Deterioration of Three Biodegradable Plastic Mulches before and after Soil Incorporation in a Broccoli Production System

Deterioration of three potentially biodegradable plastic mulch products, BioAgri, Crown 1, and SB-PLA-11, was evaluated before and after soil incorporation over two growing seasons, 2011 and 2012, with broccoli (Brassica oleracea var. italica) as the test crop. In-season mulch deterioration was evaluated during both growing seasons by estimating the percentage of visual deterioration (PVD). The PVD at the end of the growing season differed significantly by mulch product in both 2011 and 2012 (P < 0.0001), and was greatest for Crown 1 both years (63.8% and 71.3%, respectively). The PVD for BioAgri (11.8% in 2011, 10.5% in 2012) was greater than for SB-PLA-11 both years (1.3% in 2011, 0.5% in 2012). Mulches were incorporated into the soil at the end of the first growing season and soil samples were collected every three months for 13 months.  Fragments of mulch recovered from each soil sample were photographed, and mulch deterioration was evaluated based on average area per fragment, total number of fragments, and total fragment area, using ImageJ software. The average area per fragment decreased over time after soil incorporation for all mulch products. The total number of mulch fragments initially increased for all mulch products, with the number of fragments of Crown 1 and BioAgri reaching a maximum at 132 and 299 days after incorporation, respectively. As the number of fragments declined for these two mulches, the average area of each fragment did not change, suggesting that a threshold fragment size may exist for biodegradation during this time period.  At the end of the study, 397 days after soil incorporation, Crown 1 and BioAgri had deteriorated 100% and 65%, respectively, whereas SB-PLA-11 showed no appreciable deterioration based on total fragment area. Broccoli yield was only measured in the second year, after mulch had been soil-incorporated. When compared to non-mulched subplots, mulch increased total and marketable crown weights 36% and 30%, respectively (P = 0.001 and P = 0.04, respectively), and decreased days to harvest by 6% (P = 0.0002). There was no impact on broccoli yield due to mulch product.