4.1 Estimate nutrient loading from fertilizer

Key Message: Understanding the relative contribution of fertilizer to nutrient loading can help prioritize management strategies. Loads may reasonably be estimated through an approach that calculates nutrient budget coefficients based on inputs, uptake, and outflows of nutrients for differing land uses.

Importance

With more than 285,000 public and privately-owned land parcels in Sarasota using different fertilizer amounts, types, and application schedules, quantifying exact nutrient loads from fertilizer is not possible. However, loads may be reasonably estimated if land use and region-specific application rates can be determined. Understanding the relative contribution of fertilizer to nutrient loading can help prioritize management strategies.

Overview

The nutrient budget approach developed by the South Florida Water Management District (SFWMD) for the Lake Okeechobee Watershed (LOW) (SFWMD 2010) was used to estimate fertilizer loads per acre by land use. These unit loads were applied to the 2017 Land Use/Cover information based upon the latest Florida Land Use, Cover and Forms Classification System (FLUCCS) for all of Sarasota County (Figure 4.1.1 and Table 4.1.1) and then for just the Whitaker Bayou watershed (Figure 4.1.2 and Table 4.1.2).

Approach

While the approach and associated assumptions (SFWMD 2010) are believed to be generally applicable to Sarasota County and its watershed, they should be thoroughly vetted and modified/customized as needed. For example, Sarasota County is much richer in naturally occurring phosphorus than the Lake Okeechobee watershed and application of phosphorus fertilizer on golf courses is known to be much lower.

Interviews with golf course and agricultural managers in the Sarasota County region could provide better estimates of fertilizer application rates assigned by SFWMD. Likewise, tracking the tonnage and composition of fertilizer distributed (and assumed to be sold and used) in Sarasota County each year (see Chapter 4.2) could assist in informing and refining fertilizer imports for land use.

The following practices are recommended to better estimate nutrient imports from fertilizer.

1. Develop and conduct interviews with individuals and entities responsible for fertilizer application on representative land uses in Sarasota County.
2. Work with FDACS to acquire data on fertilizer distribution within Sarasota County. To the extent possible, these data could be correlated with general land use categories used to estimate fertilizer imports (see Chapter 4.2).
3. Given some similarities in the hydrologic setting and hydrogeology, the nutrient budget coefficients developed for the South Florida Caloosahatchee River and Lake Okeechobee watersheds could be reviewed and revised for specific applicability to land uses in the Sarasota County region.