…with the use of potassium nitrate (Ultrasol K® Plus*)

On studying leaf analyses, one will find that the greatest amounts of nutrients, in percentage of dry matter, are calcium, followed by nitrogen and potassium. Nutrient-removal studies in harvested citrus fruit show that potassium is removed in the greatest quantity, followed by nitrogen and to a lesser extent calcium. These three elements are thus required in the greatest quantities, either supplied by the soil or by fertilizer applications. Sufficiency will be indicated annually in the leaf analyses.

It is important to take note that even though a leaf level may be considered optimal, one needs to carefully consider the plant and fruit removal quantities on an annual basis. In 2008, in a time of economic crisis and very expensive fertilizer, many growers cut down on potassium applications because the leaf analysis was “normal”. The following year was heralded by dramatic decreases in leaf potassium levels with many dipping into low and critical levels, quite possibly lowering yield and quality levels that year (The Co-op – personal communication). Cutting down on nutrient elements can only be considered if soil and/or leaf analyses show excesses.

What growers strive to achieve are yield increases from more and larger marketable product per unit area, quality improvements (whether it be increased brix, specific gravity, TSS, acidity, vitamin C, less peel defects as well as improved colour), and by reducing crop losses by increasing disease resistance. Potassium nitrate has been shown to improve all the above-mentioned parameters**.

Fertigation programs including potassium nitrate

It has been well documented in many crops and many soil conditions that the combination of a positive potassium cation (K+) wit®®Using potassium nitrate to maximize the supply of negatively charged nitrates is thus the ideal way of optimizing K+ uptake, N uptake and the uptake of the other cations, Ca2+ and Mg2+, while at the same time, nitrate (NO3-) limits the uptake of chlorides.

Excess chlorides have been shown to be detrimental to quality and yield.

Furthermore, excessive ammonium (NH4+) nutrition is well documented to induce calcium deficiencies due to cation antagonism (positive charge competition) in many crops, even though calcium is supplied in adequate quantities.

Maximizing nitrate-N (NO3-) maintains an optimal pH close to the roots (rhizosphere pH). If the plant is fed ammonium-N (NH4+), along with all the other cations, then there is an excess of H+ given off by the plant that causes the rhizosphere pH to drop by as much as 1.5 units (between 10 and 100x more acidic). This acidity can be toxic to the roots per se and induce pH-dependent nutrient deficiencies.

Foliar applications with potassium nitrate alone or in combination

Foliar applications are required in times of peak nutrient demand, poor rooting conditions, high salinity, cold temperatures, waterlogging, drought, incorrect fertilizer applications and where root diseases are prevalent.

Periods of rapid fruit/crop filling are periods of peak nutrient demand. Fruit set, or prevention of fruit drop, is the greatest factor influencing yield. Flowering and fruit set are periods of high nutrient demand and often fruit set occurs when soil temperatures are low. Foliar potassium nitrate applications in combination with auxins in Nova, Valencia and Shamouti have been shown to improve fruit set and soluble solids, increase fruit size, weight, yield and decrease split fruit. In Nules and in Valencias, potassium nitrate applications have shown increased fruit set, increased TSS, increased fruit yield and gross income.

Potassium nitrate foliar applications have been shown to increase the resistance of a plant towards diseases (systemic acquired resistance (SAR)). This occurs on both the metabolic level (phenols, auxins, phytoalexins and short chain carbohydrates) as well as on a physical level (firmer cell walls and faster recovery). According to the International Potassium Institute, Perrenoud (1990) extensively reviewed the effect of potassium applications on disease severity in crops. With bacterial infections, potassium application decreased severity in 99 out of 144 cases with 14 unchanged. For fungal infections, potassium application decreased disease severity in 1080 of 1549 cases with 112 cases unchanged.

More specifically, the Haunglongbing (HLB) bacterial disease has been found in most citrus-growing areas of the world. The disease is severe, resulting in bitter, inedible, misshapen fruit. The trees show severe deficiency symptoms often leading to tree death in severe cases. The vector is the Asian citrus psyllid. The bacteria, introduced by the psyllid, cause blockages in the phloem transport system affecting nutrient and carbohydrate distribution within the plant. Aggressive psyllid control and balanced nutrition are ways to control the disease.

Additionally, in a five-year study (2008-2012) by the University of Florida, a heavily HBL infected citrus orchard was treated (foliar) with various nutrient cocktails containing potassium nitrate (Maury Boyd cocktails) and other compounds known to promote SAR in plants, such as phosphorous acid, salicylic acid and/or the Bacillus subtilis antibiotic. All the treatments containing potassium nitrate doubled the citrus yield; the most cost-effective in terms of return on investment was the stand-alone potassium nitrate treatment. The yield improvements were not ascribed to any bactericide properties of potassium nitrate but to its ability to enable a plant to regenerate new phloem vessels to compensate for those blocked (infected) by the HBL bacteria.

SAR in citrus has also been reported against the Florida wax scale with potassium nitrate foliar applications. With soil applications, SAR against phytophthora spp. has been reported by applying nitrogen in the nitrate form. Nitrate-N can be optimized by using potassium nitrate along with other nitrate-containing fertilizers.

Potassium nitrate is the preferred N and K source in horticultural crop nutrition often resulting in: optimum K and N uptake, stronger healthier plants, better set and fruit yields, increased fruit/crop size, better colour intensity, increased disease resistance and enhanced stress tolerance such as drought, frost and salinity. For both fertigation and foliar applications, consult your agronomist regarding rates and timing of applications. n

* Reg No K5020 Act 36/1947. Registration holder Sociedad Quimicay Minera (Africa)(Pty)Ltd

** All claims in this article can be substantiated with references which can be made available on request.

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