Nitrogen, calcium and potassium are required in greatest quantities by stone fruit crops. Large amounts of calcium are needed within the tree to support crop production. Most of this is taken up during early periods of growth. As a result, annual crop removal is around 0.25kg/t of fruit, with sour cherries having a greater need for calcium than other types of stone fruit.
Nitrogen is critical to maximize early season tree, leaf and fruit growth, and stone fruit orchards contain over 200g of nitrogen/tree. Removal in stone fruit is higher than that of calcium with up to 1.5kg of N per tonne of fruit lost annually. Common practice is to apply nitrogen at two main stages of growth. The first N application is made during the spring at flowering to maximize fruit set and fruit and leaf growth in the summer. The second, is in the autumn after harvest, to maximize reserves in the tree.
Maximum potassium uptake is later than that of nitrogen and calcium, peaking during the fruit filling stage. It is a key driver for fruit yield and is critical for fruit quality – particularly sweetness.
Apricots are particularly responsive requiring nearly 3kg/t - around 1kg/t more potassium than cherries or plums. Relatively low levels of phosphorus are used by stone fruit crops and uptake occurs steadily throughout the season. Soil supplies can commonly meet needs, however during peak periods of demand, fertigation, foliar, or fruit applications may be necessary. Magnesium and sulfur are important to maintain good growth, but removals are low at less than 0.2kg/t of fruit.
While much lower levels of micronutrients are needed to satisfy yield and quality fruit production, the correct balance of these nutrients is essential. The key micronutrients needed in greatest quantities are iron, boron, zinc and manganese.
Boron and zinc are particularly important for flowering and fruit set. Copper, while needed in smaller quantities, is important for skin quality, minimizing fruit cracking.
Soil testing can be used to estimate the supply of nutrients available in the soil to support growth. It is particularly important for assessing soil phosphate and potassium levels, as well as pH.
Leaf tissue analysis is now more commonly used to provide a snapshot of nutrient status at specific stages of growth. Nutrient levels will vary according to the ability of the tree to recycle or remobilize nutrients during the annual growth cycle. As a consequence of this seasonal variation, sampling is normally carried out at standard times. Traditionally this is during mid to late summer.
Whenever analysis is made, leaf samples must always be taken from the same section of the tree - normally the midshoot position of the current season’s growth – so as to ensure consistency in sampling across the orchard and to test new tissue.