Development of the Press Extraction Method for Plug Substrate Analysis

Abstract

The goal of this research was to develop and assess the effectiveness of the press extraction method (PEM) for nutritional analysis of bedding plant plug substrates. Conventional testing methods for soilless substrates are either unsuited for plug production or interpretive standards have not been published for pH, soluble salts, or specific nutrients. With the PEM, the rooting substrate is irrigated to container capacity and after a period of one hour the plug surface is pressed with a finger or thumb to expel the solution. The first series of experiments examined potential variation in pH, electrical conductivity (EC), and extractable nutrients (NO3-N, NH4+-N, P, K, Ca, Mg, Na, B, Cu, Fe, Mn, and Zn) associated with differing extraction forces. A press modified from a fruit penetrometer was designed to apply a range of force (similar to what could be exerted manually) to a 35 g sample of soilless substrate. Testing done on a peat-based substrate fertilized with a single fertilizer rate (20N-4.3P-16.6K at 75 mg.L-1 N) and then fertilized with a range of rates (75, 125, and 175 mg.L-1 N). A coir-based substrate was also tested at one rate (125 ppm mg.L-1 N). For all experiments, the range of extraction forces within a fertilizer rate did not affect chemical properties. When testing included a range of fertilizer rates, the method's ability to detect changes in pH and EC was demonstrated. The second study compared the PEM to the saturated media extract (SME) and the 2:1 water : substrate (v/v) suspension method (2:1). The extraction methods were used on plug trays of a peat-based germination mix treated with 20N-4.3P-16.6K at 50, 100, 150, and 200 mg.L-1 N. Sample sizes of 20 and 60 plugs were used to determine if similar results could be obtained with the smaller, less destructive sample size.. Values for pH were similar among the three methods. The PEM method resulted in the highest EC, NH4+-N, NO3--N, K, Ca, and Mg. Testing 20 plugs was sufficient since sample size accounted for little variation in the results. In a second experiment comparing peat- and coir-based substrates, the coir extract pH averaged 1.5 units higher with much higher levels of K and Na regardless of extraction method when compared to the peat. Solution from the peat substrate was higher in NH4+-N, NO3--N, P, Ca, and Mg. Within each substrate, pH, EC, and nutrients tested were similar between the PEM and SME. The similarity between the PEM and SME nutrient analysis in the second experiment suggested existing SME interpretation standards may be used for the PEM but further investigation was needed. The third study was designed to provide the necessary correlations between the methods to allow for development of pH, EC, and nutrient interpretive ranges for actively growing plugs. First, a wide range of bedding plant plugs grown at various commercial greenhouses were collected and the substrate solution extracted with the PEM, SME, and 2:1 methods. This quickly and inexpensively provided the varied population necessary to establish relative comparisons and correlations between the methods when performed on actively growing plugs. A second experiment provided comparisons of the three extraction methods performed on plugs grown at low, medium, and high fertility rates with a variety of bedding plant species; celosia (Celosia argentea L. var. cristata (L.) Kuntze Plumosa Group), impatiens (Impatiens wallerana Hook. f.), marigold (Tagetes erecta L.), petunia (Petunia x hybrida Hort. Vilm.-Andr.), and salvia (Salvia splendens F. Sellow ex Roem.& Schult.) Thirty days after planting (DAP), shoots were harvested and the solution extraction methods were performed on each flat. For both experiments, the PEM EC was consistently higher and the pH equal to or lower than the SME. The pH with the 2:1 method was similar to that for the PEM, however EC was consistently low ith this method due to the dilution effect. Solution NO3--N, P, and K were well-correlated between the PEM and SME. However, between the two experiments, there were anomalous results for most nutrients extracted by the PEM and SME methods. As found in previous studies, Ca and Mg levels were consistently higher in the PEM extracted solution. The pH and EC relationships between the three extraction methods remained consistent throughout the study. As a result, quantitative interpretation ranges for these two analyses were calculated between the PEM and SME. Plug growers can now use the PEM for on-site pH and EC testing and compare their results with the previously published sufficiency ranges for the SME.