Your observation that pH only dropped in peat not in mineral soils after droughts is probably because droughts led to remarkable organic matter accumulation in peat not in mineral soils. The accumulation of organic matter may contribute the decrease in pH.

Role of soil moisture in the soil medium influence the size and quantity of microbial world. Microbial biota play significant role in the decomposition of organic matter. Decomposition rate of organic matter is influenced by the quality of organic matter. If water (moisture level is completely evaporated or possibly removed from it ) content is completely absent from mineral layer of soil, then ionic imbalances will takes place in the soil. Retention of bases in the peat litter (litter chemistry is very important) and slow rate of decomposition may cause a drop in PH in the mineral soil.

Does the drop occur in both pH(KCl) and pH(H2O) or only in the latter? Just above a redoxfront, it is possible to have a drop in pH(H2O), probably due to (re-)oxidation of e.g. reduced iron, releasing protons in the process. The pH(KCl) remains unaffected and the soil's EC increases when this happens. I can imagine that a similar effect may occur when a relatively wet soil (containing reduced iron) dries out significantly...I don't know of any publications describing such a thing for present-day mineral soils, but we describe the redoxfront-drop in palaeosols on loess (Vancampenhout et al., 2013 in Catena 107 (2013) 118-129 and Catena 110 (2013) 8-23)... maybe this can inspire you to find a possible mechanism for your soils?

This is a common observation in mineral soils dominated with variable-charged minerals such as Al and Fe oxides and hydroxides. Moisture has an impact on the concentration of counter-ions at the surface of minerals (i.e. the ionic strength), which has an impact on the charge of surface. The effect of ionic strength on surface charge and pH depends on the point of zero charge (PZC) of the variable charged mineral itself. The result is that if the pH of the system is greater the PZC, then an increase in ionic strength (i.e. increase in "salt" concentrations) will cause the surface charge to become more negative, and the pH to decrease. This is partly why for most soils, we find that a pH measured in KCl will have a lower pH than if the pH was measured in water. However, if the pH of system is less than the PZC, then an increase in ionic strength will actually increase pH. This is only true for variable charged minerals, not for permanent charged minerals such as vermiculite. A drought-stricken soil often has a higher ionic strength. Usually, pH is greater than PZC. I recommend reading Uehara and Gillman, 1983; van Olphen, 1977; Dyer, Kopittke, Sheldon, and Menzies, 2008.

There's the historic work of Brinkman (1970) that should be appreciated (related to Vancampenhout), work that was recently discussed in Fimmen et al. (2008).

Brinkman R (1970) Ferrolysis, a hydromorphic soil forming

process. Geoderma 3:199–206.

Fimmen, R.L. et al. (2008) Biogeochemistry

I have seen a substantial drop in measured soil pH in a forest soil horizon relatively high in organic matter. But these were stored for a period of several years in a dry (air dry) state. Both salt exchangeable hydrogen and aluminum ( 1N KCl) increased substantially. One assumes the driving process was slow oxidation of the organic fraction. Dan Richter will remember one of these soils as a quality control soil we used in the forest response program here in the United States (circa late 1980s and early 1990s). Since you reference drought conditions, you may have experienced simply more oxidation of the peat. The only other thought that comes to mind is possible large release of N in dried organic horizons that rewet, usually with fall rains. We have also seen large pulses of nitrates in disturbed organic soil systems with accompanying decrease in pH as well, even if the samples are kept at 4C from the field to the laboratory.

Yes. Drying of soil results in a net decrease in pH in mineral soil, especially, the one with higher amount of clays.

Cause: Drying cause accumulation of cations (e.g. Ca2+, Mg2+, Na+) in solution as well as increase the "effective" polarity (more positive) of these cations in clays. Thus, these cations polarized the water near surface of clays resulting hydrolysis (loss of proton). These added proton increase the acidity of water.

I would be careful in making it general statement, because the rate of hydrolysis depends on other soil property such as organic carbon content (which can adsorb the proton), presence of metal oxides etc.

