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 be reasonably estimated using nutrient budget coefficients based on inputs, plant uptake, and outflows for different land uses.

Importance

With over 300,000 public and private land parcels in Sarasota County, each using different fertilizer types, rates, and schedules, precise nutrient load quantification is challenging. However, reasonable estimates can be made if land use and region-specific application rates are known. Understanding fertilizer’s relative contribution to nutrient loading helps prioritize management efforts.

Overview

The nutrient budget approach developed by the South Florida Water Management District (SFWMD) for the Lake Okeechobee Watershed estimates fertilizer loads per acre by land use to quantify overall nutrient inputs. This method was applied to the 2017 Land Use/Cover dataset for Sarasota County using the Florida Land Use, Cover and Forms Classification System (FLUCCS) (Figure 4.1.1, Table 4.1.1) and to the Whitaker Bayou watershed (Figure 4.1.2, Table 4.1.2).

Countywide results show that medium-density residential areas cover 25% of developed lands yet contribute 45% of nitrogen inputs from fertilizer. Golf courses apply nitrogen at more than twice the rate of residential lands (111 vs. 49 lbs/yr/acre). Citrus groves comprise just 2% of agricultural lands but receive 23% of agricultural nitrogen inputs.

Approach

While the nutrient budget approach and associated assumptions (SFWMD 2010) are generally applicable, they should be vetted and customized for Sarasota County. For example, Sarasota soils are richer in naturally occurring phosphorus than the Lake Okeechobee watershed, and fertilizer application rates for golf courses may be lower locally.

Interviews with golf course and agricultural managers in Sarasota County could refine the application rate estimates. Similarly, tracking the tonnage and composition of fertilizer distributed (and assumed to be sold and used) within Sarasota County (see Chapter 4.2) would further inform and refine nutrient input estimates.

Recommended practices to improve fertilizer nutrient import estimates include:

• Develop and conduct interviews with individuals and entities responsible for fertilizer application across representative land uses in Sarasota County.
• Review and revise the nutrient budget coefficients developed for the Caloosahatchee River and Lake Okeechobee watersheds for applicability to Sarasota County.