Don´t over apply phosphorous or potassium!


By: Scott Eckert, County Extension Agent, Horticulture


Plants need nutrients to live and grow.  Nitrogen is the nutrient needed most because it is used faster than any other.  Phosphorus and potassium are also needed but not as much as nitrogen.  In most soil test recommendations I make, the phosphorus and potassium readings are high or too high.   In fact, applying phosphorus and potassium when not needed will cause a buildup of these nutrients.


I have seen soil test measurements so high in phosphorus that it isn´t needed for the next 20 years.  Excessive soil phosphorus reduces the plant´s ability to take up required

micro-nutrients, particularly iron and zinc, even when soil tests show there are adequate

amounts of those nutrients in the soil.


Excess potassium causes nitrogen deficiency in plants and may affect the uptake of other positive ions such as Mg and Ca


What do these major nutrients do?


N (Nitrogen)-This nutrient element provides dark green color in plants. It promotes rapid vegetative growth. Plants deficient in nitrogen have thin, spindly stems, pale or yellow

foliage, and smaller than normal leaves.


P (Phosphorus)-This nutrient promotes early root formation, gives plants a rapid,

vigorous start, and hastens blooming and maturity. Plants deficient in this element

have thin, shortened stems, and leaves often develop a purplish color.


K (Potassium)-Potassium or potash hastens ripening of fruit. Plant disease re-sistance as well as general plant health de-pend on this element. It is also important in developing plump, full seeds. Plants deficient in this element have graying or browning on the outer edges of older leaves.


The content of N, P, and K is specified on bags of chemical fertilizers. The analysis or grade refers to the percent by weight of nitrogen, phosphate, and potash in that order. Thus, a 10-10-10 fertilizer contains 10 percent nitrogen (N), 10 percent phosphate (P205) and

10 percent potash (K20).


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