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Contents
Introduction
Importance of potash for potatoes
deficiency
Forms of potash
Potash uptake and removal
Potash supply
Varietal differences
Soil analysis
Deciding the need for potash
Potash and yield
Potash and quality
Forms of potash and quality
Priciples of potash manuring
Removal and replacement
Potato responses to potash
Timing
Placement
Manures
Foliar application & top dressing of potash
Potassium: Magnesium imbalance

Potash for Potatoes

Potash for potatoes

Introduction

Potash is a very important input for potatoes affecting yield, quality and profitability. Different recommendations are required for crops produced for different markets, for different yields, for different soil types etc.

This leaflet reviews all the previous and new information available on this topic and provides guidance on best practice for potash use.

Importance of potash for potatoes

Potash is an essential nutrient for all plants and has a major effect upon yield and quality of potatoes as well as the general health and vigour of the crop. It is involved in the regulation of water in the plant and the uptake and movement of nutrients around the plant. In the absence of sufficient potassium crops do not use water efficiently. Because of its importance in turgor maintenance, potassium is essential to get maximum leaf extension and stem elongation. This helps in achieving rapid ground cover so maximising interception of sunlight and thus rate of growth in the critical early stages of the season. This is of particular importance for spring sown crops such as potatoes. Potassium supply also contributes to various aspects of quality which may be vital for a marketable sample. Nitrogen:potassium balance is of particular importance for this crop.

Potash shortage leads to :

Symptoms of deficiencyDeficiency

Yield loss & quality effects may occur without any visible deficiency symptoms. Where symptoms are identified, potential yield loss is likely to have occurred which cannot be recovered by treatment. The visual signs of potassium deficiency are that leaf margins especially on the older leaves become brown and leaves die prematurely. In severe cases, growth is much retarded and the leaf canopy may not close between the rows. Growth may be very uneven across the field.

Back to topForms of potash

Potassium does not exist naturally in elemental form, but as various, mainly soluble "salts", which separate into their component parts when dissolved in water. Potassium occurs in soils and plants as the cation K+. The other part of the different salts occur as anions such as Cl-, SO42-, NO3-. Measurement of potassium in soils or plant material is expressed as elemental K. Potassium fertilisers are commonly referred to as potash and their content is measured as K2O.
1kg K = 1.2kg K2O.

Potassium chloride (Muriate of potash) KCl

This is by far the commonest form of potash fertiliser and contains 60% K2O. The other component is chloride which exists as the anion Cl- which is also an essential plant nutrient but very rarely in short supply in the UK. Chloride levels in soils are maintained through fertiliser application and natural deposition in rain being balanced by natural leaching.

Sulphate of potash (SOP) K2SO4

This provides two nutrients 50% K2O and 45% SO3. Sulphur fertilisers are increasingly required by UK crops as other sources are reduced and this form is highly effective being readily soluble. If it is not taken up by the crop it is soon lost from the soil and hence should be applied in the spring and not the autumn. If the sulphur content is not required the potash content is more costly per kg K2O than muriate. SOP provides specific quality benefits.

Potassium nitrate KNO3 46% K2O 13% N

This is a speciality fertiliser normally used for high value crops where a fully soluble form of nitrogen is needed in conjunction with potash. It is sometimes used as a top dressing for potatoes to secure quality benefits, but its very high cost demands a high return to make it cost effective.

Back to topPotash uptake and removal

potash uptake and removal

Potatoes take up more potash than most other crops. In the six weeks after plant emergence, two thirds of the total season requirement is taken up very rapidly. During peak vegetative growth, potatoes may require 10kg K2O/ha per day from the soil. Maincrop potatoes contain the maximum quantity of potash in late July - early August and this may be more than 500 kg K2O/ha for high yielding crops. As the tops die back and the plant matures, some potash is returned to the soil. By harvest more than 75% of the maximum uptake is found in the tubers, which typically contain around 5.8 kg K2O per tonne of tubers. For the determination of nutrient removal this figure is assumed to be constant over the normal yield range. Thus a crop yielding 40t/ha will remove 40 x 5.8 = approx 230 kg K2O/ha but will have needed a maximum supply of over 300kg K2O/ha and a crop yielding 60t/ha will remove 60 x 5.8 = 350 kg/ha which will have needed a maximum supply of over 450 kg/ha.

