Soil PH; What’s the Scale of the Issue?

The issue of soil pH and its importance receives a lot of publicity.  The key message is that at lower pH the efficacy of chemical fertiliser and availability of soil nutrients is reduced.  However despite a huge effort by Teagasc to promote the significance of the issue some 63% of soils are suboptimal pH.

The scale

The scale is open to misinterpretation.  When reading the pH of a soil sample it is important to remember that pH is based on a scale between 0-14. 7 is neutral, lower than 7 is acidic and above 7 is alkaline. 

The target for grassland is a pH of 6.3.  The trap that many people fall into is that we read the result of a soil test as % of the target that we wish to achieve.  For example one field tests at pH of 6.  This could be interpreted that we are 95% of our target pH. Which many of us would be delighted with in an exam situation. 

In reality, however, it is not that simple because of the logarithmic scale used to measure pH.  In simple terms, every 1 unit drop in pH (e.g. pH 5 compared to pH 6) is 10 times more acidic.  Every 0.1 drop in pH (e.g. 6.2 compared to pH 6.3) is 26% more acidic.  So, the pH of 6 on our soil test is actually twice as acidic than a pH of 6.3 when you compound the impact of each 0.1 pH drop.  It sounds complicated, but its just the way the maths work!

The way the scale works also means that the absolute difference in the level of acidity between soils as they drop each 0.1 pH unit gets higher as the pH gets lower.  This means that it takes more lime to shift soil pH when the pH is lower.  Hence, it takes a lot less lime to maintain soil pH around the target than it does to lift it from a low level of say 5.5

The effect of a suboptimal pH can drastically reduce the efficiency of chemical fertiliser that is spread and the availability of soil nutrient reserves.  At lower soil pH levels, the efficiency of N, P K and other nutrients can be dramatically reduced.  This is a serious issue for fertiliser efficiency, especially as both N and P are heavily regulated under the nitrates directive. 

The cause 

Much of the soils in Ireland naturally have a requirement for lime.  However rain is a huge factor attributing to soil acidity, and it usually is not in short supply.  Increased use of chemical fertiliser, increased use of slurry generated by ever increasing stocking rates also cause increased acidity.

Fertiliser spreader

The solution

The solution is to use lime.  Don’t apply more than 3 tonnes per acre per application as nutrients can be locked up by excessive lime in a single application.  Lime can be applied in a grazing situation at any time of year and once the lime has been washed fully from the leaf the grass can be grazed.  One of the few things to be careful of is not to apply it too close to the harvest date in advance of silage, as it could impact on the Ensilability in the pit.  For spreading on silage ground, try to get it spread before early March.  Otherwise it is better to wait until after cutting silage.

Maintaining good soil pH

While it’s all very well to get the soil pH right, it’s also worth thinking about how to keep it right!  Remember, it will take a lot of lime to lift the soil pH, but it takes relatively less in terms of tonnes/acre of lime to keep it right. To maintain soil pH, physiolith soil conditioner is a perfect solution.  Physiolith is also a useful product to use on high Molybdenum soils where the application of lime would cause a large increase in soil pH and increase molybdenum levels which may subsequently result in animal health issues.  Physiolith also increases soil health and stimulates root activity.

Author:  John O’Loughlin; Grassland Agro Specialist Advisor