Fertilizer International 515 Jul-Aug 2023
31 July 2023
The mineral nutrition of nut tree crops
NUTRIENT NEEDS OF NUTS
The mineral nutrition of nut tree crops
Tree nuts are an important dietary source of unsaturated fatty acids, vitamins and minerals. Orchards require balanced fertilization to promote healthy root growth, maintain tree vigour, and achieve yield targets in response to growing market demand. ICL agronomists Cristian Filote, Gali Carmi and Patricia Imas review their nutrient needs.
Nut crops are a group of speciality crops – generally trees – that produce edible seeds within a shell. These seeds are typically consumed in dry form and provide an important dietary source of unsaturated fatty acids, vitamins, and minerals such as potassium, phosphorus and magnesium. The long-term consumption of nuts, as part of a healthy human diet, contributes to a lower risk of cardiovascular disease, reductions in blood cholesterol levels, and a decline in all-cause mortality.
Production
World tree nut production has risen over the past decade to reach 5.3 million tonnes in 2022/23. The most common tree nuts in order of their production volumes are: Almonds; Walnuts; Cashew; Pistachio; Hazelnuts; Pecans; Macadamia; Pine nuts; Brazil nuts.
Tree nut plantations (orchards), whether cultivated under irrigated or rain-fed conditions, need maintaining with a specific and adequate fertilization regime. This is essential for the sustainable and economically-viable management of plantations over their long life cycle.
The United States is the world’s leading tree nut producer, with a global market share of around 36 percent. Almonds, pistachios and walnuts are the most widely grown US nuts, accounting for 59 percent, 22 percent and 15 percent, respectively, of domestic tree nut production. Turkey is the second largest world producer, being responsible for an 11 percent share of global production, with hazelnuts and pistachios accounting for 63 percent and 30 percent of Turkish output, respectively.
Nutrient requirements
Nut trees require all three macronutrients – nitrogen (N), phosphorus (P) and potassium (K) – in significant amounts. Their roles are as follows:
- Nitrogen promotes vegetative growth
- Phosphorus supports root development and fruit production
- While potassium enhances overall tree health and disease resistance.
Deficiencies in any of these major nutrients negatively affects tree physiology – resulting in visible leaf symptoms (Figure 1) – and ultimately nut yields.
Sulphur (S) is also an important nutrient: sulphur deficiency in the soil can cause slow growth rates, yellowing leaves and lower fruit yields. Deficiency is also becoming more common. Growers must therefore be aware of the need to add sulphur on a regular basis – preferably as sulphate, the plant-available form of sulphur.
Calcium (Ca) is required for the growth and function of root tips and is also a component in leaf cell walls. Magnesium (Mg) is also a vital nutrient, primarily as a component of chlorophyll. It also aids in enzyme function, protein synthesis and the formation of sugars and starches.
Nut trees also need micronutrients – including zinc (Zn), boron (B), iron (Fe), manganese (Mn), and copper (Cu). These micronutrients play vital roles in various metabolic processes and ensure proper tree growth.
CASHEW: THE INDIAN EXAMPLE
The average productivity of cashews in India is 900 kg/ha. On the subcontinent, the average macro-nutrient removal rates for mature, six-year-old, fruit-bearing cashew trees is around:
- 339 g/plant/year for nitrogen
- 55 g/plant/year for phosphorus
- 182 g/plant/year for potassium.
In Indian orchards, fertilizer use efficiency is maximised by timing fertilizer applications between September and December as this period coincides with tree flushing, early flowering and high root activity.
Ideal application timings are either at the onset of the monsoon, or immediately after the monsoon when soil moisture is still adequate. However, application during the post-monsoon period is advised when only a single application is possible, due to labour constraint or other reasons. In any case, fertilizer applications should be done after weeds and foliage have been cleared from the base of individual trees.
In the laterite soils found in the heavy rainfall areas on India’s west coast, circular trenches (25 cm deep and 15 cm wide) are opened at distance of 0.5, 0.75, 1 and 1.5 metres away from trunk during the first, second, third and fourth year after planting, respectively. In the loamy soils of the low rainfall east coast, fertiliser is instead generally applied in 50-centimetre circular strips. Trenches should be closed immediately after fertilizer application with green leaves used as mulch.
Fertilizer recommendations
In orchards, required fertilization rates can be estimated from the quantities of nutrients removed from the soil during nut harvesting, i.e. the amount of nutrients remover per one tonne of ‘in-shell’ nut yield (Tables 2-5). Efficiency coefficients (nutrient ratios) are also valuable for calculating the relative application rates of the three major nutrients. For tree nuts, N:P:K coefficients are typically 0.45:0.25:0.70.
As well as knowing the overall amount of nutrients removed per tonne of crop yield, it is also necessary to know the right application timing and nutrient requirements by phenological stage (Tables 2-5).
Leaf analysis is a reliable tool for diagnosing the nutritional status of any crop. Leaves are generally analysed from the end of July to early August in the Northern hemisphere. Nutrient deficiencies can be corrected for by comparing results to optimal values (Tables 1-5).
