The Residual Effects of Nitrogen,Calcium and Soil Management Treatments on Yield,Fruit Size and Composition of Citrus

A long-term field experiment testing the effects of 4 soil management treatments and 4 levels of ammonium sulphate on Washington Navel and Late Valencia oranges, budded on rough lemon or sweet orange rootstocks, was terminated in 1965. At this stage the difference in crop yield between the worst and best treatments was about 150%; average annual yields for the 4-year period 1962–65 varied from 18.3 to 46.2 t/ha according to treatment. After 1965 remedial N and Ca treatments were applied to restore the experimental area to a uniformly high level of productivity.

For the 2-year period 1968–69, average annual yields from plots previously receiving the 4 ammonium sulphate treatments varied from 39.1 to 44.6 t/ha; uniformity in terms of yield had thus largely been achieved. Slightly less success, however, was achieved in terms of fruit quality.

Lime, applied to counteract soil acidity induced by the previous ammonium sulphate treatments, produced greater responses on tilled plots than on non-tilled plots. The smaller response to lime on non-tilled plots was largely due to citrus roots being close to the surface and able to absorb applied fertilizer P before fixation could occur.

Some trees, receiving a bare-surface chemical weed control treatment and no N since 1947, were in severe decline in 1965 ; however, they were restored rapidly to full vigour by a midwinter application of 255 kg N/ha in 1966. Furthermore, these trees produced high yields in 1968 when the rest of the orchard was in an ‘off’ year, thereby suggesting that suitable annual adjustments to levels of applied N could be a means of limiting or controlling biennial bearing.

Soil management treatments had no significant effect on the size distribution of the fruit, but the nil and highest N treatments, applied until 1965, had an adverse effect on the size grade (63–75 mm diam.) preferred commercially.