BioCycle January 2010, Vol. 51, No. 1, p. 27
Soil sampling on farms applying compost to row crops and orchards found reduced bulk density, improved water holding capacity and higher microbial activity and nutrients.
Sally Brown and Matthew Cotton
THE California Integrated Waste Management Board (CIWMB) funded a life cycle analysis (LCA) to determine the best options for managing organics that currently are landfilled. A life cycle assessment includes a wider range of costs and benefits than a traditional cost analysis. For example, environmental costs and benefits of particular practices are calculated and added to traditional capital costs. As part of this study, composting costs were estimated/calculated, including transport to composting facilities, energy use during composting and delivery of final product. A portion of CIWMB’s budget for this analysis was dedicated to determining the benefits associated with use of compost on farms in California, which included actual soil sampling on farms.
Farms are a major industry in California, the world’s fifth largest supplier of foodstuffs. According to the California Department of Agriculture, sales of fruits, nuts, vegetables, melons, nursery and floriculture crops in California totaled $25.8 billion in 2007. Understanding the potential impact that broader use of compost can have on the sustainability of this $26 billion industry – especially given water shortages in some of the most productive regions of the state – is a critical component to fully understanding the costs and benefits of diverting organics currently landfilled in California.
SCOPE OF TESTING
An LCA conducted by the Recycled Organics Unit (ROU) of the University of New South Wales was used as a roadmap for the sampling (http://www.recycledorganics.com/publications/reports/lca/lca.htm). This LCA modeled the benefits associated with compost used as a mulch in vineyards and as a soil conditioner in irrigated cotton fields. The benefits associated with compost use were calculated based on a review of published literature. This review covered a wide range of variables including soil carbon sequestration, nutrient value, increased water holding capacity, reduced erosion potential, improved soil tilth and increased productivity.
Within the time and budget constraints of the California sampling, it was possible to test for a limited number of the variables included in the ROU study. These included measures of soil carbon, nitrogen, nutrient availability, bulk density, soil microbial activity, and water holding capacity and infiltration rates. The full results from the sampling are available on line at http://www.ciwmb.ca.gov/Temp/Climate/default.htm. In general, results showed benefits associated with compost use that were comparable to or greater than those reported by the ROU. Benefits increased with increasing application of composts with no significant benefits seen when cumulative application rates of compost were low.
COMPOST APPLICATION AND SAMPLING
Our one-week sampling expedition involved visits to farms from the desert outside of Palm Springs to farms on the coast just south of the San Francisco Bay area. This field trip covered only a small portion of the agricultural operations that currently use compost.
Two of the sites can be used to illustrate the benefits of compost for different types of high value agriculture: an organic fruit orchard in Riverside County and tilled, high value row crops in Monterey County.
Bruce Rucker’s farm in Riverside County is representative of the benefits associated with use of high rates of compost over an extended period on organic orchard crops. Compost had been applied at a rate of 24 Mg ha for 10 years. The compost is produced by California Biomass from yard trimmings and food processing residues. Two crops (citrus and grapes) on the same soil series (Myoma fine sand) were sampled at this site, increasing the number of both treated and control samples, compared to other sites.
Compost applied to orchard crops at Rucker’s farm is managed as annual surface application under the trees or vines. The same type of application at similar rates was seen on this sampling trip for almonds, citrus, grapes, apricots and mangos. The two highest revenues crops in neighboring Kern County are almonds and grapes, with close to $1 billion in revenue annually. Grapes require approximately 5 acre feet of water per year to grow, so any increases in soil water availability would have a significant impact. Riverside County receives less than 5 inches (12.5 cm) of rainfall annually. It is also likely that the benefits observed with surface application to orchards would be similar to those observed in landscaping where compost is surface applied to ornamentals annually or at high one time rates of application (Cogger et al., 2008).
