Fertilizer International 503 Jul-Aug 2021
31 July 2021
Potassium nitrate: the more productive option
NITRATE FERTILIZERS
Potassium nitrate: the more productive option
Potassium nitrate is an efficient speciality fertilizer, being able to produce more for less. Whether applied to soils, via fertigation or through foliar application, it improves water use efficiency while boosting the uptake of potassium and other nutrients. Katja Hora and Harmen Tjalling Holwerda of SQM International highlight the main advantages of supplying potassium nitrate to crops.
To meet the needs of future generations, we need to produce more food with less inputs. This will require higher yields of staple food crops. These can be secured in two main ways:
- Firstly, by prevention of plant nutrient deficiencies, particularly in the critical stages of crop growth.
- Secondly, by preventing excess application of nutrients which are not needed by the crop and can cause soil salinity.
Additionally, the logistical disruption caused by the Covid-19 pandemic has illustrated the advantages of more local food production.
The application of fertilizers can also contribute more to human health. Fresh foods such as fruits and vegetables need to supply sufficient mineral nutrients – essential components of our daily diet – as do grains biofortified with micronutrients.
Proven crop benefits
Rice and potatoes are two examples of staple food crops that benefit from the use of potassium nitrate 1 . It is easy for farmers to make timely fertilizer applications with prilled potassium nitrate (Qrop KS). ® Applications are especially valuable during crop stages when the availability of potassium and/or nitrogen in nitrate form is critical to achieving the highest yields – such as during the booting of rice or tuber-bulking in potatoes, for example.
For rice, on-farm field trials in Ecuador, Mexico and China have demonstrated how the use of Qrop KS ® – as a replacement for potassium chloride (MOP) and urea – can increase production when applied at exactly the right moment in the crop cycle.
The need for potassium in potato crops is highest during the tuber-bulking stage. Potassium nitrate can be applied as Qrop KS ® prills to soil or as water-soluble Ultra-sol K Plus ® via pivot irrigation. This supplies potassium and nitrate nitrogen at those moments in the growing cycle when the crop benefits the most from their direct uptake.
Scientific trials in the US and Belgium, and on-farm field trials in Brazil, have all shown that the use of potassium nitrate improves early potato crop development and increases marketable tuber yield, in comparison with a schedule based on potassium sulphate (SOP) or MOP.
Late season application of potassium nitrate, besides providing readily-available potassium, also improves the storage quality of potato tubers. This is linked to the uptake of exchangeable calcium and magnesium (Ca2+ and Mg2+ ) from soils which is boosted in the presence of the nitrate anion (NO3– )2 .
Water use efficiency
Horticultural growers who supply crop nutrients via irrigation systems (fertigation) will select potassium nitrate as the preferred source of potassium and nitrogen, usually because it is 100 percent composed of the two nutrients that crops need most. Application of potassium nitrate ensures that potassium and nitrate nitrogen are directly available to the plant from the nutrient solution surrounding the roots.
In areas where good quality irrigation water is becoming scarce or expensive, saving water for crop production has become an important issue – both for the environment and for the commercial viability of agriculture. Water use efficiency shows how much of the agricultural water supplied is actually taken up by crops. This indicator can be improved by measures such as the recirculation of irrigation water, precision irrigation and avoiding excess irrigation to wash out soil salts.
The minimum water needs of crops are influenced by how much water is lost from the plant in the form of water vapour. This is known as water transpiration. Water needs are also directly related to the capacity of the plant to convert carbon dioxide to sugars, for a given amount of water loss by transpiration. This is usually referred to as intrinsic water use efficiency. Plants produce less sugar when transpiration is inhibited by water scarcity. Conversely, too much transpiration, without matching CO2 conversion, may lead to loss of water without increasing yield.
Nutrients such as potassium and calcium are known to act as an important control on intrinsic water use efficiency. This is because they play a role in the opening and closing of stomata and – when deficient – may lower the intrinsic water use efficiency of the plant.
The type of nitrogen present in the root solution is also important, as a high ammonium concentration, with correspondingly low levels of nitrate, can lower water use efficiency. Recent plant physiology studies have demonstrated that a high NO3 :NH4 ratio improves water use efficiency, confirming earlier empirical studies3 .
Boosting nutrient uptake
The improvement to water use efficiency that results when NO3 – is supplied as the main nitrogen form to fertigated crops has several explanations. Overall, nitrate appears to improve plant growth and yield by boosting the uptake of cations (K+ , Ca2+ , Mg2+ ) while using the same amount of water. The presence of NO3 – has also been shown to drive up water uptake in the root zone4 . In contrast, the lower uptake of cations in root solutions, observed when NH4 + is the main nitrogen source, has been found to be detrimental to plant growth5 .
“Potassium nitrate can successfully supply potassium to crops via foliar application… preventing lodging and other deficiency symptoms.”
The foliar advantage
Reduced water availability has a direct impact on uptake of nutrients by the roots, and can be hampered by:
- Drought
- Saline soil conditions
- Reduced root system activity due to senescence linked to the ripening of seeds.
The absence of potassium is most likely to lead to yield loss if it is deficient during the generative stage of the growth cycle. This is when the crop starts flowering or filling its fruits, tubers or seeds. However, foliar applications to leaves during these critical crop stages is an effective tool for supplying plants with nutrients when they are unable to access these from the soil.
Potassium nitrate can successfully supply potassium to crops via foliar application. Several applications during the growing season improve potassium status and helps crops achieve their full yield potential. SQM’s Speedfol® formulations – which are potassium nitrate based – have been shown to increase crop yields in many experimental and field trials. These have demonstrated its value in preventing lodging and other symptoms linked to potassium deficiency.
Potassium nitrate also acts in synergy with other plant nutrients by boosting the uptake of micronutrients such as zinc or iodine, for example. It can therefore be added to any foliar application as a routine booster to solve micronutrient deficiencies in crops.
In a study for the former Potassium Nitrate Association, the plant uptake and translocation of foliar-applied nutrients – in sulphate, nitrate-based and chelated forms – was promoted by the addition of potassium nitrate to the spray tank solution. As well as boosting nutrient uptake from the surface of leaves into the leaf tissue, potassium nitrate also aided nutrient translocation via the phloem to newly developing organs and seeds.
Iodine is an essential element for human health. The co-application of iodine with potassium nitrate was examined as part of a wheat biofortification study. This showed that their joint application markedly increased the uptake of iodine to new leaves and seeds6.
Authors’ note
Information cited in this article on potato and rice crop nutrition is sourced from an extensive library of free-to-access agronomic articles and advice available on SQM’s website. Video resources are also freely available via the company’s online Specialty Plant Nutrition (SPN) Academy.
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