Background soil potassium (K) levels vary drastically across the prairie provinces. A significant portion of land has adequate levels of K, but there are areas of the province, particularly in the Northern grey and grey-black soil zone where we often see areas within fields that are testing below the critical level for potassium (Figure 1). Some zones within fields we can find extremely low soil test levels. Soils testing below the critical level means that there is a higher likelihood of seeing a response to that added nutrient. What the critical levels of K are can be complex depending on the soil type and texture, but soils below 150 ppm would be considered low. Levels below 100 ppm are very low. The soil test level and crop dictates how much additional K needs to be fertilized for.
This spring, we came across far more cases of what we could call severe K deficient barley than we have in the past (Figure 2). Barley is among the most responsive to K, followed by wheat then canola. So it makes sense that barley was the crop we most often saw issues with. Add to that the cool and drier spring we had, it made the deficiency symptoms show up very early this year. These cases were confirmed with comparative tissue tests and referring to the zone’s soil test results. In each of these cases, potash was applied to the crop in the amounts of 8 – 15 lbs K2O, but it was not enough.
The Influence of Dry Conditions on Potassium Availability
Extended periods of dry conditions can reduce availability of several nutrients, including potassium. This is likely why we saw more cases of K deficiency showing up in barley this year than we have in the past. As soils dry, clay minerals become dry and shrink, trapping K tightly between mineral layers. When the K is trapped between these mineral layers, they become unavailable for plants to uptake them. The K is released when the soil moisture increases.
Where to find Potassium Deficiency
You are not going to see K deficiency every year on every crop. If you have peaty land, it could show up in those areas (depressions, often labelled zone 9 or 10 on our zone maps). If you have sandy land in the grey soil zone, it will be in areas often labelled zone 1 on our zone maps. If you have sandy-loamy soil in the grey or black-grey soil zone, it could show up in zones 1-4 in a particularly dry year. Those zone 1-4 areas of the field are the driest areas of the field, and as such, K is more likely to become plant unavailable in these zones as it becomes trapped in clay minerals.
The biggest giveaway, and 95% of the time the way we first identify the issue in the field, is you will see strips where the crop looks good (Figure 3). These strips correspond to where swaths (often canola) have laid for an extended period of time in a wet fall in the past. Potassium is highly leachable from straw, so when those canola swaths laid there and got rained on, the potassium readily leached into the soil. In future years, the crop grown in those areas takes up the additional K in the soil resulting in better looking crops compared to the areas right next to it that did not have swaths laying.
Seeing subtle differences where past swaths or heavy trash areas are is not uncommon. Areas like that can look better in dry years because they have higher soil moisture (less evaporation from soil surface), or they can look worse because heavy trash caused more frost damage. But there is a distinct difference between some of those observations and that of K deficiency. Grab plants from both the good looking “swath” area and the poor looking area to compare the plants (Figure 4). K deficient plants will look yellower, in particular on the older leaves, and have overall less growth than healthy plants. They can also appear to have more of a scorched appearance on the older leaves. Refer to your SWAT MAP and soil test result for the zone you are in as a further clue if K could be the culprit. A comparative tissue test collecting samples from the good plants to compare to the poor plants will give the final confirmation.
Potassium Variability
Since we often find K deficiency symptoms either on hilltops or depressions (especially if peaty), this is one reason, of many, why composite soil tests don’t give a good enough picture of what could be going on in a field. This can be said with any nutrient, but potassium is no exception. Often, the midslope areas of the field are not where you will come across K deficiency. So a composite soil test taken from that area of the field might tell you have adequate K levels. Meanwhile, we see all the time in our SWAT zone soil samples that many fields could have up to 40% of the land base showing low K levels that should be addressed (Figure 5).
Do I Need More Potash?
This is a discussion for the fields that have marginal to deficient levels of K in their soils. Virtually all farms with these kinds of soils are applying some level of potash with their crops already. But some may need to consider increasing potash rates to better match what the crop requires that the soil is not able to provide. Logistics and drill/tank type begin to factor into this. If you are restricted by the amount of seed-placed fertilizer you are applying as it is, it becomes hard to increase potash levels. Broadcasting, pre-banding or banding with the drill (higher rates down the side-band or MRB) during seeding might need to be considered.
There are a number of potash fertilizer charts available that vary slightly in their recommendations. They are all based on soil test levels and expected yield, but some don’t take into account the effect moisture conditions can play on response to potash. We have seen benefits of small amounts of potash in fields with high testing soils, either due to the response to chloride or the response to K itself if on very dry, heavy clay soils. The example potash fertilizer chart in Figure 6 comes from the University of Minnesota, but other research would show similar amounts of potash recommended. Another good resource can be found on this Alberta Ag website: https://www.alberta.ca/wheat-nutrition-and-fertilizer-requirements-potassium.aspx. In marginal and deficient soils, the main thing to take note of is the higher recommended potash rates than what would typically be applied with the seed and/or most crops in general on a given year. For our clients with adequate levels of K, this is usually no issue. But for those with low levels, we may need to consider applying more potash. To summarize these graphs and tables, if a zone is testing 100 ppm, the potash recommendation is to apply up to 30-40 lbs K20 to that crop.
Summary
Not every field or farm will have potassium issues in their crops, but some areas of the prairies may run into K deficiency symptoms when growing certain crops (like barley) in certain years (dry and cool springs). The first step is to pay attention to your crops throughout the season to determine if you see any deficiency symptoms showing up. From there, we can address the problem. Even better is being proactive and applying enough K based on the crop and soil test levels to avoid any deficiency symptoms showing up in the first place. Looking for differences where previous years’ swaths have laid during wet falls is the biggest clue that there is a potassium response in the crop.
Kerrie de Gooijer
Agronomy Operations Manager
kerrie@swatmaps.com