Soil Community Structure, Function, and Spatial Variation in an Organic Agroecosystem

View the project final report

• 2008 annual report

Commodities

• Agronomic: potatoes
• Fruits: berries (brambles), melons
• Vegetables: beans, beets, broccoli, cabbages, carrots, cauliflower, celery, cucurbits, eggplant, greens (leafy), onions, parsnips, peas (culinary), peppers, rutabagas, sweet corn, tomatoes, turnips, brussel sprouts
• Additional Plants: herbs

Practices

• Crop Production: no-till, organic fertilizers
• Education and Training: on-farm/ranch research
• Natural Resources/Environment: indicators
• Soil Management: organic matter, soil analysis, soil quality/health

This project examined the spatial variation of soil organisms and other edaphic properties across a large organic vegetable farm with diverse soil physical properties. Our study site, Full Circle Farm (FCF), is located near Carnation, WA. In 2006 we sampled across a 62-acre area to examine spatial variation of edaphic properties at a farm scale. In 2007 we intensively sampled two separate fields with near identical management to examine spatial variation at a field scale. In 2007 we presented research results at a farm walk at FCF (with over 80 participants) and at the WA Tilth Producers Conference.

Introduction

Sustainable farm management requires the maintenance of ecosystem functions such as nitrogen mineralization, aggregate formation, and pathogen control. Given the importance of biological processes in production, growers are increasingly interested in information about the biological status of their soil. While several labs offer soil food web analyses (Diver, 2002) effective guidelines for interpretation of soil biological data remain elusive (Bengtsson, 1998). In particular, the importance of variability to interpreting soil biological data is seldom addressed. Farmers have limited resources with which to monitor soil quality. Many wonder whether to pay for biological sampling and if so how should they sample and leverage results in site-specific management.

Most soil properties exhibit high degrees of spatial structure; “hotspots” of biological activity ebb into areas of little or no activity often over predictable distances (Klironomos et al., 1999). While spatial heterogeneity has typically been viewed as a hindrance to understanding soil biogeochemical phenomena, Ettema and Wardle (2002) suggest “spatial variability may be the key, rather than the obstacle to understanding the structure and function of soil biodiversity.” Ignoring spatial variability compromises our ability to competently describe soil communities through typical sampling plans. A spatially-explicit research approach can strengthen our understanding of biological diversity and abundance and better connect those parameters to edaphic properties and biological functions.

One indication that significant agronomic soil functions are being provided is a healthy crop. Likewise, an ailing crop and decreasing yields are possible indications, albeit untimely, that functions, such as good soil structure, nutrient availability, and pathogen control are inadequate. Soil organisms mediate or contribute to all of these essential functions and biological populations are therefore potential indicators of productive soil and good management practices. Doran and Zeiss (2000) identified soil functions that can be influenced by management decisions as “dynamic soil quality” and those properties not easily changed or influenced by management decisions, e.g. climate, clay mineralogy, texture, etc., as “inherent soil quality”.

One challenge to the development of biological-based indicators of soil health is that soil organisms are influenced by both dynamic and inherent soil quality. The importance of soil texture and other edaphic properties on biological properties has been demonstrated in several studies. Franzluebbers et al. (1996) found increasing soil microbial activity in coarser textured soils. This finding is in agreement with the general recognition that organic matter decomposes more rapidly in sandy soils than in fine textured soils (Hassink, 1994). However, Thomsen et al. (1999) found more rapid turnover of organic matter in clay-amended soils when the soils were adjusted for soil water potential. Understanding how inherent soil properties affect potential biological indicators will help growers interpret results and make site-specific management adjustments accordingly.

Robertson and Freckman (1995) found that sand and silt were positively correlated with bacterial- and fungal-feeding nematode density, but not with abundance of omnivore/predators or plant parasites. Though the relationship was weaker, pH was also positively correlated with both microbial-feeding nematodes. Avendaño et al. (2004) found that soybean cyst nematode density was positively correlated with sand content, but negatively correlated with silt and clay. Noe and Barker (1985) found that clay content was a factor that determined the population density of three parasitic nematodes at one site, but was not relevant for any of the species at another site. The authors note that where edaphic variables occur at or near biologically limiting levels their specific concentrations can influence local populations (in this case influencing the host-pathogen interaction), but where the edaphic variable is above a threshold other variables play more of a part.

Knowledge of the spatial properties of parameters of interest as well as relationships to other spatially structured and predictable edaphic properties can potentially be used to leverage sampling plans directed at improving management. Researchers have documented associations between specific pathogenic nematode species and soil texture (Avendaño et al., 2004), texture and organic matter (Wyse-Pester et al., 2002) and combinations of multiple chemical and physical soil properties (Noe and Barker, 1985). These results suggest the potential for identifying pathogenic nematode infestations based on edaphic properties. Understanding how edaphic properties can affect other soil biological properties will strengthen their use as soil health indicators and their efficacy for directing management.

There were five primary objectives for our project: 1) use geostatistics to develop maps for soil organisms and edaphic properties, 2) develop biological indicators that correlate to N-mineralization potential and aggregate stability, 3) recommend general biological sampling methods, 4) make recommendations to optimize farm productivity and profitability, and 5) share results with other growers and agricultural professionals.

Important performance targets:
Spring 2006 Establish sample locations & map management zones at FCF
Fall 2006 1st sampling at FCF
Spring 2007 2nd sampling at FCF
Summer 2007 Presentation at Puyallup Field Day, 50 people expected. Farm walk at FCF.
Fall 2007 1.) Presentation at WA Tilth Producers Conference. 2.) Post report on Puyallup Soils Group Webpage. 3.) Submit to WSU Small Farms Newsletter