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Potassium for crop production

Daniel E Kaiser, Carl J. Rose and John A. Lamb

Potassium (K) is an essential nutrient for plant growth and is classified as a macronutrient due to large quantities of K being taken up by plants during their life cycle. Minnesota soils can supply some K for crop production, but when the supply from the soil is not adequate, K must be supplied in a fertilizer program. This publication provides information important to the basic understanding of K nutrition of plants, its reaction in soils, its function in plants, and its role in efficient crop production.

Role in plant growth

Potassium is associated with movement of water, nutrients, and carbohydrates in plant tissue. Potassium is involved with enzyme activation within the plant which affects protein, starch and adenosine triphosphate (ATP) production. The production of (ATP) can regulate the rate of photosynthesis. Potassium also helps to regulate the opening and closing of the stomata which regulates the exchange of water vapor, oxygen, and carbon dioxide. If K is deficient or not supplied in adequate amounts, growth is stunted and yield is reduced. For perennial crops such as alfalfa, potassium has been shown to play a role in stand persistence through the winter. Other roles of K include:

Potassium in soils

The total K content of soils frequently exceeds 20,000 ppm (parts per million). Nearly all of this K is in the structural component of soil minerals and is not available for plant growth. Because of large differences in soil parent materials and the effect of weathering of these materials in the United States, the amount of K supplied by soils varies. Therefore, the need for K in a fertilizer program varies across the United States.

Three forms of K (unavailable, slowly available or fixed, readily available or exchangeable) exist in soils. A description of these forms and their relationship to each other is provided in the paragraphs that follow. The general relationships of these forms to each other are illustrated in Figure 1.

Primary minerals (unavailable potassium)

Depending on soil type, approximately 90-98% of total soil K is found in this form. Feldspars and micas are minerals that contain most of the K. Plants cannot use the K in this crystalline-insoluble form. Over long periods of time, these minerals weather (break down) and K is released. This process, however, is too slow to supply the full K needs of field crops. As these minerals weather, some K moves to the slowly available pool. Some also moves to the readily available pool (see Figure 1).

potassium cycle

IPNI

Figure 1. Generalized soil potassium cycle representing K in the soil and where K may be applied or removed on an annual basis.

Secondary minerals and compounds (slowly available potassium)

This form of K is thought to be trapped between layers of clay minerals and is frequently referred to as being fixed. Growing plants cannot use much of the slowly available K during a single growing season. This slowly available K is not measured by the routine soil testing procedures. Slowly available K can also serve as a reservoir for readily available K. While some slowly available K can be released for plant use during a growing season, some of the readily available K can also be fixed between clay layers and thus converted into slowly available K (see Figure 1).

The amount of K fixed in the slowly available form varies with the type of clay that dominates in the soil. Montmorillonite clays are dominant in many of central and western Minnesota soils. These clays fix K when soils become dry because K is trapped between the layers in the clay mineral. This K, however, is released when the soil becomes wet. Illite clays are dominant in most of the soils in southeastern Minnesota. These clays also fix K between layers when they become dry, but do not release all of the fixed K when water is added. This fixation without release causes problems for management of potash fertilizers for crop production in the region.

Solution potassium (readily available K)

Potassium that is dissolved in soil water (water soluble) plus that held on the exchange sites on clay particles (exchangeable K) is considered readily available for plant growth. The exchange sites are found on the surface of clay particles. This is the form of K measured by the routine soil testing procedure.

Plants readily absorb the K dissolved in the soil water. As soon as the K concentration in soil water drops, additional K is released into the soil solution from the K attached to the clay minerals. The K attached to the exchange sites on the clay minerals is more readily available for plant growth than the K trapped between the layers of the clay minerals.

The relationships among slowly available K, exchangeable K, and water-soluble K are summarized below.

slowly available K

exchangeable K

water-soluble K

Notice that when the arrows go in both directions, one form of K is converted to another. The rate of conversion is affected by such factors as root uptake, fertilizer K applied, soil moisture, and soil temperature.

Potassium uptake

Potassium uptake by plants is affected by several factors.

Soil Moisture: Higher soil moisture usually means greater availability of K. Increasing soil moisture increases movement of K to plant roots and enhances availability. Research has generally shown more responses to K fertilization in dry years.

Soil Aeration and Oxygen Level: Air is necessary for root respiration and K uptake. Root activity and subsequent K uptake decrease as soil moisture content increases to saturation. Levels of oxygen are very low in saturated soils.

Soil Temperature: Root activity, plant functions, and physiological processes all increase as soil temperature increases. This increase in physiological activity leads to increased K uptake. Optimum soil temperature for uptake is 60-80°F. Potassium uptake is reduced at low soil temperatures.

