Potash, magnesium & sodium, fertilisers for grass |
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Potash
See PDA leaflet 14 Potash for Grassland & the PDA Grassland Kalculator for detailed recommendations.
Recommended rates of potash for grazed grass are 60kg/ha and 30kg/ha of K2O for soil index 0 and 1 respectively. For soils with higher soil fertility levels no potash required.
Very large quantities of potash are removed in grass silage which must be replaced to maintain soil fertility and yield potential. The following is guide to typical potash offtakes from multi-cut silage systems.
| Total annual fresh yield (25% dry matter) |
Potash offtake (kg K2O/ha) |
| 1 cut system (23 t/ha) | 140 |
| 2 cut system (38 t/ha) | 230 |
| 2 cut system (47 t/ha) | 280 |
| 4 cut system (54 t/ha) | 320 |
Timing of application
Grazing
Avoid applying potash between March and June except as small dressings of about 10 Kg K2O/ha
Cutting
Apply up to 80-90 Kg K2O/ha for each cut according to soil analysis. Make allowance for potash contribution of slurry or manure used. Where larger amounts of potash are required to improve soil reserves additional potash should be applied in the autumn/winter.
Magnesium
Low soil magnesium levels will affect grass yield as well as mineral balance in the animal. Apply 50-100 kg/ha MgO every three to four years at Mg index 0. Yield response is less certain at index 1 but magnesium application is justified in terms of insurance for grass yield and mineral balance for the animal, to maintain a soil magnesium index of 2.
Where lime is required and Mg levels are 0 and 1, use magnesian limestone as the most cost effective magnesium source. Where pH is satisfactory, specific magnesium fertilisers (see table below) should be used for soil improvement. Use water soluble forms of magnesium where rapid plant uptake is required.
Epsom salts applied as a foliar spray may be used to help boost magnesium intake by the animal. Alternatively calcined magnesite may be dusted on pastures for direct grazing and ingestion by the animal at critical times.
Sodium
Sodium is very soluble and is rapidly taken up by plants but levels cannot be built up on a long term basis as Na is not firmly held in the soil.
To improve herbage mineral balance
Apply up to 140 kg/ha Na2O as an early spring dressing. A split application may be preferable at higher rates.
To improve pasture palatability
Apply regular dressings of about 10kg/ha Na2O through the season.
Guide to Potash, Magnesium & Sodium
content of a range of fertilisers
| PRODUCT | %K2O |
%Na2O |
%Na |
%MgO |
%Mg |
| Muriate of Potash | 60 |
- |
- |
- |
- |
| Korn-Kali ® | 40 |
4 |
3 |
6* |
3.6* |
| Agricultural Salt | - |
50 |
37 |
- |
- |
| Magnesia Kainit ® | 11 |
27 |
20 |
5* |
3* |
| Sylvinite TM | 21 |
25 |
19 |
1.5* |
1* |
| Meadowsalt TM | 21 |
25 |
19 |
1.5* |
1* |
| Nitrate of Soda (16%N) | - |
36 |
27 |
- |
- |
| Kieserite (Granular) | - |
- |
- |
25* |
15* |
| Calcined Magnesite | - |
- |
- |
73-85 |
44-51 |
| Magnesian Limestone | - |
- |
- |
5-20 |
3-12 |
* water soluble
Korn-Kali ® & Magnesia - Kainit ® are registered trade marks of K+S KALI GmbH
Sylvinite & Meadowsalt are trade marks of Cleveland Potash Ltd
Nutrient description
Levels of potassium, sodium and magnesium as analysed in soil or herbage are expressed in elemental terms ie. K, Na, Mg.
Nutrient content of fertilisers and application rates of these nutrients to grass are expressed in the oxide form ie. K2O, Na2O, MgO.
To convert:-
K |
to |
K2O |
Multiply by 1.205 |  |
K2O |
to |
K |
Multiply by 0.830 |
Mg |
to |
MgO |
Multiply by 1.658 | MgO |
to |
Mg |
Multiply by 0.603 | |
Na |
to |
Na2O |
Multiply by 1.348 | Na2O |
to |
Na |
Multiply by 0.742 | |
Na |
to |
NaCl |
Multiply by 2.542 | NaCl |
to |
Na |
Multiply by 0.393 |