Wisconsin Phosphorus Index

Reduce Field Runoff - Increase Infiltration

If all the water that falls on the land infiltrated, there would be no runoff, and much of the P pollution that we see would not occur. However, soil factors-at the surface or deeper in the soil-can cause the surface soil to become saturated rapidly, leading to runoff during a rainfall. Consequently, a major objective in lowering P losses from the land should be to increase infiltration. Natural systems like forests and grasslands usually have very high rates of infiltration. Pastures, crop land, and urban lands have dramatically lower infiltration rates. Many of the same practices used to reduce soil loss will also reduce runoff. In addition:

1. Tillage can at least temporarily increase infiltration rates and reduce runoff, particularly in heavier soils and when tillage is done on the contour. However, once the disturbed surface becomes wet, the surface may crust up again if the soil aggregates disintegrate. Fall tillage, particularly on the contour, can reduce runoff of rain and snowmelt over the following winter.
2. No-till systems encourage the formation of large, continuous pores that are open at the soil surface and reach deep into the soil. These include earthworm holes and root channels.

Several factors result in undisturbed lands having high infiltration rates:

1. Abundant large pores ("macropores"), open at the surface of the soil, provide large, easy pathways for water to enter. These are predominantly large and small animal burrows and root channels.
2. Any litter layer-dead leaves, stems, and wood-on the soil surface intercepts the energy of rainfall or flowing water and keeps it from dislodging and carrying sediment.
3. Soil particles that are aggregated into larger particles are harder to move. Clay particles and silt particles especially can form stable aggregates.
4. Continuous plant canopies intercept raindrops and keep them from hitting the ground at high speed. Plant canopies intercept raindrops and keep them from hitting the ground at high speed. The more complete the plant cover is, the more effectively it will reduce erosion. With perennial cover, there is always something above the soil to reduce rainfall energy.
5. Deep-rooted plants like alfalfa and some trees and shrubs can open up deep root channels into the soil, increasing rates of internal drainage.