By Thom Weir, Senior Precision Agronomist

For this third installment of this series, will focus on Potassium. Potassium or Potash 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, whereas nitrogen is contained mainly in the seeds. There are approximately

5-million acres of prairie soils are deficient in potassium. These tend to be concentrated in east central Alberta and the northern agricultural regions of Saskatchewan and Manitoba. Potassium-deficient soils tend to be light to medium textured, alkaline, carbonated and poorly drained in their natural state. Organic soils are also frequently deficient in potassium.


Soil Potassium Distribution in Western Canada

Let’s look at the three laws I proposed in the first part of this series.

Liebig’ Law of the Minimum

The Law of Diminishing Returns – This law can be illustrated with the following data from a malt barley study done by Westco in 1987. The study was carried out at four locations in Alberta. Four rates of potassium (K20) were applied with the Nitrogen rate being held constant at 54 lbs / acre.

The Law of Diminishing Returns is also illustrated by looking at the results above.

On to the 4R’s of Potassium Management. The first R is The Right Source.  Unlike Phosphate, there are relatively few potash sources.

Murate of Potash (0-0-60 or 0-0-62) KCl – This is the product that is mined at many facilities across Saskatchewan and shipped abroad for use as a fertilizer. Not only is it an excellent source of K – the symbol used to represent Potassium or Potash, it contains chloride (Cl) which is also a nutrient required by plants. KCl is very flexible for application. It can be broadcast, banded or seedplaced.  However, when seed-placing, one must remember that this product has a high salt index and can be toxic to some seeds. When using it, the effect with a product like MAP is additive – meaning that the effect of both products is equal to that of either product being used at the combined rate. Murate of Potash (0-0-62) can also be dissolved in water and applied as a liquid.

Sulphate of potash-magnesia (0-0-22-10.5Mg-22S)  K2SO4•2MgSO4 is mined from deposits in New Mexico and is commonly referred to as K-Mag or Sul-Po-Mag.  Potassium-magnesium sulphate has a higher cost per unit of K than the muriate form. It also contains 10.5% magnesium and 22% sulphur in water-soluble form and therefore readily available to plants. It is useful as a source of soluble magnesium in fields where lime is not required.

Sulphate of Potash (0-0-50-17S) • K2SO4 is considered a premium-quality potash. It contains two key nutrients for growing crops: potassium and Sulphur. It has a lower salt index and is more expensive than Murate of Potash. It is used mainly on crops sensitive to chloride, such as tobacco, potatoes and some vegetable crops.

Again, let’s now look at the Right Rate. Like phosphate, the Right Rate is going to depend on three factors.  Factor number one 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. Canary seed and certain varieties of winter wheat have shown to respond to the chloride component of KCl.

There are a lot of growers that apply 15 lbs K2O with several crops. This practice is an artifact from a number of years ago when a few studies indicated 25 – 30 lbs of seedplaced K2O occasionally showed a yield response. The recommendations were to apply 0 or 30 lbs with the seed. When automation took over in the printing of recommendations, the recommendation became an average of 0 and 30 or 15 lbs for all cereal recommendations. There weren’t any studies that showed 15 lbs K2O applied in the seedrow gave a response!

The Right Rate is going to depend on the soil test. As stated above, there are soils that may be deficient in potassium. However, most of western Canada has inherently high levels of potassium – those being above 125 ppm or over 250 lbs/acre. In these soils we can apply 25 – 30 lbs in the seedrow to provide potassium to young plants when the soils are cold and wet. However, the chances of getting economic payback is low – approximately 30% 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, seedrow potassium is about as toxic to germinating seeds so any additional K will have to be subtracted from the amount of P you seed-place. If you 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 lbs removal for every ton of hay.

Over the past few years, some agronomists and agronomy organizations have been using the base-cation saturation ratio or BCSR approach to develop fertilizer recommendations from a soil analysis. This concept was developed by soil scientist Dr. F. E. Bear and his co-workers and introduced in papers published in 1945 and 1948 based on a greenhouse experiment in New Jersey. They suggested that in the ideal soil Ca, Mg, K and H should occupy 65, 10, five and 20 per cent of the cation exchange capacity and that the ideal ratio between Calcium (Ca) and Potassium (K) should be 13:1 and with Magnesium (Mg) and Potassium (K) be 2:1.

For the past three decades researchers 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 shown to be an effective way of predicting potassium recommendations in the Northern Great Plains. Dr. Johnson and Dr. Karamanos wrote a newsletter in 2015 that discusses BCSR and its use in the Norther Great Plains and Canadian Prairies.  You can read it here.

 International Plant Nutrition Institute (IPNI) Median Soil Test K Levels 2015

The International Plant Nutrition Institute (IPNI) analyzes soil test results across North America every five years. The most recent analysis in 2015 showed median soil test K levels in Alberta was 158 (down from 173 ppm in 2010), Saskatchewan at 233 ppm (static with 2010 levels) and Manitoba at 211 ppm (again static with 2010 levels. Based on research and the relatively high cation exchange capacity (CEC) of Prairie soils, 160 ppm 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-in soil test K levels also indicates a possible shortfall in K applications to crops in Alberta.  Silage crops, fields where straw has been removed and Forage fields are most likely to show a reduction in K levels. Always confirm suspected K deficiencies with soil and tissue analysis.

The Right Place for Potassium will be determined by what your soil potassium levels are. For most efficient use by cereal crops, K 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% that of potash banded with the seed or side-banded. However, in deficient soil, a seedplaced or side banded application might be combined with a broadcast application to get the desired rates applied. If all your potash is broadcast, the recommended rate for seed placement or side banding should be doubled to obtain equal crop response. Broadcast K fertilizer should also be incorporated into the soil.

And finally, the Right Time for Potassium fertilizers will also depend on your soil levels. As previously stated, the most efficient use by cereal crops, K fertilizer when seedplaced or side banded at seeding.  Broadcast applications can be made in the fall or spring prior to seeding. For forage crops, potassium is usually applied at rates equivalent to the anticipated removal for the first 2-3 years. Annual applications can then be applied in following years. These applications may or may not result in a crop response so a strip trial in crop of 50 – 100 lbs K2O 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.

Stay tuned for the next part of this series where I’ll tackle Sulphur.