For nut trees grown on alkaline soils, ammonium sulphate is the preferred nitrogen fertilizer, while ammonium nitrate performs best for nitrogen applications on low-pH soils. All nut trees react positively to the incorporation of manure into the soil just before flowering (April-May in northern hemisphere). This can be sourced from husbandry animals or as green manure derived from legumes.
Pistachio’s nutrient needs
Pistachio trees are prone to alternate bearing. This is the tendency to produce a much greater-than-average crop in one year, followed by a much lower-than-average crop in the subsequent year. This drastically affects both crop productivity and fertilizer requirements.
Pistachio’s yields in an ‘off’ year, for example, are 25-33 percent lower than those in an ‘on’ year. Correspondingly, the amount of the nutrients applied prior to and during the ‘on’ year need to be greater than during the ‘off’ year. A total nitrogen application rate of 224252 kg/ha is recommended for pistachio during an ‘on’ year, for example, while during an ‘off’ year only 50 percent of that amount is typically necessary. Likewise, while a total potassium application of 118-224 kg/ha is advised for pistachio during an ‘on’ year, the application should be reduced to just 33-50 percent of this rate during an ‘off’ year.
EFFECTIVE NUT NUTRITION WITH ICL PRODUCTS
Water-soluble fertilizers
ICL has a specific range of water-soluble and liquid fertilizers designed for drip irrigation and foliar application. The company’s Novacid range is made from highly pure ingredients and all the micronutrients present are chelated. Valuably, it is a granular water-soluble solid and therefore – unlike liquid acids – is safe to handle.
Novacid products are perfect for growers needing to deal with high alkalinity in their irrigation water. Their acidic nature means that all the nutrients dissolve efficiently, even in hard water, and the drip lines are kept free of limescale build-up.
Previously, research trials have proven the effectiveness of foliar feeding in the fertilization of almonds and other nuts. ICL’s innovative foliar technologies, such as Nutrivant and Agroleaf, are particularly well suited to tree nuts.
The Nutrivant foliar product line combines crop-specific nutrient mixes with a proprietary adjuvant (Fertivant) that can break through the leaf cuticle. The innovative Agroleaf foliar product range is designed for both the correction and prevention of nutrient deficiencies. Agroleaf, developed by ICL researchers, is equipped with two proprietary technologies: M-77 for chelating micronutrients and DPI (Double Power Impact), a plant stimulant that optimises nutrient uptake through the foliage.
Controlled-release fertilizers (CRFs)
ICL’s Agroblen CRF range is ideal for establishing nut plantations (Table 6, top). These fully coated NPK granules deliver crop nutrients over a period of 8-9 months or 12-14 months, depending on the region where the plantations are being established.
By placing only a few grams directly into the planting holes, Agroblen supplies nutrients in a controlled manner and according to plant needs. Because of this, NPK losses are significantly reduced, thereby ensuring more nutrients are available for plant uptake. The controlled release of nutrients over time also eliminates the labour, equipment and material costs associated with the multiple application of conventional granular fertilizers.
ICL also offers Agromaster – a CRF product line that combines both coated and uncoated NPKs – for young and fruit-bearing plantations (Table 6, bottom). These powerful fertilizers partly deliver nutrients in an uncoated form for immediate take-up by plants to stimulate intensive growth. At the same time, they also provide NPKs in coated form. This prevents nutrient losses via run-off, leaching from light soils and/or losses under heavy rainfall conditions.
The use of Agromaster products allows fertilizer rates to be reduced without affecting crop yields. They are also versatile and, by creating a buffer supply of nutrients in the soil, especially valuable in circumstances where fertigation is not possible.
Nonetheless, the perfect fertilization plan is always based on fertigation with water-soluble fertilizers, as this precisely supplies nutrients to nut trees according to both the age of the plantation and the tree’s phenological stage.
Polysulphate
Polysulphate® is a unique multi-nutrient fertilizer rich in four key plant nutrients: sulphur, potassium, magnesium and calcium. It contains the natural mineral polyhalite and is exclusively mined in the UK by ICL.
Polysulphate is provided to growers in its natural state, with no industrial processing, being delivered directly from mine to field. As a natural mineral fertilizer, it is widely approved for use in organic agriculture – holding organic certification in many countries internationally.
Polysulphate is a low environmental impact fertilizer with the lowest carbon footprint on the market, when compared to competitor products. Its use therefore helps farmers to lower greenhouse gas emissions (GHGs) and manage their carbon targets.
Polysulphate applications supply nutrients in a balanced way that nurtures nut trees and maintains soil fertility. Research shows that nutrients (S, K, Mg and Ca) are gradually made available to the crop over time due to Polysulphate’s mineral composition. The product’s gradual and prolonged nutrient release profile maximises nut yields by promoting healthy tree growth and development.
Polysulphate also provides a low-chloride source of potassium, as well as being a reliable source of sulphur. Trials conducted on Californian almond trees in the US have demonstrated that applying Polysulphate can deliver a 5-11 percent increase in kernel yield in comparison to growers’ standard practice.
