A three-step approach to tackling the most common soil fertility issues


They don't come cheap, but fertiliser programmes to address soil fertility issues are not complicated
They don't come cheap, but fertiliser programmes to address soil fertility issues are not complicated

Richard Hackett

New research from Teagasc shows that while there is finally some improvement in in terms of pH, Phosphorus (P) and Potassium (K) levels, our soils are still far from optimum fertility and the requirement for significant investment in fertilisers continues.

While they don't come cheap, fertiliser programmes to address soil fertility issues are not complicated.

There are three layers to a fertiliser programme: the soil pH level; the Phosphorus (P) and Potassium (K) status, and the appropriate nitrogen (N) rate to be applied.

Unless the soil pH level is at a workable level, growth response to any nutrient applied, whether that is N, P or K will always be disappointing. If the soil P levels or soil K levels are deficient, response to applied N will also be muted.

There is no point applying more nitrogen on a soil that is deficient in lime. The crop, be it grass, cereal, potato or vegetable, just won't respond.

A soil sample costing €30 will determine the soil pH level. One tonne of lime costs slightly more than one 50kg bag of compound fertiliser. If the soil is lime deficient, that bag of compound won't work the way it should. If a soil is seriously lime deficient, it is better to spend money on rectifying the soil pH first and spend little or nothing on any other fertiliser.

The application of lime often has an added benefit of releasing previously applied nutrients that were unavailable as they couldn't be 'activated' because of the soil pH issues. So in the absence of applying other nutrients, lime application can result in the soil supplying other nutrients to the crop from reserves, so there is a 'double whammy' benefit from applying lime.

Once the soil pH has been addressed, the next step is to build up the P and K levels.

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There are two main sources of P and K: application of inorganic or chemical fertiliser and through recycling of organic nutrients.

For a well-run livestock farm, the recycling of nutrients is normally a simple process: apply the slurry or farmyard manure back on to the land from which the silage or hay that was fed to the livestock came from.

Where soil P and soil l K level are at optimum levels, the only P and K offtake from a livestock farm is through sales of livestock, meat or milk. Concentrate feed brought on to the farm can replenish a lot of this nutrient loss, as concentrate feed contains quite a lot of P and K.

So for a farm with good overall soil nutrient status, the overall P and K requirement is relatively modest on an ongoing basis.

Where soil P and K levels are high in a highly stocked holding, it may not be the best economic or environmental use of these expensive and useful nutrients to recycle them back on the farm.

A well-designed nutrient management plan is invaluable in determining where the excesses are and where more nutrients can usefully be applied.

Organic manure sources, whether from cattle slurry, farmyard manure, pig and poultry slurry, or spent mushroom compost, are very viable sources of P and K to meet tillage crop requirements. The benefit in terms of nutrient loading is only part of the picture.

Biological activity

Organic manures bring biological activity to our increasingly damaged tillage soils, and the benefit from applications of manures over and above the nutrient content is becoming more and more apparent to those that make the effort.

This brings us to the third layer of the process: nitrogen application.

We are in a fortunate position this year in that many grass and cereal crops are coming into the spring in a very strong position.

In many cases Nitrogen applications will have to be adjusted to take this strong growth into account.

Whether grass or cereal, initial Nitrogen application may be delayed, reduced or eliminated altogether. This can reduce costs, which is a straight benefit.

However, it is also crucial to take over-winter uptake into account as excessive early growth may cause lodging, which will reduce grass quality and be hugely detrimental to cereal crop yields.

Richard Hackett is an agronomist based in north Co Dublin and is a member of the ITCA and ACA

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