In Monterey County, soils were sampled from high production, tilled row crop farms. Two of the sites were owned by Tanimura & Antle (T&A), a produce company which leased out the fields to contract growers. A requirement of the lease was that fields receive an annual application of compost. T&A purchases compost from the Z-Best Composting Facility in Santa Clara County. Most of Z-Best’s compost is made from green waste that comes primarily from collection programs in the City of San Jose.
One of the fields was certified organic and compost had been applied at 11 dry Mg ha for over nine years. The other was managed conventionally, with compost applied at 5.6 dry Mg ha for more than 10 years. The control soils for this series were sampled from a neighboring field that was also used for row crop production, was managed conventionally, and was the same soil series, Pico fine sandy loam. Crops (lettuce and cauliflower) had just been harvested from all three sampling sites. The soils produce two to three crops per year and are tilled several times each year.
The Monterey sites were the only row crops in our sampling where compost application rates were known and the control soil was the same soil series. Monterey County has about 17 inches (45 cm) of rainfall each year. Extensive use of irrigation water in Monterey has resulted in saltwater intrusion into the ground water table. As a way to minimize dependence on groundwater, reclaimed water from wastewater treatment plants is now used extensively in Monterey to irrigate truck crops. Any increases in soil water holding capacity would further reduce dependence on groundwater.
Compost application improved the soils for almost all variables tested. For organic carbon content, compost increased carbon concentrations at the orchard site from about 0.4 percent to 2.7 percent. In the row crop, soil total carbon in the high rate of compost was 1.1 percent in comparison to 0.7 percent in the control.
Compost application also reduced bulk density – a measure of the weight of the soil per unit volume. The weight of rock is about 2.65 g cm3. A highly compacted soil will have a bulk density approaching 2.0 g cm3 while a soil with good tilth, high organic matter and stable aggregates will have a bulk density less than 1.0 g cm3.
Compost also improved soil water. We measured both total water in the soil at 1 bar tension (the stage of dryness when irrigation would normally be turned on) and the infiltration rate (how quickly the soil can absorb water). Water generally infiltrates into sandy soils more quickly than finer textured soils. It also drains out of these soils more quickly than finer textured soils. The orchard site had very sandy soil and the row crop site had a much finer textured soil, a sandy loam. Compost improved total water holding capacity for both soils with much more significant increases in the sandy soil.
Infiltration rate in the sandy soil was slightly slower in the compost-amended areas but was significantly faster in the row crop soil that had compost applied. The complete data set for both sites along with the percent change is shown in Table 1. The soils that had received compost application had better physical properties, higher microbial activity and higher nutrient status than control soils. It is easy to understand why so many of the farmers that we spoke to on the trip said that they used compost to improve the quality of their crops.
For the variables we measured on the sites with high loading rates, our results were generally within the range of those predicted by the ROU study. These results suggest that broader use of compost on farms in California can have a significant impact on the sustainability of agriculture in that state. Compost will help to store carbon in soils, increase soil tilth, replace synthetic fertilizers, and reduce water usage. In a state where water shortages are commonplace and where over 80 percent of the potable water is used to irrigate crops, water savings are critical.
The benefits associated with compost use will vary by type of soil, type of crop, local climate and total compost applied. The results from our study confirm that the results in the ROU study are a good guide for agriculture in California. They also confirm that the land is a much more productive home than the landfill for organics.
Sally Brown is Research Associate Professor at the University of Washington in Seattle. Matt Cotton is with Integrated Waste Management Consulting, LLC in Nevada City, California.
Cogger, C., R. Hummel, J. Hart, and A. Bary. 2008. Soil and Redosier Dogwood response to incorporated and surface- applied compost. Hort Sci., 43:2143-2150.
Recycled Organics Unit. 2006. Life cycle inventory and life cycle assessment for windrow composting systems. The Univ. of New South Wales, Sydney, Australia. Available at (verified 5 Mar. 2008).
January 19, 2010 | General
Compost Boost To California Agriculture
BioCycle January 2010, Vol. 51, No. 1, p. 27