Influence of Weed Mat and Surface Sawdust Mulch On Soil Nutrient Availability and Soil Chemical Properties Under Organic Blueberry Production

Weed control represents one of the most important cultural management aspects for organic blueberry production. Two of the most common ways to control weeds in blueberries is by the use of surface sawdust mulch or by landscape fabric, often referred to as weed mat. Soil temperature and soil moisture frequently differ under these two mulches due to differences in their physical and chemical properties. Because mineralization of organic amendments is intimately tied to temperature and moisture, the type of mulch used may affect nutrient availability to plants. Thus, to better predict crop growth and productivity in organic blueberry systems, a proper understanding of how mulches influence the availability of soil nutrients is critical. Data were collected during the third and fourth year after planting from an organic trial of ‘Duke’ blueberries fertilized with fish emulsion. Plants were spaced 0.75 x 3.0 m apart on flat or raised beds covered with either a 5-cm deep layer of Douglas-fir sawdust mulch or a single layer of black, woven weed mat fabric. Soil solution was collected bi-weekly using suction lysimeters installed at two depths under each mulch and bed type and measured for pH, EC, and NH₄-N and NO₃-N concentrations. Soil macro- and micronutrient concentrations were also measured periodically using ion exchange resin membranes. Results indicated that solution pH and EC were higher under sawdust (5.7-6.1 and 151-202 µs·cm^-1) than under weed mat (5.3-5.7 and 125-179 µs·cm^-1) while nutrient availability, including NH₄-N, which is considered the preferred form of N for blueberry, was often greater under weed mat than under sawdust. Weed mat also increased soil temperatures by as much as 5 °C during the day compared to sawdust, which likely promoted higher decomposition and mineralization of the fish fertilizer. Overall, weed mat is an effective weed control option that resulted in soil conditions considered better for blueberry through increased soil nutrient concentrations and lower soil solution pH and EC.