For more details: https://dl.sciencesocieties.org/publications/sssaj/abstracts/44/4/SS0440040721

I like to keep things simple (when it makes sense). Dessication of peat> ^ oxidation> lower pH. That said the references cited above are good reads for anyone interested in the historically much neglected area of organic soils geochemistry.

I want to thank everyone that suggested organic soil refs for my research. I have obtained the articles and am going through them and as always marking the refs within for more acquisitions

Let's think

In the soil solution has concentration of salts. If you lost water the concentration up. If concentration has more H, the pH down, if more OH the pH up. The losses is just water, the concentration of salts is the same.

Many good answers and thoughts have been provided above. I would think that, generally, as soil profiles dry out due to drought, there would be an accumulation of basic cations (Ca, Mg, Na, K) in the surface layers, hence, the pH should increase in mineral soils. However, for peat or swampy areas, as others have pointed out there could be a concentration of organic matter which could lead to a slight lowering of the soil pH.

In my opinion, nitrification can drop soil pH in all soil, expecially soils with long term of nitrogen input. In northern China, it is not uncommon that nitrogen or nitrogen with potassium treatments have a slight low pH when compared with treatments of nitrogen with phosphorus or nitrogen with animal manure. This is even true in soils of the north-west China, eg., in grey desert soil of Xinjiang, China. For the mechanisms, maybe the irreversible collapse of minerals around roots takes effect. Anyway, plant growth is important for drop of soil pH.

Dear all,

many thanks for the number of detailed responses! I will explore some of these references and see how they can help me explain this observation. I have plans to investigate this a little further with a manipulation experiment to test some of my initial ideas, and I will certainly benefit from including some of the above comments too.

Many thanks, and I hope I might return the favour.

rob

You already got many answers, and the simplest one looks excellent to me: drought leads to oxidation and thus acidification.

You probably have to take into consideration the possible presence of pyrite in your mineral soil. Oxidation of one mole of pyrite (FeS2) produces 4 moles of sulfuric acid.

Under drought condition the deficit of Hydrogen and oxygen from the undecomposed organic matter more specifically the organic acid groups results negative charge which further chelates/complexes with the basic cations in the mineral soils resulting in the drop of pH.

Robert

Here in the Great Plains of the US (semiarid/arid) we tend to have neutral pH soils. However when organics are land applied (biosolids , manures) or with any of the conservation tillages practices utilized (leaving plant residuals on the surface and in the soil) we develop an acid layer of soil at the surface to ~15 cm depending on tillage depth. This is due to N minerialization of the organic material releasing excess H+ to the soil solution. If you sample the soils at various depths you'll find the acid soils to ~15 cm (pH ~5.0) and more neutral soils below that (7.0 -7.5).

It may depend on the region. In semiarid regions where drought is an usual phenomenon due to limited rainfall, soil pH is normally high due to greater presence of basic cations, such as Ca, Mg, and Na. In humid regions with low soil pH, occasional drought may not make much difference in pH.

In our experience soil organic matter left on top of the soil ( Ultic Haploxeroll) in conservation agriculture leads to a slight decrease of soil pH in the upper (0-2 cm) of soil. This is a case of accumulation of un-decomposed organic matter on top of the soil and a drop in pH. ( Martínez, E., Fuentes, J.P., Pino, V., Silva, P, Acevedo, E. 2013. Chemical and biological properties as affected by no-tillage and conventional tillage systems in an irrigated Haploxeroll of Central Chile. Soil & Tillage Research 126:238-245).

Hello,

I don't know exactly what is your "mineral soil", and what is the OM content. Even if it is a very poor organic soil, biofilms can develop during wet phases on mineral surfaces and die when a threshlod minimum water content is reached. Then, when you measure the pH, you solubilize this "died" OM that contains hydroxyl, carboxyl and amin functional sites susceptible to raise the proton content in the solution and drop the pH.