Varietal and husbandry improvements have resulted in a continual increase in potato yields - for second earlies & maincrops, yields have increased from around 20t/ha in 1960 to 48t/ha in 2000. This has an important implication for potassium supply which needs to match this increased production.

Back to topPotash supply

Potash supply

All soils contain large quantities of potassium but only a small proportion of this is available for crop use in a season. Potash is held by the clay minerals and organic matter and is not leached or lost in the same way as nitrogen. On the lightest textured and shallow soils some potash may move down the profile to a greater depth and be lost especially to shallower rooting crops, but in the majority of soils if more K is applied than taken up, the excess will simply increase reserves for following crops. If offtake is greater than the quantity supplied, soil reserves will be depleted. Hence normal manuring policy is to identify the appropriate level of soil K for the individual soil type and calculate nutrient use to maintain it.

Most crops take up nutrients mainly from the plough or cultivated depth of soil. Where crops root more deeply some nutrient may be taken up from supplies at greater depth. Potato root systems are generally far less extensive than crops such as winter cereals or sugar beet and deeper nutrient uptake is therefore of much less importance with this crop. However modern cultivations and de-stoned bed systems do encourage greater root development than in the past and thus increase the potential soil volume available to the crop for nutrient & water uptake. Medium and heavy soils may also release some potash from non exchangeable reserves which are not measured by a normal soil analysis, however the potato crop is increasingly being grown on lighter soils which do not have this capacity.

Potash may be taken in via foliar uptake but only a very small proportion of the large total requirement can be supplied by this route and foliar feeding tends to be a more expensive source of K than soil application.

The potato crop needs a supply of potash from the soil which can satisfy both the peak rate of crop demand during vegetative growth and the total quantity of maximum uptake in late summer. Where soil reserves are insufficient to meet either of these two requirements, the supply needs to be supplemented by fertilisers and manures but these additions may not be as efficient in providing nutrient as the reserves in the soil. Long term experiments at Rothamsted and Woburn have shown that yields from impoverished soils may not match yields on fertile soils even if higher levels of fertiliser are applied and that it takes a long time to restore low fertility once reserves have been run down.

Effect of soil K residues
Effect of soil K residues

Back to topVarietal differences

The increasing recognition of varietal differences in nitrogen requirement and response has lead to interest in whether there should be similar discrimination for potassium. It has been suggested that indeterminate varieties (e.g. Cara) develop a better root system which may improve uptake & utilisation of K from the soil and so affect potash requirements. However there is insufficient evidence to differentiate recommendations for variety except where certain varieties produce higher yields and require larger potash dressings to replace the greater offtake.

Back to topSoil analysis

Soil analysis has been developed over many years in the UK to indicate the ability of a soil to supply nutrient to the crop and to show the need for any additional nutrient. Samples are normally taken to 15cm and thus the analysis does not indicate possible depth supplies nor does it predict the release of non exchangeable K which can supplement available K on some soils. As explained earlier neither of these sources is as important for potatoes as some other crops.

Soil analysis is not a precise measurement because there are large variations in soil and because the laboratory technique cannot exactly mirror actual plant uptake. However the index system has been developed to provide a practical and worthwhile guide relating to crop response as shown below. With the publication of the 7th edition of the MAFF Fertiliser Recommendations Book RB209 in 2000, the system is modified with K index 2 being divided into an upper half (2+) and a lower half (2-):

Soil K Index
mg/l
Potato yield response to added nutrient
0
0-60
Large response normal
1
61-120
Response likely
2-
121-180
Possible response
2+
181-240
Response less likely
3
241-400
Response unlikely
4
401-600
No response

 

Back to topDeciding the need for potash

There are three reasons why potash should be applied for potatoes :-

It is important to identify the relevance and interaction of these different justifications.

