Potassium has a direct effect on flower number and consequent fruit set. High levels of potassium of 3- 4 kg K per tonne of fruit - are required for strong crop growth.
Compared to other nutrients (e.g. nitrogen), potassium is hardly diluted in the fruit during ripening, so supplies need to be maintained throughout maturity as the fruit changes colour. Peak uptake is between fruit formation until the end of harvest.
A good supply of boron is important for root growth, pollen germination, seed formation and fruit set. Boron also has a direct effect on membrane integrity, maintaining cell wall structure.
Trials show a clear benefit from boron in terms of fruit set, fruit number and fruit weight, leading to improved yields. This may in part be due to boron’s influence on pollen formation and better pollination.
Typical requirements are around 5 g/t of fruit (figure above on the left). Uptake increases during flowering and leads to good accumulation in the fruit. The B requirement varies between varieties and this needs to be taken into account when devising nutrient programmes.
Applications of zinc can increase fruit numbers per plant leading to higher yields, but too much zinc leads to fruit deformation.
Zinc is found in relatively high quantities in the flowers and fruits compared to other micronutrients. Over 30% of all zinc is contained in the fruit and 2.5 g/t is taken up by the plant to produce 1 ton of fruit.
Zn deficiency usually occurs at leaf contents below 10-20 ppm Zn. Too much zinc - at leaf concentrations above 50 ppm – results in toxicity. Deficiency problems are worse on high pH soils (>pH 7.0) and exacerbated in highly or recently limed soils. When insufficient zinc is available, foliar zinc can help increase yield.