It is recommended that soil be tested once every 4-5 years. However, some intensive farmers carry out soil tests on a yearly basis to build an accurate picture of the soil nutrient balance.

Back in the 1950s, most soil types in Ireland were low in P (phosphorus) and K (potassium); this limited the productivity of soils. The decades thereafter saw an increase in the use of chemical fertilisers to increase productivity.

As a result the levels of P and K increased up to and into the late 1990’s. With the high participation in Rural Environmental Protection Scheme (REPS) and rising fertiliser costs, the use of P and K has reduced in recent years.

Trends in soil fertility

Soil indexes for P and K range from index 1 meaning very poor, to index 4, which is high. The ideal soil P and K index is 3; this level of P and K ensures that soils are fertile for growing crops, while P loss from the soil is maintained at a low level.

An analysis of soil sample results which were submitted through Teagasc advisers from 2001 to 2011 was undertaken last year by Teagasc. This shows that the percentage of soils with poor fertility for P (index 1 and 2) increased from 39% in 2008 to 54% in 2011. At the same time, the percentage of soils with a P index of 4 decreased from 31.9% to 21.6%, a drop of 10.3%.

Teagasc have said that a similar trend has appeared for the levels of K in soils. The percentage of soils with a K index of 1 or 2 has increased from 36% in 2001 to 55% in 2011.

In regards to soil pH, Teagasc analysis shows that over 60% of drystock farms would benefit from some level of lime application. Typically, a higher percentage of tillage land is of higher pH than dry stock farms.

Targeting index 3 for P and K

Soil index 3 is required to achieve optimum plant growth. P is extremely important for grass rooting and growth, particularly in the shoulders of the year. Participants in the Teagasc/Irish Farmers Journal BETTER Farm Programme have found that the use of compound fertilisers containing P and K has increased grass growth rates and has resulted in a thicker, denser sward also.

pH

Often farmers forget of the importance of maintaining a correct soil pH. Clay soils should have a pH of between 6 and 6.5, and ideally over 6.3. Peat soils should have a pH of approximately 5.5.

By increasing the pH of clay soil to 6.5 from 5.5, up to 40 units of nitrogen can be released from the organic matter in the soil per year. The value of this extra nitrogen released can help to offset the cost of lime. It is not recommended to apply more than 2.5t/acre of lime at a time.

Liming will help to break down an old sod and can lead to ground becoming more tender. For this reason, Teagasc advise the little-and-often approach to lime application. When reseeding, approximately 2.5t/acre of lime will be required to neutralise the acidity associated with the break-down of the sod.

In high molybdenum areas, lime application can lead to increased availability of molybdenum in grass, which in turn will reduce the uptake of copper by grazing livestock.

Having a soil pH of less than 6.5 can result in reduced levels of P availability in soil, thus exaggerating a problem caused by low soil P levels.

Sulphur levels

Sulphur (S) deposition from the atmosphere has declined steadily due to reduced use of fossil fuels. Since 1990, S deposition has more than halved. Areas around large towns and cities have the highest rates of sulphur deposition, while in the west and north-west, low levels of sulphur depositions occur.

In many cases, and especially on sandy soils, S application is required. Where S levels are low, applying S can result in improved rooting of grass and better growth, particularly in late season and in the autumn. Applying S on a deficient soil can result in a much stronger green colour, increased herbage production of up to 20% and increased carbohydrate content.

Teagasc studies have also shown that S fertilisation on deficient soils can reduce the molybdenum concentration in grass and reduce the copper levels in the blood of grazing livestock too.