This experiment has several treatments, and soil management has changed several times over the last ~180 years since the beginning of the experiment.  Not all treatments have behaved same way.  The decline in soil pH has happened largely in ‘NPKNaMg treatment’ (Fig 3 in Silvertown et al. 2006).  This decline in soil pH has been largely attributed to continuous application of ammonium fertilizer and atmospheric N deposition (which was in peak around 1970 to 1990 according to Fig 1a in Storkey et al 2015), in the absence of lime application.

Silvertown et al. (2006) http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2745.2006.01145.x/abstract

Storkey et al. (2015) http://www.nature.com/nature/journal/v528/n7582/full/nature16444.html

This experiment has several treatments, and soil management has changed several times over the last ~180 years since the beginning of the experiment.  Not all treatments have behaved same way.  The decline in soil pH has happened largely in ‘NPKNaMg treatment’ (Fig 3 in Silvertown et al. 2006).  This decline in soil pH has been largely attributed to continuous application of ammonium fertilizer and atmospheric N deposition (which was in peak around 1970 to 1990 according to Fig 1a in Storkey et al 2015), in the absence of lime application.

Silvertown et al. (2006) http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2745.2006.01145.x/abstract

Storkey et al. (2015) http://www.nature.com/nature/journal/v528/n7582/full/nature16444.html

Is there any relationship between soil acidification the articles ascribe to nitrogen  oxides and ocean acidification generally ascribed to carbon dioxide?

Dear Susantha Jayasundara, 

Thank you for the helpful information. But, I would like to receive information about possible natural causes of soil acidification. By the way, this is indicated by Rothamsted researchers.In the conclusions of the article” Soil Analyses in the Rothamsted Park Grass Experiment”  the authors (Zoltán GyőRi · Keith Goulding · Lawrence Blake · József Prokisch) write about the natural causes acidification soil experience ... What it could be the reason that so drastically reduced in the control soil pH (see figures Rt )? https://www.researchgate.net/publication/303633209_Soil_Analyses_in_the_Rothamsted_Park_Grass_Experiment

Sincerely

Chapter Soil Analyses in the Rothamsted Park Grass Experiment

Interesting question and equally interesting answers . How far , you account increasing menace of soil acidification to climate changes-related issues, especially increase in precipitation causing leaching of bases..?

I think the soil acidification is a natural process resulting from combination of 2 factors: 1) metabolic processes occurring in soil (mainly respiration); 2) water balance in the particular region.

Ad. 1. Soil respiration (animals, plant roots, many microorganisms), and possibly other metabolic processes produce CO2 which combines with water producing carbonic acid. It is a weak acid, but is produced all the time in the soil. This acid dissociates producing H+ ions, which replaces basic cations in sorption complex. But this is not sufficient to acidify soil;

Ad. 2. In the climatic zones, in which precipitations are greater than evapotranspiration, the water moves downward in the profile causing leaching of cations. It includes basic and acid (H+, Al+) cations, however in natural soils (without liming) the acid cations are input to the soil all the time (respiration), and the basic no. That's why in humid regions - both tropical jungle and Taiga the soils are acid.

On the other hand, not all natural soils in humid climate are acid, probably due to the fact of their parent materials, with high calcium carbonates content and high buffer capacities.

The human's activity may accelerate soil acidification (e. g. fertilization with urea or ammonium N fertilzers, but also K fertilization, emission of the S and N oxides to the atmosphere and many other activities) or temporally reduces it's results (liming). But basically, it is a natural process....

You can find description and also very interesting table showing acidifying power of various materials in Daniel Hillel (ed.) 2005 Encyclopaedia of soils in the environment.

http://www.sciencedirect.com/science/referenceworks/9780123485304

page 14, table 2: "Acidity production by various fertilizers...". For me it was strange, that N2 fixation is as or even more acidifying factor as compared with ammonium sulfate applications....

You should remember, that Great Britain has very humid climate ("English weather").

Best regards, Michal

Interesting response Michal . When we look at natural  main causes of soil acidification , which is comparatively slower process compared to acidification induced through anthropogenic reasons. Leaching of bases due to rainfall exceeding evaporation ( typical pf humid/humid tropical climate ), acceleration in  weathering of parent material and decay of organic matter  etc could be predominantly picked up . 

Global change drivers such as rising atmospheric carbon dioxide (CO2) levels, elevated temperature, altered precipitation (rainfall) and atmospheric nitrogen (N) deposition could cause temporal changes in soil acidity. Acid depositions from air pollution induced through anthropogenic activities contribute to natural acidification processes.

Highly interesting question and interesting answers. The soil pH is decreasing constantly in humid regions and increasing in arid regions. Same processes of soil genesis remained active which gave rise to acid and alkaline soils separately. As pointed out by Dr. Kumar, leaching of bases under very high rainfall results in to decrease of soil pH in acidic soils of humid and sub-humid regions whereas incomplete leching results in slow but gradual accumulation of bases in arid regions due to lesser or scanty rainfall. Additional factors pointed out by Mehmood also play their role and contribute to acidification process.  

