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Crop nutrition assignment by Melissa Gorst of Newcastle University

August 2016

This year we sponsored the Crop Nutrition essay at Newcastle University and Melissa Gorst, a 2nd year agricultural degree student, won our prize with the essay below. It describes the benefits and planning of Potash use. The scenario was set so that it would be written as an adviser not as an academic, and Melissa has achieved this admirably. I’d like to congratulate her and thank all at Newcastle for their efforts.

The Scenario

You are an adviser on a conventional mixed farm with a rotation of 1st wheat, 2nd wheat, winter barley and oilseed rape followed by a 4 year grass-ley.  Arable crop yields have been below average over recent years. The soil type is a clay loam and all the cereal straw is baled and used for animal bedding in the beef unit. You are looking to address the issue of low yields and you recently had soil samples taken and analysed in the spring of 2015 where the following results were obtained pH 6.4, OM% 3.42, soil P concentration range of 19-32 mg/litre (Olsen’s P) and soil K concentration range of 56-85 mg/litre.  This supports your belief that the essence of the problem is a lack of K caused by K application holidays, lack of regular analyses etc.

Your job is to advise the farmer how he can get greater production in relation to Potash use.

  1. How do you persuade the farmer to address the lack of K in his soils?
  2. What processes do you go through?
  3. Develop a strategy for the farmer to address the issue for the future encompassing both short- and long-term benefits.

The Assignment

Potassium is an essential nutrient for crop growth, as it is fundamental for many plant processes which impact upon crop yield and quality. It is required in large quantities by the plant, and therefore a deficiency can have a severely detrimental impact.

Your soil test indicated that your soil has a potassium concentration range of 56-85 mg/litre. Therefore, your soil has a potassium index of 0 or 1 which means that your crops are deficient in potassium (as the optimum potassium index is 2- ). Plants that are deficient in potassium have much lower yields, slower growth rates and poorly developed root systems (Barker and Pilbeam, 2015). At index 0, there are yield losses of up to 1.3t/ha compared to index 2, with the wheat price at £102/t this loses you £133/ha. Therefore, the cost of applying potassium fertiliser to increase your potassium index would certainly be worth the investment, due to the range of benefits potassium brings to plant growth and yield.

Potassium can increase the efficiency of use of other nutrients by plants, particularly nitrogen (Syers, 1998). If a plant is deficient in potassium it will not be fully able to take up and utilise the added nitrogen – therefore, you are wasting money applying nitrogen that will not be utilised by the plant.

Potassium has also been shown to improve the quality of crops. For example, Glatzel (1983) found that protein content of wheat grain could be increased by 4% by correcting potassium deficiency, this is due to potassium assisting in the conversion of nitrate to protein. The quality of the proteins produced also improves when the plant is provided with enough potassium (Munson, 1985).

Potassium also increases stem strength (Skogley, 1977) due to its role in synthesising cellulose which thickens cell walls. This provides resistance to lodging and greater potential for high grain yields. K+ also helps to maintain cell turgor due to its role in osmosis, also helping the plant resist lodging. Lodging reduces yield and quality (such as the Hagberg falling number), as well as increasing combining and drying costs. Extra drying can cost £3-£7.50/tonne (HGCA, 2005).

Haeder and Beringer (1981) found that potassium fertiliser extended the grain filling period in wheat by as much as 3-4 weeks. This means that potassium fertiliser increases yields and grain weights. Potassium deficiency also leads to fewer grains per ear (36), compared to plants with sufficient potassium (43).

Furthermore, application of potassium decreases the incidence of plant diseases and pests, especially under high nitrogen conditions. This is due to the thicker cell walls providing resistance to pests and also potassium reduces soluble nitrogen compounds and simple carbohydrates which are a food source for pathogens. An example of this is that potassium decreases the levels of powdery mildew and rusts in wheat and barley (Prabhu, 2007).

Potassium increases the plant’s tolerance of abiotic stresses such as drought, heat and cold (Rowland et al, 2010). For example, plants with sufficient potassium can resist wilting for longer in drought conditions as stomata are able to close more quickly since potassium controls the guard cells in stomata. So if they can close more quickly, there is reduced water loss via transpiration. Therefore, with sufficient potassium applications, your crops will become hardier resulting in fewer yield losses.

Potassium also plays a critical role in photosynthesis as it regulates stomatal opening which is needed to move carbon dioxide into the leaf (Matts, 2015). Also, because potassium prevents wilting, it does not reduce the surface area available for absorbing sunlight. Therefore, if potassium is deficient there will be reduced levels of photosynthesis.


