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Plastic film soil mulches (i.e., protective soil covers) and plastic covered low tunnels (i.e., enclosure) have the potential to alter crop microclimate, lengthen the growing-season, and increase plant productivity. A plastic film’s ability to alter microclimate is related to its shortwave (S) and longwave (L) radiative properties (reflectivity (ρ), transmissivity (τ) and absorptivity (α)). This thesis examines the effect of plastic films with different radiative properties on 1) surface energy balance, 2) crop microclimate, and 3) crop productivity. A study of nine plastic film mulches with various radiative properties (Chapter 2) showed that all films increased daily soil heat flux density, including a high ρs value film (0.45), due to its insulating effect at night. A comparison of three black plastic films with high shortwave absorptivity (αs ≈ 0.95) but different αL values showed that low and high αL value films achieved the highest and lowest daytime soil temperatures, respectively. A study of vegetation-free plastic film low tunnels with similar τs but different αL values (Chapter 3) showed that a high αL value cover (i.e., glass-like) increased net longwave radiation inside the low tunnel compared to a low αL value cover, and increased inside air temperature (Tain) by 5 and 2°C during the daytime and nighttime, respectively. A model to predict daytime and nighttime Tain is presented and validated. A study of Padrón peppers (Capsicum annuum) grown inside and outside low tunnels (Chapter 4) showed that low tunnels increase pepper growth, productivity (10%) and growing-season length (~2 weeks), but CO₂ depletion and high water vapour density occurs when leaf area index is high. A study of summer squash (Cucurbita pepo) grown within black plastic mulch showed that the addition of a perforated low tunnel increased yield 27%. A study of broccoli (Brassica oleracea) showed that low tunnels increased yield due to wind protection in spring, which conserved soil moisture and increased Tain during low temperatures. This thesis shows that plastic film covered low tunnels and soil mulches are an effective tool for altering crop microclimate and increasing yield, but trade-offs regarding microclimate exist that crop producers should consider. |
