Fertilizer International 515 Jul-Aug 2023
31 July 2023
Tree nuts: the importance of calcium and sulphur
NUTRIENT NEEDS OF NUTS
Tree nuts: the importance of calcium and sulphur
Tessenderlo Kerley International recently published a tree nut crop brochure. This article draws on the brochure to highlight the importance of two essential nutrients, calcium and sulphur, as part of a balanced fertilization programme for nut crops.
The most common nuts produced worldwide include almond, walnut, pistachio and hazelnut. These are cultivated under either irrigated or rainfed (often water stressed) areas and, consequently, large variations in nut yield occur1 .
Yield is also affected by differences in crop practices, particularly the fertilization regime. A common denominator across all nut crops is therefore the importance of balanced fertilization. Indeed, a balanced fertilization programme is the prerequisite for healthy root growth, the maintenance of tree vigour, as well as the crop productivity needed to achieve target yields and meet growing market demand1 .
This article highlights the role of calcium and sulphur in the fertilization of nut crops – and the benefits of applying these nutrients together within a single product. A more comprehensive overview of nut cop fertilization is provided in Tessenderlo Kerley International’s crop brochure for nuts1 .
Calcium and sulphur – two essential nutrients
Fruit and nut trees thrive on calcium (Ca) as it improves both leaf and fruit quality. This essential macronutrient is involved in many plant processes, including cell elongation, cell division, germination, pollen growth and senescence. Adequate leaf calcium content ranges from two percent in almond to four percent in pistachio1 .
Calcium has an impact on the interior quality of nuts. Calcium fertilization in deficient soils is a necessity – this being very important in acidic soils. Calcium’s immobility in the plant also requires periodic supplementation to maintain adequate levels in the upper canopy1 .
Sulphur (S) is widely recognised as the fourth major plant nutrient. It is essential for promoting the healthy tree growth needed to deliver high nut yields. The uptake and availability of sulphur is less sensitive to soil pH and this nutrient can therefore usually be taken up readily across a range of orchard soil conditions1
Sulphur fertilization is a beneficial way of enhancing the uptake and use efficiency of nitrogen, phosphorus, potassium and zinc – due to sulphur’s synergistic relationships with all these four nutrients. Adequate leaf sulphur content for nuts varies from around 0.2 percent in walnut to 0.6 percent in almond1 .
Within the tree, sulphur is incorporated into certain amino acids (cysteine, methionine) and subsequently becomes part of vitamins, oils, and lipids such as sulpholipids. Deficiency symptoms generally occur in the younger plant tissues first. That is because, once locked within these complex molecules, sulphur is not easily mobilised within the plant1 .
Early calcium application essential
“While the total calcium content of soils often appears to be satisfactory, the level of plant-available calcium present in the soil solution can often be below the threshold necessary for adequate crop supply. The calcium demand from nut trees can be surprisingly high too. A walnut orchard in France, for example, will export between 25-35 kg/ha of CaO through harvested nuts at a yield of 3-4 t/ha,” comments Régis Muteau, lead agronomy manager at Tessenderlo Kerley International.
For walnut, as in many trees, calcium is essential for the regeneration of rootlets and secondary roots after the winter period. Somewhat paradoxically, it is during this largely dormant period – before vegetative growth begins – that calcium deficiency can be the most detrimental. This is despite the fact that the early growth stage at the start of the season is when the calcium needs of walnut trees are at their highest, this stage being responsible for 30 percent of total uptake.
“Calcium inputs (7-10 kg/ha of CaO) at the very beginning of the growth cycle, even in the few weeks before bud break, have a strong influence on the annual rooting of the walnut tree – and therefore on the uptake of all nutrients provided by soil-applied fertilizers from the very start of the season,” comments Régis Muteau.
Ultimately, these early additions of calcium will also have positive repercussions for walnut size (caliber) at harvest. This effect was verified by Tessenderlo Kerley International in crop trials that applied calcium thiosulfate on Fernor variety walnuts in the Dordogne and Lot regions of France in 2020 and 2021. The increase in walnut size from applications of CaTs® to the soil (150 l/ ha) resulted in a 26 percent yield surplus.
“Remember too that liquid calcium is also a beneficial soil flocculant. This means that early application of a liquid calcium fertilizer such as CaTs® will improve soil structure very quickly,” comments Régis Muteau.
Dual benefits delivering sulphur with calcium
The multiple effects of a liquid fertilizer such as CaTs® on nut yield – and on the size of the nuts in particular – is explained by the fact that sulphur in the form of thiosulphate (S2 O3 ) is supplied alongside calcium.
The presence of sulphur in this reducing form improves nitrogen use efficiency (NUE). This is a fundamental to the effectiveness of calcium thiosulphate. This is linked to the well-known fact that sulphur and calcium are key enablers of the nitrate reductase reaction. This allows nitrates to be metabolised by plants into amino acids and proteins. A liquid calcium and sulphur fertilizer such as CaTs® , by providing these two nutrients together, therefore significantly increases the efficiency of nitrogen fertilizer applications too.
“Finally, it should also be noted that nuts, such as hazelnuts and almonds, are fruits that typically require sulphur in large amounts. This is because their kernels are very rich in sulpholipids. These are essential for walnut oil production, for example, and it is estimated that the sulphur input of 80 kg/ha (SO3 ) will have to be supplied to walnut orchards to cover the export in nuts at harvest for a yield of 3.5-4 t/ha,” sums up Régis Muteau.
Calcium thiosulfate
Calcium thiosulphate products such as CaTs® , due to the positive rooting effects of calcium on orchard trees, coupled with the benefical presence of sulphur, are leading liquid fertilizers for the cultivation of nut crops such as walnut. Because of their ability to improve nutrient use efficiency (NUE), they also contribute to the goal of reducing nitrogen applications in agricultural – especially nitrate use. Overall, calcium thiosulphate, as a precision liquid fertilizer, can improve both the yield and quality of nut kernels while in parallel helping deliver improvements to soil health. Its flocculating behaviour is particularly beneficial to soil structure.
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