Local Fertility: Municipal and Industrial Waste as a Source of Organic Matter and Nitrogen, and Its Effect on Vegetable Yields

Locally available industrial and municipal residual wastes are a potential source of organic matter (OM) and nitrogen for peri-urban farmers. Land application would not only divert residuals from landfills or incineration but could also improve soil fertility and quality, thereby increasing yields. Factors which may limit the suitability of residuals for agricultural use include heavy metal and salt content, as well as carbon to nitrogen ratio. We evaluated (1) paper fiber sludge/chicken manure, (2) biosolids/yard waste co-compost, (3) multi-source compost, (4) leaf compost, (5) dehydrated food waste and (6) gelatin waste against a mineral fertilizer control for their effects on soil inorganic N levels and yield of sweet corn (Zea mays cv. Applause), butternut squash (Cucurbita moschata cv. JWS 6823 PMR (F1)), and potatoes (Solanum tuberosum cv. Eva). The experiment was conducted in 2013 on a silt loam in southern Rhode Island, with crops and amendments arranged in a randomized block design (n=4). Residual amendments were spring applied at a rate of 10,200 kg OM/ha. Soil samples were collected periodically from May to October and analyzed for ammonium and nitrate. Total weight of crop yield from the center of each plot was recorded. Soil ammonium and nitrate levels were generally significantly higher in plots amended with gelatin waste. Levels of potentially mineralizable N (PMN) were significantly higher in gelatin waste amended plots later in the season, indicating its potential as a slow-release source of N. Negative PMN values for some amendments indicated net immobilization of inorganic N. Yields varied by crop, with high yields (by weight) observed in plots amended with biosolids co-compost and gelatin waste. The high levels of inorganic soil N in plots amended with gelatin waste (up to 1,268 µg N/g dry soil in August) were not reflected in proportionately higher yields, indicating N supply may have surpassed the point at which any additional benefit was obtained and that application rates could be reduced without reducing yields. Only yard waste compost consistently gave poorer yields across all three crops when compared to the control, although corn yields were not significantly different from the control. All residual amendments evaluated appear to have some potential to act as a substitute for mineral fertilizer. This, when considered alongside their potential to improve soil quality through the addition of OM, may make them a viable local source of soil fertility.