Effects of Biosolids Based Soil Products on Soil Physical and Chemical Properties in Urban Gardens

Abstract

A study was conducted to evaluate the effect of two biosolids based soil products on soil physical properties in urban community gardens in Tacoma, Washington. The Tagro soil product is made from Class A biosolids cake mixed with sand and sawdust. The GroCo biosolids compost is produced from biosolids cake and sawdust. Both products meet regulatory requirements for unrestricted use and are locally available to gardeners. Plots were established in 3 community gardens and maintained for 2 growing seasons. Amendments were applied at 200 Mg ha−1 dry weight in the first year of the study. Plots were split for the 2nd year with half of the amendment plots receiving an additional 200 Mg ha−1 of amendments. Although lower in most metals than the soils they were added to, biosolids did not have a significant effect on total soil metal concentrations. Biosolids addition increased the water infiltration rate from 10.1 ± 0.95 ml min−1 in control soils to between 51 ± 6.1 and 212 ± 34 ml min−1, depending on the product used. Bulk density was decreased from 1.07 ± 0.01 g cm−3 to between 0.51 ± 0.08 and 0.77 ± 0.03 g cm−3. Total carbon, nitrogen and available phosphorus were increased in the biosolids amended plots in comparison to the control plots. Biosolids addition resulted in decreased pH (5.35-5.75) in comparison to the control soils (6.43). The results from this study indicate that biosolids-based soil amendments can be a suitable amendment for urban community gardens.