Over the last 10 years, Teagasc has analysed an average of 38,694 soil samples annually for its farmer clients. Given the geographical spread (all counties) and large numbers of soil-sample data included, the Teagasc soil nutrient data has provided valuable insights into soil fertility trends at a national level. This information has helped to highlight the areas and farm enterprises where action on nutrient management is most required.

In this article, we review soil fertility trends for soil pH and the major nutrients – phosphorus (P) and potassium (K) – emerging from these soil sample results.

Overall soil fertility

Overall soil test results for 2015 (Figure 1) indicate that 11% of soil samples have the optimum mix of soil pH, P and K, which is the same as the previous two years.

A closer look at this data indicates that soil P and K levels have declined compared with the previous two years, which is of major concern. Soil pH has shown a small improvement, with an increase in the percentage of soils achieving a pH of at least 6.2.

Grassland soils

Trends in pH, P and K levels in grassland soils from 2007 to 2015 are shown in Figure 2.

Currently, 64% of grassland soils have a soil pH of below 6.2 (target pH threshold for efficient grassland production). In 2015, the soil data indicates a slight reduction in the percentage of soils with less than a soil pH 5.9. The percentage of soils analysed in the pH range just below the target (ie pH of 5.9 to 6.2) remained stable and soils with a pH above 6.2 increased by 3%.

Since 2010, there has been a trend of decreasing proportions of soil samples with a soil pH below 5.9. This is positive but the pH results continue to indicate that there is a large requirement for lime applications on most grassland farms.

Over the last decade, the percentage of soils with P Index 1 and 2 has increased from 40% in 2007 to 61% in 2015. Between 2009 and 2012, there was a sharp increase from 40% to 59% in the number of soils that are sub-optimal for P (ie Index 1 and 2 combined). This is likely to be connected to the reduced fertiliser P usage in the previous three years from 2007 to 2009.

Between 2012 and 2014, there was an indication of a potential recovery in soil P test levels on grassland farms. However, the most recent soil test results (2015) show a return to 2012 levels of P deficient soils at 61% (ie Index 1 and 2 combined). This indicates that there are insufficient P fertiliser applications annually on Irish grassland farms and that P offtakes are exceeding P inputs, resulting in a further decline in soil P levels.

These trends in grassland soil P fertility between 2007 and 2015 clearly show that the production potential of our grassland soils is being slowly eroded. The declining soil P fertility trends are serious and, if allowed to continue, pose a serious threat to the expansion of our national livestock sector (dairy and meat output).

Soil K fertility trends somewhat mirror those of soil P. Since 2007, the percentage of soils Index 1 and 2 increased from 40% to 54% up until 2011. Between 2011 and 2015, soil K levels have somewhat stabilised with approximately 54% of soils with sub-optimal K levels (Index 1 and 2). The same is true for soils at Index 3 and 4 where there was a rapid decline between 2007 and 2011 (60% to 46%), followed by stabilisation between 2011 and 2014 (46% of soils). This stabilisation in soil K levels may be due to the better management and targeted use of organic manures (cattle slurry and farmyard manure) on grassland farms.

Tillage soils

Trends in pH, P and K levels in grassland soils from 2007 to 2015 are shown in Figure 3.

On tillage farms, the percentage of soils below the target pH of 6.5 decreased by 6% in 2015. Currently, approximately 45% of tillage soils have a soil pH of at least 6.5. Again, these results indicate that a large percentage (55%) of tillage soils have a lime requirement. Phosphorus fertility trends for tillage soils are similar to those described for grassland. Across tillage soils, there has been a more gradual decline in soil P levels since 2007, with 59% of tillage soils having sub-optimal P levels (Index 1 and 2 combined).

Over this time, there has been a large proportion of tillage soils (currently 36%) at Index 2. There has been a relatively small reduction in the proportion at Index 3 from 26% in 2007 to 21% in 2015, representing a 0.5 percentage point decline per year. The largest changes are in the P Index 1 and 4 category soils where the proportion of high P soils has declined by 9% and the low P soils have increased by 10% – a rate of change in both of these categories of approximately 1% per year.

Between 2007 and 2011, the percentage of tillage soils with low K levels (ie Index 1 and 2) has increased from 50% to 57%. This is related to the reduction in K usage in fertiliser applications during that period from 84,000t to 71,000t. Between 2011 and 2015, the proportion of soils with sub-optimal soil K levels (Index 1 and 2) has decreased from 57% to 47% (slight increase in 2015 on 2014). Soils with K Index 3 levels have increased from 24% in 2007 to 30% in 2015.

There was a rapid decline in the proportion of soil with K Index 4 between 2007 and 2011 from 26% to 17%, respectively. Since 2011, soils with K Index 4 have increased from 17% to 23%.

Overall, in the last five years, soil K on tillage farms has shown a marked improvement and positive trend in terms of the percentage of soils at Index 2 and 3.

Teagasc K advice for cereals was changed (2008) to take account of higher-yielding cereal crops. Fertiliser practice has change at farm level where fertiliser compounds now have altered P to K ratios to improve the supply of K requirements to crops.

Productivity

Soil fertility is a key driver of the productivity of our farms and where soil fertility continues to decline it erodes farm productivity, profitability and competitive advantage in the marketplace. More attention needs to be placed on soil test results from individual farms.

These national soil test results indicate a large percentage of our grassland and tillage soils would benefit from an application of lime. Correcting soil pH is the primary step towards improving soil P and K availability.

Utilising organic manures and selecting a suitable P to K fertiliser is critical to delivering the required nutrient balance for both productive grassland swards and high-yielding cereal crops.

To read the full Fertilizer Focus Supplement click here.