October 19, 2011 | General

Climate Change Connections: The One-Acre View

BioCycle October 2011, Vol. 52, No. 10, p. 60
Sally Brown
I’d like to go big picture with this column, say from one ton to one acre. One ton is a manageable amount. It is possible in a day to shovel a ton of dirt. An acre is a little harder to visualize, particularly if you are from a city where apartments and home lots are more often described in square feet. For my PhD defense exam for example, my advisor (who is from a farm in Ohio) asked me (who is from a house on a 40-foot by 100-foot lot in Queens) how big an acre was. I stared at him dumbfounded. I had no idea (43,560 sq. ft.). Luckily I was much better at answering the soil chemistry questions and so I passed. Since that time I have committed that 43,560 sq. ft. to memory.
Municipal managers often think of things using the ton framework. How many tons of food waste, yard trimmings, biosolids a day? Tonnage is translated more often into number of trucks rather than number of acres. Farmers and land stewards, on the other hand, think in acres. How many acres in wheat? How many acres returned to forestry or in set aside reserve programs? Tonnage comes to mind if you are a farmer when you think about yield. With dryland wheat in Washington State for example, 1 to 3.5 tons/acre is what you might expect. In order to make appropriate decisions about tons of residuals, the per ton mindset folks need to look at things from a different perspective every once in a while, say a 43,560 sq. ft. kind of perspective.
Life cycle assessment (LCA) is a tool that is frequently used to evaluate the broad range costs and benefits of different residuals management practices. A number of studies have been published that use LCA to guide municipalities on the best way to use or dispose of waste materials. Common categories in LCA include greenhouse gas emissions, water use, environmental hazards and resource conservation. End use options for organics in these analyses generally include energy from combustion or anaerobic digestion, landfill disposal or land application.
Life cycle assessment is a wonderful tool – much, much better than the traditional simple accounting of figuring out how much the trucking company is going to charge and what the landfill tip fee might be. However, LCAs of urban organic residuals often consider costs and benefits only on a per ton of materials basis and often neglect important costs and benefits in their analysis. What happens is the results, while much improved over traditional dollar based assessments, present only a glimpse or a part of the big picture. Here I want to present results from a study focusing on land application of organics that looked at some of these benefits using both lenses – per ton and per acre.

The study was based on extensive field sampling done by Kate Kurtz, who completed her masters with Craig Cogger of Washington State University (WSU) and I as her main advisors. The sampling was designed to evaluate the long-term benefits of land application of biosolids and composts across a range of sites in Washington State. It was funded by the Washington State Department of Ecology (WA DOE). The WA DOE has traditionally looked at organics in the state using the ton lens, namely the BTUs/ton lens. They were highly skeptical that land application of these materials could have any long- term carbon storage benefits. The sites, ranging in age from 2 to 18 years, were primarily long-term experiments, put in by Craig well over a decade ago. There were some commercial farms too, fruit orchards and hops fields in eastern Washington. In total, the sites ranged from turf grass and landscaping shrubs, to a highway right of way, dryland wheat, and organic pears and cherries.
Across all sites but one we saw increased soil carbon reserves when composts or biosolids were added to the soils. Increases ranged from 0.014 tons of carbon per ton of amendment added to one turf grass site (not significant) to 0.54 tons of carbon per ton of compost applied in a commercial orchard. We saw more carbon stored per ton of amendment applied in poor soils in comparison to carbon-rich soils. This may be because the high C soils are already close to equilibrium carbon concentrations and only a limited amount of what is added remains. In all of the sites, total soil nitrogen increased in comparison to both control or fertilized soils. In a majority of the sites, we saw increased available phosphorus (more so with biosolids amendments in comparison to composts) and increases in soil water holding capacity. Carbon for the long- term sites didn’t decrease with time, but showed signs of either stabilizing or increasing. This is pretty cool.
To illustrate more specifically, let’s focus on my favorite site: dryland wheat fertilized with biosolids. This is a replicated field trial, set up in the 1990s by WSU. The soils receive biosolids or fertilizer every other year when wheat is grown using conventional tillage and no irrigation. The interim year is a fallow year, so that the soils can rest. The site is called Boulder Park because there are giant boulders scattered in the flat wheat fields. On a per ton of biosolids basis, for every ton that was applied, the soil stored from 0.34 to 0.43 tons of carbon. Total nitrogen and phosphorus was higher in the biosolids amended soils than in the fertilizer treatment plots. The soil also held more water. That is pretty impressive.
But to me, it is much more impressive when the benefits are calculated on a per acre basis. If 25 tons an acre of biosolids are applied to a field at Boulder Park, over the course of several years, based on the results from our sampling, you could expect to sequester 8 to 10 tons of carbon. In addition, you save carbon emissions because no nitrogen and phosphorus have to be added. N and P fertilizers require a lot of energy to produce – about 4 kg CO2/1 kg for N – and a little less than half that for P. That provides another 1.5 to 2.5 tons/acre of carbon. Finally, because the soil water holding capacity is improved, you can expect higher yields.
And that is just what has been seen in Boulder Park and elsewhere where biosolids have been added to soil instead of synthetic fertilizer. In fact, Craig just had a paper published in Applied and Environmental Soil Science where he reported yield increases of up to 47 percent in comparison to synthetic fertilizer. So looking at these benefits through the acre lens rather than the ton lens, you get about 10 to 13 tons of carbon per acre plus you get a lot more spaghetti (winter wheat grown in this region is generally used to produce unleavened products like pasta). The best other land management tool we have is no-till management. Increases in soil carbon for no till average about 0.15 tons of carbon per acre per year. That is a whole lot of years to get you to substituting biosolids for synthetic fertilizer.
We saw similar results for compost – also high on the soil carbon storage and water holding capacity, a little lower on the nutrients. As I said earlier, much lower storage was seen on carbon rich soils for both biosolids and composts. I am happy to share the full report. Just send an email. But the point of this column is to encourage you to look at these issues from a broader perspective. And as you are pacing out that 43,560 sq. ft., you’ll have some time to think about how much broader those benefits seem versus that one ton pile of dirt. Think of it as opening your mind, and saving your back.

Sally Brown – Research Associate Professor at the University of Washington in Seattle – authors this regular column. E-mail Dr. Brown at

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