Sugarcane Production Related to Extractable Soil Phosphorus in Florida Organic Soils.

Sugarcane is grown on 163,000 ha in and adjacent to the Everglades Agricultural Area (EAA) of Florida. Organic soils comprise approximately 80% of this sugarcane hectarage. Phosphorus (P) concentration in water draining from the EAA to the Everglades is a major environmental concern. An important best management practice (BMP) is the use of a calibrated P soil test. The current UF/IFAS soil test P calibration for Florida sugarcane uses water as an extractant. However, the water extractant is highly pH-dependent and was developed primarily for short-season crops such as vegetables. Phosphorus fertilizer rate studies have been conducted at five field locations on organic soils with the objective of determining relationships of fertilizer P rates and soil-extractable P with sugarcane biomass ha^-1 and sugar ha^-1. Randomized complete-block designs were used at each location with 5-7 replications of 6 banded P rates: 0, 9, 18, 36, 72, and 144 kg P ha^-1. There were significant responses of sugar ha^-1 to P fertilizer at four of five locations. Responses were generally greater in ratoon crops than with plant cane. There was no production response above the 36 kg P ha^-1 fertilizer rate. Relationships of extractable soil P using Bray 2, water, 0.5 M acetic acid, and Mehlich 3 extractants with sugar ha^-1 and relative sugar ha^-1 were examined. Acetic acid has previously performed favorably in relation with sugarcane response to fertilizer P, but one of the five test sites in this study had high initial acetic acid-extractable P and so would not have predicted the observed response. Mehlich 3-extractable P correlated best with relative sugar ha^-1 and is the basis for a new proposed soil test calibration.