Potassium for crop production (2024)

Depending on soil type, approximately 90 to 98 percent of total soil K is found in this form. The minerals feldspars and micas contain most of the K. Plants cannot use the K in this crystalline-insoluble form.

Over long periods of time, these minerals weather, or break down, and K is released. However, this process 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).

This form of K is thought to be trapped between layers of clay minerals and is frequently referred to as being fixed. Other things to note about slowly available K:

• Growing plants cannot use much of it during a single growing season.

• It isn’t measured by routine soil-testing procedures.

• It can serve as a reservoir for readily available K.

• While some of it can be released for plant use during a growing season, some of it can also be fixed between clay layers and, thus, converted into slowly available K (see Figure 1).

• The amount of it varies with the dominating type of clay in the soil.

Variances among Minnesota clay types

Montmorillonite clays are dominant in many central and western Minnesota soils. These clays fix K when soils become dry because K is trapped between the layers in the clay mineral. When the soil gets wet, this K is released.

Illite clays are dominant in most southeastern Minnesota soils. These clays also fix K between layers when they become dry, but they don’t release all of the fixed K when the soil gets wet. This fixation without release causes problems when managing potash fertilizers for crop production.

Potassium that’s considered readily available for plant growth is potassium that is:

• Dissolved in soil water (water soluble).

• Held on clay particles’ exchange sites, which are found on the surface of clay particles. Called exchangeable K, 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 soil water’s K concentration drops, additional K is released into the soil solution from the K attached to the clay minerals. The K attached to clay minerals’ exchange sites is more readily available for plant growth than the K trapped between layers of clay minerals.

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

Notice that when the arrows go in both directions, one form of K is converted to another. Factors that affect the conversion rate include root uptake, fertilizer K applied, soil moisture and soil temperature.

Root activity, plant functions and physiological processes all increase as soil temperature increases. And increased physiological activity leads to increased K uptake.

The optimum soil temperature for uptake is 60 to 80 degrees Fahrenheit. Potassium uptake reduces at low soil temperatures.

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 rest of the leaf. The entire leaf has a very distinct light green appearance when viewed from a distance. The striping can be easily confused with deficiency symptoms for sulfur (S), magnesium (Mg) and zinc (Zn).

For soybean, the margins of the leaflets turn light green to yellow when K is deficient (Figure 3). Deficiency symptoms first appear on the lower leaves, as with corn. With maturity, deficiency symptoms expand to leaves closer to the top of the canopy.

It’s not uncommon to find K deficiency symptoms near the plant tops in isolated field areas with intense soybean aphid pressure. In this case, the deficiency isn’t necessarily related to a deficiency of K in the soil.

Yellow or white spots on the margins of the leaflets (Figure 4) characterize potassium deficiency in alfalfa. Symptoms first appear on the older plant tissue. Potassium deficiency in alfalfa can be easily confused with damage caused by the potato leafhopper.

Potassium deficiency in potato occurs as scorching of the leaflet margins, which first occurs on the older leaves (Figure 5 and 6).

You can usually first notice symptoms during tuber bulking (mid-July), as the tuber is a strong sink for potassium. Potassium-deficient potato vines will die back prematurely, which can often be confused with diseases causing vine death.

By using the plant analysis management tool, you can look back at nutrient supplies during the growing season. You cannot use this tool to predict the amount of potash needed for crops 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. Table 2 summarizes the definitions for the relative levels of soil test K.

The relative level classifications estimate the soil’s ability to supply all the needed K for a crop:

• Low and very low ranges: You can expect an increase in production if you add potash fertilizer to the fertilizer program.

• Medium and higher ranges: Added yield may or may not be observed if you add potash.

• High or very high range: Do not expect a response to potash fertilization.

If K is fixed by clay, the soil test will extract less K and underestimate the available K, leading to the overapplication of potash fertilizer. Similarly, with clays that release K, availability tends to be overestimated and fertilizer needs underestimated.

Influencing factors

Factors that determine whether K is fixed or released:

• The type of clay that dominates the soil clay fraction.

• The soil test K level.

• The native level of K available from parent material.

• Recent K fertilizer applications.