Tillage System: Availability of soil K is reduced in no-till and ridge-till planting systems. The exact cause of this reduction is not known. Results of research point to restrictions in root growth combined with a restricted distribution of roots in the soil.

Potassium deficiency symptoms

corn with potassium deficiency

Figure 2. Potassium deficiency symptoms in corn showing necrosis along the leaf margin.

Some crops exhibit characteristic deficiency symptoms when adequate amounts of K are not available for growth and development. Potassium is mobile in plants and will move from lower to upper leaves. For corn, the margins of the lower leaves turn brown (Figure 2). This development of dead tissue is accompanied by a striped appearance in the remainder of the leaf. The entire leaf has a very distinct light green appearance when viewed from a distance. The striping associated with K deficiency in corn can be easily confused with deficiency symptoms for sulfur (S), magnesium (Mg), and zinc (Zn).

The margins of the leaflets turn light green to yellow when K is deficient for soybean production (Figure 3). As with corn, these deficiency symptoms first appear on the lower leaves. With maturity, the deficiency symptoms expand to leaves closer to the top of the canopy. It is not uncommon to find K deficiency symptoms near the top of the plant in isolated field areas with intense soybean aphid pressure. In this case the deficiency is not necessarily related to a deficiency of K in the soil.

Potassium deficiency in alfalfa is characterized by yellow or white spots on the margins of the leaflets (Figure 4), with symptoms first appearing on the older plant tissue. Potassium deficiency in alfalfa can be easily confused with damage caused by the potato leafhopper.

soybean showing potassium deficiency symptoms

Figure 3. Potassium deficiency symptoms in soybean.

alfalfa leaves showing potassium deficiency symptoms

Figure 4. Potassium deficiency symptoms in alfalfa. Note the white spots in the margins of the leaves.

Potassium deficiency in potato occurs as scorching of the leaflet margins on the older leaves first (Figure 5 and 6). Symptoms are usually first noticeable during tuber bulking (mid-July) as the tuber is a strong sink for potassium. Potato vines deficient in potassium will dieback prematurely, which can often be confused with diseases causing vine death.

 potato leaves showing early potassium deficiency symptoms

Figure 5. Early potassium deficiency symptoms in potato.

potato leaves showing late potassium deficiency symptoms

Figure 6. Late potassium deficiency symptoms in potato.

Predicting the needs for potash

The K status of soils can be monitored with either plant analysis or routine soil testing procedures. Plant analysis can be used to either confirm a suspected deficiency indicated by visual symptoms or routinely monitor the effects of a chosen fertilizer program. An interpretation for K levels in plant tissue is provided in Table 1.

Table 1. Sufficiency levels of potassium for major agronomic crops, vegetables, and fruit grown in Minnesota.
Crop Plant part Time Sufficiency range
(% K)
Alfalfa Tops (6" new growth) Prior to flowering 2.0-3.5
Apple Leaf from middle of current terminal shoot July 15 - August 15 1.2-1.8
Blueberry Young mature leaf First week of harvest 0.4-0.7
Broccoli Young mature leaf Heading 2.0-4.0
Cabbage Half-grown young wrapper leaf Heads 3.0-5.0
Carrot Young mature leaf Mid-growth 2.8-4.3
Cauliflower Young mature leaf Buttoning 2.6-4.2
Corn Whole tops Less than 12" tall 2.5-3.5
  Leaf at base of ear Initial silk 1.8-3.0
Edible bean Most recently matured trifoliate Bloom stage 1.5-3.3
Grape Petiole from young mature leaf Flowering 1.5-2.0
Pea Recently mature leaflet First bloom 2.0-3.5
Potato Fourth leaf from tip 40-50 days after emergence 4.0-6.0
  Petiole from fourth leaf to tip 40-50 days after emergence 8.0-10.0
Raspberry Leaf 18" from tip First week in August 1.1-3.0
Soybean Trifoliate leaves Early flowering 1.7-2.5
Spring wheat Whole tops As head emerges from boot 1.5-3.0
Strawberry Young mature leaf Mid-August 1.1-2.5
Sweet corn Ear leaf Tasseling to silk 1.8-3.0
Sugar beet Recently matured leaves 50-80 days after planting 2.0-6.0

Source: Bryson et al. (2014), Plant Analysis Handbook III; Rosen and Eliason (2002), Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota.

If amounts of K in the root zone are more than enough to meet crop needs, K will be absorbed by plants in amounts higher than required for optimum yield. This can lead to higher than normal concentrations of K in plant tissue and is referred to as "luxury consumption." Luxury consumption has no known negative effect on plant growth. Plant analysis is a management tool that can be used to look back at nutrient supplies during the growing season. This tool cannot be used to predict the amount of potash needed for any crop in the next growing season.

