Making the Most of Potash Applications

In many fertilizer programs, the attitude towards potash was “put on lots…it’s cheap”. In the last year, however, potash prices have increased sharply. This, along with the tight margins in crop production, suggests we should look carefully at where we do, or don’t need to add potash. This is not to suggest that potash is not important for crop production. It is critical for maintaining proper moisture balance in the plant, and plays an important role in disease resistance, standability, and grain yield and quality. We do, however, have opportunities for fine tuning applications.

Step 1: Know what your soil can provide

Most of the potassium taken up by your crop will come from the soil. It is tough to manage potash if you don’t know what is available from the soil. This means you have to have a soil test. In general, tests from within the past three years will give a pretty accurate picture of what is in the soil. The exception is on very sandy soil, where crops like alfalfa, silage corn or tomatoes, that remove large amounts of potash, are grown. These soils should be sampled more frequently.

Low testing soils will respond to added potash almost every year, and the yield losses from inadequate potash can be large. It would be false economy to cut K rates on these fields. High testing soils will still occasionally respond to added potash, but only rarely would the response be large enough to pay for the fertilizer.

Do I need more on clay soils?

Some jurisdictions recommend higher rates of potassium on clay soils than on loams or sands, so we often get questions about why Ontario does not include this in our recommendations. There are two reasons why the recommendations for a clay soil might be higher:

1. the recommendation is based on building up soil tests (and it takes more potash to raise the soil test in a clay soil), or
2. there is evidence that crops actually respond to higher rates of K on the particular clay soils within a region.

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One neighbouring state that does recommend higher potash rates on clays is Ohio. However, their field trials showed different response patterns depending on the part of the state. In southeast Ohio, the clay soils needed more potash fertilizer to reach optimum yields. In northern Ohio, on the lacustrine soils that are most similar to our soils in Ontario, there was no difference in potash requirements with clay content. Ohio decided to have one recommendation system for the whole state that included the clay content factor. Their trial results actually supported the Ontario studies that showed no difference in K requirements on clay soils.

The clay minerals in Ontario soils actually contain a huge reserve of potash that is slowly released. It could be argued that we will suffer less yield loss from cutting back on potash on clay soils than on lighter soils. The caution is that you need to know the fertility status of any soil before you start cutting back.

Will my crop fall down if I cut back on K rates?

An important role of potassium in the plant is maintaining cell turgor. It is well-known that stalk strength decreases, and lodging increases, where potassium is deficient. High nitrogen fertility increases lodging even further in this situation. This has led to recommendations for high rates of potash on any crop with high nitrogen requirements, often at a fixed ratio of nitrogen to potash. The trouble with these recommendations are that trial results don’t show any differences in standability with higher potash rates, as long as the crop is not deficient. Providing adequate K for yield will also provide adequate K for standability.

Can I cut rates by banding?

There have been some studies showing an advantage to banding potash fertilizer, particularly in no-till. The difference between banded and broadcast, however, is small. The biggest difference was between no fertilizer, and fertilizer applied by any method. The conclusion is that you cannot expect the advantage to banding with potash that we get with phosphorus. You will need to balance the small advantage we do get against the cost of applying the potash in a band, and the increased risk of salt injury to the crop.

Are there other sources of K that I can use?

High potash fertilizer prices focus our attention on alternative sources of this element. A rich source of potassium is livestock manure, particularly from cattle. Liquid dairy manure, for example, contains about 30 pounds of available potash per thousand gallons. Applying this manure to meet nitrogen requirements will provide enough potash to meet the requirements of even the most deficient soil. Sewage biosolids, on the other hand, have almost no potash.

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