wheat field

By Eliza Smith, Oregon State University

The Soil Fertility Specialist at Oregon State, Amber Moore, and research assistant Eliza Smith, in collaboration with Dan Sullivan, Nutrient Management Specialist at OSU, and Andy Bary, Soil Scientist at WSU, are working on a project entitled, “Plant-available S release from biosolids”.  The objective of the project is to investigate the amount and timing of plant-available sulfur release from biosolids produced by different technologies. 

Sulfur is an essential plant nutrient that can contribute to the positive effects that growers have seen following biosolids applications. As with all plant nutrients, it is important for growers to know how much plant-available sulfur is provided by biosolids that are applied to crop fields. Sulfur is available in two forms in biosolids, as rapidly mineralizable sulfur (from sulfides in biosolids) and as slow-release sulfur (from organic matter in biosolids). Limited research has been done on this topic. This project is intended to help farmers make decisions about biosolids applications by giving them information about how much plant-available sulfur is in the material. 

Biosolids samples were collected by Andy Bary from 30 facilities in Washington State. The processes sampled include; various aerobic and anaerobic processes, air dried, lagoon, and compost.  He shipped the samples to OSU where they were dried, ground, and the percent moisture of each sample was calculated. 

Biosolids before drying (left) and after drying (right)
Biosolids before drying
biosolids post drying
Biosolids after drying

The samples will be analyzed for total sulfur content to help determine which ones will be used in the full trial, with the goal of including products with a wide range of sulfur contents. A Walla Walla silt loam from a dryland wheat production system was collected to use in the trial. That soil type was chosen because it is common in an agricultural system where biosolids are commonly land applied in Washington and Oregon.

wheat field
Wheat field where soil was sampled for the trial.

Eliza Smith is leading a preliminary incubation trial this summer to determine the best methods to use for the full incubation trial that will begin in winter 2019. During both the pre- and full trials, bags of soil are moistened and biosolids are added based on the amount of sulfur they contain so all the bags start out with the same amount of sulfur. 

Ground biosolids being added to a bag of moist soil in the pre-trial
Ground biosolids being added to a bag of moist soil in the pre-trial

The bags are placed in a low-temperature incubator at 25°C for 12 weeks to promote the breakdown of organic sulfur into plant-available forms of sulfur. Every 4 weeks, the bag contents are evaluated for sulfate amount, moisture content, and pH. 

This project will provide information to growers and biosolids producers about the amount of plant-available sulfur that is in a variety of biosolids products made in the Pacific Northwest.