Seasonal factors can affect phosphorus leaching

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Editor's note: A special thanks to Dr. Monica Ozores-Hampton for coordinating The Immokalee Report.

Phosphorus (P) is required for the successful commercial production of vegetables and melons in Florida. However, a surplus of P can result in leaching into the groundwater, affecting surface water and adversely impacting water bodies.

Although considered less mobile than nitrogen, P leaching does occur in Florida due to its sandy soils. Best Management Practices—BMPs—have been developed to reduce the environmental losses of nutrients including P.

But limited information exists on the main factors that control P losses to Florida groundwaters.

A BMP study was conducted over six seasons at the Southwest Florida Research and Education Center, UF/IFAS research farm, in Immokalee, Fla. The study’s dataset included a wide range of crop-soil-water-management factors for tomato and watermelon production systems.

Two irrigation methods were used—seepage and sub-surface drip—with varying fertilizer-P rates of 0 to 75 pounds per acre.

During the study, plant available P—measured as Mechlich-1 extractable soil P or M1P—varied from 15 to 145 parts per million, groundwater concentrations of total P (GWP) ranged from 0.53 to 1.27 ppm, and daily rainfall reached a maximum of 8 inches with Hurricane Wilma. Data were analyzed by season and year.

Watch fertilizers more closely during wet fall

The analyses showed that M1P is a good indicator of GWP for the fall growing season as well as on an annual basis. Fertilizer P was also important as it directly affects the M1P in the soil. Water table depth was important for dry spring seasons.

Statistical analyses of the data resulted in the following equations for predicting seasonal and annual GWP.

These models estimate GWP (µg/l) above the spodic horizon for spring, fall and annual periods. Seasonal totals are required for rainfall (cm) and fertilizer-P (kilograms/hectare), while seasonal averages are required for WaterTable (cm) and Mehlich-1P (ppm).

These equations are valid for similar production systems and environments, and illustrate that M1P and fertilizer-P are the two most important factors that govern subsurface P losses for vegetable production in shallow water table regions of Florida.

The fall and annual equations do not include water-related factors and is likely due to large amounts of rainfall received during the start of each fall season. 

This study shows that greater focus on fertilizer (versus water) management is required during the wet fall season. For dry spring season, greater focus is required for irrigation management as water table and rainfall became dominant factors.

The models presented here require readily available data already used by vegetable growers to manage their farms. Rainfall and water table depths are used to manage irrigation and drainage and M1P measurements help determine fertilizer P rates.

These models are unique but simple and can be used as management tools by growers. Although the models represent a comprehensive long-term data, they may not necessarily work for all farms.

The relationships are for dual-cropping systems with plastic-mulched beds and shallow water table conditions. Once similar data are available for different farms, a generic equation may be derived for its use as a screening tool by the growers to manage P losses from their farms. 

Sanjay Shukla is an associate professor of agricultural and biological engineering at the Southwest Florida Research and Education Center. He can be reached at Gregory Hendricks is a senior engineer and research coordinator at the SWFREC.

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