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Understanding Phosphorus Behavior in Soils

Wade Thomason

Posted Jan. 1, 2002

Many of the soils in the Noble Research Institute service area are low in phosphorus (P). Some of this is due to low P level in the soils' parent material and otheris due to cropping history and nutrient removal. Either way, since P is anessential element for plant growth, we must supply additional fertilizer Pif high crop yields are desired and soil test P levels are low.

In nature, phosphorus is always found in combination with oxygen in the phosphate form.

This chemical form allows the molecule to react with up to three single positive ions such as hydrogen (H+), potassium (K+), or ammonium (NH4+) or with other positive ions with a 2+ or even 3+ charge. Phosphorus is absorbed by plants in the orthophosphate form, generally as H2PO4- or HPO42-. The amounts of these ions in the soil solution are determined by soil pH (Figure 1). At pH 7.2, there are approximately equal amounts of these two forms in solution. Maximum solubility of calcium phosphate minerals occurs at about the same pH, therefore maximum plant available P occurs at approximately pH 7. As pH changes in either direction, P availability is decreased. You may notice that other forms of phosphate are equally prevalent at the extreme ends of the pH scale, but this occurs outside the range of normal soil pH. While the portion of P in these forms is high, plants cannot survive the other conditions resulting from these extreme pH levels.

Figure 1.

Figure 1.
Influence of pH on the distribution of orthophosphate speciesin solution.1 1Tisdale, S.L., W.L. Nelson, J.D. Beaton, and J.L. Havlin. 1993. Soil Fertility and Fertilizers. 5th ed. MacMillan, New York, NY.

At normal soil pH (5.0-8.0), the concentration of H+ available for reactions is so low that the phosphate forms mentioned earlier are only a small and transient component of the total soil P reserve. This means that phosphates react with other positively charged ions to form stable components that can both bind and release P. Most soils can bind much more P than plants can use. This explains the concept of P sufficiency and the reason why P may not be required for many years on some sites. The soil can be viewed as a giant warehouse for P. The resupply of P comes from the P bound to the soil and only refills the warehouse at a slow rate. Some warehouses start out full (high P soils) and some start out almost empty (low P soils). Those that are full have more P supplying potential than those that start out almost empty. The good news is that P fertilizer can be used to fill the warehouse to provide sufficient P for plant growth. It is important to note that while soil test P levels increase with P fertilizer applications, it takes about 15 pounds of P2O5 applied per acre to increase soil test P one point. This makes it impractical to attempt to raise soil test P levels over a short time; it would just take too much fertilizer. It is possible, however, to satisfy the crops immediate needs with reasonable rates of fertilizer. It is crucial that P fertilizer be applied annually at reasonable rates to satisfy plant needs if soil tests levels indicate a deficiency.

At a soil pH above 5.5 most of the phosphates react with calcium to form calcium phosphates. Below pH 5.5, aluminum (Al3+) is abundant and will react more readily with the phosphates. Calcium phosphates are relatively more water-soluble than aluminum phosphates. The lack of water solubility of aluminum phosphates means that these compounds are not readily available for plant use. In other words, in strongly acid soils, most of the P is bound and not released. This means the warehouse is locked and no P can get out. Liming to raise pH will open the warehouse and reverse this reaction by supplying carbonates to neutralize acidity and calcium to react with phosphorus. Calcium, when supplied at adequate rates, "knocks off" the aluminum and replaces it with calcium, rendering the phosphate water soluble once again.

You may be asking yourself what this all means.

The nuts and bolts are this: When the P soil test index indicates a deficiency, then the likelihood of response to P fertilizer is high. Choosing not to soil sample or to ignore soil test recommendations for P means missing out on potential yields. If your soil test P level is only 50 percent sufficient then you can only achieve 50 percent of the maximum potential yield. This is before any other yield-limiting factors are encountered. Lack of rainfall only exploits deficiencies. In drier years, it is important to apply fertilizer to eliminate deficiencies to take full advantage of the moisture that is present.

The highest solubility for calcium phosphates is around pH 7.2. The highest level of plant available phosphorus parallels this solubility range. Letting pH fall below 5.5 "locks the warehouse" and severely reduces the amount of water-soluble P components in the soil and increases the amount of insoluble aluminum phosphates. This in turn directly reduces plant available P.

There are really only two solutions to this problem.

  1. Lime to acceptable pH.
  2. Band P with the seed at planting. This reduces the amount of soil the phosphorus must react with and hopefully creates an area of concentrated P that doesn't react with aluminum.

A complete discussion of how and why P banding works and on plant P uptake mechanisms in general will require another article.

The things to remember are:

  1. Soil test and apply P fertilizer at recommended rates to maintain a warehouse of P available for plant uptake, and
  2. Keep soil pH at levels (5.5-7.0) where phosphates are mainly bound to calcium and are thus water soluble and available to restock the warehouse as plants remove P for growth.

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