AGRI-BRIEFS
  AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Spring 1997, No. 4

FERTILIZING AFTER THE FLOODS...OR JUST A WET YEAR

The floods are over in California and other western states, so it is business as usual…or maybe not. Heavy rainfall, whether it leads to flooding or not, will have an impact in the coming season and beyond. Damage caused by soil erosion is obvious. But there are also negative effects of wet soils and flooding that are not so visible.

How are soils impacted by flooding and prolonged water logging? Consider the factors listed below.

Nitrogen. Do not expect much nitrate to be left in the soil profile. Whatever was present in the fall most assuredly has been leached from the crop rooting zone or denitrified (that is, lost to the air as a gas). Some fall applied ammonium may have survived the floods. The question is, was there time for much of it to be converted to nitrate before flooding?

Another source of nitrogen comes from crop residues and soil organic matter. Nitrogen is released as these materials decompose. The prolonged wet conditions will slow the decomposition process, so this source also will provide less available nitrogen than usual.

Phosphorus. Phosphorus does not leach from soil as does nitrate, but is lost through erosion of fertile topsoil. Also, reduced activity and chemical transformations in saturated soils reduce phosphorus availability.

Most crops have beneficial fungi…called mycorrhizae…colonizing their root system. These fungi enhance phosphorus absorption by crop roots. Mycorrhizae populations are often reduced after flooding, resulting in severe phosphorus deficiency.

Potassium. Flooding of soils over the winter will not directly affect potassium availability. The exceptions are that some potassium was undoubtedly lost through leaching of sandy soils and some was lost from erosion of topsoil.

There is a problem waiting to happen. Reduced potassium availability will be created when anxious growers return to their fields and attempt to work them while they are too wet…causing compaction. Compaction reduces availability of potassium to plants. This is compounded by cool, wet conditions contributing to poor root development.

Besides the obvious nutritional benefit of supplying potassium in adequate amounts, potassium also enhances the ability of crops to resist disease. It could be especially important to build up soil potassium for tree and vine crops weakened by the prolonged flooding and especially prone to development of disease problems.

In summary…there is no doubt that the 1997 season will offer many challenges for nutrient management on soils damaged by flooding and erosion. It may be a cliché, but it is so true: A fertile soil is not always a productive soil, but a productive soil is always a fertile soil. Right now a lot of farmers in the western U.S. and other regions are going into the 1997 season with less fertility in the field than they have had in many years. So do not get caught short. A program of balanced fertilizer inputs will likely give excellent returns…even more so than in normal years.


—AEL—

For more information, contact Dr. Albert E. Ludwick, Western Director, PPI, P.O. Box 1326, Bodega Bay, CA 94923. Phone (707) 875-2163.
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