Potash for Grassland |
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Why a leaflet on potash
Many farmers are not getting the best from their grassland because of a lack of potash.
Around 50% of grassland has below-target soil K levels and is very responsive to added potash. The
problem is increasing, because the average application of potash fertilisers to grassland has fallen by
30% in the last ten years. Far too little soil analysis is undertaken on grassland farms and fertiliser and
manure usage is therefore often not soundly based. This can result in costly penalties from either under
or over use. A major problem is that this is not evident with deficiency symptoms or identifiable poor
growth. Whilst precision farming techniques can now provide arable farmers with increasingly
sophisticated information for every part of every field, most grass farmers still have very little knowledge
of individual field performance because forage harvesting equipment generally provides no information
on crop yield. This is of vital importance for both phosphate and potash because manure and fertiliser
use should be related to crop offtake. For the same reason of lack of measurement or estimation, there
is frequently enormous scope to improve the effectiveness of manure use, again resulting either in very
valuable yield improvements or fertiliser cost savings.
New varieties and grass mixtures are constantly improving forage potential but this is not being realised in practice where potash supply is inadequate and this Cinderella input has become the limiting factor to grass performance.
The objective of this leaflet is to improve understanding of potash and provide recommendations to ensure more cost-effective usage which will result in better forage performance and lower unit costs of production.
Potash is essential for full yield & high quality feed value.
Potash is used to regulate the movement and storage of solutions throughout the plant which might be compared to the blood system in animals or humans. This is clearly a very wide ranging and vital role, affecting nutrient uptake, photosynthesis, rate of growth and feed value of the forage. These functions require larger amounts of potash in total in the plant than any other nutrient including nitrogen. If adequate amounts are not present grass will not grow or yield as it should.
There is also a particular relationship between nitrogen and potash. The large and worthwhile growth response of grass to nitrogen is dependent upon a balanced supply of potash both to assist N uptake by the roots as nitrate and to support the conversion of this to the complex proteins needed by the animal. Potash is also very important in the microbiological fixation of nitrogen by root nodules in legumes. If clover is required to play a significant part in the sward the supply of potash is of even greater importance.
The practical implications of shortage of potash are summarised in the following table:-
| Deficient K | Satisfactory K |
| Low yield | Full yield |
| Deterioration of sward composition | Maintenance of productive species |
| Inefficient N response | Full N response |
| Increased risk of N loss | Minimum N loss |
| Less vigorous legumes | Full potential legume contribution |
| Reduced N fixation by clover | N fixation related to management of sward |
| Increased risk of poor silage fermentation | Normal fermentation |
| Reduced protein production | Full protein production |
| Ranker, weak growth | Strong healthy growth |
| Greater susceptibility to frost | Less winter damage |
| Slower growth in spring | Earlier spring growth |
| Open swards, more sod pulling | Denser vigorous sward |
| Increased susceptibility to drought | Normal drought resistance |
| Increased disease susceptibility | Disease risk according to season |
| Poor recovery especially in dry conditions | Quicker aftermath growth |
N:K partnership
Grass requires a balance of nitrogen and potash to obtain full response to applied nitrogen.
Careful optimisation of nitrogen is a waste of time if potash supplies are not adequate. As shown in the diagram both the level of yield and shape of response curve are radically altered by potash limitation. Application of potash alongside optimal nitrogen increased annual yield from 9 to 14 t/ha.
If potash supply is limiting, the uptake and utilisation of nitrogen will be restricted. If soluble forms of nitrogen remain in the soil and are not taken up there is increased risk of leaching giving environmental concern when through-drainage occurs. Ready availability of both nutrients helps the uptake of the large requirements of N and K necessary for full yield. During rapid vegetative growth, the large uptake of nitrogen as negatively charged nitrate ions (NO3-) is normally balanced by a similar uptake of positively charged potash ions (K+) which maintains the electrical neutrality of the plant.
| Silage yield at 325 kg N/ha | ||||
 |
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| No Potash | 150 kg K2O/ha |
300 kg K2O/ha |
450 kg K2O/ha |
|
| Silage at 25% DM |
34 t/ha |
42 t/ha |
47 t/ha |
55 t/ha |