Previously we had looked at the full list of nutrients and spent some time understanding nitrogen (N) as a major nutrient for plants. This time will press forward and talk about phosphorus (P) and potassium (K). This will round out the three macronutrients that are found on every fertilizer bag and are known by their initials, N-P-K.

 Phosphorus can be a difficult nutrient to manage in the soil. First, the total supply of phosphorus in many soils is usually slow, and yet, even when present, its availability to the plants that need it may be limited. In our area with acidic soils, the phosphorus availability may be even lower because of the formation of chemical compounds which make phosphorus very slowly available.

A few of the many ways that phosphorus is important to plant growth is that it stimulates root formation and growth, encourages flower development, seed formation, and makes plants more winter hardy.

But as much as it is needed, too much phosphorus can get in the way. Excessive phosphorus in the soil chemically binds with iron and zinc and keeps those vital micronutrients away from the plants that need them. Phosphorus excessive phosphorus can build up from too much inorganic or commercial fertilizer. In the soil phosphorus is very immobile – it doesn’t move through the soil like nitrogen. If that top layer of soil, where phosphorus has accumulated, runs off into lakes and streams, it can be a real problem.

If phosphorus had a personality, it would be like that friend that you invited over to watch a football game. He would sit in “your chair” and take over the remote. Then when the game is over, he would ask what else you got to eat in the kitchen, and then would overstay his welcome looking for more games to watch.

For soils that have an accumulation of too much phosphorus, there is no simple solution. The first step is to eliminate any new fertilizers that contain phosphorus. This would include eliminating organic materials such as compost and manure. This elimination of fertilizers and soil amendments which contain phosphorus may need to continue for a period of years. And as always, the way we will know that the nutrients or at the proper level is through a soil test.

The last of the three macro nutrients needed by all plants is potassium. Potassium is represented in the periodic table as the letter K. Potassium also has several benefits to plants such as imparting vigor and aiding in resistance to disease, providing winter hardiness, and it aids in the translocation of sugars starches and other carbohydrates.

Potassium is crucial to plant metabolism and is a part of the regulation of water loss and is needed to adapt to stress such as severe cold or extremely dry weather. Plants that are deficient in potassium will exhibit a reduced yield before any symptoms are noticed. Those who produce Bermuda hay know that potassium is a hugely important nutrient for a successful field of Bermuda.

Potassium is somewhat mobile in the soil, falling between the fickleness of nitrogen and the immobility of phosphorus.

Both phosphorous and potassium is mined and packaged for all types of fertilizer applications.

Finally, if you are waiting for any recommendation on how much fertilizer to put out on your yard, garden, or field, you won’t hear any from me. To get it right, really correct, you’ll need to fill out a soil sample. Texas A&M and many other land-grant universities have a form online that you can download and print, or you can easily get the form from your local Extension office. 

Next, I’ll cover the three secondary nutrients, Calcium (Ca), Magnesium (Mg), and Sulfur (S).