Apple quality as related to nitrogen and phosphorus nutrition

Starking apple trees on M793 rootstocks were grown in a low phosphorus (P) soil treated prior to planting in 1982 with different rates of P (as superphosphate) and, commencing three years after planting, nitrogen (N) as limestone ammonium nitrate (LAN). The object was to improve vigour, yields, and fruit quality. Trunk circumferences increased progressively with superphosphate application rates up to 4,730 kg/ha. Susceptibility to bitter pit also increased as did fruit size, and leaf P and magnesium (Mg) levels. Some improvements in colour were also apparent. Yields increased at superphosphate applications up to 2,240 kg/ha, but at 4,730 kg/ha were little different to the yields at 2,240 kg/ha. Leaf potassium (K) tended to decrease with increasing rate of applied superphosphate. The levels of Bray 2-extractable P in the soil decreased with time. Reapplying superphosphate to the 0, 1, 180, and 2,420 kg/ha treatments at the rate of 1,286 kg/ha in 1989 resulted in some of the differences between the initially applied superphosphate treatments becoming non-significant. No interaction between N and P was observed, and there were no consistent, significant differences between treatments which received 110 kg LAN/ha at full bloom, 50 kg LAN/ha 40 d after full bloom, and 110 kg LAN/ha post harvest and treatments which received LAN at double these rates. In the 1989/90, 1990/91 and 1991/92 seasons, the LAN application rates in the low and high N treatments were reduced to 80, 40, 80, and 160, 80, 160 kg/ha at full bloom, 40 d after full bloom and post harvest, respectively. In 1991 and 1992, leaf and fruit N contents were significantly lower in the 80, 40, and 80 kg LAN/ha treatments than in the 160, 80, and 160 treatments. The incidence of bitter pit was lowest in the higher N treatments. Under South African orchard conditions, Starking apple trees appear to require a Bray 2-extractable soil P level of 30-40 mg P/kg. Applications of LAN equivalent to 160 kg/ha at full bloom, 80 kg/ha 40 d after full bloom, and 160 kg/ha after harvest would appear to be sufficient for a 25 t crop.