Hello. CO2 has a very small contribution to soil acidity. A fraction of a kmol of H+ per hectare per year. It can generally be ignored. Acid rain can be a greater input, especially where SO2 emissions are high.

For most agricultural systems, the export of product is a large source of acidification. Consider the ash alkalinity of products and the export rate, and it can be seen that a kmol or more of H+ per ha per year can be generated. In legume-based systems, particularly with annual growth patterns, leaching of NO3- produces considerate H+. One kmol per 70 kg NO3-/ha/year leached. Similarly, fertilisation with NH4+ fertiliser is acidifying (1kmol H+ per kmol NH4+ incorporated into organic matter as NH3 equivalent). 

Organic matter accumulation is acidifying via the incorporation of organic anions into associations with base cations, leaving unconjugated H+. The ash alkalinity of organic matter is a good estimate of the residual H+ generated in its production. The oxidation of some neutral organic compounds, such as methoxyl groups, produce highly acidic compounds, such as those containing carboxylic acids. Lactic acid fermentation is an example of such a process.

Also some oxidation and reduction processes of minerals produce or neutralise acids. These may be important where inundation occurs.

Several high-quality books review these processes. (Helyar and Porter is an example relating to agriculture in Australia). I am a little disappointed to see some ill-informed suggestions here. 

Best wishes for your studies.

Dear Jeffrey, can you inform, if you mean CO2 from atmosphere, or CO2 produced in respiration of all organisms living in soil?

I will be very grateful if you provide data and good references estimating how many CO2 is produced in soil as result of it's biological activity?

Do you have data comparing the CO2 input with precipitations (in my opinion small) with CO2 input by soil organisms at yearly basis?

Regards, Michal

An excellent issue Michal to be debated in the context of whole discussion..CO2 input with precipiation will be much smaller than CO@ input through microbial population of the soil , on annual basis , since the role of soil as ecosystem service provider emerges  primarily through the microbial abundance of the soil . ..

Dear all,

when discussing about Rothamsted and ParkGrass experiment we have a lot of data on internet:

http://www.era.rothamsted.ac.uk/Park

so everyone may analyse it. Certainly it has rather fine soil (silty clay loam according to the text, but silt loam according to provided results of analyses) overlying chalk - in other words soil with high buffer capacity due to presence of chalk in subsoil and rather high (22%) clay content. Despite of this the soil of this site is well drained, and this favours leaching.

We should remember, that the pH of water being in equilibrium with atmospheric CO2 (0,03%) is about 5,65, but the pH of water in equilibrium with soil CO2 (for this example we assume 0,3%) is about 4,65 (page 43, Fotyma and Mercik, 1992, Chemia Rolnicza - it is a textbook on agricultural chemistry written in Polish. As Ukrainian and Polish languages are very similar, I think Zenon may verify it).

We should also remember, that there were about 50-60 species, including red clover on unfertilzed plots, although without predomination of any species.

I maintain my opinion, that the main natural cause of soil acidification on unfertilized and unmanured plots is biological activity of soil, comprising:

-respiration of all organisms living in soil (note, that N-fixation has high energy demand - and this is obtained through respiration of Rhizobia);

-decay of organic matter, which is also produces a lot of CO2 and much less NH4 (it is slightly alcaline, but it is subject to nitrification, which is a very acidifying process);

- production of organic acids by roots and microorganisms.

Actually, we should ask the people working in Rothamsted for data which allow to quantify the production of CO2, NH4+, organic acids and other substances at yearly basis, to assess their contribution to soil acidification.

My opinion on decisive role of metabolic activity (mainly, but not only respiration) in soil acidification is supported also by the knowledge of climate of England - very humid, with moderate temperatures, frequently without frequent droppings of temperatures below zero centigrades during winter. These factors (moisture and temperature) are favourable for biological activity and leaching during all the year. The good drainage also favours leaching, although the soil texture and underlying chalk oppose to acidification.

Best regards.

In our experience the primary cause of soil acidification is excessive use of N. Farmers are frequently over optimistic with regard to yield potentials in a country where droughts or yield limiting short, dry periods are common and induced subsoil acidity, particularly in no-till situations, is a frequent occurrence.

Nice input , thoroughly debated explanations, appreciate it , Michal..

To complete my answer, why the soil pH dropped on unfertilized plots so sharply after 1975, supposing, that the soil biological processes were similar before and after that year:

1) the exhaustion of calcium carbonate in shallow subsoil due to leaching - to be verified directly in Rothamsted - it would be natural cause;

2) more acid rains - probably it is also possible to verify.

Certainly, other explanations may be better.

Yes, Michal, there may be many reasons and causes; natural and man-made.

