Farm Ireland

Saturday 20 January 2018

Tackling soil fertility issues

Incorrect fertiliser application rates could affect farm output

Mark Plunkett

THE RECENTLY published Irish Fertiliser Use Survey shows falling fertiliser usage over the years it covered (2004-2008), compared to the previous survey in 2003.

Phosphorus (P) and potassium (K) usage has dropped by around 50pc, which is now similar to levels applied in the 1950s (see graph, below). This is a big concern as lower fertiliser usage will run down soil reserves and cut animal and crop production.

For example, the average use of nitrogen (N), P and K on grass during 2004-2008 was around 85kg/ha, 5kg/ha and 14kg/ha respectively. These figures indicate a 30pc reduction in N application, 55pc in P and 48pc in K compared to 2003.

These effects will all depend on the soil's fertility status and the production system. Where soil fertility levels are high (indexes 3 and 4) and there is good recycling of farm manures, effects will not be large as soils will supply crop nutrient needs.

But where soil fertility levels are low (index 1) and there is poor redistribution of farm organic manures, farm output can be reduced significantly. For example, a crop of grass silage will remove around 20kg P/ha and 120kg K/ha in a single cut. Where a field receives no organic manure due to location, the removal of P and K will need to be replaced by chemical fertiliser.

Reduced fertiliser usage has been driven by the difficult economic conditions in the past few years, coupled with new nutrient legislation, which has placed limits on N and P applications and improved usage of organic manures.

A similar trend is also shown for spring barley, where the average N, P and K application rates were 118kg/ha, 21kg/ha and 45kg/ha respectively.

The usage of N was similar to that advised, while the P and K usage was below advised rates. Since 2003, N usage has fallen by 4pc, while P and K usage shows larger drops of 19pc and 18pc respectively.

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Nitrogen usage has remained close to advised rates, while P and K applications have been reduced in order to cut fertiliser costs. This is not a good strategy as N alone will not be sufficient to produce top-yielding crops.

For example, a crop of spring barley will remove around 25kg P/ha and 85kg K/ha (grain and straw removed), which is greater than fertiliser input.

In the past decade there has been a trend to apply more high N compounds, such as 27-2.5- 5.0 or 24-2.5-10, rather than a 0-10-20 or 0-7-30. In addition, straight N has been applied as urea/CAN to reduce costs.

Where P and K usage continues to decline in grass and tillage systems, it will reduce the output from poor grass growth in the spring to low-yield crops.

Insufficient P and K usage creates a negative balance between crop off-takes and inputs. This, in turn, will reduce soil fertility levels, which will have to be replenished at a later stage and prove more costly as fertiliser resources decline.

To ensure that correct decisions are being made in relation to fertiliser applications, it is essential to have an up-to-date set of soil samples for your farm. Soil samples should be taken once every five years to ensure good, effective nutrient advice.

In the absence of soil test results, apply crop maintenance requirements and take soil samples over the coming months to plan and optimise crop fertiliser requirements.

Irish Independent