Potash Basics Revisited

In the past three years, potash has gone from the cheapest ingredient in the fertilizer blend to the most expensive. This has many growers rethinking their old attitude towards potash of "put on lots…it's cheap". This is not to suggest that potash is not important for crop production. It is critical for maintaining proper moisture balance in the plant. It plays an important role in disease resistance, standability, crop yield and quality. However, we have opportunities to fine tune applications.

Step 1 - Know What Your Soil Can Provide

Most soils contain a lot of total potassium (K) - up to 40,000 pounds per acre - but most of this is bound up in unavailable forms. Slowly available forms that are released gradually as the soil weathers will make up 2-10% of this total. Only 1-2% will be immediately available to the crop, either as K in the soil solution or held on exchange sites. Soil testing measures this available portion. In general, tests from within the past three years provide an accurate picture of what is in the soil. The exception is on very sandy soil, where crops are grown that remove large amounts of potash, such as alfalfa, silage corn or tomatoes. These soils should be sampled more frequently.

Low testing soils will respond to added potash almost every year. Yield losses from inadequate potash can be large, so it is a false economy to cut K rates on low testing 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. This is clearly illustrated in the chart "Maximum Yield Response of Forages To K Fertilizer At Various Soil Tests". A similar pattern of response can be expected with grain and oilseed crops.

Dr. Bob Sheard, University of Guelph

Do I Need More K 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.

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.

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. For example, liquid dairy manure contains about 30 lbs available potash per 1,000 gallons. Applying this manure to meet nitrogen requirements will also provide enough potash to meet the requirements of even the most deficient soil. Sewage biosolids, on the other hand, have almost no potash.