Long-Term Organic Farming Impacts On Soil Fertility.

Very few long-term organic farming studies exist, especially in the central Great Plains. Data was gathered from annual soil tests from Grant Family Farms (GFF), the largest organic mixed crop farm in Colorado, located in the Great Plains in Northern Colorado.  The land that now makes up GFF was purchased in the early 1970s and sustainable agricultural practices were used until the mid-1980s when organic farming practices were implemented.  GFF uses leguminous cover crops as green manures and dairy manure as their only fertility inputs.  Crop rotations among over twenty different vegetable and herb crops are standard practice.  Soil test data from twelve fields, 7 to 20 hectares in size, were compiled and analyzed for changes over time in soil chemical properties for ten soil fertility components: pH, electrical conductivity (EC), soil organic matter (SOM), nitrate (NO₃-N), phosphorus (P), potassium (K), zinc (Zn), iron (Fe), manganese (Mn), and copper (Cu).  The number of years of data per field ranges from to 5 to12 over the period of 1985 to 2000.  Annual soil tests revealed a significant increase in P, K, SOM, Zn, and Fe levels in ³ 33% of the fields.  Soil pH decreased significantly (p<0.05) in four fields.  P reached a level that could be an environmental risk if soils are not managed carefully.  NO₃-N and Cu did not change significantly over time in organic production in any of the observed fields. Overall, there was an observed increase in soil fertility components over time in organic production.