The soil test for K is the best management tool for predicting the amount of potash needed in a fertilizer program. Available K in soils is estimated by measuring the total of solution K (water = soluble K) and exchangeable K. The definitions for the relative levels of soil test K are summarized in Table 2.

Table 2. Relative levels of soil test values for K for Minnesota soils for air-dried soil samples.
Soil test potassium (ppm) Relative level
0-40 Very low
41-80 Low
81-120 Medium
121-160 High
161+ Very high

The relative level classifications represent an estimation of the soil's ability to supply all the needed K for a crop. An increase in production can be expected if potash fertilizer is added to the fertilizer program when soil test values are in the low and very low ranges. Added yield may or may not be observed if potash is added when the soil test values are in the medium range or greater. A response to potash fertilization should not be expected if soil test values for K are in the high or very high range.

Potash fertilizer suggestions are listed in BU-6240-S, Fertilizer Recommendations for Agronomic Crops in Minnesota and in BU-5886-E, Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota.

Effect of soil sample drying on soil test results

Soil testing labs commonly air dry soil samples prior to analysis. Drying of soils high in clay can affect the amount of K extracted. If K is fixed by clay, less K will be extracted by the soil test resulting in underestimation of the available K and an over application of potash fertilizer. Clays that release K will tend to over-estimate availability and underestimate fertilizer needs. The type of clay that dominates the soil clay fraction, the soil test K level, the native level of K available from parents material, and recent K fertilizer applications can dictate whether K is fixed or released.

Soils in Minnesota contain a mixture of clays. Smectite and illite are two of the most abundant clay types. Iron is a structural component of clay minerals and the reduction of Fe3+ to Fe2+ can affect fixation and release of K. As Fe3+ is reduced, K can be trapped (fixed) between clay layers for smectite and K will be released from illite. As soils wet and dry, the two types of clays will affect the availability of K to the crop.

Examples of how drying various soils prior to analysis affect the amount of K extracted are given in Figure 7. Soil samples were taken from a silt loam soil at Red Wing, a loam soil at Rochester and Lamberton, and loamy sand at Becker. At Rochester and Red Wing, the moist K test extracted more K than the air dried test, and the amount of K extracted by the moist test was greater when the rate of K applied increased. The two tests were similar for the sandy soil at Becker and when no K was applied at Red Wing. At Lamberton, the moist K test extracted less K than the air dried test when 100 lb. K2O or less was applied.

Four soil type K extraction comparison

Figure 7. Comparison of soil test K extracted on air dried and field moist soil samples at four field locations across Minnesota with differing soil types.

The effect of drying on the K soil test can make assessing where a deficiency may occur difficult. No fertilizer is suggested when the air dry K test is above 160 ppm. Uncertainty in whether a release of fixation occurs may change the interpretation of this critical level. Figure 8 summarizes the relationship between the amounts of K extracted on moist samples versus the difference between K extracted by the moist test versus the conventional air dried test.

Lamberton vs Morris soil comparison

Figure 8. Relationship between soil test K concentration from field moist soil samples and the difference between the K extracted with 1M ammonium acetate by the field moist versus air dried test for a loam soil near Lamberton, MN and a silt loam soil near Morris, MN.

Data suggests that the moist test will extract less K than the air dry test when the moist soil test is less than 200 ppm and will extract more K than the air dried test when greater than 200 ppm. For soils where overestimation of K is likely, utilizing a critical level of 200 ppm for build or maintenance purposes may be warranted to reduce the risk of a yield reduction due to an overestimation of available K by air drying soil samples

The moist K test uses the same chemical extraction as the conventional air dried test but should be considered a separate test. Soil test interpretation classes for the air dried test in Minnesota should not be used for the moist K test. Research is currently underway to calibrate the moist K test for Minnesota.

Sources of potassium

There are a limited number of fertilizer materials that can be used to supply K when needed. These materials are listed in Table 3.

Table 3. Some common potassium fertilizer sources.
Material* Chemical formula Approximate K2O (%)
Potassium chloride KCl 60-62
Potassium sulfate K2SO4 50
Potassium-magnesium sulfate (K-mag® or Sul-Po-Mag) K2SO4·2MgSO4 20
Potassium thiosulfate K2S2O3 17
Potassium nitrate KNO3 44

*Mention of brand name materials does not constitute endorsement by the University of Minnesota over similar products that might be commercially available.

Potassium chloride is the most common K source used in Minnesota. Red, pink, and white forms are available. These materials are equivalent as sources of K. The color in 0-0-60 is due to iron impurities that have no effect on the availability of K for crop growth. Most of the potassium chloride used in the United States is mined from underground deposits in Saskatchewan. Some are mined in the western United States.

