We can’t. This is just one more illustration of the importance of achieving proper nutrient balance in the soil. Anytime there is too much of one nutrient in the soil, another will be lacking. Having analyzed thousands upon thousands of soils and from all parts of the U.S. and around the world, we find no exceptions to this rule.
Although most well made composts have a significant content of phosphorus, there is more to the story concerning how that compost will increase phosphorus levels in the soil. This has to do with the biological aspect of the soil. As the nutrients and organic matter from the compost begins to feed the biological life in the soil (equal to the weight of one average sized cow per acre), the formation of mild organic acids begin to etch away at soil bound phosphorus, making it available for use by the plants growing there.
This biological activity is extremely effective at releasing previously unavailable forms of phosphate. When considering the amount of phosphate taken up by properly grown plants each year, it becomes important to keep track of the amount of compost added annually as levels of phosphate in the soil will steadily increase by more than the amount of phosphate in the compost added in subsequent years. This can result in too much phosphate in the soil and the problems just discussed. In general, three to four tons of composted poultry manure or six to eight tons of composted cattle manure can generate such problems after two to four years of being applied at such rates.
Potassium
In parts of the Midwest and Mid-South, potassium is called “the poor man’s irrigation.” It is true. Potassium is a key component for water uptake by plants. It also enables fruit and seeds to grow larger, and increases tolerance to colder weather.
For many food plants, potassium is needed in greater amounts than nitrogen, so providing adequate potassium is a basic requirement for plant growth and nutrition. But growers need to realize that the number of pounds of potassium contained in a soil is not the real measure of nutrition, yield and quality. It is the specific percentage of the soil’s nutrient-holding capacity of needed potassium that ensures best results. The soil requires an absolute minimum of 2% saturation of potassium. 5% is normally considered excellent for most growing plants, but woody plants such as fruit and nut trees, grape vines, shrubs and even timber respond even better to 7.5% potassium saturation.
However, building potassium levels above 7.5% is detrimental to all plants and especially those we grow for food. The most common method of causing an excess of potassium in the soil is the excessive application of manure or compost. Most private soil testing laboratories now provide the saturation percentage of potassium as a part of the soil analysis. If you want to grow good quality organic food, find a laboratory that at least shows the percentage of potassium, calcium, magnesium, and in many areas sodium should also be tested.
The comments concerning use of too much compost are applicable to potassium as well as phosphate. The difference is that when potassium exceeds 7.5% it ties up boron availability in the soil. Most soils are low to deficient in boron without the problem of too much potassium, but excessive potassium just makes it worse. Boron is necessary to get calcium into the plant, for nitrogen conversion in plant growth, and it also takes the starch out of the leaf and puts it in the fruit or grain, thus increasing the size.
So too much potassium in the soil can reduce the size of the seed or the fruit that you are trying to grow.