Welsh Poultry Centre

Why Potash deficiency is known as the ‘hidden hunger'

If your crops are yellow even though the soil had adequate phosphate and nitrogen levels then potash deficiency could be the problem. The only way to tell is by soil testing. Once you find out it is too late to correct, hence it is known as the ‘hidden hunger'.

Potash (K) is the last of the three macronutrients, the others being Nitrogen (N) and Phosphate, (P). Potash or potassium was first discovered, or identified by Sir Humphrey Davy, (probably better remembered for inventing the Davy safety lantern for miners), in 1807. For many year people had been using wood ash as a fertiliser. They had also been boiling wood ash to produce lye, an essential ingredient in soap. Incidentally, if you don't have any washing up liquid try using wood ash. Because it has lye in it you will find it efficiently cuts through grease. The word potash is thought to come from this process of producing lye as it was boiled in big pots. The K symbol for potash comes from the Latin for potassium, kalium which means alkali.

Why is Potash essential to plants?

Potash is a regulatory nutrient, which means that it is essential for the plant to carry out the processes needed to sustain plant growth and reproduction. This includes photosynthesis, the regulation of water use by the plant, the activation of some 60 enzymes that control plant growth and many other processes within the plant. Potash is also known as a quality nutrient, having an effect on the colour shape and size of the harvest. Plants deficient in potash are less able to withstand drought or excess water or extremes of temperature and they are more susceptible to pests and disease.

Potash also affects the ability of the plant to take up other nutrients. A deficiency of potash would, for instance, affect the ability of a plant to take up Nitrogen; an application of N could be a waste of time if there was inadequate potash available.

Potash uptake by plants

Not only do different plants require different amounts of potash, they also vary in when they require the potash. Potatoes take up two thirds of their requirement of potash in the first six weeks after emergence, while cereals only require a small amount initially and take up most between tillering and earing. At a peak time a crop can be taking 10kg/ha each day of potash out of the soil.

Potash availability

Like phosphate, there is no shortage of potash in the soil. Depending on the type of soil and the soil bedrock there can be between a few kgs to several tonnes of the stuff. Clay type soils would generally have more potash than sandy type soils. The trouble is, again like phosphate, it is not all available to the plant.

In the soil Potash exists in four different forms fixed potash, as a part of the soil minerals, water-soluble potash and exchangeable potash. The latter two, water soluble and exchangeable, provide the readily available potash, which plants can utilize. The fixed and soil mineral potash does become available to the plants over time through chemical reactions and weathering. This process can take many years. However, as potassium is one of the 8 elements forming 98% of all rock there is, potentially an inexhaustible supply of potash. Little work has been done on weathering and it is unknown how long the process actually takes. However, it is likely that small amounts of potash move into the available side in a constant process.

There will be some loss of potash through leaching, although how much depends on the type of soil and the potash reserves. In general, lighter, sandy soils will tend to lose potash through leaching more readily than heavier soil. It is therefore important, especially on lighter soils, to keep a vegetation cover at all times. In this way the plants will take up the potash and once turned back in will be readily available for following crops.

Soil Structure

Just as with Phosphate, good soil structure is important to enable plants to develop good root systems. In this way the plants are better able to find the nutrients in the soil. The return of FYMs and the incorporation of green manures will not only improve the soil organic matter levels they also return potash to the soil in an available form.

Potash Cycle

Although potash is an essential regulator of plant growth and reproduction it doesn't actually go anywhere. When a crop is harvested potash leaves the field with the crop. If that crop is then fed to an animal it will tend to come out the other end of the animal. Humans and animals don't store potash, it merely passes through them. If you wanted to deplete your farms reserve of potash then the ideal way would be by selling the forage off your farm rather than putting it through your animals. If you were an organic farm this would be discouraged as you can't readily build up potash reserves from a bag. Ultimately all potash is part of a very long-term cycle and is not lost to the planet.

It should also be noted that the manures will contain potash from outside your farm system. If you buy in straw for bedding and or animal feed, these are effectively introducing potash onto your farm.

If your forage crop is going through your animals it is important to minimise the loss of potash while the manures are being stored. This is best done by covering your muck heap. An uncovered muck heap will, through rainfall, see up to 60% of the potash content washed out.

It is also important to apply your farmyard manures (FYM) at an appropriate time, preferably when there is a growing crop that can utilise it. Given the need to reduce compaction, which could affect the soil structure and therefore the plants ability to get at the nutrient, the application should also be at a time when the ground is not going to be damaged.

Potash Deficiency

When deficient in Potash plants tend to grow slowly and have poorly developed root system. Probably the most obvious sign is a yellowing of the leaves, although this can also indicate drought, water-logging or wind damage.

       Potash deficiency in a cereal crop     

By the time the symptoms of potash deficiency are evident it will be too late to avoid yield loss, hence potash deficiency is often called "the hidden hunger". Soil testing is the most reliable way to determine if potash is sufficient.

Where Does Potash Fertiliser Come From?

The vast majority of potash is used for fertiliser (94%), with the rest being used by the chemical industry, soaps, explosives and interestingly, de-icers. In recent year the price of potash has increased. This is because of world population increases and changes in diet as countries, such as China and India, become wealthier.   While potash fertiliser is used in most countries it is only produced (mined) in around 12, with the most significant reserves being found in Canada, being controlled by the Potash Corporation of Saskatchewan, (known as the Potash Corp). Unlike phosphate, there does not appear to be a lack of potash reserves in the world. What there is, however, is controlled by only a few companies with the Potash Corp being by far the largest.