Economic and environmental consequences of overfertilization under extreme weather conditions

Abstract

Higher-than-recommended fertilizer application is considered by farmers as insurance to maintain sufficient levels of plant nutrients during periods of excessive rainfall, or to capitalize on favorable market conditions and facilitate an extended harvest period. In addition to increasing the likelihood of eutrophic waters, excess fertilizer application may exacerbate soil salinity that reduces yields and economic benefits after extreme weather events such as hurricanes. Research-based fertilizer recommendations are needed to address global eutrophication of fresh water systems. A three-year, four-season study evaluated the effects of higher-than-recommended nutrient and water inputs on tomato (Solanum lycopersicum) yield and farm profitability under normal and extreme weather conditions in subtropical Florida, a major fresh vegetable producing region. Three nutrient-water management treatments were considered: conventional high fertilizer rate with subirrigation (High-Sub), recommended (best management practice) fertilizer rate with subirrigation (Rec-Sub), and recommended fertilizer rate with subsurface drip irrigation (Rec-Drip). There were significant differences in yield between seasons (i.e., Fall, Year 1 = 52,742 kg ha⁻¹; Fall, Year 2 = 22,045 kg ha⁻¹; Spring, Year 3 = 78,926 kg ha⁻¹; and Fall, Year 3 = 61,905 kg ha⁻¹). Yield reductions in Year 2, an unusually wet season affected by Hurricane Wilma (21 cm rain in 11 hours), were related to high nitrogen (N) losses and soil salinity (electrical conductivity > 2,500 μS cm⁻¹). Average groundwater N, seasonal groundwater electrical conductivity, and soil N indicated high salt concentrations and leaching in High-Sub especially during Fall Year 2. There were no statistically significant yield differences among treatments. Growers, however, tend to look at the differences in average yields for trends in farm profits and losses. With this perspective in mind, we conducted a grower-type economic calculation using prevailing market prices that showed additional revenues (US$12,000 ha⁻¹) from Rec-Sub and Rec-Drip during Fall Year 2, which almost entirely balanced the lower revenues for remaining seasons. National market prices for tomato were highest during Fall Year 2 due to extensive crop damage (wind and flooding) from Hurricane Wilma. Given the average frequency of a tropical storm/hurricane being three years, growers may enjoy an economic benefit once every three years from not applying excess fertilizer, and at the same time, avoid external environmental costs that include publicly funded treatment systems. Reduced groundwater nutrient concentration (N = 61% and phosphorus [P] = 42%) in Rec-Sub could achieve similar reductions in nutrient loads in regions such as Florida, which have significant surface water and groundwater interaction. Future increases in frequency and magnitude of extreme events (e.g., 2017 Hurricane Irma, rainfall = 37 cm) may exacerbate the impacts on production and water quality. Therefore, excessive fertilization is not likely to be economically or environmentally sustainable.