What's the best way to deal with high pH soils?

Wednesday, August 10, 2011

In most cases, high pH is a symptom of a problem elsewhere in your production system, so addressing it without dealing with the root causes will be ineffective

by KEITH REID

Occasionally, farmers will complain that their soil pH levels are increasing and demand some way to bring the levels back down. They are generally quite disappointed when I tell them that anything they try will be expensive and not likely to be effective anyway. 

To avoid this unpleasantness, it is worth considering what may be causing soil pH to increase (whether real or apparent), and how changes to farm practices may prevent soil pH changes.

Factors in pH change. Most natural processes tend to cause soil pH values to decline over time. Rainfall is naturally slightly acidic, because dissolved carbon dioxide forms carbonic acid. The nitric and sulphuric acids added from car exhausts and industrial pollution increase the amount of acidity, but not the fundamental nature of precipitation.     

Breakdown of organic matter also creates organic acids. The conversion of ammonium (from fertilizers or organic sources) to nitrate releases a significant amount of acidity.  Growing crops with high nitrogen requirements accelerates the natural acidification of the soil.

Increasing levels of soil pH indicate that something is happening to counter these natural processes. The most common culprit is tillage, and particularly the moldboard plow.  Many of our subsoils are alkaline, so tillage that brings subsoil or parent material to the surface will cause an increase in soil pH. This occurs where erosion has thinned the topsoil, so tillage at a constant depth is going to bring subsoil to the surface.

On some knolls, the topsoil and subsoil have been completely depleted, so each tillage pass spreads alkaline parent material over a larger and larger area. Aside from having poor soil structure and low native fertility, the high pH in these areas can tie up phosphorus and micronutrients, creating deficiencies for crop growth. The purchase of larger tillage equipment will also often lead to deeper tillage and an accompanying increase in soil pH, although this will tend to stabilize quickly at the new levels.

Occasionally, some external addition of material will cause soil pH to increase. The dust from gravel roads or laneways is often high in calcium carbonate, effectively liming the areas where this dust falls. Rarely, the seepage from springs or around oil wells may be high in minerals that raise soil pH, although this effect is very localized.

Preventing or mitigating these situations means eliminating the source of alkalinity. Tile drainage to intercept mineral-rich spring water, paving roadways and reducing tillage can all prevent further increase in soil pH in the affected areas.

Apparent increases in soil pH. I find that, for every case of a genuine increase in soil pH, there are probably two that are artifacts of either the sampling or analytical technique. These can often be sorted out by looking at the records of past soil tests to see if there has been a gradual but consistent increase in pH over time (suggesting a real change), or if there has been a sudden spike in values.

If tillage bringing subsoil to the surface can increase soil pH, then deep sampling that includes subsoil in the sample can do the same thing. Consistent sampling depth is important, but it can be difficult to achieve on some soils.

The way that pH is measured in the lab can also change pH values. On very sandy soils, the measured pH will increase as more water is added, since the pH meter is measuring the concentration of hydrogen ions in the solution and the wetter sample is more dilute.  This effect is much smaller on loamy or clay soils, since there is enough acidity bound on the soil to buffer this effect.

There is also a certain amount of variation allowed (and expected) within the lab, so some apparent differences may reflect the natural variability in the soil and the analysis. If you get results from the lab that don't seem right, don't be afraid to call and ask them to re-analyze the samples.

Can I "fix" high pH? In most cases, high pH is a symptom of a problem elsewhere in your production system, so addressing pH without dealing with the root causes will be ineffective. Trying to lower soil pH, particularly if it is above 7, is not likely to be effective and may not provide any agronomic benefit, since most crops will tolerate high pH much better than low pH. 

You can focus on the direct impacts of elevated soil pH on nutrient availability by banding phosphorus fertilizer rather than broadcasting it, and scouting fields regularly for deficiencies of micronutrients like manganese or zinc. BF

Keith Reid is manager (Eastern Canada), Soil Nutrient and GHG Management,
Agriculture and Agri-Food Canada, Guelph.