Nitrogen, water BMPs for tomatoes and watermelon can cut leaching

05/15/2012 12:55:00 AM
Sanjay Shukla, Gregory Hendricks and Gene McAvoy

Recommended best management practices for water and fertilizer nitrogen (N) inputs for seepageirrigated tomato and watermelon in South Florida can reduce water use and N leaching into groundwater without adversely impacting the yield under average weather conditions.

These are the findings of a comprehensive University of Florida/Institute of Food and Agricultural Sciences study conducted from 2004-2008. It not only evaluated the environmental effects but also considered the crop yield and economic impact of three production systems using different irrigation and nitrogen inputs.

The study was conducted at the research farm of the IFAS’s Southwest Florida Research and Education Center in Immokalee. Two of the systems represented the industry average and BMP-based inputs with seepage irrigation, whereas the third system used the BMP N rates with subsurface drip, an alternative to traditional seepage irrigation.

The industry N rates for tomato and watermelon, determined from a survey of southwest Florida growers, were 373 and 265 pounds per acre, respectively. The BMP N rates were 200 pounds per acre for tomatoes and 150 pounds per acre for watermelon.

The target soil moisture level for the industry system was determined from actual measurements at selected farms whereas the BMP system was based on measured soil moisture in the bed.

Water use and N concentrations in the plant, soil and groundwater were measured.

The water table depths, artificially raised for the seepage irrigation, were shallower for the industry average fields compared to the BMP fields.

Yield results

Tomato yields over the four seasons for the three rates were no different, meaning no adverse impact of reduced N rates could be established. The effect on watermelon yield varied widely over the two seasons tested.

In one season, there was no effect. In the other year, yields for the industry average fields were higher, mainly due to an unusually wet spring that resulted in almost 4 inches of rainfall within the same week when watermelon were transplanted.

Heavy rainfall resulted in bringing the water table to the surface, washing out N and potassium (K) from the beds. Results from watermelon leaf tissue analyses showed that N and K concentrations for the BMP system for the second season were lower than the industry average; however, the N concentrations were within the sufficiency range.

The soil N concentrations, measured below the crop beds to evaluate N leaching, were lower for BMP (76 ppm) fields than industry (112 ppm) fields.

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