"Response requirement"

In the short term a grower needs to know how much potash to apply for the best economic return and this is related to the yield response likely for the site and the year.

"Replacement requirement"

If the response requirement is less than the amount of potash removed by the crop it would run soil reserves down and would not be sustainable in the longer term. As soil K reduced, response to potash fertiliser would rise and the optimum "response" dressing amount would become larger. This may be a theoretical ideal economic optimum approach but assumes far more precision than is possible in practice and ignores the fact that potash held in historically accumulated reserves is more efficiently taken up and used by the plant than fresh fertiliser.

It is also argued that a "response" dressing is applicable for the increasing area of the crop that is grown on land rented for a single year. Again if the "response" dressing was less than removal, soil fertility would be depleted by this approach. This is not equitable for the landlord and cannot be generally recommended as a sound soil management practice.

It is worth noting that potash is completely different to phosphate in respect of removal and response amounts. For potash the requirement for optimum response is generally less than the very large quantities removed. Phosphate removal is typically less than 20% that of potash removal, but responses to phosphate are often obtained at much higher application rates than required to replace removal.

"Quality requirement"

Potash supply also contributes to quality characteristics which may affect marketability of potatoes such as dry matter, specific gravity, sample size and tuber number, starch content, fry colour, fat absorption, internal blackening, susceptibility to mechanical bruising, cooking quality and flavour. Many other factors also affect these characteristics (often to a greater degree than potash) and additional potash supply will not improve the characteristic where it is being controlled by an excess or limitation of another factor.

If adequate potash is available to the crop for full yield, additional nutrient is unlikely to provide cost effective additional quality benefits.

Back to topPotash and yield

A very large number of experiments from many countries have confirmed that potash supply has a major influence on yield. For the UK, current recommendations are based on trials data linking yield response to K soil analysis

Fig A Yield response & soil K

Fig B Yield increase & fertiliser K

Yield response & soil K Yield increase & fertiliser K

Whilst there is considerable variation for different sites, responses to added nutrient are generally large at low levels of soil K and tend to reduce as soil K levels increase. The likelihood of a response also gets less with higher levels of soil K. As Fig A shows there are variations from this general interpretation and it is important to attempt to identify site specific factors which may result in responses being obtained at high soil K levels so that potential yield is not lost because of inadequate potash use. There are also cases where little or no response is obtained at very low soil K levels and these have been used to argue that potash recommendations need to be reduced or ommitted. This would ,however, represent a negative potash supply/removal balance and lead to reducing soil reserves. Whilst there is no precise critical value for soil reserves, the 60mg/l band represented by index 2- is still seen as an appropriate target range to aim for on most soils. True sands and loamy sands have so little K retention ability that lower target values are set for these very light soils.

There is a narrow cost/benefit ratio for potash and potatoes. Assuming a cost of 20p/kg of potash and a value of £80/t for potatoes, 1kg of potash requires only 2.5kg of tubers to cover the cost.

Fig B shows the results of 9 recent trials at K index 1. Fertiliser potash rate is plotted against yield increase and the "cost" of potash is shown in terms of tuber yield - the red line.

Two trials gave large yield increases of 15 & 18t/ha. Two trials gave little or no increase and the rest gave small but worthwhile increases. The results are highly statistically significant. The green line shows the average response for all sites.

Back to topPotash and quality

The amount and form of potash can affect various aspects of quality but other factors are involved and often have greater impact. Requirements for different markets will require particular attention to specific aspects of quality. As indicated above, extra potash beyond response or replacement recommendations is not normally justified by quality benefits.

Dry matter

Dry matter contentDry matter content

Higher potash supply generally reduces dry matter (DM) of tubers by up to 2%. Where DM is important and levels need to be improved when muriate of potash (potassium chloride) is being used, splitting the total requirement to half in the autumn and half in the seedbed can help. Switching from chloride to sulphate of potash (applied in the spring) may have a greater effect and may improve DM by up to 2%. DM is affected to a greater extent by factors such as N supply, irrigation and variety.

Both specific gravity and starch content tend to follow dry matter and where these characteristics are important it is necessary to ensure the correct rate of K and consider the use of sulphate rather than muriate.

