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Yields and quality will disappoint if potash is short

Yields and quality will disappoint if potash is short Crops have a large requirement for potassium (K) and the K content will be considerably greater than the N content at the peak of vegetative growth. If the crop cannot obtain all the potassium it needs the yield and quality will suffer, it will have a greater susceptibility to disease and pest attack, and is more likely to lodge.

The importance of potassium stems from its multiple roles in the plant:

  • The transport of nitrate within the plant, essential for the efficient use of N.
  • The activation of more than sixty enzyme systems that ensure cell function and plant growth.
  • The synthesis of proteins, vitamins, starch and cellulose.
  • The process, photosynthesis, by which carbon dioxide (CO2) and water is converted to sugar.
  • The opening and closing of the leaf stomata (pores that regulate the water status in the plant).
  • The translocation of sugars to storage organs such as grain and tubers where the sugars are converted to starch.

Potassium not only increases yields, but also enhances crop quality. It improves the nutritive value of:

  • Grain, by increasing the protein and oil content.
  • Tubers and seed, by increasing the starch content.
  • Fruit, by improving the vitamin C and sugar content.

When there is a potassium deficiency, many of the metabolic processes in the plant, like the photosynthetic rate and the different enzymatic systems, are impeded. As a consequence, plant development slows down and plant growth rate decrease, resulting in a reduction in yield and quality.

The figures show potassium uptake by spring barley in a field experiment. Open circles indicate measurements of uptake from a soil well-supplied with reserves of potassium and the crosses indicate uptakes from a soil deficient in potassium. The upper figure shows daily uptake rate, kg K/ha/day; the lower figure shows cumulative uptake, kg K/ha. For clarification, 6 kg K/ha/day equals 7.2 kg K2O/ha/day, and 160 kg K/ha equals 193 kg K2O/ha. The yield on the deficient soil was 37% down on the well-fertilised soil, and straw yield was down by almost 60%.

The upper figure shows an uptake by barley of over 7 kg K2O/ha/day, but higher-demanding crops such as potatoes can require 10 kg/day. This quantity of potash has to be immediately available for release into the soil solution from the exchange sites every day if the crop is not to suffer. If soil reserves are low this amount of potash will not be immediately available and the crop will suffer. It will not necessarily show potassium deficiency symptoms, but performance will disappoint: yield and quality will be jeopardised and profit will be lost.

Accumulating potash balance per hectare on major arable crops in England and Wales, excluding inputs of manures
fig 2: Accumulating potash balance per hectare on major arable crops in England and Wales, excluding inputs of manures
source: British Survey of Fertiliser Practice and Defra statistics

The annual British Survey of Fertiliser Practice is an extremely valuable source of information for farmers and advisors in Britain. It is a large and highly representative survey of actual practice on farm, and its findings are also without doubt indicative for other northern European countries. We are very fortunate to have the results of this unique survey, which go back for many seasons, and to 1974 for England and Wales in this precise format.

The survey tells us what we are doing on average, and the chart above shows that over the last 15 years we have potentially mined our arable soils of about 200 kg/ha of potash reserves. The data refer to all cereals, oilseeds, potatoes and sugar beet, taking account of the fate of straw etc, but not including inputs of manures. The calculation is made on a per hectare basis, using inputs of potash and offtakes in yield. The implications are that many soils are likely to be so low in potassium reserves that crop performance suffers, as outlined on the previous page. Unfortunately there are often no clear symptoms of this shortage, simply a harvested crop which does not come up to expectations.

The cost of soil analysis is less than 15 pence/ha/year (based on a 15 ha field sampled every 4 yrs), a small price to pay to find out if soil K reserves in your fields are sufficient: less than 2 kg of grain at £75/t!

For further information please contact:
PDA info@pda.org.uk
Potash Development Association

update: November 2006

 

 
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