Identification of soil and biological factors in crop rotation systems with significance to wheat crop performance in the Overberg production area of South Africa
A two year experiment (2004-2005) was conducted at the Tygerhoek Experimental Farm near Riviersonderend in the Western Cape Province of South Africa. The effect of different crop rotation systems on soil properties, disease and insect pests, weed populations, wheat growth, yield and quality in the wheat crop phase, included in these crop rotation systems, was determined. This trial was part of a long term crop rotation experiment started in 2002. This trial was laid out as a block design with four replications. Crop rotation systems included wheat, barley, canola, lupins and pasture phases which consisted of medics and clovers planted collectively. Soil samples were taken at each replication for N-incubations for determination of mineral N (NO3 - -N plus NH4 + -N) at 0-150 mm soil depth. A basic soil chemical analysis was done at 0-150 mm and 150-300 mm soil depths, respectively. Each sub-plot (replication) consisted of a 3 m2 block that was divided into a 1.5 m2 block for harvest and smaller 0.25 m2 blocks for samples that were taken at different growth stages throughout both seasons. Dry mass and nitrogen (N) content of different plant components, leaf area index, disease symptoms and pest damage were recorded from each sample. Trends in basic soil chemical properties mostly differed between crop rotation systems during different seasons while similar trends in soil mineral nitrogen occurred. Highest soil mineral N levels occurred after one or two consecutive years of pasture while levels after a lupin phase were disappointingly low in both seasons. These high soil mineral N levels showed similar trends to wheat grain quality and some wheat yields, while the most influencing factors on wheat grain yield were probably soil physical properties. Soil mineral N after canola was high during plant after which levels were much lower than many other crop rotation systems. This occurrence will probably need a re-evaluation of N fertilizing programs if the same trends are found in similar, but longer trials. Lolium spp. was the most prominent weed that occurred in both seasons at some crop rotation systems seemingly with no direct effect from crop rotation. Highest disease incidence mainly from Septoria spp. and Puccinia spp. occurred, particularly in wheat/wheat rotations, except for Puccinia which showed high ratings of disease symptoms in all crop rotations in the drier 2004 season. Lower ratings occurred in crop rotation systems when wheat was preceded by nonwheat crops. Insect pest damage showed no similar trends indicating no direct effect of crop rotation on these pests and/or effective control from applied pesticides in both seasons. It was concluded that climate was one of the most influencing factors affecting differences and seem to be the main cause for different trends found between these two seasons in similar crop rotation systems. A similar trial with longer duration than this one is thus needed to obtain conclusive trends. This also indicates the importance of integration of crop rotation and management practices that are most optimal during dry and wet seasons, thus limiting risk.