Balancing slurry effects with appropriate lime application

RUMINANT animals pro-duce a great deal of slurry. This slurry has to be disposed of and most slurry will be applied to fields on the farm. With the controls and restrictions governing the application of slurry, we can be forgiven for sometimes forgetting the value and benefits that it contains.

It will provide a good source of N, P, K, Mg and trace elements to the soil to assist in the growing of both quality and quantity.

However, it is fair to say that like everything in life, ‘too much of a good thing is not good for you.’

So it is with slurry. As we all know slurry is acidic and most Northern Ireland soils are acidic, and together they form a tag team that will inhibit grass growth by not allowing all of the nutrients in the slurry available to the growing crop. (See Table 1 below.)

They will remain locked up in the soil, increasing index levels, therefore restricting future slurry application rates.

Thus it is good practise to apply a high Calcium lime either as bulk lime or as Granucal granulated lime to the pasture that is receiving a regular dressing of slurry.

To neutralise the acidifying effect of every 1,000 gallons of slurry that is applied, it will take an application of approximately 20 kilos of bulk lime each year.

This takes no account of other acidifying effects from other sources such as fertiliser applications and rainfall.

Silage fields are not ploughed on the same rotational basis as arable fields and this can lead to a build-up of acidity on the surface (see Table 2, above).

The pH scale is a logarithmic scale so fields at pH5.5 are ten times more acidic that fields at pH6.5. (The difference between pH5.0 and pH7.0 is 100 times.)

Further research work (Murphy, Stevens & Christie at Hillsborough, County Down) shows that slurry treated fields will suffer from the levels of Salt (Na) also building up at the surface of the pasture.

This disruption to the balance within the soil of the nutrients will result in the break down of the soil structure and the formation of a barrier, that will lead to the field not being able to drain as well and to the very visible poaching of the field.

The Hillsborough work has also highlighted the unseen result of slurry application, that there will be the larger risk of surface run off of Phosphate, that will also increase the possible contamination of waterways and all the trouble that brings.

If the Phosphate is running away it will not be available to grow the next crop of grass.

With an annual application of lime it is possible to avoid this by adding Calcium to the soil to rebalance the soil content driving off the excess salt and allowing the Phosphate and other major elements to be available for plant up take at the correct time in the spring and through the growing season.

Slurry can either be a great asset or a great hindrance, with the regular use of lime it can be a means to overcome some of the potential issues encountered.

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