By Liz Kalaugher
For many years, scientists have known there's a link between the amount of rainfall in a region and soil pH. Now, for the first time, a team from the US and China has investigated the soil pH–water-balance relationship on a global scale.
"Soil pH is important because it controls many aspects of soil fertility," Eric Slessarev told environmentalresearchweb. "We show that soil pH tends to be controlled by two chemical reactions, and that climate tips the balance between these two reactions, driving an abrupt switch from alkaline pH in dry climates to acid pH in wet climates."
Two minerals in soil – calcite (CaCO3) and gibbsite (Al(OH)3) – generally have the strongest effect on pH. Calcite precipitates from calcium ions and carbonate ions, which can form from dissolved carbon dioxide. In the lab, if soil is in equilibrium with calcite and atmospheric carbon dioxide, it will have a pH of 8.2, while if soil contains exchangeable Al3+ it will have a pH of around 5.
According to earlier, local studies, if there's more evaporation than rainfall, leaching rates are low and fewer calcium ions are washed from the soil, allowing calcite to accumulate, which raises the pH. But if there's more precipitation, leaching and run-off will increase, removing calcium ions and letting the relatively immobile aluminium ions accumulate. That buffers the soil pH to 5.1.
"Other factors – such as topography and rock chemistry – can also tip the balance, but climate appears to be a dominant control across the planet," said Slessarev. "This understanding has existed in pieces for years, but until now, the pieces hadn't been assembled and tested with direct observations at the global scale."
The most fertile soils generally have a moderate pH, but since climate tends to drive soil pH towards extremes, moderate pH soils are less common than acid or alkaline ones, the team discovered. "Because moderate-pH soils are limited in their extent, we need to tend them carefully to maintain their fertility," said Slessarev. "Soil pH can be actively managed, for instance, acid soils can be improved through limestone addition, but active management of soil pH represents an additional cost to farmers. Preservation of naturally fertile, moderate-pH soils may become especially important as climate change applies new pressures to global agricultural production."
That said, soils respond to climate slowly, over centuries to millennia. Slessarev reckons this could be a mixed blessing for global agriculture. "On the one hand, we don't expect fertile soils to suddenly become less fertile as climate changes; on the other hand, we can't expect to find fertile soils in places where the climate may become more favourable for agriculture in the future," he said. "For instance, many northern conifer-forest soils are acid and infertile because of their historic climate, and they will likely stay poorly suited for intensive agriculture even as high latitudes become warmer."
To come up with the results, the researchers used a 20,000 strong spatially random sample of soil-pH measurements taken from 60,000 data points combed from recordings from the International Soil Research Information Center in the Netherlands and agencies in the US, China, Canada, Australia and Brazil. "This allowed us to build a global-scale picture of soil pH, observing the effects of climate while allowing for all of the other sources of variation," said Slessarev.
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