Ace the test

Substrate testing for container nursery crops is usually performed using the pour-through or leachate collection procedure. This procedure is easy to conduct in the field and does not require substrate removal from containers. The electrical conductivity (EC) and pH of leachate (liquid that exited the container during procedure) can be determined immediately or the leachate can be sent to a laboratory for analyses of specific plant nutrients.

Details about how to conduct the pour-through are given in the Substrate Nutrition video linked from the manual titled Water Quality/Quantity Best Management Practices for Florida Nurseries (http://bit.ly/1LeDGdJ). An optional procedure is to remove substrate from several containers to make a composite sample that is sent to a laboratory for nutrient, pH and EC analyses.

Regardless of the procedure, reasons for testing can be grouped into three broad categories of intensity: Frequent, intermittent and infrequent.

Frequent testing, for example weekly, is done to ensure nutrition is optimal — not too high or too low — for plant growth. Because of the frequency of testing, substrate nutritional status is indicated by monitoring EC only. Although EC does not indicate the concentration or amount of a specific nutrient, it indicates the overall nutritional status of the substrate, thereby helping the grower avoid potential nutritional issues before they are visually evident and plant quality is affected. (See Table 1 for optimal EC ranges, page 50.)

When conducting frequent sampling of container substrates, collect leachates from the same group of plants (plants treated similarly in same location) each time. This repetitive sampling from the same group of plants improves consistency and confidence in your technique. Reducing variability by monitoring the same plants will make it easier to evaluate trends in EC over time. Strategic sampling of additional groups of plants at various stages of growth and with different fertilization programs will help growers monitor the overall nutritional program in the nursery.

Intermittent testing entails monitoring substrate nutritional status at specific stages of production, typically where potential problems can occur. As an example, compare two scenarios where containers filled with substrate amended with controlled-release fertilizer (CRF) are placed in the production area. In the first scenario, containers are left in the field for a given period of time before liners are planted; in the second liners are planted immediately. For the first scenario, consider testing the substrate before planting begins as nutrient release that occurred before planting may have resulted in high EC levels. Ensuring that EC levels are not excessive may help prevent newly planted liner roots from being damaged. Once liners are planted, check the EC one to two weeks later. In the second scenario, when liners are planted immediately monitoring EC one or two weeks after planting should be sufficient.

Another reason to sample intermittently is to verify the longevity of CRF for the production conditions in your nursery. Substrate sampling times will vary, but sample near the end of expected nutrient release to avoid exhausting the existing substrate nutrition before adding additional fertilizer. Also, results from samples taken near the end of expected nutrient release may reveal that additional fertilizer is not needed. It is best to have a laboratory conduct nitrate nitrogen, phosphorus, and potassium analyses for this verification.

Infrequent testing is usually done to confirm or diagnose a nutrient deficiency or toxicity. Samples should be taken from substrate of plants exhibiting symptoms as well as from substrate of plants that appear unaffected. A laboratory analysis for all macro- and micronutrients required by plants is recommended so that ratios of nutrient concentrations can be compared as well as the individual nutrient concentrations. Optimal nutrient concentrations as well as pH are given in Table 1. Additional information about substrates and nutrient management can be found in the Best Management Practices manual mentioned earlier.

Dr. Tom Yeager is an Extension Specialist for Nursery Crop Production in the Department of Environmental Horticulture at the University of Florida, IFAS in Gainesville; yeagert@ufl.edu.