Product characteristics

The declared nutrient content, as stated on the POLY4 label, is 14% K₂O, 19% S, 6% MgO and 17% CaO.

Label declared analysis of a fertilizer is the minimum content of its nutrients or, in the case of undesirable elements, the maximum content of that element. It is most commonly expressed as a percentage by weight.

POLY4’s chemical formula:
K₂SO₄MgSO₄2CaSO₄2H₂O

Nutrient content of major potash fertilizers 

All fertilizers are characterised by their solubility in water at a given temperature.

Solubility of POLY4 in water (g L^-1) over a range of temperatures compared to other commercial fertilizers

The solubility of POLY4 was tested over a range of temperatures against the water solubility reported for other common fertilizers. POLY4 has a solubility of 27 g L^-1 at 25°C, which corresponds to the amount of POLY4 that would dissolve in the plough layer of a moist, medium textured soil at a 10 t ha^-1 application rate. With this solubility, POLY4 effectively delivers K, S, Mg and Ca at commercially-required rates.

Summary of commercial fertilizer solubility at 25°C

Dissolution rate characterises the transition of a solid fertilizer into a solution. This rate is largely governed by physical parameters controlled during the patented granulation process. POLY4 was compared to MOP, SOP and SOP-M at 20˚C. Its dissolution rate was similar to MOP, but slightly faster than SOP during the first 15 minutes.

Since POLY4 is a mineral, dissolution resulted in simultaneous nutrient release. Both tests demonstrated that all nutrients are available, with near full dissolution of its nutrient content within six hours. The format of material clearly impacted the release profile. Crops benefit from this characteristic as nutrients are delivered at a pace that is more compatible with their metabolic requirements, supporting plant growth throughout the growing season.

Dissolution of granular POLY4 in water over time compared to other potassium-based fertilizers at concentrations of 10 g L^-1

The availability of nutrients to plants is as important as the timing of fertilizer application. Dissolution is the rate of phase-change from solid into solution. Many factors control and influence this such as temperature and physical form. Fertilizer needs to effectively deliver its nutrients over a suitable timeframe allowing a plant to capture them when required. The fertilizer industry uses artificial barriers to control dissolution rate. POLY4’s natural dissolution rate effectively regulates release of nutrients to crops without the need for artificial barriers.

Dissolution rate of POLY4 granules and crushed polyhalite rock was tested in the laboratory. Material was added to deionised water (0.5 g/500ml) to test for solublised potassium, sulphur, magnesium and calcium over time.

The percentage of nutrient recovery was quicker from POLY4 granules. Testing results showed that nutrients were released from the POLY4 granules in 40 hours, whereas it took up to 180 hours to recover nutrients from a similar size chip of crushed rock.

Nutrient recovery

A plant’s nutrient requirements change as it develops so farmers must find a fertilizer option that aligns with those needs. POLY4 has a sustained nutrient delivery, which more closely meets the requirements of the plant.

Common fertilizer practice tends to apply nutrients such as nitrogen, potassium and sulphur at sowing. These nutrients can be lost through erosion, runoff or leaching, and so become unavailable towards the end of the plant’s life cycle where nutrient uptake is at its highest. Sustained nutrient release is especially important for crops on sandy and other soils with limited capacity to retain and release nutrients.

In this laboratory study soil columns (30 cm of sandy loam topsoil) were used to demonstrate nutrient release from POLY4 and other products. The test aimed to quantify the release of K, S and Mg to demonstrate the additional benefits of controlled nutrient release achieved from multi-nutrient POLY4.

The sulphate of potash magnesia (SOP-M) treatment quickly released K, with 70% of the added K removed from the topsoil in two pore volumes of simulated rainfall. The POLY4-fertilized soil also had a large peak of K mobilisation but expressed it over a longer period of time. This means it had both high K availability and sustained release of nutrients.