Tuber size and number

Yield response to potash is normally associated with an increase in average tuber size and weight. The use of sulphate of potash instead of muriate may be beneficial where higher tuber numbers of smaller-medium size tubers is required such as for seed, canning, salad etc. The benefit will be more pronounced under dry or stressed growing conditions.

% yield by tuber size

Fry Colour

Lighter colours on frying can be very important for crisping potatoes and adequate potash supply can improve this aspect of quality. Muriate appears to be marginally superior to sulphate for this characteristic, however sulphate does appear to reduce the quantity of fat absorbed on frying and this has important process cost implications.

Tuber damage

Tuber bruising and damage is caused by mishandling but will be worse where potash is limiting. Additional potash beyond the optimum for yield or replacement will not help to cure a damage problem.

Internal blackening/Black spot

This is aggravated when DM is high and is alleviated with higher rates of potash. Muriate is somewhat more effective in reducing the occurrence of this problem than sulphate but adequate rate of K is the more important factor.

% of tubers with Black Spot

K2O kg/ha
0
125
250
375
500
625
KCl
35
30
25
17
13
13
K2SO4
35
31
26
23
18
20

Example of internal blackening

Cracking on high pH soils

It is desirable to avoid growing potatoes on high pH soils but there is some evidence that where this is practised, SOP can reduce the severity of tuber cracking.

Flavour

This is a very subjective characteristic but there have been a number of taste experiments which suggest that the use of sulphate of potash results in a better flavoured product.

Cooking quality

Many other factors especially the method of cooking and variety are more important for cooking quality than nutritional factors, but there is some evidence to suggest that potash reduces disintegration on boiling and mealiness of the cooked product.

Physiological stress

There is little doubt that crops deficient in potash are less able to handle stress caused by drought, waterlogging, frost, heat, wind, etc. Again no advantage is gained from using higher rates than are justified for yield or replacement requirements. Whilst yield variation will occur according to the growing conditions of different years, the depression of yields in "poor" years can be minimised by ensuring adequate potash supply.

Back to topForm of potash and quality

 
MOP
SOP
Yield
Increased
Increased
Dry Matter
Decreased
Decreased less
Specific Gravity
Normal
Higher
Starch
Lower
Maintained
Fry colour
Improved
Improved less
Fat absorption on frying
More
Less
Tuber numbers
Normal
Increased
Tuber damage
Reduced
Reduced
Internal blackening
Reduced
Reduced less
Flavour
Normal
Better
Cracking on high pH soils
Normal
Less

Bag of chips

Back to topPrinciples of potash manuring

The normal principles of potash manuring are :-

to replace nutrient removed where soil reserves are adequate
to apply more than removed on soils which are below an adequate status
to reduce or omit applications on soils which are above an adequate level.

The accepted approach as defined in MAFF Fertiliser Recommendations for Agricultural & Horticultural Crops RB209 7th edition 2000 and PDA leaflet 8 Principles of Potash Use is based on soil K index and replacement of potash removed by the expected crop.

Soil K Index
Potash recommendation
0
Replacement + 50 kg/ha
1
Replacement + 25 kg/ha
2-
Replacement
2+
Replacement - 25 kg/ha
3
Replacement -70 kg/ha
4
Nil

These are general guidelines which in most situations ensure adequate nutrient for full yield. Soil analysis should be used as a starting guide rather than a final definition of requirement. Site specific consideration should also be given to soil structure, soil depth/volume, stone content, organic matter level, likelihood of drought, possibility of K leaching and any other relevant factors.

For a maincrop potato yield of 50 t/ha the replacement value is 300 kg K2O/ha. For a 30 t/ha early and seed potato yield, a potash replacement of 170 kg K2O/ha is required.

Back to topRemoval and replacement

There are large variations in uptake and removal in practice but typical figures for calculation of replacement requirements by multiplying by yield are given below. Thus the requirements of crops with very different growing periods and yields (e.g. earlies, canners, main crops, seed, etc.) are covered by the same approach.

