Composted Dairy Manure and Alfalfa Hay Mulch Affect Soil Ecology and Early Production of ‘Braeburn’ Apple on M.9 Rootstock

Abstract

A variety of organic mulches and amendments have been observed to improve soil quality and productivity of apple orchards. Alfalfa hay and composted dairy manure solids (CDS) are readily available in the apple-growing region of the Pacific Northwest and British Columbia and could be used to improve orchard performance. The objective of this research was to determine the influences of CDS amendment and alfalfa hay mulch on populations of soil microfauna, soil chemical properties, and early growth; nutrient uptake; and yield of apple planted into a fumigated coarse-textured soil in central Washington State. Alfalfa mulch significantly improved tree vigor and fruit yield with minimal adverse effects on fruit quality, whereas the CDS amendment had minor effects on vigor and yield. Both alfalfa mulch and CDS amendment increased availability and uptake into leaves and fruit of most key nutrients but plant nutrient concentrations were not deficient in control plots, suggesting that increased nutrient availability was probably not the primary reason for the increased vigor attributed to alfalfa mulch at this site. The alfalfa mulch resulted in elevated populations of microbivorous nematodes and protozoa that persisted through later years of the experiment, indicating greater overall microbial activity, mineralization of nutrients, and possible direct stimulation of root growth under mulch; the CDS amendment did not consistently enhance populations of microbivorous nematodes. The alfalfa mulch, but not CDS amendment, suppressed the buildup of populations of root-lesion nematodes, which are important components of the replant disease complex that was suppressing tree growth at the site despite the preplant fumigation. Accordingly, we speculate that the reduced impacts of root-lesion nematodes contributed to the improved vigor and fruit yield of trees grown with alfalfa hay mulch.