The first thing you need to do is to begin raising the potassium index closer to the target index of 2- (121-180 mg/litre). This is the target index as, below this, there is a reduction in yield due to potassium deficiency. To raise the index, enough potassium to match the current crop requirement has to be applied, with extra to increase the K index. This will be done year on year, steadily increasing the index to 2- then after that, only maintenance applications will be applied to maintain this index, giving a long term benefit.

At the moment, your soils contain 56-85 mg/litre of potassium, equating to index 0/1. Due to the large amounts of potash required to raise the potassium index by one, it would be too expensive to apply in one dressing. Therefore, smaller “build up” applications are used in addition to the maintenance dressing – this will result in increasing the potassium index over 10-15 years.  (PDA note: the PDA PK calculator on our website allows advisers to choose the number of years to build and then calculates the recommendation accordingly.)

According to DEFRA RB209 (2010), these are the maintenance and build up applications required for the crops grown on your land (using standard yields):-

Table 1 – annual K2O requirements per hectare, costs and timings to build up soil index from 0 to 2- over 10 – 15 years

Crop Maintenance application (Kg K2O) Build up application (Kg K2O) Total (Kg K2O) Potassium Chloride (Kg)[1] Cost per ha[2] Timing of application
Winter Wheat/Barley – straw incorporated 45 60 105 175 £35 Half at drilling, half as top dressing in spring.
Winter Wheat/Barley – straw removed 85 60 145 240 £48 Half at drilling, half as top dressing in spring.
Oilseed rape 40 60 100 170 £34 Work into seedbed.
Grass establishment 60 60 120 200 £40 At planting.
Grazed grass 0 60 60 100 £20 Any time, apart from spring.[3] Split into two applications.
First cut 0 60 60 100 £20 Previous autumn.[4]
First cut 80 0 80 135 £27 Spring.[5]
Second cut 30 90 120 200 £40 After first cut.
After cutting 60 0 60 100 £20 After last cut of the year.

[1] Contains 60% K2O. So divide K2O requirement by 60 and multiply by 100 to work out KCl required.
[2] Using the current price of £201 per tonne of potassium chloride (Index mundi, 2016).
[3] To prevent hypomagnesaemia in grazing animals.
[4] In the first season after sowing, deduct amounts that were applied in the seedbed.
[5] No more than 80kg K2O should be applied in spring for first cut to minimise luxury uptake.

Since the K index is so low, the potassium for the arable crops should be applied at planting and worked into the seedbed, followed by a top dressing in spring (Barker and Pilbeam, 2015). This is due to the large amounts of potassium required, so split applications are necessary.

The nutrient offtake by the crop is higher when the straw is removed as this part contains a lot of the total potassium content (Fageria and Gheyi, 1999) so more potassium needs to be applied if removing the straw. Alternatively, you could start to incorporate the straw as this would allow plant potassium to be recycled for the next crop and mean that less fertiliser would need to be applied, reducing costs.

I suggest that you apply the amount of K2O that appears in bold in the table for each crop type – the amount of potassium chloride this would require if the K2O requirements were applied solely as inorganic fertiliser are in the next column.

However, I advise some of the K2O to be supplied by organic fertilisers. Applying organic fertilisers such as manure will increase the soil organic matter (SOM), and are also a cheaper way to provide potassium compared to inorganic fertilisers. Ideally, your manure should be tested so you know how much potash it contains so you know how much to apply. Usually, cattle FYM contains 7.2kg potash/tonne that is available to crops (DEFRA, 2010). Increasing your SOM is important as it holds potassium ions very weakly so they are readily available for uptake by roots (it increases the Cation Exchange Capacity) (Barker and Pilbeam, 2015).

I would also advise that you re-test your soils every 3-5 years so that the correct nutrient indexes are maintained in the future (DEFRA, 2010). This will avoid wasting money on applying excess amounts of fertilisers and also prevent deficiencies in the future.

Whilst the potassium index is low I would recommend growing a wheat variety with a high resistance to lodging, such as Skyfall or KWS Barrell which have lodging resistance values of 8 (AHDB, 2016).


Over the next few years, it is imperative that you increase your potassium indices to 2- due to the wide range of benefits it will bring to your crops. The cost of applying potassium fertiliser will, without a doubt, be covered by the extra yield it will bring; this, combined with the simplicity of correcting the deficiency, means that you should unquestionably correct the potassium indices.

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