Potassium Fertilizer Management

Table of contents

    Important tips for best management

    • Most Western Canadian soils have enough potassium (K) to meet canola needs, soil test levels are unlikely to change much over time. Most K taken up by plants remains in the plant biomass, not the seed, and is returned quickly to the soil. Eroding clay particles restock available soil potassium levels. Soils that may show K deficiency are sandy soils with low clay content, especially if fields have been in forages where a large percent of the biomass is removed each year.
    • Cereals are more likely to show symptoms when soil levels drop below 300 lb./ac. of K. Symptoms may not be obvious in canola until soil reserves drop below 150 lb./ac.


    Canola response to fertilizer potassium

    Yield

    Canola needs 2.5 pounds per acre of potassium (K) for every bushel of seed yield. A 35 bu./ac. canola crop will take up 60-75 lb./ac. of K.

    Many fields do not show a potassium response because:

    • Soil reserves are high, in general, across the Prairies. Prairie mineral (non peat) soils often have 17,000-56,000 kg/ha of K (15,000-50,000 lb./ac.) in the top 15 cm (6”). About 1% of that is available for plant uptake at any given time.
    • Eroding clay particles restock available soil potassium levels.
    • Canola has a strong ability to find and absorb K.
    • 75% of K taken up by canola, wheat, barley and peas is returned to the field in the straw. The ratio is higher for oats, and lower for lentils and flax. K releases quickly from plant residue because most is in inorganic form.

    If soil tests show less than 300 lb./ac. of K (150 ppm), then canola may show a yield response to a K application. Response is more likely in soils with less than 150 lb./ac.

    Quality

    Potash applications have not been shown to help with canola seed quality (oil content or meal protein content). They have not shown to help with disease resistance or lodging either. Some of these benefits, particularly with regard to reduced diseases like root rots, have been shown with KCl (0-0-60) applications for cereals, but not for canola.

    Economic

    Canola rarely responds to applied K, even under conditions where cereals normally respond. Critical levels are often stated to be around 280 kg/ha of K (250 lb./ac.) or 112 ppm in the top 15 cm (6"), but research indicates that canola will not consistently or economically respond to fertilizer K unless the soil test is very low — 78 to 112 kg/ha of K (70 to 100 lb./ac.) or 35 to 50 ppm.

    Fate of applied fertilizer K

    Soil interactions and availability

    Very sandy or peaty soils are the most likely soil types to have very low K soil test values. Other factors that increase the likelihood of K deficiency are:

    • free lime in the rooting zone
    • acid soil
    • poor drainage
    • cool temperatures
    • soil compaction
    • shallow root zone

    Identifying deficiency symptoms

    Potassium deficiencies look like:

    • A yellowish brown “scorched” look at leaf margins edges.
    • Potassium is mobile, so plants will move the nutrient from bottom leaves to top leaves, so bottom leaves will yellow off first.
    • Uneven pod maturity.


    Potassium deficiency reduces overall canola growth but to a lesser degree than N or P deficiency. Since K is mobile within the plant, deficiencies are first visible in older leaves. The edges and areas between veins of older leaves tend to turn pale green or yellow, followed by withering. The yellowing can occur first in middle leaves before older ones if observed at bolting to flowering stages. In severe cases, leaves die but remain attached to the stem. Small white spots can develop on leaves. Plants are prone to wilting during midday. Potassium deficiency symptoms in canola are rather nondistinct and can be easily confused with other problems.

    How to confirm

    Look at cereal crops.Cereals are more sensitive than canola to K deficient soils. Symptoms on cereals appear as a burning or scorching of the lower leaves. Burning begins at the tip of the leaf and continues down the leaf margin.

    Soil tests.Soil tests are imperfect because K pools in the soil are in dynamic equilibrium — available K is constantly replenished. As K+ is removed by plant uptake and through leaching on sandy soils, additional K is released from the mineral soils to become available. Available K moves to plant roots by diffusion through the soil only up to 6 mm (1/4").

    Therefore, the equilibrium process that repeatedly moves K from the slowly available to readily available pool is very important for K nutrition. The rate of movement from the slowly available to readily available pool varies among soils due to differences in minerals and clays. This variation in K dynamics creates problems for soil testing. An extractant that measures plant available K in soil solution and exchangeable K over a short time period does not assess the replenishment power. Unfortunately, tests that measure the replenishment power are time-consuming and cost-prohibitive.

