Fertilization management – SOP versus MOP

LOW-CHLORIDE CROP NUTRITION

Fertilization management – SOP versus MOP

Many crops are sensitive to chloride due to a genetic susceptibility. Similarly, crops can become stressed when soil salinity reaches high levels, a situation that typically occurs in water scarce regions. So, when is low-chloride crop nutrition needed? Dr Heike Thiel of K+S Minerals and Agriculture GmbH provides some answers.

A flowering potato plant. Potatoes prefer potassium sulphate (SOP) over potassium chloride (MOP) as this can increase the starch content of their tubers.

BEASTFROMTHEEAST/ISTOCKPHOTO.COM

Introduction

Although some crops are chloride-tolerant, many others – notably potato and other fruit and vegetables – are only partly tolerant or chloride-sensitive. Farmers will therefore require low-chloride fertilizer sources, or fertilizers with no chloride content, in certain situations if they are to grow and harvest the best quality and highest yielding crops. This is especially true under saline growing conditions when water availability is low.

Potassium chloride (MOP) is produced and sold on a massive scale globally with potash products such as 60er Kali® proving popular with the world’s famers, particularly in broad acre agriculture. Low-chloride potassium sources, although less widely used and generally more expensive, are also available. The most common types are:

• Potassium sulphate (SOP) products such as KALISOP®
• Potassium magnesium sulphate products such Patentkali®
• Potassium nitrate (NOP).

Table 1: The main functions of potassium (K) in plants

The SOP sourced from natural mineral deposits in Germany provides crops with a valuable source of both sulphur and potassium. Sulphur is an important macro nutrient required for healthy plant growth but is often lacking in soils. SOP produced in Germany and elsewhere is also much less likely to exacerbate existing salinity problems in soils, compared to chloride-based products such as MOP.

As the role of potassium in crop nutrition is well-known (Table 1), the focus of this article is instead on chloride and sulphate and their effects on plants.

Chloride’s effects on soils and plants

Chloride can accumulate in soils or be introduced via irrigation water, making strict fertilization and water management plans necessary due to its deleterious effects. This negatively-charged anion is notable for being water-soluble and highly mobile in soils.

We should not forget that chloride (Cl-) is actually an essential micronutrient with vital functional roles in plants.

Table 2: The functions of sulphur (S) in plants

Table 3: Classification Which crops love of crops chloride based and on which theirdo susceptibility not? to chloride

It is essential for splitting water molecules during photosynthesis, for example. It is also important in osmotic regulation and, in combination with potassium, has a valuable part to play in maintaining the water balance within plants.

Being necessary for growth, plants will display classic deficiency symptom when chloride is missing due to metabolic problems. These symptoms hardly ever appear in the field, however, due to the small chloride requirement of most crops. While plants have an average Cl content in the 2-20 mg/g range (dry matter basis), the optimal growth requirement is typically 10 to 100 times lower for most plant species.

In chloride-sensitive crops, excessively high chloride content will lower germination rates, cause seedling injury, and result in lower yielding, poorer quality crops. The accumulation of excessive amounts of chloride in cells is toxic to chloride-sensitive plants and the resulting strong osmotic effects reduce the transport and storage of assimilates.

Sulphur – essential for growth

Sulphur, the second macronutrient in SOP, optimises plant growth and is present in highly efficient sulphate (SO4 ) form. It is essential for protein synthesis, improves nitrogen efficiency and helps plants produce their own defensive substances (Table 2).

Sulphur is associated with significant yield increases in oilseeds and legumes. Onions, leeks and garlic also require sulphur due to its presence in their characteristic flavour-forming compounds.

Sulphur is exclusively taken up by plants from the soil in the form of sulphate anions. The adsorption capacity of soils for sulphate is poor due to its negative charge. This means sulphate, similar to the nitrate anion, is at risk of leaching, especially during the winter months.

The risk of severe sulphur deficiency exists on all light soils. Additional risk factors include above-average precipitation, structurally weak soils, as well as all conditions that lead to a restricted root system. However, sulphur supply for the entire vegetative growth period is usually ensured by fertilizing with sulphate to satisfy spring demand, as evaporation in the spring and summer generally exceeds precipitation, thereby avoiding leaching.

When to avoid chloride’s negative effects?

SOP is generally favoured over MOP (KCl) as a potassium source for chloride-sensitive crops, including potatoes, certain fruits and vegetables, tobacco, almonds etc (Table 3).

Citrus fruits, for example, become bigger and heavier and contain more sugar and juice when they are fertilized with SOP. Pineapple also benefits from potassium fertilization with SOP – whereas fruit yield, fruit size and fruit quality characteristics, such as the sugar acidity ratio and flesh colour, are negatively affected when chloride fertilizers are applied.

Avoiding fertilizers with high chlorine levels – and favouring SOP instead – can have beneficial effects. When applied to potato crops, for example, SOP helps to start the enzymatic browning process by raising the plant’s free amino acid content and levels of phenolic compounds. Additionally, SOP increases the starch content of potato tubers by improving the translocation and storage of assimilates.

“In chloride-sensitive crops, excessively high chloride content will lower germination rates, cause seedling injury, and result in lower yielding, poorer quality crops.

Potatoes are leafy crops that require large quantities of potassium. When growing potatoes, farmers should focus on selecting the mildest possible type of potassium fertilizer with a low salt index – while avoiding those with high chloride levels due to their potentially damaging effects. The translocation of assimilates formed within leaves and the development of a fine root system are both impaired, for example, in potato plants which receive larger amounts of chloride. The potato plant’s leaf system can also become stressed when the fertilization regime is inadequate.

Chloride affects plants by increasing the osmotic potential of the soil water. Because of this, applying chloride-based products in regions facing water scarcity should be avoided as this can lead to higher soil salinity levels. The use of these products can also reduce the ability of plants to take up water.

Conclusion

Summing up, fertilizer applications need to be tailored to both the individual requirements of the crop and the environmental conditions of the growing region. In practice, nutrient management needs to take account of two key factors: on the one hand, there are chloride-sensitive crops and, on the other, there are regions with little precipitation. In these low rainfall regions, the accumulation of chloride from fertilizers needs to be specifically avoided, as this can add to existing soil salinisation, making the soils either barely usable or completely unusable.

In these circumstances, fertilization with chloride-free fertilizers based on sulphate such as SOP (K2 SO4 ) offers an ideal solution. While these products are usually more expensive, they provide extra value as they contain sulphur – another vital plant nutrient – in addition to potassium. Indeed, farmers benefit twice by fertilizing with SOP: not only have they eliminated the potentially negative impacts of chloride on their crops, they also gain sulphur, a nutrient involved in many important growing processes.