Mulch effects on highbush blueberry under organic management

Authors: LARCO, H - Oregon State University; SULLIVAN, DAN - Oregon State University; STRIK, B - Oregon State University; Bryla, David

Submitted to: Acta Horticulturae
Publication Type: Proceedings
Publication Acceptance Date: 10/15/2011
Publication Date: 1/31/2014
Citation: Larco, H., Sullivan, D.M., Strik, B., Bryla, D.R. 2014. Mulch effects on highbush blueberry under organic management. Acta Horticulturae. 1018:375-382.

Interpretive Summary: We initiated a field trial in 2006 to evaluate organic production practices for blueberry, including the use of different mulches. Mulch treatments are imbedded in a long-term trial allowing evaluation of mulch interactions with raised beds or flat ground, organic fertilizer rate and source, and cultivar. Mulch treatments were selected in consultation with a grower advisory committee and included sawdust, weed mat, and compost. Sawdust mulch has been used in conventional blueberry production for many years. Geotextile weed mat is a newer mulching technique. It is usually very effective in weed control and typically has a lifespan of 4-5 years. Yard debris compost is produced by municipal composting programs and typically costs 1.5-2 times more than sawdust. Because organic sources of nitrogen (N) are expensive and laborious to apply, the potential for a slow-release N benefit from compost is of great economic significance. This paper focuses on the effects of mulch treatments on plant and soil nutrients and berry production during the first two growing seasons after planting. Compost had a positive effect on berry yield, likely by maintaining soil pH in the optimum range for blueberry and by providing nutrients essential for plant growth and production. We conclude that the positive effect of this yard debris compost on blueberry yield is more than just a N substitution effect. The maintenance of higher organic matter levels in soil via compost application is likely the key factor related to compost benefit. Compost may also provide a more favorable soil microbial environment or improved soil water relations for blueberry. The mechanism(s) of the “organic matter benefit” to blueberry await discovery in later years of this field trial.

Technical Abstract: A long-term organic blueberry trial was planted in October 2006 in Aurora, Oregon to investigate the effect of mulch on soil and plant nutrient status, plant growth, berry yield, irrigation requirements, and weed control efficacy. Mulch treatments were applied at planting and included 1) weed mat (geotextile), 2) Douglas-fir (Pseudotsuga menziesii) sawdust, and 3) yard debris compost + sawdust (compost applied to soil, then covered with sawdust). This paper addresses mulch treatment effects on soil and plant nutrient status during the first two years of the trial, and the first berry harvest in year 2 (2008). Yard-debris compost used in the field experiment had a pH of 7.3 and low soluble salt (EC < 1 mS/cm). The yard debris compost + sawdust treatment produced greater berry yields than sawdust alone in the first bearing season. Relative berry yields were 100% for weedmat, 90% for sawdust + compost, and 70% for sawdust mulch. The positive effects of compost on berry yield were observed across two cultivars, flat or raised beds, and for two organic fertilizer sources (fish emulsion or feather meal) applied at two rates (29 and 57 kg/ha N). Soil pH underlying the mulch was lowest (most acidic) with weed mat and highest under sawdust + compost mulch. After two years, compost increased soil organic matter (OM; 0-20 cm) by 9 g/kg vs. weed mat, while sawdust mulch did not increase soil OM. In summary, compost maintained soil pH in the optimum range for blueberry, provided plant-available cations, increased soil organic matter, and increased berry yield (relative to sawdust alone). The mechanism(s) behind the compost benefit deserve further investigation.