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ISSUE 7   June 15, 2000



    The volume of water that can enter an area of surface soil is called the infiltration rate. We have unfortunately recently had a demonstration of its limits in some parts of the state and the result was rainfall much higher than the soil ability to absorb it. The amount of rainfall that can enter the soil is sometimes greater initially than later on during the rain. If the soil is well aggregated, as in a no-till field, water enters more quickly until larger pores are saturated. If the soil contains more clay and is cracked, filling the cracks with water initially results in initially high infiltration until the clay becomes saturated, swells and closes the cracks.

    Once the pores are filled and cracks are closed, the texture of the soil becomes the most limiting factor in water infiltration. The force of the rain, the presence of shallow limiting layers, such as compacted tire tracks or tillage pans also restrict the downward flow of water. But in the absence of these factors, texture is the key to water infiltration. The following chart contains general numbers from a "An Introduction to Soil Physics", by D. Hillel, 1982. The values are useful to demonstrate the
magnitude of water infiltration. Individual soils will vary around these values.

Textural class

Steady infiltration rate inches/hour



Sandy loams


Loams, fine sandy loams


Clay loams, silty clay loams and clays


Sodic clay soils


    So in St. Thomas, with 2 inches of rain over about 8 hours on a fine sandy loam, hardly any runoff occurred. But in areas with 10 inches in 8 hours in clay soils- perhaps one inch entered the soil through cracks and normal infiltration, but the other 9 inches ponded or ran off the field. Because of the size of the area affected, the drainage-ways that we call "rivers", but to many of us are more aptly called creeks or brooks, cannot handle the kind of water that 8-14 inches of rain over a several county area can provide. So the water backs up and is held on the field until downstream levels recede. In a clay soil, 24 hours of ponding may result in only inch of water infiltration. Several days are needed to absorb 3-4 inches of ponded water.



    The heavy rain recently experienced will result in many problems. In areas where crops are covered with water for several days, many acres will be lost. In areas where rainfall was not quite so heavy and the crops survive, the nitrogen status of these areas will be questionable. In ponded areas, warm soil temperatures and active bacteria populations will result in denitrification
in eastern areas, particularly the Valley. In sandy areas, most of our nitrogen is now nitrate and significant leaching will have occurred. In row crops, these sandy ridges can be supplemented through sidedressing. In small grains, such as spring wheat, leaf analysis at early heading will indicate N status to determine whether protein enhancement through post-anthesis application
of foliar N would be worthwhile.

    Irrigators should also be vigilant and conduct plant analysis to determine the nitrogen status of their crops and supplement as needed through the pivot.

Dr. Dave Franzen
NDSU Extension Soil Specialist

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