The trend was similar for Mg, with a quick, though short, release from SOP-M but a prolonged release from POLY4 that gave a broad peak of available Mg. When expressed as a percentage of the Mg applied by each of these treatments, the POLY4 quickly exceeded the Mg released from that of SOP-M. Less than 80% of the Mg added in SOP-M was mobilised.

The POLY4 treated soils also mobilised more sulphate (SO₄^2-) than soils treated with SOP or SOP-M. As with the K and Mg, adding POLY4 created a large and sustained peak of mobilised sulphate. The POLY4 added a little more sulphate than the SOP-M, but dramatically increased the available sulphate.

POLY4 provided a very strong and unique combination of prolonged K, Mg and S release with increased total availability of these nutrients: 115% K added (MOP = 64%) and 90% Mg added (SOP-M = 61%). The sustained release of nutrients allows plants to take advantage of this availability for a longer period of time, more closely matching the requirements of the plant.

Nutrient availability in soil

The demand for food will escalate in line with population growth. It is expected to double within 30 years equivalent to a 2.4% compound annual growth rate. Nutrient use efficiency (NUE) is a critically important concept in the evaluation of crop production systems. It can be greatly impacted by fertilizer management as well as by soil- and plant-water management.

The amount of nutrients taken up by plants relative to the amount applied in fertilizer can be used to measure NUE. We evaluated 32 trials globally including those conducted in Asia (China), Europe (UK & France), Latin America (Brazil) and North America (US) covering both high-value and broad-acre crops such as tea, tomato, soybean, wheat, potato and corn. Normalizing the nutrient uptakes to that delivered from MOP showed the value of POLY4 fertilizer plans to micro and macro-nutrient uptake.

Trials showed that POLY4 supplied its nutrients over an extended delivery period. Our product promoted nutrient release, and enabled and supported a broad spectrum of macro and micro-nutrient uptake, creating a favourable nutritional support for crop production.

A nutrient-balanced field trial, in partnership with the University of Kentucky, compared a POLY4-based fertilizer plan with a locally typical programme including one with a similar nutrient profile.

As part of the fertilizer plan, POLY4 elevated crop stalk and leaf biomass of the trialed Solanaceae crop. Nutrient uptake drove biomass generating 7% to 28% higher yields compared to MOP options.

Low-chloride POLY4 delivered nutrients such as K, S, Mg and Ca through to harvest influencing and improving the quality of crops.

Biomass

“Fertilizer is still one of the best investments a farmer can make. Reducing rates below optimum means less profit for the grower and also results in mining of soil nutrients. Maintaining soil fertility is easier and less costly than building it up.”  

Dr. Terry Roberts, International Plant Nutrient Institute

Independent testing examined the impact of POLY4 on the characteristics of compacted, steam and chemically granulated NPK fertilizer products. This handbook gives a summary of the actual testing results using real data and displays physical and chemical properties of MOP, urea-based and AN-based NPK products.   

Read the handbook by clicking on the picture below.

Good crop nutritional health supports several physiological and disease defense functions. Reduction in tissue potassium leads to:

• Yield and quality penalties;
• Lodging risk;
• Inefficient nitrogen use;
• Drought and/or cold susceptibility.

POLY4 supports plant disease resilience resulting in a healthier crop

We assessed the effectiveness of POLY4 on tomato bacterial spot caused by Xanthomonas bacteria. This pathogen affects tomato yield and quality in many major tomato-producing countries.

In this second year trial, in partnership with University of Florida, fertilizer treatments were incorporated into the soil at the start of the trial. Plants were inoculated with a bacterial spot suspension at 53 and 67 days after planting. Disease severity and plant height were assessed 64 and 98 days after transplanting.Various fertilizer sources were used to apply a range of K, S, Mg and/or Ca combinations.

In this NPK-balanced trial we observed that: 

• Supplying potassium significantly reduced the severity of bacterial spot;

• POLY4 supplied potassium, sulphur, magnesium and calcium ensuring the highest reduction of infection.

