Effects of the amount and timing of fertilizer application on tree growth, yield and fruit quality in an extremely early maturing cultivar of Satsuma mandarin [Citrus unshiu], 'Hiroshimakaken 7gou'

The effects of the amount and timing of fertilizer application were investigated from 2003 to 2005 using young trees of an extremely early maturing cultivar of Satsuma mandarin (Citrus unshiu Marc.), 'Hiroshimakaken 7gou'. This variety was grown using reflective sheet mulch from 70 days after full bloom to harvest, and we measured the nitrate level in the soil, leaf nitrogen concentration in spring shoots, canopy volume, yield and fruit quality. The concentration of nitrate ions in the soil remained low year round in plots receiving split applications of fertilizer, in spring and autumn, in comparison to that in plots receiving a single large quantity of fertilizer in autumn. In addition, leaf nitrogen concentration from a split application at the end of February and at harvest (harvest being the end of September or the beginning of October) was lower than with a single autumn application, and both fruit sugar content and the ratio of good fruit with high coloration increased. The rind was redder and the percentage of pulp also tended to increase. On the other hand, when the quantity of fertilizer applied was increased, the leaf nitrogen concentration at harvest rose, and a tendency was noted for less alternate bearing. In addition, the yield was greater in plots that had fertilizer application of normal quantity or of 130 % of normal quantity than in plots receiving 70 % of normal quantity. In plots where fertilizer application was 130 % of normal, both sugar and acidity tended to be slightly higher than where fertilizer use was 70% of normal. From the results obtained, the best way to hasten the development of color in 'Hiroshimakaken 7gou', and so to market fruit with a high sugar content early in September when the price is high, is to follow the standard fertilizer recommendation for Hiroshima Prefecture, and apply the standard quantity in both spring and autumn.