Kiwifruit Nutrition diagnosis of nutritional disorders
Kiwifruit are relatively tolerant of excess phosphorus in the soil. To date phosphorus toxicity has not been observed in the field despite the very high phosphorus status of some New Zealand soils. For example, vines have been grown in soils with MAF quicktest phosphorus values in excess of 500 ppm; average soil phosphorus values in kiwifruit orchards usually range from 20 to 30 (Table 7).
In solution culture experiments symptoms of phosphorus toxicity did not appear despite applying phosphorus at concentrations which were 100 times that which produced maximum growth. In the field the concentration of phosphorus in the leaves appears to be largely independent of the phosphorus status of the soil. Phosphorus concentrations in the leaves of healthy plants sampled at mid season vary only over a very narrow range of between 0.18 and 0.22 per cent of the dry matter. It should be noted that it is usual for phosphorus concentrations to exceed 1 per cent of the dry matter of the leaves of healthy plants shortly after bud break, the high concentrations resulting from a release of phosphorus stored in the plant from the previous season rather than from recent additions of phosphate fertiliser. These high phosphorus concentrations at this early stage of growth do not appear to be damaging to the plant. As the season progresses these high phosphorus concentrations in the leaf decline rapidly so that by December they have reached a minimum. They then remain relatively constant for the rest of the growing season.
