AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Spring 2001, No. 2
Under saturated soil conditions, losses of soil nitrogen can be substantial. Nitrate nitrogen can be lost by leaching down and out of the reach of crops. While leaching occurs rapidly on coarse textured sandy soils, it is a slower process on loam and clay soils due to slower water movement. The gaseous loss of nitrogen by denitrification occurs when soil microorganisms reduce nitrate under saturated conditions, leading to loss of nitrogen gas. In addition, soil microorganisms are not very effective at decomposing crop residues and organic matter when the soil is saturated, slowing the release of nitrogen from this source.
The mobility of sulfate makes it subject to leaching, similar to nitrate. However, unlike nitrate, losses of sulfate by volatilization from the soil are negligible. As a result, in the absence of leaching, soil sulfur is not affected to any great extent under conditions of excess water. Being a nutrient that comes mainly from the soil organic matter, sulfur is impacted by any physical factor that restricts microbial assisted release.
Due to its precipitation with calcium and magnesium in calcareous soils, phosphorus is not leached from the soil. However, its availability to plants can be dramatically reduced under wet soil conditions. Reduced microbial activity and chemical transformation in saturated soils reduce phosphorus release from the organic and mineral complex. In addition, the beneficial fungi, mycorrhizae, which normally enhance the absorption of phosphorus and other nutrients, are often reduced after flooded conditions. This leads to limited plant access to soil phosphorus supplies due to the restricted volume of soil being explored by the plant root system.
Flooding of soils has little direct effect on potassium availability. However, indirectly, soil potassium and phosphorus may be less available in soils that remain wet and cold in the spring of the year. Wet soils are more prone to compaction, which also restricts plant root growth and uptake of soil potassium. In addition, the role that potassium plays in helping crops to resist plant disease can be negatively affected when saturated soil conditions limit crop uptake.
When challenged by adverse environmental conditions, like saturated soils, we need to rely on some of our basic tools. In particular, we need to keep in mind the reactions that nitrogen, sulfur, phosphorus, and potassium are subject to with excess water. Also, spring soil sampling will help us clarify what impact the wet soil conditions have had on those nutrients subject to large potential loss, like nitrogen.