The crush strength of a fertilizer determines its suitability for spreading by agricultural machinery. A crush strength greater than 3 kgf is recommended to ensure that the fertilizer will resist stress during transportation and spreading.

Testing showed that POLY4 had a crush strength of 6.5 kgf which should be optimal for handling, distribution and field application. This strength means POLY4 is spreadable using a spinner at speeds of up to 900 rpm and also via boom spreaders.

Crush strength of POLY4 and other fertilizers

The CRH of fertilizer is the value of relative humidity, above which a fertilizer will absorb moisture and below which it does not absorb moisture. Water absorption influences caking propensity which leads to difficulties in handling and spreading.

A typical curve from which CRH is determined compares uncoated POLY4 with coated MOP in the graph below. CRH values of 70% for MAP, DAP, MOP and urea, and lower values — near 55% — for blends have been reported.

Uncoated POLY4 has a CRH of 70%. This is similar to other substitute products such as MOP (72%), giving blenders and growers confidence in the product’s shelf life.

Fertilizer is often applied to fields using mechanical spreaders to ensure an even distribution of the required nutrients. Ineffective spreading leads to uneven fertilizer application, resulting in strips of nutrient-deficient crops and a subsequent loss of farmer income.

POLY4 granules were tested using spreading machinery set to spreading widths of 24m and 36m, typical distances for fertilizer application. Uniformity of application is expressed as the coefficient of variation (CV). A CV of more than 20% generates stripes in the crop. Subsequently, uneven spreading increases the cost of crop production due to yield penalties and required corrective actions.

A lower CV means a more even distribution of fertilizer. The results of the POLY4 spreader testing were within the 20% tolerance limits with, for example, 3mm granules showing a CV of 5.52%. POLY4’s quality spread pattern reduces the risk of additional expenditure.

Consistent particle size is also important for fertilizer practice use, for optimal spreading and for preventing segregation in bags and blends. A tighter grade pattern production of POLY4 granules allows them to be manufactured within 2mm to 4mm in diameter. This consistency in particle size is important for fertilizer spreading and for meeting customers specifications. 

Most fertilizers are salts containing macro and micro nutrients which, when added to soil, cause an increase in the osmotic pressure of the soil solution. Fertilizers vary with regards to their osmotic effects and their potential for crop injury. They are measured in comparison to a standard reference material, sodium nitrate, giving a ratio referred to as the salt index.

The Jackson (1958) method is the current industry standard and was used to assess a range of potash fertilizers. Results from seven independent laboratories show a lower salt index for POLY4 than MOP, SOP and SOP-M.

POLY4’s relatively low salt index supports a plant’s ability to absorb water and the nutrients contained within the soil solution.

Salt index (SI) +/- standard deviation for MOP, SOP, SOP-M and POLY4 using the Jackson (1958) method

The compatibility of POLY4 is essential since many fertilizers are sold as blends. Incompatible products often result in caking, which is affected by humidity, particle shape and size, composition, storage duration, temperature and pressure.

Blending fertilizers require consideration of all components’ compatibilities to prevent caking and ensure safety. International Fertilizer Development Center (IFDC) is an established industry specialist. Using a wide range of likely fertilizer combinations, IFDC carried out advanced testing on POLY4 to determine blend compatability.

Testing took place over an extended period in order to mimic industrial conditions. Materials were combined and blended such that the total sample (100 grams), occupied two-thirds of a 200 mL glass bottle. The samples were then tightly capped and placed in a convection oven at a temperature of 30°C for 30 days. The mixtures/bulk blends were inspected daily for signs of incompatibility. The chemical analyses of tested materials were performed according to the Association of Official Analytical Chemists (AOAC) methods.

After testing, all mixtures remained free-flowing through the duration of the test. The results showed that POLY4 is a compatible input for blending into NPK fertilizers.