A Brief Tale of Phosphorus

A brief post about Phosphorus continues our series of Plant Nutrient posts. Yesterday, we spoke about Nitrogen as the primary macronutrient needed by plants. Today we pick up the theme with the second most important nutrient, Phosphorus.

Phosphorus is another element on the periodic table of chemicals. It is second only to Nitrogen as a very important nutrient needed by plants. In fact, all organisms need it as it is the base structure of cell membranes, and the energy carrying molecule in living things. It is usually found in commercial fertilizer in the form of P2O5. naturally occurring rocks such as apatite contain large amounts of phosphorus and are the source, in soil of the element in nature.

The thing to remember about Phosphorus is that it is never water-soluble. As opposed to Nitrogen which is always leaching away with water, once Phosphorus is in the soil, it stays where it is put and binds tightly to soil particles. It is therefore commonly abundant to most soils and additional fertilizers containing it are seldom needed. A soil test of your garden soil will tell you whether the native soil is already well supplied with Phosphorus and if it is, you will not need to worry about the concentration in your fertilizer.

If you add Phosphorus that is not needed, it can bind to silt and run off into natural waterways causing troublesome extraneous levels of fertility in lakes and streams. Foaming along the edges of waterways is usually from Phosphorus runoff. This can cause an overgrowth of algae and seaweed that cause an imbalance in the body of water leading to depletion of Oxygen for fish, etc. Extra phosphorus is not only a source of pollution but is an unnecessary expense. In limestone based soil, which is most of the cultivable United States, you usually do not need to use anymore than 5% Phosphorus in your fertilizer. If your soil tests low in Phosphorus, use of up to 12% may be warranted but seldom more than that. Remember, pouring fertilizer on the top of the soil leaves a heavy layer of insoluble Phosphorus at the very top and not much penetration down into the rest of the root zone. A good way to apply it is when you are turning over the soil with a rototiller or plow and can incorporate Phosphorus deep into the field.

Phosphorus is the second number in the three number sequence on the fertilizer bag. It is an expensive component and makes up the majority of the cost of production of a fertilizer. The most expensive sources of Phosphorus are water-soluble forms specially created to be watered in. An example of this is Rapid Gro or Miracle Gro where the product is applied to individual plants by a watering can. These are only economical when used in very small batches applied to specialty crops such as vegetable gardens of flowers. They have been advertised over the years as extra good for flowers and fruit. Although plants do use a little more to make these structures, not a whole lot more are used versus a plant you are growing for foliage such as grass. The main response you see from applying water-soluble fertilizers to flowers and such is from the Nitrogen, not the Phosphorus.

While we are on growth responses from fertilizers, I’ll throw in another old wives’ tale. There is one stating that if your flowers do not flower and grow only green leaves, your fertilizer is out of balance and you have given too much Nitrogen and not enough Phosphorus. Usually the reason that your flowers do not flower is lack of sunlight. Both Nitrogen and Phosphorus are need for flowering and seldom is Phosphorus deficiency to blame for lack of flowers. At the most, the flowers may be stunted and smallish, but if it is going to have flowers, it will. More tomorrow on the third macronutrient, Potassium.


1 Comment

  1. Kevin said,

    June 23, 2010 at 11:55 AM

    Phosphorous is also a major part of DNA (Gel electrophoresis relies on all the negative charges in the phosphates, after all).
    Another mineral that is rich in phosphorus is phosphorite, surprisingly enough. If I remember correctly it’s very valuable since it’s so full of phosphorus.

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