fertilizer is in a band approximately 2.5 cm (1 inch) to the side and possibly 2.5 cm (1 inch) beneath the seed. This separation of fertilizer and seed reduces the possible detrimental effects on germination when high rates are applied. There are a number of different openers that have slightly different placement positions.

Banding

Banding, also referred to as deep banding, places the K into the soil in a concentrated band prior to seeding. It is believed that this placement method should provide similar results to side-banding K.

Broadcasting

Broadcasting K before seeding is less efficient than applying K in a band with or near the seed. The major role for broadcast applications of K fertilizer will be in building up soils extremely deficient in K or for use with forage crops.

Perennial crops

For perennial forage crops, potassium is best applied by broadcasting and incorporating at higher rates of 100 lb K₂O/acre before seeding. Higher rates should be used on K-deficient irrigated land. This approach will overcome the problem of limited movement of K into the soil when applications are made after stand establishment.

Where established stands require K fertilizer, broadcast applications are the only option, and relatively high rates may be required on severely K-deficient soils. Fall or spring applications could be made, but fall applications would likely be preferred in dry areas because of the additional winter moisture available to move the K into the root zone. For K-deficient soils, K fertilizer application in early fall may help reduce alfalfa winterkill and help maintain the proportion of alfalfa in mixed forage stands.

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