Evaluating Visual Assessments of Mulch Deterioration to Predict Changes in Mulch Mechanical Properties

Visual assessments can be a quick and inexpensive tool to evaluate mulch deterioration in the field. Laboratory evaluation of mulch mechanical properties provide an objective measure of the physical status of mulch, but the equipment necessary is inaccessible to growers and some researchers. Unfortunately, no comprehensive comparison of these methods has been reported. This study evaluated the relationships between visual assessments and mulch tensile properties. Using tomato (Solanum lycopersicum L.) as a model crop, visual assessments of five mulch treatments were made twice monthly over the 2010 through 2012 growing seasons at Mount Vernon, WA. Mulch treatments included black polyethylene plastic and four biodegradable mulches: BioAgri, BioTelo, WeedGuardPlus, and an experimental spun-bonded polylactic acid (SB-PLA) non-woven fabric. Mulch samples were extracted from field plots at three times: mulch installation, tomato first flower, and tomato final harvest. For each sample, breaking force and elongation at break in the machine and transverse directions were measured in the laboratory. Regression analyses evaluated the linearity of relationships between visual assessment parameters and mulch tensile properties. The strongest relationship overall (P < 0.0001; r² = 0.41) was between the percentage of initial breaking force in the machine direction (BF-M-PD) and log₁₀ of the percentage of visual deterioration (PVD-L10). For each unit of increase in PVD-L10, BF-M-PD decreased by 31.78%. However, when analyzed separately by mulch treatment, the relationship between BF-M-PD and PVD-L10 was only statistically significant for WeedGuardPlus (P = 0.0001; r² = 0.30). The strongest relationship for BioAgri and BioTelo was between elongation at break in the machine direction (El-M) and PVD-L10 (P ≤ 0.03; r² = 0.10 and r² = 0.22, respectively); for WeedGuardPlus the strongest relationship was between elongation at break in the transverse direction (El-T) and the percentage of visual deterioration (PVD; P = 0.009; r² = 0.42); and for polyethylene the strongest relationship was between the percentage of initial elongation at break in the transverse direction (El-T-PD) and log₁₀ of the area under the deterioration progress curve (AUDPC-L10; P = 0.002; r² = 0.22). No statistically significant (P > 0.05) relationship was identified for SB-PLA. While moderate relationships were detected overall, it is important to note that visual assessments and tensile tests each measure different properties of the mulch materials, and thus are measures of different factors of deterioration. To better assess the ability of visual assessments to predict mechanical properties, independent evaluation of mulch products and increased sampling frequency is recommended