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Act now to stop trend of declining soil fertility

A recent review of the soils analysed by Teagasc highlights an alarming drop in soil fertility, especially in phosphorous (P) and potassium (K) levels. In addition, nearly two-thirds of grassland samples tested below the optimum soil pH 6.

The decline in soil P and K levels has only become noticeable in the last 3-4 years and is probably a direct result of the steady decline in fertiliser usage from 1.7m tonnes in 2000 to 1.4m last year. While declining fertiliser usage has benefits from both cost base and environmental perspectives, it is still critical that we maintain fertile soils if we are to have any hope of competing with other farming nations as Ireland embarks on its Food Harvest 2020 plan for growth.

Soil Fertility Trends

Figure 1 (right) shows the different soil indices for P over the past four years of soil samples. From 2001-2008, the proportion of soils in each P index were relatively stable. However, since 2007-08, there has been an increase in low fertility soils at index 1 and 2 and a decrease in high fertility soils at index 3 and 4. The 5pc drop in the proportion of soils in index 4 is a positive, since it is more economical to reduce fertiliser applications on such fertile soils in addition to environmental benefits. However, soils tested in the last three years at the optimal index 3 have also declined from 30pc to 25pc and this is bad news for a farm sector hoping to compete with the best. The 25pc increase in the soil samples showing a low fertility index 1 and 2 is the final nail in the coffin.

Trends in soil K levels over the same period are shown in Figure 2 (below right). Similar to the trends in our P, the levels of K in our soils are on the slide, albeit at a less dramatic rate. Soils in index 1 and 2 have increased from 40 to 45pc in the last three years, although soils reading index 3 have increased slightly during the same period. Soils tested at index 4 have declined significantly from 33pc in 2008 to 24pc, which is positive in terms of fertiliser savings and the environment, but indicates a greater problem of declining fertility on Irish farms.

Implications of low P and K fertility:

•Soils in P index 1 will produce around 1.5t/ha less of grass dry matter compared with soils maintained at the target soil index 3. This is a potential loss of €345/ha. Worse still, the most valuable growth early in the spring is particularly affected by low soil P, while silage yields are very sensitive to soil and fertiliser K levels.

•Low fertility soils will have an increased need for grass reseeding as more productive rye-grasses and clovers will be less persistent on low fertility soils.

•Low fertility soils will also reduce grain yield by 1t/ha in a crop of spring barley.


•Establish soil fertility on your farm through regular soil testing every 3-5 years.

•Target livestock slurry and manures on fields with the lowest soil P and K levels to help build soil fertility to index 3.

•Apply lime as recommended on soil test report. Where soil magnesium levels are low, apply magnesium limestone.

•Develop a fertiliser spreading plan to build soil fertility levels to index 3 for maximum production. Be prepared to invest in P and K for this purpose.

•Soil fertility is a long-term investment and needs to be reviewed every 3-5 years to plan future fertiliser applications.

Over the past four years, soil pH trends have been relatively stable, with around 60pc of soil samples tested from dairy and drystock farms below a soil pH 6 (see the table, above). The target soil pH for grassland farms is pH 6.3-6.5. Soils maintained within the optimum range have the following benefits:

•Produce more grass each year.

•Use applied fertilisers and manures more efficiently.

•Ensure the persistence of more productive rye-grasses and clovers in swards.

•Grassland soils maintained at pH 6.3-6.5 will release around 60-80kg N/ha annually, which is worth €60-80/ha.

The majority of tillage soils are above pH 6.0, with around 25pc of soil below it. The aim is to maintain tillage soils at a pH 6.5 for high crop yields and efficient use of fertilisers. Lime should be applied to tillage soils based on the most sensitive crop to lime in the rotation. Again, apply magnesium limestone where soil magnesium levels are lower than 50ppm. Now is a good time to consider lime application to either tillage or grassland soils as it will allow time for lime to work over the winter period.

Soil fertility should be managed on a long-term basis with the aim of maintaining soils at index 3 for optimum production. Now is a good time to consider the soil fertility on your farm. It is worthwhile comparing old soil test results to current readings. This will provide a sound basis for tailoring a fertiliser spreading plan for the soils on your farm.

It will also help identify fields that possibly need extra nutrients in the form of slurry or manure which is a cost effective way of replenishing soil fertility levels. Soil fertility changes very slowly over time so a small annual investment in P and K now will pay dividends in terms of production in the future.

Mark Plunkett and Stan Lalor are Teagasc soil specialists based at Johnstown Castle

Indo Farming