Tests can be conducted prior to planting to evaluate the potential for crop injury. Many times soil samples are taken and sent to a lab for analysis. These tests are usually done using a High Performance Liquid Chromatograph (HPLC) or a Gas Chromatograph (GC) and can be costly, time-consuming and difficult to interpret. A simpler and more direct test can be done by using a bioassay or growing sensitive plants in pots containing soil from the questionable field. This technique is less expensive, requires little equipment and can be done by anyone. Bioassays are often conducted by growing species of plants that are known to be sensitive to a specific herbicide or class of herbicides. It is reasonable, however, to use the crop that is to be planted. If, for instance, lettuce will be grown, the seed used in the bioassay should be from the variety and lot number that will be used. Bioassays can be more accurate than more sophisticated lab tests in predicting potential crop injury. These tests are only good, however, if the soil sample collected is representative of what is in the field. Sample collection is very important and small amounts should be taken from several areas of the field. Herbicide concentration often varies within fields and separate samples should be collected and labeled to indicate in which part of the field they came from. Samples are normally taken from the top 2-4 inches from the surface. Soil must also be collected from similar soil types in the same vicinity that have not been treated for comparison.
We are again conducting a bioassay program this season to help growers evaluate potential herbicide injury using the greenhouses at the Valley Agriculture Center. You can contact either Barry Tickes (928-580-9902) or Marco Pena (928-782-5871) for guidelines on how to collect and drop off soil samples.
Effects of an excessive rate of Prefar® on lettuce root growth.