Adaptation of the PourThru Nutrient Extraction Procedures to Greenhouse Crop Production

Abstract

The purpose of this research was to adapt the PourThru nutrient extraction technique, which is the displacement of the bulk solution from the production container without a destructive harvest, to commercial greenhouse crop production. PourThru is a quick and easy nutrient sampling technique that is not laboratory oriented and can help prevent costly nutritional problems for greenhouse crop producers. Additionally, time domain reflectometry (TDR) was evaluated as an experimental tool to measure substrate moisture content, which may affect PourThru extraction. Time domain reflectometry is quick, non-destructive and has potential for use in automation of moisture content determination in greenhouse production. Previous PourThru research had focused on techniques for use on large nursery containers (≈ 3800 cm3) versus the smaller floriculture containers (≈ 1500 cm3) and no exact calibration of PourThru nutrient values to saturated media extract (SME) values, the current standard for nutrient testing, had been completed. Therefore, studies were implemented to examine the relationship of PourThru to SME, evaluate irrigation systems and timing effect on PourThru results as well as develop recommended influent and leachate volumes to ensure an unadulterated sample. Calibration curves were developed between PourThru and SME values and r2 values ranged from 0.91 to 0.99 for linear relationships. Irrigation systems did affect electrical conductivity (EC) values and alternative interpretive standard values were developed dependent upon irrigation system. Timing of the PourThru was important to ensure adequate leachate was collected for sample analysis and the amount of influent affected EC values and the amount of leachate collected. The use of TDR was effective in small containers (980.6 and 2177.5 cm3); however, care should be taken to match probe size to container size to ensure representative sampling. The largest limitation to TDR use was the bulk density of the substrate being analyzed. Approximately a 50 to 75% underestimation occurred when substrate bulk densities were below 1.2 g.cm-3, but material specific calibration improved TDR accuracy to within 4%.