Soil Testing and Information for Organic Transition

Farmers wishing to transition from conventional to organic management practices face numerous technical and logistical problems. Following conversion to organic management, crop yields frequently decline while weed pressures increase. Farmers, encouraged to build soil organic matter to overcome these constraints, are rightfully frustrated as they seek information about soil management. Even though soil organic matter is reputed to be a defining factor in organically managed systems, soil tests that document organic matter targets or provide useful management handles have not been developed. Even though organic standards require a soil building program, regulated components of the plan list things not to do instead of detailing how to manage soil effectively. We investigated two assays, particulate organic matter (POM) and amino sugar N plus hydrolyzable NH4 quantified by the Illinois N test (IL-N). Particulate organic matter is a sensitive and readily measurable index of soil C metabolism while IL-N can be used to predict soil N supply. The objective of this project was to develop soil test strategies and informational resources for soil fertility management. Post-transition soils were obtained from numerous long-term studies that included organic and conventional production systems. Organic systems that rely on animal inputs including manure or compost were considered separately from those that relied exclusively on mineral amendments, and plant based inputs including legume N. Use of organic practices increased organic carbon and total N contents and the concentrations of POM-C, POM-N and IL-N. Organic systems that relied on animal-based production systems did not do a better job of building total or labile SOM than organic systems that were purely plant based. Organic management increased the proportion of organic matter in POM but not in the IL-N fraction and the quantity of N in POM and the IL-N fraction were similar. POM appears to be a more sensitive indicator of organic management. It may be a more effective soil testing parameter than the IL-N fraction provided its contents can be linked to soil N supply, soil physical fertility, and/or diseases suppression.

Introduction:

The International Federation of Organic Agricultural Movements (IFOAM) standards, which were the first formalized organic standards, state "Soil and soil management is the foundation of organic production. Organic growing systems are soil based, care for the soil and surrounding ecosystems and provide support for a diversity of species while encouraging nutrient cycling and mitigating soil and nutrient losses." IFOAM Norms (4.3.) require that certification follow a 36-month minimum period since prohibited substances have been applied and note a longer transition period could be required depending upon past land use, ecological context, and operator experience. The purpose of the conversion period is to establish an organic management system that builds soil fertility and re-establishes the balance of the ecosystem. Implementation of organic standards in a manner that ensures they achieve these lofty goals may be a challenge, given how general these standards are and how limited the research on this topic has been. The efficacy of organic practices is bound to be questioned by many, including producers, advocates, and enemies of the industry. This challenge may be greatest in the United States because the USDA National Organic Program (NOP) rules are even less proscriptive than those that preceded it (ATTRA, 2003).

In practical terms, what 'enhanced' organic matter characteristics means is a critical question. Enhancement of biologically-based fertility may have as much or more to do with changes in SOM quality as they do with quantity. Even though numerous studies have documented differences between organic and conventional soils in SOM and microbial properties, few measures of SOM or microbial biomass have found their way into practical use. Two fractions that hold particular promise as fertility indices are particulate organic matter (POM) and amino sugars. Measures of POM have been tied to microbial growth and nutrient supply and suggest it is closely related to biologically mediated C, N, and in some soils P availability. Amino sugars are derived from chitin and from bacterial and fungal residues. Newly immobilized N is disproportionately incorporated into the acid-soluble fraction that contains microbially-derived amino compounds. Related fractions are sensitive to organic amendments and have been related to plant N acquisition.

The ability of organic management to enhance soil fertility varies with context and crop. For example, cereal yields are frequently reduced in organic systems due to lower availability of nutrients and delayed timing of N release. In systems of organic farming without livestock, with no nutrients imported in feed or as bedding, or no manure to transfer nutrients within the farm, supplying adequate nutrients for crop growth, particularly nitrogen (N), is a major challenge. In cooler climates, cereals must complete their growth rapidly and have a high requirement for nutrients early in the growing season. Diagnosis of oversupply of nutrients might be as or more important to organic producers as is diagnosis of limitation. It is critical for organic producers to get a better handle on how to efficiently use manures and composts, which are the principle fertility tools in many systems. Inefficient use of manures is a key concern for the organic industry. Over application of amendments is a greater problem in intensive- high value systems or where livestock is concentrated.