    Recent research in Alberta indicates that soil tests do not adequately reflect residual fertilizer K where high rates have been applied to very potassium-deficient soils. The study applied up to 670 lb./ac. of K on very potassium-deficient soils. Following this, soil test K was still in the deficient range, but subsequent crops showed little or no response to additional K fertilizer. Therefore, when high rates of K fertilizer are applied to very potassium-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.

    Placement of fertilizer

    Potassium is relatively immobile in the soil (less mobile than N but more than P), so placement near the roots is important. Banding is probably the best choice since seed-placed K can damage seedlings.

    Potassium is relatively immobile in the soil since the K+ cations are readily adsorbed to the negative surface charges on clay particles and organic matter. Potassium can also be fixed into the clay lattice structure of certain clay types. Potassium is more mobile in sandy soil and thus can be leached in these soil textures. In most soils, K+ is much less mobile than nitrate but somewhat more mobile than phosphate. This relative immobility means that fertilizer placement will greatly affect uptake efficiency. Ensure application methods minimize contact with soil and increase root contact. Banding and seed-placed methods can achieve good uptake efficiency. Since canola responses to K fertilizer are rare on the Prairies, there has been limited K placement research in this crop.

    Seed row

    The high salt index of potash fertilizer limits the amount that can be safely applied near the seed. Canola has a much lower tolerance to seed-placed potash than cereals, and stands will be reduced if seed-placed K rates exceed 14 kg/ha of K (12.4 lb./ac.) with drills that have low seedbed utilization (such as double disc drills). Higher rates of potash fertilizer can be safely seed-placed as the seedbed utilization is increased. If other nutrients such as N or P are also seed placed, this reduces the safe rate of seed-placed K. Good seedbed moisture, higher clay and organic matter contents help reduce the severity of seedling damage from seed-placed K fertilizer. However, most K deficient soils are sandy, and are sensitive to seed-placed K.

    Band

    Due to canola’s sensitivity to seed-placed K fertilizer, a band placement away from the seed row is more advisable. Sideband placement is an efficient method and the separation between fertilizer and seed reduces the risk of germination damage. Openers with side-band capability are becoming available, especially for direct seeding implements. Deep banding prior to seeding should also be an efficient and safe method of K fertilization. Potash fertilizer can be banded together with other nutrients. Banding efficiency should not differ greatly between fall and spring unless the soil is very sandy and subject to leaching loss under conditions of high snowmelt or spring rainfall.

    Broadcast

    The broadcast-incorporation application method is less efficient, and probably requires rates double that of banding to achieve a similar crop response. However, in situations where banding equipment is not readily available and seed placement is too risky, broadcast incorporation may be useful and not overly expensive due to the relatively low cost of potash fertilizer. The higher fertilizer rates necessary for broadcast K may also benefit subsequent crops with a higher K response than canola.

    Foliar

    Foliar K products are available but with very little research, it is unknown whether this is an effective rescue treatment for K deficiency.

    Timing of application

    Fall or spring

    Banding efficiency should not differ greatly between fall and spring unless the soil is very sandy and subject to leaching loss under conditions of high snowmelt or spring rainfall. In that case, spring is preferred.

    Product choices

    Available forms

    Potassium chloride (KCl) 0-0-60This is the most common K fertilizer due to lower relative cost and high K analysis. It comes from potash.

    Potassium sulphate (K2SO4) 0-0-50-17 This tends to be a more expensive K product, but it provides some sulphur as well. The salt measurement from a K2SO4 solution is less than a third of a similar concentration of a KCl solution.

    Special formulations

    Potassium thiosulphate 0-0-25-17This product is sold as a liquid for top dress applications. If also using for sulphur, keep in mind that thiosulphate must convert to sulphate to be available to plants. This can take a week or two in warm soils.

    Inhibitors and coatings

    None for K

    Inoculants

    None for K


    References

    Canadian Fertilizer Institute, “Nutrient uptake and removal by field crops,” 2001. CFI actually uses 73-89 lb./ac. of K20. This has been converted to 60-74 lb./ac. of K based on a K:K20 ratio of 1:1.21.

    Alberta Agriculture and Rural Development, 2000, “Potassium Fertilizer Application In Crop Production” Agdex 542-9. The original document used 810 lb./ac. of K2O. This has been converted to K based on a K:K20 ratio of 1:1.21.

    International Plant Nutrition Institute, “Potassium sulfate,” Nutrient Source Specifics factsheet No.5