The importance of quality for water soluble SOP
Given that some water-soluble fertilizers (WSF) are traded like commodities, it would be an easy mistake to assume that all SOP products are the same. However, given the variation in production processes, the quality of end product can vary dramatically, and this is an important consideration for end-users, particularly those running the product through a drip irrigation system.
So how can you start to distinguish between the products? When given a sample of product there is an easy test to check the level of insoluble material and the speed of dissolution. The presence of insolubles is highly undesirable and can lead to clogging of both the filters and the drippers. Such clogging can have economic, in the sense of time consuming and costly cleaning operations, as well as agronomic consequences. An acceptable level of insoluble is generally considered to be 0.05% or less. SoluPotasse, the class leading water-soluble SOP produced by Tessenderlo Kerley, has an average score of 0.02%, according to the company’s data from 2020.
In the company’s monitoring of other water-soluble SOP products available in the market, almost 50% of analysed samples were above the 0.05% limit and 12% were even above 1%.
SoluPotasse (left) dissolves rapidly and completely unlike poor quality soluble SOP (right)
Photo: Tessenderlo Kerley
The concentration of nutrients in a product directly benefits the grower since it means there are more nutrients per kilogram of product, which ultimately means the grower needs to handle fewer tonnes, saving time and logistics. In SOP, growers are of course looking for high concentrations of K2O and SO4. There is also variability in the market among soluble SOP products, according to Tessenderlo. The company says that it has identified many examples where the level of K20 is less than 51%. SoluPotasse samples achieved a score in 2020 on average of 51.6% and never lower than 51.5%.
One of the agronomic benefits that growers are seeking when choosing SOP is low chloride content for crops that are chloride intolerant and also in conditions where the use of chloride containing fertilizers may contribute to salinity problems. And it is often the high-value cash crops are chloride intolerant. But in order to get the best crop quality, growers need to be extra careful about the chloride content of the WSF. The chloride (Cl-) content of products can also vary, like the solubility. In research conducted by Tessenderlo Kerley, only around half the samples taken from the market and analysed had a chloride content below 0.6%.
There were even some samples (almost one quarter of those analysed) with chloride content above 1%, a level considered by many as too high for a water-soluble form of SOP. SoluPotasse averaged 0.35% in 2020 and never scored above 0.5%, according to the company. The sodium (Na+) level, also a good indicator of purity, typically varies between 0-2% for products found in the market. For SoluPotasse, the sodium level in 2018 never exceeded 0.4% with an average of 0.35%. A higher content of chloride or sodium will lead to a difference in performance.
An acidic solution can be positive since it will help prevent the build-up of deposits in the irrigation system. This can help to minimize costly maintenance downtime.
The acidic solution will also regulate the pH level in the rhizosphere. This can help to optimize the uptake of the full range of plant nutrients. Different nutrients have different pH windows for their optimum uptake.
The speed at which the product dissolves in water is a key metric for the usability of a product. From a practical point of view, a faster dissolution speed saves the grower time and money. In the company’s research, Tessenderlo Kerley found that some forms of SOP claiming to be soluble did not dissolve to more than 90%, even after 10 minutes of stirring. Tessenderlo Kerley’s definition for a quality product is one that is 90% dissolved after 3 minutes of stirring.
An important parameter when it comes to application rates is maximum solubility. The quantity of SOP that can be dissolved in a certain volume of water is important to the grower. The higher the quantity the higher the concentration. If a grower is looking to conserve water, they might want a higher concentration. Tessenderlo Kerley has found a range of maximum solubilities. A product such as SoluPotasse has a maximum solubility of 12 kg/100 l in pure water at room temperature, while some other products barely reach 11 kg /100 l. It should be noted that maximum solubility can also be impacted by water quality and temperature.
Dust content of a product is an important factor. Not only does high dust content make the product unpleasant to handle, it can make dosing imprecise. Ideally the maximum dust level, according to Tessenderlo Kerley, is 0.2%. SoluPotasse has been independently verified as typically containing only 0.05% dust.
Being able to score a product, thus picking out the best from the worst, is vital to an efficient operation. Selecting the most important quality parameters also increases efficiency and allows products to be easily and speedily compared. In this way a quality index (QI) can be developed.
In order to develop their quality index for water-soluble SOP products, Tessenderlo Kerley conducted workshops and surveys with distributors and growers from across the globe. Eight quality parameters were identified as being the most essential, and agreement
was reached on the order of importance. The parameters are listed in the Table 1, showing the top 8 shared between the most important chemical and physical parameters.
