Table 2. Description of flow calculations for urban agriculture P budget.
| Flow letter (in Fig. 3) | Flow name | Equation | Assumptions and Specifications |
|---|---|---|---|
| a | P fertilizer and soil amendments imported applied soil | Sum for all gardens in type n [(total P inputs from off-island source/ area of garden)*(area of garden/total area of UA type n surveyed)] estimated area for type n | Weighted P application by area of farm or garden, and by the estimated area for the 3 types of management, so type n is type of management (see x, y, z). See Table 3 for types of inputs considered |
| b | P in harvested crops (feed and food) | Sum for all gardens in type n [(total P harvested/ area of garden)*(area of garden/total area of UA type n surveyed)] *estimated area for type n | Weighted P application by area of farm or garden, and by the estimated area for the 3 types of management, so type n is type of management (see x, y, z) |
| c | P compost and manure from on-island sources applied to soil | Sum for all gardens in type n [(total P inputs from on-island sources/ area of garden)*(area of garden/total area of UA type n surveyed)] *estimated area for type n | Weighted P application by area of farm or garden, and by the estimated area for the 3 types of management, so type n is type of management (see x, y, z). We combined recycled inputs (plant residues, compost, vermicompost, and animal manures) into one flow in order to maintain anonymity of survey respondents |
| d | P imported as animal feed and supplements | Sum for all types [(Feed or supplement imported type n*P concentration type n)] | Did not scale to estimated area of UA because we surveyed all known farms that raise animals and P concentrations were obtained by survey respondents or by manufacturers |
| e | P exported off island (food, feed, and manure) | P as exported manure + P as exported feed | Did not scale to estimated area of UA because we surveyed all known farms that export |
| f | P consumed by on-island residents | (P harvested—P harvested for animal feed) + P in animal products (milk and eggs) | P harvested is scaled to total UA area but P in animal feed and P in animal products are not because we surveyed all known farms that raise animals |
| x | ** Estimating total area: UA private and community garden type | (% of households practicing UA* % of practicing households doing UA in back-, side-, front-yard *# of households on island * average size of vegetable garden)+ (% of households practicing UA* % of practicing households doing UA on roof or balcony*# of households on island * area of 4 alternatives containers (0.96m2))+ (area of community gardens) | References: Household participating in UA: [41], Area of private backyard gardens: [42], Community garden area: [21] |
| y | Estimating total area: UA collective garden type | (Area surveyed collective gardens)+(area of missing collective gardens with known area)+(average area of known collective gardens reporting area*# of collective gardens with unknown area) | Reference: Area of collective gardens not surveyed: [43] |
| z | Estimating total area: UA farm type | Known area of farms from survey + reported area of the 2 farms we did not survey | Reference: Area of farms not surveyed: [43] |
Data are from surveys, and if P content was not provided by the survey respondent values in Table 3 were used. Note that we did not include flows relating to runoff and erosion losses or inputs from soil and soil mixes if P content was not available from the survey respondent (e.g., soil, potting-mix, vermiculite, perlite, or coco fiber).