Agronomic and sustainability outcomes from compost application in south Australian citrus orchards

SARDI has undertaken trials with compost mulch, grape marc and animal manure at sites on three citrus orchards in South Australia for a number of years. The trials were established to evaluate the potential of soil amendments as part of an integrated management program for Kelly’s citrus thrips (KCT). Experimental sites were designed to obtain best possible data for pest management within budget limitations, subsequently, the agronomic and environmental data were in some cases restricted to one site and selected representative treatments. A range of data were collected, including fruit yields, and fruit, soil, leaf, and water quality measures and biological changes, and the potential for water savings assessed. The field trials involved application of various rates of compost to crops, ranging from 40-200 m3 ha-1 for compost mulch, 100-200 m3 ha-1 for grape marc, and 10-40 m3 ha-1 for animal manure. The recycled green waste and composted animal manure have provided significant pest management and agronomic benefits through suppression of KCT and improved yield and fruit size. There were considerable economic net benefits from every type of trial application to citrus at both sites over the four year period assessed. For example, for every dollar invested in an application of 40 m3 ha-1 compost mulch at Loxton North, a return of about $ 5 dollars was realised. Returns ranged from $ 1.91 to 4.96. There also is the benefit of improved water efficiency that could provide significant cost savings. The longevity of these benefits remains unknown and is currently being evaluated. Whilst the grape marc treatments improved yields, and are cheaper than the composted green waste, the level of suppression of KCT was not as good as that provided by the compost, and the increased acidity that resulted from the high phosphate levels associated with the grape marc treatments could result in reduced quality. It seems that higher levels of application for compost mulch create more overall benefits in terms of fruit quality and tree health.