 

 
Typical removal figures for potatoes
 
P2O5
K2O
kg per tonne of tubers
1.0
5.8

In a recent replicated trial on a loamy sand soil of K index 1 (K 79mg/l) at ADAS Gleadthorpe in 2002 (funded by BPC), 0 and 325 kg K2O/ha (RB209 recommendation for the site) were applied to 8 potato varieties and their tuber yields measured.

Back to topPotato responses to potash

Cultivar

 

Yield t/ha

Estimated

0 kg K2O/ha

325 kg K2O/ha

K2O removal kg/ha

Ambo

71.1

85.3

494

Cara

58.7

77.6

452

Cultra

80.6

91.1

528

Desiree

70.7

78.5

455

Maris Piper

62.1

72.8

422

Navan

69.4

85.2

494

Pentland Crown

72.6

79.9

463

Russet Burbank

52.5

63.2

366

Mean of all vars.

67.2

79.2

459

For comparing means of all cultivars, s.e.d. = 2.23 (3 df), P=0.013
Coefficient of variation = 8%

Although not a potash response trial and whilst the study did not identify optimum potash rates, the data clearly demonstrate large yield responses for all the varieties to potash fertiliser on this K index 1 site. Mean response of all varieties was 12 t/ha giving an excellent cost: benefit ratio of 12:1.

Potash removal rates have been estimated using actual yields and standard potash contents (see above). Average offtake of 459 kg K2O/ha for all varieties was 134 kg/ha greater than the 325 kg/ha fertiliser application, giving a range of K2O soil deficits from 41 to 203kg/ha for different varieties. Thus, there was a depletion of soil reserves for all varieties. The current principles of potash use recommend that this shortfall should be avoided by matching potash use to offtake plus an addition of 25kg/ha for this loamy sand at the low end of Index 1 (459 + 25 =484 kg K2O/ha).

High yielding potato crops remove very large quantities of potash and a manuring policy based on replacement, with supplements where reserves are low, will generally provide adequate potash to ensure full yield.

Back to topTiming

Apply up to 190kg/ha in the seedbed. For higher rates some should be applied earlier in the spring or in the previous autumn/winter. Where rates of over 300kg/ha are required, half the total should be applied in the autumn/winter and half in the spring. On lighter soils the winter application should be made after Christmas.

Back to topPlacement

Placement of nutrient adjacent to the seed can improve fertiliser efficiency and this can be important on soils of low nutrient status. However, placement of large amounts of soluble nutrient close to the seed can cause damage especially under dry conditions. It is not recommended to place more than a total of 250 kg/ha of nitrogen and potash near to the seed.

Back to topManures

Around a third of the national potato crop receives some manure but there appears to be no allowance for potash contribution from this source. Indeed average potash application to crops receiving manure is actually higher than for those not manured ! Estimates of nutrient contribution from manures can be easily made using typical values (see PDA manures card) and fertiliser rates should be adjusted accordingly. An excess of nutrient not only wastes money but may provide additional problems.

Back to topFoliar application & top dressing of potash

The enthusiasm for foliar phosphate has lead to interest in foliar potash. However the large quantities of potash required and the lower responsiveness of the crop to potash compared to phosphate does not lend support. There is inadequate evidence to support any general recommendation here.

Where large quantities of potash are needed because of high potential yields on light soils with low potash reserves, a case could be made for top dressing potash in conjunction with nitrogen at tuber initiation, especially where irrigation avoids droughting. Again there is insufficient evidence at present to support this with firm recommendations.

Back to topPotassium: Magnesium imbalance

There is growing evidence that potash availability and uptake can be seriously reduced on soils where magnesium levels are excessive, even though the level of soil K may not be deficient. This is most frequently associated with long term use of magnesium limestone to correct pH. It is suggested that where soil concentration of Mg is more than double that of soil K, potash applications need to be increased above the normal recommended rate to achieve adequate potash nutrition of the crop. The problem is greater in dry seasons.


Further information from a review of recent work on potash for potatoes can be found in "Profitable use for potatoes" in the Technical Notes section of the website.

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