Out of the Landfill and Into the Field: Suitability of Wastes as Agricultural Amendments

Wastes, from municipal and industrial sources, can be used as agricultural amendments and many are locally available. As an alternative to mineral fertilizers they provide plant nutrients and organic carbon, the main component of soil organic matter. Land application prevents the need to landfill or incinerate wastes, sequesters carbon in the soil, and recycles nutrients that would otherwise be lost. However, the properties of waste amendments are less consistent than mineral fertilizers, and there is concern about their potential to contribute problematic levels of salt or heavy metals. To establish their suitability as agricultural amendments, I determined the properties of six wastes: paper fiber sludge/chicken manure (PF), biosolids/yard waste co-compost (BS), multi-source compost (MS), yard waste compost (YW), dehydrated food waste (FW), and gelatin waste (GW) (n=1-2). All amendments had consistent electrical conductivity from year to year (< 10 mS/cm), whereas pH was more variable and ranged from 4.9 - 7.9. The most variable pH was for BS, which ranged from 5.1-7.9. The presence of seashells (CaCO₃) in DF and MS meant that 16% and 47%, respectively, of the total C was not available to microorganisms, which influences estimates of carbon and nutrient availability. Waste amendments varied in texture, density and moisture content, which affect their decomposition in the soil, as well as practical considerations like transportation and spreading equipment. While none of the wastes had heavy metal concentrations that exceeded the U.S. EPA's limits set for land application of biosolids, the arsenic content of YW exceeded the agency’s more restrictive limits. The nutrient levels of waste amendments also differed: most had a low to moderate N concentration (0.4 – 3.7%), although GW contained 4.9% total N. All wastes had a C:N ratio below 25:1, the threshold above which N immobilization is likely, except for PF. Unlike mineral fertilizers, over 95% of N in the waste amendments was in organic forms. Amendments were not good sources of P, except GW (3.9%), which contained almost equal parts N and P. Finally, wastes contained varying amounts of K but were all low in comparison to mineral fertilizer. My results showed that the amendments studied did not have problematic levels of salt, heavy metals or acidity, and had low to moderate concentrations of plant nutrients. Amendment properties varied across amendment type and year-to-year from the same amendment source, stressing the importance of regular testing prior to application.