Sustainable sweetpotato farming systems to improve soil health and reduce losses caused by root-knot nematode

Abstract

Australia produces some of the highest sweetpotato yields in the world but the health of the soil used to produce the crop is declining because it is repeatedly tilled to kill volunteer sweetpotatoes, incorporate cover crop residues and prepare beds for planting. Additional disturbance occurs when the swollen roots are harvested. An on-farm study showed that it is possible to replace this tillage-dominated farming system with an alternative that is not only productive, but is also likely to be sustainable in the long-term. When the soil was amended with organic matter, beds were formed 10 months prior to planting, two cover crops (forage sorghum followed by oats) were grown on the beds, residues from those crops were retained on the soil surface as mulch, and sweetpotato cuttings were planted into the mulched, undisturbed beds using strip tillage, the crop established well and yielded 93 t/ha. The only detrimental effect was that root-knot nematode (Meloidogyne incognita) caused severe damage. In contrast, a trial in the same field showed that when organic amendments were placed in a furrow in the centre of the bed so that the swollen roots were surrounded by the amendment as they developed, nematode control was much better. Sawdust and a mixture of sawdust and chicken litter were the most effective amendments, as they both reduced root-knot nematode populations and the severity of nematode damage, and increased marketable yield by 29%. The wide range of natural enemies found in soil collected from the amended furrow at harvest, and the results of a suppression assay in the laboratory suggested that the soils amended with sawdust and sawdust /chicken litter were biologically suppressive to root-knot nematode.