The Effects of Tillage, Non-Tillage and Nitrogen on Yield and Fruit Composition of Citrus

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Abstract
Over the period 1947 to 1965 studies were made of the effects of four cultural treatments and four levels of ammonium sulphate on crop growth and fruit composition of Washington Navel and Late Valencia oranges, budded on to either Rough Lemon or Sweet Orange rootstocks. In terms of yield and quality, the best treatment combination was weed control by kerosene oil sprays together with 4 lb. of ammonium sulphate per tree per annum. For this treatment combination the average annual yield over the five-year period 1961–65 approached 500 lb. per tree. Almost all the nitrogen requirements of mature citrus trees were obtained from clover grown during March to September and then rotavated into the soil. Repeated ammonium sulphate applications caused soil acidity, aggravating an inherent soil phosphorus deficiency. Basal superphosphate applications, made since 1955 at an average annual rate of 4 lb. per tree, gradually improved fruit quality. From 1955 to 1965 small annual calcium carbonate applications of up to 30 lb. per tree were made to half the trial area. Although these applications only partially neutralized soil acidity there was a consistent tendency since 1961 for yield and fruit quality to be higher on limed plots. As there was no evidence of calcium deficiency, it is considered that liming treatments stimulated crop growth mainly by raising soil pH and increasing soil phosphorus availability. Soil analytical results showed that considerable differences existed between the fertility status of inter-tree and under-branch-spread areas: base exchange capacities and soil pH were considerably higher under the leaf canopy than in inter-tree areas. A study of fruit quality indices, summarized for four periods each of four years duration, suggests that the juice content of an orange is a more sensitive index of fruit quality than skin thickness or sugar and acid contents.

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