Potassium sulfate can be used to supply sulfur in addition to potassium. This source is often used for potatoes if low specific gravity is a concern. Studies have shown that potatoes fertilized with potassium chloride have will lower specific gravity compared to those fertilized with potassium sulfate. Potassium-magnesium sulfate is a good source of K when there is also a need for magnesium in a fertilizer program. This product should be considered for fertilizing corn, alfalfa, and small grains grown on sandy soils. The cost of potassium nitrate and potassium sulfate is usually higher so the use of these two products in Minnesota is very limited.

Potassium thiosulfate is a liquid fertilizer that can be used for fertigation or foliar K applications. It may be phytotoxic if used as a popup fertilizer, especially when soil moisture conditions are low. Do not apply as a foliar application if temperatures are above 90°F. Potassium nitrate is a readily available source of K but is primarily used for high-value crops because of its cost.

Manure is also a source of K. The K content of manure varies with animal type, feed ration, storage, and handling practices. Manure should be analyzed to determine the amount of K that was applied. The total amount of K contained in manure should be considered readily available as K is not a structural component of organic molecules.

Management practices for potassium

Suggested management practices for K vary with each crop. There is a higher probability of successful establishment of perennial crops such as alfalfa and grasses if the soil test for K is in the medium range or higher. For these crops, the best strategy would be to apply potash fertilizers before seeding followed by annual top-dress applications. The annual applications should be based on the results of routine soil tests for K.

Potassium fertilizer can be applied either in the fall or spring for most soils in Minnesota. Sandy soils with a low cation exchange capacity have a low ability to hold K. Potassium should be considered partially mobile on sandy soils and should be applied closer to the time of planting. Recent studies for corn have shown a single pre-plant application of K to be sufficient for corn grown on sandy soils compared to a split application of K pre-plant and when the corn is 12" tall.

Any potash needed for corn and small grain production can be applied in a band near the seed at planting or broadcast and incorporated before planting. When applied in a band, the recommended broadcast rate of potash can be reduced by one-half without causing a reduction in yield. The effectiveness of banded potash for corn production is illustrated in Table 4.

Table 4. The effect of management of potash fertilizer on corn production. Goodhue County.
Rate of K applied (lb./acre) Placement Grain yield (% of maximum)
0 - 100
40 Starter (band) 125
100 Broadcast 119
200 Broadcast 124

Soil test (ammonium acetate at 0-6 in.) = 85ppm

For best results, potash fertilizer needed for soybean production should be broadcast and incorporated before planting. Banding fertilizer for the soybean crop will increase grain yield in soils where K is deficient. In contrast to corn and small grains, the greatest yield increases have been associated with broadcast fertilizer application to soybean.

Banded application of potash is essential for corn grown in ridge-till and no-till production systems. A rate of 40-50 lb. K2O per acre is suggested even though the soil test for K may be in the high or very high range. Application of the K2O in a band in the fall following soybean harvest is a popular management practice for those using reduced tillage in the corn/soybean rotation.

Liquid forms of potassium are available for in-furrow application as a starter fertilizer. These forms of fertilizer are manufactured with either potassium chloride (KCl) or potassium hydroxide (KOH). Fertilizer sources containing KCl pose a greater risk for stand damage due to salts in solution. No more than10 lbs of N + K2O should be applied in loamy soils and 4 lbs N + K2O in sandy soils if a fertilizer source contains KCl. Fertilizers containing KOH are safer for seed placement as they for potassium-phosphate compounds that do not contribute to the salt content of the fertilizer. Low salt fertilizer sources manufactured with KOH are higher in cost and may contain a very low concentration of K. Application of fertilizer on the corn seed as an in-furrow application cannot supply all the needed K in a K responsive soil.

Potassium thiosulfate is liquid fertilizer containing K and S. Application of fertilizer containing thiosulfate can also have a negative impact on seedling emergence and is not suggested for in-furrow application.

Summary

Potassium is an essential major nutrient for crop production in Minnesota. The supply of total K in soils is quite large. Yet, relatively small amounts are available for plant growth at any one time. The three forms of K (unavailable, slowly available, readily available) exist in an equilibrium in the soil system.

The need for potash in a fertilizer program can be determined from plant analysis and soil testing. Soil testing is the most reliable predictor of this need.

Suggested management practices for this nutrient vary with each crop. Top-dress applications are appropriate for perennial crops such as alfalfa and grasses. For soybeans, broadcast applications incorporated before planting are most effective. Either banded or broadcast applications can be used for corn and small grain production. Broadcast rates can be reduced by one-half if banded applications are used for these crops. This management practice does not reduce yields but results in a savings of fertilizer dollars.

Revised 2016

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