Behavior of Phosphates in the Unsaturated Zone

Phosphorus is an element dealt with as a pollutant due to its effect on algal growth and thus on the eutriphication of surface water bodies. Phosphorus is considered to be a major factor controlling algal growth in many lakes and impoundments (Wetzel 1975). Very low phosphorus concentrations lead to excessive algal growth. When total phosphorus concentration exceeds 30 μ Pl −1, the lake tends to be eutrophic (Vollenweider 1970). This concentration range is way below that found in fertilized soils. A different presentation of the problem can be given by comparing quantities of phosphorus leading to eutriphication in a water body to those applied to agricultural crops. According to Vollenweider (1970), a lake (average depth 10 m) receiving more than 0.2 g P m−2 y−1 tends to be eutrophic. The average amounts of fertilizer P applied to arable soils in developed countries are in the range of 1–2 gPm−2y−1. Assuming a contributing watershed/lake surface ratio of 10, it may be shown that a transfer of 1% of the land-applied P to the lake is large enough to induce eutriphication. The differences in orders of magnitude between concentrations and amounts of phosphorus in cultivated soils and limits for the equivalent entities demanded for high quality surface water pose a problem. From the farmer’s viewpoint, a few percent leaching of the applied phosphorus with the drained water is hardly measurable and economically insignificant. Yet, as far as water quality is concerned, such leaching can be deleterious.