Improper nutrient management is one of the primary causes of poor crop quality and plant losses in nurseries and greenhouses. However, few commercial operations routinely monitor the media solution properties related to nutritional status of their container-grown crops. There are three accepted methods for monitoring substrate pH and electrical conductivity (EC) on-site: the pour-through (PT), the saturated media extract, and the 1:2 water:substrate (v/v) suspension test (1:2) (Camberato et al., 2009; Cavins et al., 2000, 2004; Pennisi and Thomas, 2009; Ruter and Garber, 1993). Among these, the PT method is a widely accepted practice for both nurseries and greenhouses.
The PT method is a bulk solution displacement method that was developed as a simple, rapid, non-destructive, and cost-effective means of monitoring pH, EC, and nutrient availability of soilless substrates, including those that contain slow- or controlled-release fertilizers (Blythe and Merhaut, 2007; Yeager et al., 1983). The PT method as described by Wright (1986) and adapted by Cavins et al. (2008) is as follows. The crop should be irrigated 30 to 60 min before collecting leachate to ensure that the container substrate is at or near full water-holding capacity. If using constant liquid fertilization, the crop should be irrigated with fertilizer solution as usual. A collection vessel is placed under the container. A sufficient amount of distilled water should be applied to the surface of the container so that ≈50 mL of leachate is collected. Generally, applying 75, 150, or 350 mL of water to #1, # 3, and #5 containers is sufficient (Bilderback, 2001; Cavins et al., 2000). The leachate is then collected and pH and EC values are determined.
The advantages of the PT method over other methods are that it is fast, the sampling solution is easily obtained, and it is not destructive (Wright, 1986; Yao et al., 2008). In small containers, the leachate collected by the PT method should sample the solution from the entire root zone. However, as container size increases, it is increasingly difficult to displace the solution from the entire root zone without using large quantities of water. It is unclear how the ratio of applied water to displaced root zone solution affects the pH and/or EC values of the leachate used for testing. If larger amounts of water are used, channeling through the root zone may result in inaccurate and/or inconsistent measurement values resulting from non-uniformity of the substrate and fertilizer distribution within the substrate (Altland, 2006). General recommendations are to collect 50 mL of leachate each time a PT test is conducted for a variety of container sizes up to #1 (Cavins et al., 2000). However, few if any well-defined recommendations exist for leachate volumes of larger containers.
Values of EC obtained with the PT method accurately reflect EC values obtained with more direct measurements taken through media solution samples (Cabrera, 1998). This, combined with the ease of leachate collection and reproducibility, make the PT method an ideal technique for container substrate management. There is little information, however, on the use of this method in large containers.
According to Ruter and Garber (1993) and Cavins et al. (2008), growers do not use the PT method because of 1) lack of uniformity in testing procedures; 2) few recommended values for interpretation; and 3) limited knowledge on how to interpret results. Therefore, it is important to identify potential sources of variation such as leachate volume. We tested the effects of two leachate volumes (50 mL or 2.5% of container volume) on the pH and EC values obtained from leachate of four container volumes (2, 8, 11, or 27 L). Our objectives were to determine if leachate volume affected the values obtained or the uniformity of results in large containers.
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Blythe, E.K. & Merhaut, D.J. 2007 Testing the assumption of normality for pH and electrical conductivity of substrate extract obtained using the pour-through method HortScience 42 661 669
Camberato, D.M., Lopez, R.G. & Mickelbart, M.V. 2009 pH and electrical conductivity measurements in soilless substrates Purdue Univ. Ext. Serv. Bul. HO-237-W
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