The effects of time and level of application of phosphoric acid and potassium on yield and quality of grapes

Abstract

1. In sand culture of 3-year-old Delaware grape vines supplied with proper level of nitrogen (80 ppm), the effects of varying levels of phosphoric acid (80, 40, 20 and 0 ppm) and potassium (160, 80, 40 and 0 ppm) on yield and quality of fruit were observed. The treatments were applied at two stages of shoot growth (from early April to late June) and fruit growth (from early July to late August).
2. A high level of phosphoric acid (80 ppm) when supplied at the stage of shoot growth, gave greatly improved yield and better quality of fruit, by increasing the degree of color and sugar content, and by lowering the acidity. The same application made at the stage of fruit growth, gave an increase in yield, but a reduction in the degree of color and sugar content, and no striking effect on the acidity.
3. On the contrary, high levels of potassium (160 ppm and 80 ppm) applied at the stage of shoot growth produced no significant influence on the yield and quality of fruit except for an extreme increase of acidity. But, when applied at the stage of fruit growth, high levels of potassium improved both the yield and degree of color.
4. Moreover, in the potassium treatments, when the number of berries per vine in each plot was restricted equally at the beginning of fruit growth stage, the high level plots gave better quality of fruit including the color, sugar content, and acidity than the low level plots.
5. Thus, this experiment seems to show that phosphoric acid needs to be applied more abundantly at the stage of shoot growth than at the stage of fruit growth, while for potassium the reverse is true, that is, smaller applications are needed at the stage of shoot growth and larger ones at the stage of fruit growth.
6. Besides, it was observed that the higher the level of phosphoric acid given at the stage of shoot growth, the greater the number of flowers per vine, and the number of seeds per berry. The higher the level of potassium supplied at the stage of shoot growth, the greater the amount of potassium and the lower the quantity of both nitrogen and phosphoric acid contained in the leaves at the end of the stage, resulting in the shorter shoot elongation.
7. And the leaf analysis made at harvesting time, indicated that the higher the level of phosphoric acid or potassium given at any stage of shoot or fruit growth, the greater the content of those elements in the leaves. This, however, showed no distinct antagonism against the other elements, probably due to the difference in fruit load in the plots.