Articles
EFFECTS OF HERBICIDE STRIP WIDTH AND NITROGEN ON CROP AND FRUIT QUALITY
Control trees were maintained in overall grass; three rates of nitrogen (0, 50 and 100 kg ha-1) were applied on a split plot design.
Cropping and leaf composition were measured from 1972 until the completion of the trial in 1976. Storage quality was assessed from 1973 to 1976, with the exception of 1974 when there was a low yield.
The herbicide treatments caused an initial drop in soil pH but had little effect on the levels of available soil nutrients.
Herbicide strip treatments and applied nitrogen increased leaf nitrogen (the effect of applied nitrogen being greatest in complete grass cover) and fruit nitrogen in 1973–1976, although in 1976 the effect of strip was not significantly different from overall grass.
Both strip and applied nitrogen treatments tended to decrease both phosphorus and potassium in the leaf and phosphorus in the fruit, but increased potassium in the fruit.
Fruit set in 1973 and 1976 was related to the level of leaf nitrogen in the previous August and in general yield was increased by the strip and applied nitrogen treatments.
Fruit calcium concentrations were negatively correlated with mean fruit size and were consequently influenced by the levels of cropping since yield and mean fruit size were also negatively correlated.
In 1976, the increased fruit set in strip and applied nitrogen treatments led to smaller fruit which were high in nitrogen and calcium and developed less bitter pit and breakdown in storage at 3°C and 0°C respectively, than apples from grass and no nitrogen treatments which were also low in nitrogen and calcium.
In 1975 a negative correlation between fruit nitrogen and bitter pit incidence was independent of yield, fruit size or calcium concentration.
It is suggested that the increased availability of nitrogen enabled the trees to withstand the effects of water stress which were severe in the summer of 1975.
Although in 1973 bitter pit did not occur in fruit with calcium levels in excess of 5 mg/100 g fresh weight, in the two dry years (1975 and 1976) the threshold value appeared to be about 6.0 mg/100 g.
Increased fruit nitrogen caused by strip or applied nitrogen led to commercially significant reductions in fruit red colour.