Volume 43 Number 2 Summer 1996

Chris E. Welsh, Elizabeth A. Guertal, and C. Wesley Wood

Soil fumigation—the process of chemically or physically sterilizing soil to eliminate viable weeds and plant pathogenic microorganisms—is a common practice in vegetable and strawberry production. However, results of an AAES study suggest that soil fumigants may affect the availablity of certain types of nitrogen (N), implying that new fertility strategies may be needed for certain crops.

Methyl bromide and metham sodium are two common agricultural soil fumigants that chemically kill weeds and microorganisms. Unfortunately, these chemicals kill not only pathogenic microorganisms, but also eliminate or reduce beneficial soil microorganisms, such as Nitrosomonas and Nitrobacter. These bacterium are responsible for converting ammonium (NH4-N) to nitrate (NO3-N), an important step in a process called the nitrogen cycle (see Figure 1). The conversion of NH4-N to NO3-N is called nitrification.

Although most plants will take up both forms of nitrogen and use these for growth, previous research has shown that some vegetable crops may favor NO3-N forms of nitrogen. A simple management strategy for vegetable crops is to add nitrogen as a nitrate source, such as in calcium nitrate [Ca(NO3)2]. However, these sources of nitrogen may be more expensive than such sources as ammonium nitrate (NH4NO3). An AAES experiment was conducted to fmd out if it is necessary to use NO3-based fertilizers right after fumigation.

To help answer this question, an AAES laboratory study was conducted to examine soil nitrogen release in fumigated and nonfumigated soils. Portions of the soil were fumigated with either methyl bromide or metham sodium and a control group of soils was not fumigated. Nitrogen rate treatments included four rates of nitrogen at 0, 60, and 120 pounds of N per acre (120 lb. is the recommended rate for most vegetable crops in Alabama) and 240 pounds per acre. All N treatments were applied to fumigated and nonfumigated soils alike.

The treatments were enclosed in sealed canning jars. Soil samples were taken daily for 11 days and every other day for 11 more days. Samples were analyzed for NO3-N and NH4-N content, and differences in N content as affected by soil fumigant and N fertilizer were evaluated.

After 22 days of sampling, the nonfumigated soil samples contained more soil NO3-N than those fumigated with either methyl bromide or metham sodium. This indicates that soil fumigation, regardless of the type of fumigant used, did slow nitrification, and less NH4-N was being converted to NO3-N. Fumigated soils exhibited net increase in NH4-N while nonfumigated soils had net decreases (see Figure 2).

The decrease in nitrification caused by fumigating soil with these two chemicals suggests that producers using fumigants may need to adjust their starter fertilizer applications on vegetable crops that are sensitive to N form. Additional research is underway to determine if these laboratory findings will also occur in a field setting and perhaps fine-tune fertility recommendations for fruit and vegetable growers.

Welsh is a Graduate Research Assistant, Guertal is an Assistant Professor, and Wood is an Alumni Associate Professor of Agronomy and Soils.

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