Table 1: Quality Index parameters for SOP in order of priority (Source: Tessenderlo Kerley)
Modern drip irrigation equipment requires a top quality WSF in order to perform optimally. Lower quality SOP will give problems, particularly where water quality is poorer. Some SOP products are difficult to dissolve and as mentioned above contain higher levels of insolubles. This increases the risk of blockages to the filters and drippers, and will ultimately have an agronomic impact in that there is an inefficient uptake of nutrient by the plant. There is always a trade-off to be made between the price of a bag of product and its quality and efficiency. Given the investment in drip irrigation equipment, opting for a lower quality product might pay in the short-term but could lead to problems in the longer term.
Different production routes account partly for the variability in product quality. There are three principal methods for SOP production.
The evaporation process is where surface or subsurface brines undergo an evaporation and crystallisation process. SOP products tend to be pH neutral, which, according to Tessenderlo Kerley is not ideal as a soluble grade. See earlier reference to where acidity is an important characteristic for fertigation. A second route is the ion exchange process based on a mined ore known as Hartsalz, which contains potassium chloride (KCl) and magnesium sulphate (MgSO4). An ion exchange reaction occurs to give potassium sulphate (K2SO4) and magnesium chloride(MgCl2)
A third route is the Mannheim process which is conducted at a high temperature. In this process potash (potassium chloride) reacts with sulphuric acid in a furnace to produce SOP. The by-product hydrochloric acid is used in other processes and industries. Products have slightly acidic pH, which is better for soluble grade according to Tessenderlo Kerley. It should be noted that the Mannheim process is not a guarantee of a high quality, according to Tessenderlo Kerley. “The best forms of water-soluble SOP can be found where there is a long tradition of Mannheim production.”
Since soluble SOP is used in over 100 countries worldwide, with different cropping seasons, it is important that product is available throughout the year. ●
Tessenderlo Kerley International was the first company to produce a true water-soluble form of soluble SOP, commercialized as SoluPotasse®, back in the 1990s. One of the main reasons for introducing the product at the time was to give growers looking for a chloride free source of potassium an alternative to water soluble potassium nitrate.
Crops of course need both nitrogen and potassium but the amounts required vary during the plant’s growth cycle. Nitrogen is important early on for crop development but later in the season potassium becomes the most important nutrient, and the need for nitrogen decreases. Getting the N/K ratio right is extremely important for optimal results – and providing the crop with enough potassium when it is needed most will help ensure optimal yield and quality. Excessive use of nitrate-based fertilizers late on in the season can often lead to softer, lower quality fruits and vegetables that are less resistant to storage and transport. SoluPotasse is therefore the ideal chloride-free potassium source to use when the crop requires less or no nitrogen or in cases where the crop receives sufficient nitrogen from other products (for example calcium nitrate). Tessenderlo Kerley International is proud to be the world’s leading producer of high-grade water-soluble SOP and SoluPotasse today is sold in over 90 countries throughout the world. Product quality is one thing but not all producers deliver products of a consistently high quality. There can be variation in quality between batches or deliveries. This is not the case for SoluPotasse, which thanks to rigorous quality control, is always of a consistently high quality. Reliability of supply is also of paramount importance since agriculture is a seasonal business – distributors and growers want to be sure they can get soluble SOP when they need it and in sufficient quantities. Tessenderlo Kerely is the world’s biggest SOP producer using the Mannheim process and this helps ensure that the company can maintain a reliable supply of SoluPotasse throughout the season even when demand peaks. This is not always the case with other producers. It is also worth mentioning that Tessenderlo Kerley has a dedicated team of agronomists working in key markets across the globe to support customers who use its SOP products, including SoluPotasse, as well as the company’s unique range of thiosulfate based liquid fertilizers. The agronomy team provide advice and support and conduct trials to demonstrate the benefits of Tessenderlo Kerley’s products and help growers get the best results from them. To enable Tessenderlo Kerley to grow and remain the leader in this premium water-soluble potassium sulphate market, the company announced at the end of 2020 that it had signed a long-term off-take agreement with Finnish chemicals company Kemira for the marketing and distribution of premium SOP fertilizers. The agreement is for the off-take and marketing of the premium water-soluble SOP produced by Kemira at its plant in Helsingborg, Sweden. Thanks to this contract, SOP customers around the world can now be even more sure of reliability of supply, access to highest quality product, and professional service and support. Both Kemira and Tessenderlo Kelrey share the same mindset towards continuous improvement and qualitative production. The off-take agreement with Kemira effectively increases the amount of SOP that Tessenderlo Kerley can bring to market. In addition, being able to supply from two sites provides additional logistical flexibility for deliveries and enhanced output, particularly important in periods of high demand. Tessenderlo Kerley offers a complete SOP range: powder grade, granular grade and soluble grade as well as a special grade, K-Leaf®, for foliar applications. ●
Tobacco is a chloride-intolerant crop.
Fertigation using high quality water soluble fertilizers is widely practiced in Indian floriculture
Fertilizer stock solutions are prepared in tanks before injection into the drip irrigation system
