Potash, magnesium & sodium, fertilisers for grass |
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Potash in perspective
Increasing uptake of one nutrient by the plant may affect the level of others, but it is quite wrong to assume that the application of potash automatically leads to mineral disorders. The effect of potash fertiliser on herbage mineral content will vary widely in different situations. The graph below shows how magnesium and sodium % may decrease as a result of potash application. However this is not always the case and the application of potash may not have any effect on the sodium or magnesium %.
Effect of Potash on the mineral content of ryegrass |
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Data from other trials show that soil type can be important:
Magnesium % in 1st cut herbage |
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Potash rate applied in March (kg/ha) |
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nil |
40 |
80 |
120 |
160 |
200 |
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| Clay loam soil | 0.15 |
0.15 |
0.17 |
0.15 |
0.14 |
0.15 |
| Sandy soil | 0.21 |
0.15 |
0.14 |
0.14 |
0.13 |
0.13 |
Magnesium % in grass on the sandy soil was progressively reduced by higher spring potash applications whereas magnesium levels in grass on the clay loam were unchanged even up to 200 kg/ha K2O.
Potash, magnesium and sodium relationships
On farms where staggers is a recurring problem, attention should be given to the potassium, magnesium and sodium content of herbage.
The mineral content of healthy and tetany prone pasture |
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Normal magnesium concentrations in herbage are frequently below the minimum 0.20% suggested for animal diets. Magnesium % in plants is affected by a large number of factors and whilst the risk of magnesium disorders may increase with lower herbage magnesium, this is not a reliable measure of whether clinical mineral problems will occur in the animal.
The level of sodium should also be considered. The bar chart above shows that where herbage sodium levels are above the minimum dietary guide of 0.15% Na, the risk of staggers is low, but rises with lower sodium levels.
Nutrient balance
Nutrient balance is important in avoiding mineral disorders and experimental work has shown that there is less risk of staggers when potassium, magnesium and sodium levels in herbage result in K:Na and K:Mg ratios of between 10 and 20:1. The histogram indicates the greater risk of staggers at K:Na ratios greater than 20:1. Field trials and surveys have shown that maintaining a high level of sodium and magnesium in grass will reduce the risk of staggers.
