University of Idaho Extension


Water is the lifeblood of Idaho. More than 22 million gallons of water are used in the state each day. More than 97 percent of this water irrigates 4.1 million acres of farmland. Eighty percent of this water comes from surface sources (rivers and reservoirs); the other 20 percent is groundwater. Currently, the quality of water used in Idaho is very good compared with water in other areas of the United States and the world.

Because water is so vital to Idahoans, agricultural best management practices (BMPs) to protect water from phosphorus pollution are becoming more important. Phosphorus is a common water pollutant in Idaho's lakes and rivers. Phosphorus originates from many sources, including agriculture.

Phosphorus is essential to all forms of terrestrial life. It is widely distributed over the surface of the earth in biologically available forms, cycling within plants, animals, soil, and water in the phosphorus cycle. A simplified phosphorus cycle is shown in figure 1. In commercial agriculture, fertilizer is the major phosphorus addition to this cycle.

Fig. 1. Phosphorus cycle in an agricultural setting. Note that chemical fertilizers are the primary phosphorus input into the system.

P Cycle

Water quality problems associated with phosphorus are generally confined to surface waters. Phosphorus in soil is tightly held to soil particles, is immobile, and does not leach. Consequently, contamination of groundwater is rarely a problem. This publication dicusses phosphorus as a surface water quality concern.

Many human activities contribute phosphorus to surface waters. Agricultural land enriched with phosphorus by fertilization or manure can contribute substantial amounts of phosphorus to surface waters as the result of runoff and/or erosional processes. Activities associated with modern agriculture often significantly increase soil erosion and water runoff from land and transport sediment into surface waters.

Surface water pollution with phosphorus is controllable -- by reducing soil erosion and keeping soil out of creeks, streams, rivers, and lakes.

Specific BMPs for phosphorus fertilizer and manure management that should be employed to protect surface water quality in many areas of Idaho include:

  • Soil erosion control
  • Fertilizer recommendations based on research and soil sampling
  • Correct phosphorus fertilizer placement
  • Variable fertilizer management
  • Efficient manure management
  • Barnyard and/or feedlot runoff control
  • Conservation tillage and reside management
  • Buffer (filter) strips

Relevant Publications