Soil that has been farmed or gardened "conventionally," using chemical herbicides and pesticides, can usually be converted to organic standards over a period of years. How successful the process is depends on the types and quantities of chemicals used, the soil type and characteristics and the length of time allowed for the transition. Three years is considered to be the minimum period necessary.
Testing and Remediating
Test the soil to find out what chemicals it contains. Your local County Cooperative Extension office can help you find a competent soil-testing laboratory and provide instructions on how to take soil samples.
Take safety precautions if the results of the soil rest reveal hazardous chemicals. You may need to wear a face mask, or even a respirator, gloves and protective clothing when working with the soil. You should avoid breathing any dust, and you should carefully wash your hands and clothing after soil contact.
Add at least 4 inches of good-quality compost over the entire area, and incorporate it into the top 6 inches of soil. For large areas this may amount to many tons of material, but compost is necessary as a first step in restoring the soil to a healthy condition. Chemicals destroy soil life, which negatively impacts soil structure. Compost helps to support soil life that is a necessary part of healthy, organic soil.
Plant a beneficial cover crop in the spring or early summer, such as a combination of vetch (a legume) and buckwheat. Legumes have an extensive root structure and fix nitrogen in the soil, and buckwheat provides ample vegetative matter. At the end of the season, cut these crops down and incorporate them into the soil, adding organic matter. Follow these crops with a winter cover crop, such as rye, that protects the soil from erosion from wind and rain.
Repeat the cover crop routine during years two and three. Take soil tests in the spring and fall to assess your progress in ridding the soil of chemical residues. Some agricultural and horticultural chemicals, such as arsenic, will linger in trace amounts for many years.