Soil in Minnesota

Soils in Minnesota contain a mixture of clays, with smectite and illite being the most abundant clay types. Iron is a structural component of clay minerals, and reducing 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 get wet and dry off, the two types of clays will affect the availability of K to the crop.

Figure 7 gives examples of how drying various soils prior to analysis affects the amount of K extracted. Soil samples were taken from a silt loam soil in Red Wing, a loam soil in Rochester and Lamberton and loamy sand in Becker.

In Rochester and Red Wing, the moist K test extracted more K than the air-dried test. The moist K test extracted more K when the rate of K applied increased.

The two tests were similar for the sandy soil in Becker and when no K was applied in Red Wing. In Lamberton, the moist K test extracted less K than the air-dried test when 100 pounds of K₂O or less was applied.

The effect of drying on the K soil test can make it difficult to assess where a deficiency may occur. No fertilizer is suggested when the air-dry K test is above 160 ppm. Uncertainty in whether release or fixation occurs may change how to interpret this critical level.

Amounts of K extracted and soil test results

Figure 8 summarizes the relationship between the amounts of K extracted on moist samples versus the difference between K extracted with ammonium acetate by the moist test versus the conventional air-dried test.

Data suggests 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 when it exceeds 200 ppm.

If overestimation of K is likely for your soil, you may want to use a critical level of 200 ppm for build or maintenance purposes. This helps reduce the risk of a yield reduction due to overestimating 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. Do not use soil test interpretation classes for the air-dried test in Minnesota for the moist K test.

• 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 don’t affect the availability of K for crop growth.

• Most potassium chloride used in the United States is mined from underground deposits in Saskatchewan, although some are mined in the western United States.

• Can be used to supply sulfur in addition to potassium.

• 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’s also a need for magnesium in a fertilizer program. Consider this for fertilizing corn, alfalfa and small grains grown on sandy soils.

• The cost of potassium nitrate and potassium sulfate is usually higher, so use in Minnesota is very limited.

• Use this liquid fertilizer for fertigation or foliar K applications.

• It may be phytotoxic if used as a popup fertilizer, especially in low soil moisture conditions.

• Don’t apply as a foliar application if temperatures are above 90 degrees Fahrenheit.

• Potassium nitrate is a readily available source of K, but is primarily used for high-value crops because of its cost.

• The K content of manure varies with animal type, feed ration, storage and handling practices.

• Analyze manure to determine how much K was applied.

• Consider the total amount of K in manure to be readily available, as K isn’t a structural component of organic molecules.

There’s 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 is to apply potash fertilizers before seeding, followed by annual top-dress applications. Base the annual applications on the results of routine soil tests for K.

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 inches tall.

You can apply any potash needed for corn and small grain production in a band near the seed at planting, or broadcast and incorporated before planting. When applying in a band, you can cut the recommended potash broadcast rate in half. This doesn’t reduce yield, but does reduce fertilizer costs.

Table 4 illustrates the effectiveness of banded potash for corn production.

Banded application of potash is essential for corn grown in ridge-till and no-till production systems. A rate of 40 to 50 pounds of K₂O per acre is suggested, even though the soil test for K may be in the high or very high range.

For soybeans, broadcast applications of potash fertilizer incorporated before planting are most effective. Banding fertilizer for the soybean crop will increase grain yield in K-deficient soils. Unlike corn and small grains, the greatest soybean yield increases are associated with broadcast fertilizer application.

Applying K₂O in a band in the fall after soybean harvest is a popular management practice for those using reduced tillage in the corn-soybean rotation.

Fertilizer sources containing KCl pose a greater risk for stand damage due to salts in solution.

If a fertilizer source contains KCl, apply no more than 10 pounds of N + K₂O in loamy soils and 4 pounds of N + K₂O in sandy soils.

Fertilizers containing KOH are safer for seed placement as they form potassium-phosphate compounds that don’t contribute to the fertilizer’s salt content. Low-salt fertilizer sources manufactured with KOH cost more and may contain a very low concentration of K.

Applying fertilizer on the corn seed as an in-furrow application cannot supply all the needed K in a K-responsive soil.