The Fertilizer Association of Ireland, in conjunction with Teagasc and K&S Kali, has launched a very useful nutrient removal calculation wheel for Irish farmers and advisors. The wheel, which uses a dial to adjust the variables, will show the amount of P and K that a crop of that size removes.

The nutrient wheel, which is simple to use, is useful for crop and grass farmers. A similar system has been available in Britain for crops for a number of years but the Irish version uses elemental P and K and gives guidance on off-take from grass crops and grazing systems.

The tool is meant to be used by farmers. Too often we ignore the fact that off-take is nutrient, whether that be milk, beef, lamb, cereal, roots, etc. And if you remove nutrient and don’t replace it then you are reducing the levels in the soil over time and this is decreasing yield potential, of grass as well as crops.

The wheel principle

When using the wheel, the colour in the cut-out column must be the same as the colour in the circle segment. A rotating disc is sandwiched between two sheets. On the front you can check details for grass and forage crops while on the back you can look up the nutrient off-takes from cereals, oilseeds/proteins and root crops. Each category is colour coded. Rotating the wheel allows you to choose the yield level for the crop to get the off-take or your organic nitrogen range in the case of grazing livestock.

background information

The booklet also provides useful background information on soul fertility. For example, it gives the P and K levels that apply to the different indices and the requirement for different grassland uses and crop types. The P application rate for oilseeds on an Index 1 soil is maintenance (M) plus 15 kgP/ha for a 4.5t/ha crop. And the maintenance is the off-take which according to the wheel is 27kgP/ha if the straw is not removed and 30 kgP/ha where the straw is removed (as found on the wheel). So the P to apply is either 27+15 (42kg/ha) or 30+15 (45kg/ha). And for potash the requirement is M+40 kg/ha where M is 41 where the straw is not removed and 65 where the straw is removed, giving a total of either 40+41 (81 kgK/ha) or 65+41 (106 kgK/ha).

In a grass situation on an Index 1 soil the application is M+20, where the stocking rate equivalent is 131-170kg organic N per hectare, M for drystock grazing is 8 kgK/ha where 0.5t/ha equivalent of concentrate feed is used. If 2.0t/ha of concentrate feed was used the M requirement would be zero. In the same dairy grazing scenario M would be 12 kgP/ha for 0.5 t/ha of concentrate fed or 4 kgP/ha where 2.0 t/ha of concentrate was fed.

Also included in this section are the optimum pH levels for different land uses, the recommended application rates of sulphur for different land uses, conversion factors for different units and a useful table of the standardised available nutrients in different slurries and manures.

Dairy grazing

To calculate the net off-take of P and K in a dairy grazing scenario you must go to the orange coloured segment of the wheel and turn the dial to the 131-170kg orgN/ha level. The dial then shows a 14 kgP/ha requirement where no concentrate is fed, 9 kgP/ha where 1.0t/ha is fed and a net gain to the soil of 1 kgP/ha where 3.0 t/ha of concentrate is fed. In this case the K off-take is 30 kgK/ha.

Silage

To calculate the net off-take of P and K for silage go to the blue coloured segment of the wheel and turn the dial to have blue in the column background and select the silage dry matter yield level you want. For a 7.0 tDM/ha crop of grass silage the dial shows an off-take of 28 kgP/ha plus 175kgK/ha. The same column shows an off-take of 37 kgP/ha plus 177 kgK/ha from a 14 tDM/ha yield of maize silage and an off-take of 38 kgP/ha plus 98 kgK/ha from a 10t/ha crop of wholecrop silage.

Drystock grazing

To calculate the net off-take of P and K in a drystock grazing scenario you must first go to the green coloured segment of the wheel and turn the dial to the appropriate stocking rate as represented by the same colour and the level of total organic N produced on your farm. If we take the 131-170kg orgN/ha level then the dial shows a 10 kgP/ha requirement where no concentrate is fed, 5 kgP/ha where 1.0t/ha is fed and a net gain to the soil of 5 kgP/ha where 3.0 t/ha of concentrate is fed. But in all instances the K off-take is 15 kgK/ha.

Tillage crops

On the tillage page there is a useful table outlining the magnesium requirement of different crops. In the orange cereals section a 10t/ha crop of winter wheat or barley will remove 38 kgP plus 98 kgK/ha; a 9.0t/ha crop of oats removed 34 kgP plus 130 kgK/ha. A 4.5t/ha crop oilseed rape will remove 27 kgP plus 41 kgK/ha where the straw is not removed, while the same yield of beans will remove 22 kgP plus 45 kgK/ha. For root crops a 55 t/ha yield of beet will remove 19 kgP/ha plus 77kgK/ha while the same yield of potatoes will remove 50 kgP/ha plus 265 kgK/ha. In all instances the yield selected builds in the additional nutrient allowance for higher historic grain yields. Sometimes the K off-take in straw may be higher where a low grain yield threw up a big crop of straw. And it is possible that the nutrient off-take for wholecrop cereal silage may be higher than the numbers indicated because the immature straw may not have returned its full complement of nutrient to the soil before cutting.