The third instalment of my series on nutrient back-to-the-basics is about potassium.
Also known as potash, it is second only to nitrogen in the amount of our applied nutrients used by plants.
Crops contain about the same amount of nitrogen and potash. However, most of the potash is contained in the vegetative (leaves and stems) portion of the crop, while nitrogen is contained mainly in the seeds.
There are approximately five million acres of prairie soil considered to be deficient in potassium. These tend to be concentrated in east-central Alberta and northern agricultural regions of Saskatchewan and Manitoba. Potassium-deficient soil tends to be light to medium textured, alkaline, carbonated and poorly drained in their natural state. Highly organic soil is also frequently deficient in potassium.
The law of diminishing returns is well-illustrated by a malting barley study done by Westco in 1987, and the data stands up well today.
The study was carried out at four locations in Alberta. Four rates of potassium were applied with the nitrogen rate being held constant at 54 pounds per acre. After the needed levels of potassium were achieved, no economic benefit was gained with additional application of potash. Yields reached their highest levels once the crop had sufficient supplies.
The right source for potassium, unlike phosphate, is simple because there are relatively few potash sources. By far and away, the most commonly used product is muriate of potash (0-0-60 or 0-0-62) KCl.
It is mined at many facilities across Saskatchewan and shipped abroad for use as a fertilizer. Not only is it an excellent source of potassium but it also contains chloride (Cl), another nutrient required by plants.
KCl is very flexible for application. It can be broadcast, banded or seed-placed. However, when seed-placing, one has to remember that this product has a high salt index and can be toxic to some seeds.
When using it, the effect when combined with a product like monoammonium phosphate is additive, meaning that the effect of both products is equal to that of either product being used at the combined rate.
Muriate of potash (0-0-62) can also be dissolved in water and applied as a liquid.
Sulfate of potash-magnesia (0-0-22-10.5Mg-22S) K2SO4-2MgSO4 is mined from deposits in New Mexico and commonly referred to as K-Mag or Sul-Po-Mag. Potassium-magnesium sulfate has a higher cost per unit of potassium than the muriate form. It also contains 10.5 percent magnesium and 22 percent sulfur in water-soluble form and is therefore readily available to plants. It is useful as a source of soluble magnesium in fields where lime is not required.
Sulfate of potash (0-0-50-17S) – K2SO4 is considered a premium-quality potash. It contains two key nutrients for growing crops: potassium and sulfur. It has a lower salt index and is more expensive than muriate of potash. It is used mainly on crops sensitive to chloride, such as tobacco, potatoes and some vegetables.
The right rate of potash, like phosphate, is going to depend on three factors.
The first is the crop that is being grown. Potash is concentrated in the leaves of plants, so plants that produce a lot of foliage compared to seeds are going to use more potash. This would include grasses and legumes grown for forage.
Of our grain crops, barley has shown to be most responsive to potash, followed by wheat. Broadleaf crops such as canola, flax and peas are not responsive to potash. Canaryseed and certain varieties of winter wheat have shown to respond to the chloride component of KCl.
A lot of growers apply 15 lb. of potash with a variety of crops. This practice is an artifact from a number of years ago when a few studies indicated 25 to 30 lb. of seed-placed K2O occasionally showed a yield response. The recommendations were to apply zero or 30 lb. with the seed. The recommendation evolved to become an average of zero and 30, so 15 lb. became the rule for all cereal recommendations. There never were any studies that showed 15 lb. of potash applied in the seed row gave a response.
The right rate is going to depend on a soil test.
There are soils that may be deficient in potassium, but most of Western Canada has inherently high levels. Often these are above 125 parts per million, or more than 250 lb. per acre. In these soils we can apply 25 to 30 lb. in the seed row to provide potassium to young plants when the soils are cold and wet.
However, the chances of getting economic payback is low: approximately 30 percent for barley and less for wheat and oats. There is very little chance of an economic response in pulses or oilseeds in high potassium soils.
As well, seed row potassium is about as toxic as phosphorus to germinating seeds. So, any additional potassium will have to be subtracted from the amount of phosphorus that is seed-placed.
For farmers who follow a crop removal philosophy when dealing with potassium, forage crops are probably the crops with the highest demand. In these crops, we are looking at 50 lb. of removal for every ton of hay.
Over the past few years, some agronomists and agronomy organizations have been using the base-cation saturation ratio (BCSR) approach to develop fertilizer recommendations from a soil analysis.
This concept was developed by soil scientist F. E. Bear and his co-workers and introduced in papers published in 1945 and 1948. These were based on a greenhouse experiment in New Jersey.
They suggested that in the ideal soil, calcium, magnesium, potash and sodium should occupy 65, 10, five and 20 percent of the cation exchange capacity. As well, the ideal ratio between calcium and potassium should be 13:1, and with magnesium and potassium it should be 2:1.
For the last three decades, scientists have invested considerable field research dollars and effort in studying and comparing the BCSR to the sufficiency level approach to making fertilizer (and more specifically, potassium) recommendations. More on this in a future column.
To date, it has not been shown to be an effective way of predicting potassium recommendations in the northern Great Plains.
Adrian Johnson and Rigas Karamanos wrote a newsletter in 2015 that discusses BCSR and its use in the northern Great Plains and Canadian Prairies. It can be found at www.agvise.com/wp-content/uploads/2015/02/PPI-Cation-Ratios.pdf.
The International Plant Nutrition Institute analyzes soil test results across North America every five years. The most recent analysis in 2015 showed that median soil test potassium levels in Alberta were 158 p.p.m., down from 173 p.p.m. in 2010, Saskatchewan at 233 p.p.m., static with 2010 levels, and Manitoba had 211 p.p.m., even with 2010 levels.
Based on research and the relatively high cation exchange capacity of prairie soil, 160 p.p.m. is considered approximately the critical level where crops, especially cereals, are responsive to potash applications.
These results indicate that soils in Alberta are much more liable to react to a potash application than either Saskatchewan or Manitoba. The drop of soil-test potassium levels also indicates a possible shortfall in applications to crops in Alberta. Silage crops, fields where straw has been removed and forage fields are most likely to show a reduction in potassium levels. Always confirm suspected deficiencies with soil and tissue analysis.
The right place for potassium will be determined by the soil potassium levels. For most efficient use by cereal crops, potassium fertilizer should be placed with the seed. For most row crops, potash should be side-banded to the side and/or below the seed. The efficiency of broadcast and incorporated potash is about 50 percent of potash banded with the seed or side-banded.
However, in deficient soil, a seed-placed or side-banded application might be combined with a broadcast application in order to get the desired rates applied to the field. If all of your potash is broadcast, the recommended rate for seed-placement or side-banding should be doubled to obtain equal crop response. Broadcast potassium fertilizer should also be incorporated into the soil.
The right time for potassium fertilizers will also depend on the levels in the soil. The most efficient use by cereal crops is achieved when the nutrient is seed-placed or side-banded at seeding.
Broadcast applications can be made in the fall or spring before seeding. For forage crops, potassium is usually applied at rates equivalent to the anticipated removal for the first two to three years. Annual applications can then be applied in following years. These applications may, or may not, result in a crop response, so an in-crop strip trial of 50 to 100 lb. of potash per acre can be made to see if there is a response following the second year of hay removal. Depending on your soil and rainfall, this application may prove beneficial.
Next column, we’ll tackle sulfur.
Thom Weir, PAg. is an agrologist with Farmer’s Edge. He can be reached by emailing firstname.lastname@example.org.