Fertilizer International 505 Nov-Dec 2021
30 November 2021
Fertilizing potatoes with Polysulphate: the natural multi-nutrient fertilizer
POTATO CROP NUTRITION
Fertilizing potatoes with Polysulphate: the natural multi-nutrient fertilizer
Polysulphate is a new multi-nutrient fertilizer mined in the UK. Its unique characteristics and mix of nutrients make it especially suitable for potato crops, explains Patricia Imas, chief agronomist at ICL Innovative Ag Solutions (IAS).
Potato (Solanum tuberosum) is the world’s third most important staple food crop. It is commonly grown on sandy, poorly fertile soils, and therefore usually requires regular irrigation and an adequate supply of plant nutrients to achieve high yields and good tuber quality. The demand from potato crops for key nutrients increases significantly 30-45 days after planting. This is when tubers begin to form and the canopy is at its peak growth.
At harvest, a good potato crop can remove around:
- 300 kg/ha of potassium (K2 O)
- 110 kg/ha of sulphur (SO3 )
- 20 kg/ha of both calcium (CaO) and magnesium (MgO).
Supplying four key nutrients
Polysulphate is a new multi-nutrient fertilizer mined in the UK by ICL. It is composed of a natural mineral (polyhalite), has a low carbon footprint (0.034 kg CO2 e per kg of product) and is approved for organic agriculture. Four essential plant nutrients are present: sulphur, magnesium, potassium and calcium. Sulphur is the leading constituent (48% SO3 ) together with potassium (14% K2 O), magnesium (6% MgO) and calcium (17% CaO), all in sulphate (SO4 ) form.
Polysulphate readily dissolves in soil, making all four nutrients available for plant uptake. Due to its unique natural crystalline composition and solubility behaviour, Polysulphate provides the crop with a continual and fresh source of these nutrients throughout the growing season. This nutrient release pattern, by matching the timing of nutrient uptake by the crop, minimises the risk of sulphate loss through leaching. Polysulphate is also notable for having a very low chloride content, very low salinity index, neutral pH and no liming effect.
Polysulphate for potato: a perfect match
The above characteristics make Polysulphate especially suitable for potato crops, as it provides an adequate and balanced supply of four essential nutrients (S, K, Mg and Ca) in one single application. As already indicated, these four nutrients are also released gradually, supplying a continuous fresh source of each to the growing potato crop.
In general, the application of Polysulphate to potato crops will result in:
- Higher yields
- Good skin finish
- Improved dry matter
- Increased starch content.
Many sulphur, potassium, magnesium and calcium fertilizers applied at planting or pre-planting are ill-suited to the agronomic needs of potatoes. This is because the nutrients they provide can leach out of the root zone long before demand from the plant is at its peak. Polysulphate, in contrast, when incorporated preplant or at planting, delivers a natural and sustained release of all four nutrients which closely corresponds to the demand for these from potato plants.
Total potato crop uptake of potassium (tubers and haulm), for example, is around 400 kg/ha K2 O. Maximum uptake occurs 95-120 days after planting, when the daily uptake reaches 6.6 kg K/ha/day.
Quality in potato is more critical than with many other crops as it holds the key to securing the best economic returns. In particular, an adequate supply of K, S, Mg and Ca is needed to ensure quality is delivered – whether that is the desired size, uniformity, colour or shelf life of potatoes. Calcium is particularly crucial for skin quality. Nutritional disorders such as internal brown spot and hollow heart in potatoes, for example, are all caused by low Ca supply to the tubers.
Expected benefits of Polysulphate
Polysulphate is especially suitable for potato as it can supply all of the S, Mg and Ca needed by growing potato crops – and can also replace a significant proportion of the potassium removed at harvest (Figure 1).
Potatoes remove very large amounts of potassium at harvest. The proportion not supplied by Polysulphate can be applied as MOP (potassium chloride, KCl) which can be ploughed or worked into the soil a month or two before planting. This allows rainfall to move unwanted chloride down through the soil and away from the roots of the potato plant.
The solubility behaviour of Polysulphate means the Ca, K, Mg and S it contains are released gradually, providing the potato crop with a continual and fresh source of these nutrients throughout the growing season. This is critical for tuber formation in light, irrigated soils with low nutrient holding capacity. Importantly, fertilization with Polysulphate increases profits by improving quality as well as delivering high yields.
Making a world of difference
In our global trials, potato crops have shown a very good response to Polysulphate – in terms of yield and/or quality. As a consequence, farmers are now regularly including Polysulphate in their fertilization schedules. Crop experiments in Brazil, China, France, Germany, India (see Figure 2), Israel, the Netherlands, Peru, Sweden and the UK have all shown positive results.
For example, in a trial on dark volcanic soil (andisol) in Colombia, Polysulphate increased total potato yield by up to 27 percent compared to the control. The additional K, Ca, Mg and S supplied by Polysulphate in this trial also improved tuber size by up to 35 percent.
Similarly, in a trial in Peru, Polysulphate increased total yield by 7-12 percent, compared to the control, and improved tuber size by 5-13 percent.
The use of Polysulphate in a trial in Wisconsin in the United States achieved a 15 percent increase in marketable yield, versus the conventional grower practice of MOP plus gypsum.
Demonstration plots in Gujarat State, India, showed 6-12 percent yield increases with Polysulphate, as compared to the farmer’s practice, depending on the potato crop variety and location (Figure 3).
Superior potato quality
Polysulphate has delivered consistent improvements in yield, specific gravity and dry matter while reducing hollow heart in potatoes, as shown by a US trial with the University of Minnesota (Figure 4).
In addition to the above harvest improvements – which are clearly of direct benefit to growers – Polysulphate improves fry colour by reducing sucrose levels. A lighter fry colour is a desirable quality characteristic as it is directly linked to lower acrylamide formation during frying. Different potato varieties have all shown less sucrose and acrylamide levels when Polysulphate was applied to the crop, based on trials conducted by ICL in the UK. This results in lower penalties from potato processors and provides consumers with a healthier product.
Nutrient knowledge delivers crop benefits
A Polysulphate application rate of 400-700 kg/ha is generally suitable for potato and other vegetable crops. Straight Polysulphate can be incorporated into the seedbed before planting, or instead applied as a constituent of a fertilizer blend at planting.
This article illustrates how Polysulphate, as a fertilization option, is becoming key to high quality, sustainable potato farming around the world. In general, its application to potato crops will result in higher yields, good skin finish, improved dry matter and increased nitrogen use efficiency.