Dr Nazir and other colleagues , you will agree with me , while debating on natural soil acidification , climate change-induced shifts in precipitaion will have most pronounced effects.?

I agree. The recent el-Niño affected season in South Africa will almost certainly have aggravated acidity in our major cropping areas. My apologies to Michal for addressing induced acidity rather than natural acidity.

Hello. To explain the situation concerning CO2, I will separate the discussion into issues of acid deposition in rainfall and acid production from respiratory CO2. 

Concerning rainfall, CO2 as weak carbonic acid is well known to be corrosive to limestone and neutralises alkaline soils, particularly highly alkaline soils that are buffered in the carbonate pH range. This is because at normal partial pressures of CO2 the pH of rainfall is approx 4.7, only 1 or 2 orders of magnitude of H+ concentration from the pH of most soils, but 4 to 5 orders of magnitude from the pH of carbonates. 

From this it can be seen that extremely high partial pressures of CO2 must be present to create low pH with respect to soils. Consider that human breath is 4% CO2 and is only weakly acidifying of neutral materials. It is not surprising, therefore, that CO2 from soil respiration is rarely a significant source of soil acidification.

Please note, however, that some respiratory processes, such as lactic acid fermentation, produce strong carbon-based acids (carboxylic acids and others) and can be the source of high rates of acidification.

Cheers

Apologies. I referred to the CO2 at in rainfall producing a pH of 4.7. I meant 5.7.

Colleagues,

Dr. Goulding (at Rothamsted Research) and his co-workers have several publications relating to this experiment.  The relative importance of different sources of soil acidification at this site over the course of the experiment is well explained in one of those papers available at this link:

http://onlinelibrary.wiley.com/doi/10.1111/j.1475-2743.1986.tb00669.x/full

For convenience, here is a section from the abstract that may be very relevant to this question.

“…We have calculated the contributions to soil acidification of natural sources of acidity in the soil, atmospheric deposition, crop growth and nutrient removal, and, where applicable, additions of fertilizers. The relative importance of each source of acidification has changed as the soils have become more acid. Acid rain (wet deposited acidity) is a negligible source, but total atmospheric deposition may comprise up to 30% of acidifying inputs at near neutral soil pH values and more as soil pH decreases. Excepting fertilizers, the greatest causes of soil acidification at or near neutral pH values are the natural inputs of H+ from the dissolution of CO2 and subsequent dissociation of carbonic acid, and the mineralization of organic matter.

Under grassland, single superphosphate and small amounts of sodium and magnesium sulphates have had no effect on soil pH, whilst potassium sulphate increased soil acidity slightly. All of these effects are greatly outweighed under grassland, however, by those of nitrogen fertilizers. Against a background of acidification from atmospheric, crop and natural inputs, nitrogen applied as ammonium sulphate decreased soil pH up to a maximum of 1.2 units at a rate in direct proportion to the amount added, and nitrogen applied as sodium nitrate increased soil pH by between 0.5 and 1 unit.”

Regards!

I think that acid rain in cold temperate zones resulting from industrial activities wastes and wet environment have a strong effect in soil pH depletion in these countries in contrast  with arid or semi arid countries where we faced a raise  in soil pH and salinity problems year by year.  

The main reason for the soil acidification in the past 50 years are the following: the athmospheric deposition of SO2 by heavy industry which are eleiminated by today. The application of liquid Ammonia for fertilization. The application of mineral fertilizers that has acidic dissociation. And finally the plant production is dangerous for the soil PH because the remains of ionic exchange, hydrogene irons are in the soil. In my article the main acidic agent was the Ammunium Sulfate. Because when started in 1848 the experiment this form was very popular.

Clearly, there are a number of factors that can be implicated and many are environment specific. In our semi-arid environment  over generous use of nitrogenous fertilizers is undoubtedly the primary factor. The consequences include Al and Mn toxicity and poor moisture utilization --- a primary consideration in our environment. Ammoniated N sources are clearly more damaging than other N fertilizers, but limestone ammonium nitrate is also an important causal factor. NO3 leaches rapidly with Ca and Mg as counter ions and the net effect is the same.

Lets concentrate on natural causes of soil acidification , that will be enriching to all of us , an interesting debate already picked up..

I feel the climate change is the most predominant factor for soil acidification due to leaching out of bases. 

I'm not quite sure of the effect of climate change on soil acidification. On the one hand, increased rainfall predicted for some areas will favour leaching of alkali and alkaline earth cations, and therefore acidification (aluminium replaces these cations in the exchange complex). On the other hand, an increase in temperature and precipitation will favour weathering of the parent material, which counteracts acidification. The latter process will not occur in highly evolved soils (humid tropics), where the original material is at great depth.

Acidic rain due to the expansion of industries (releases NOx and SOx compounds) and/or long time uses of fertilizers.....and also high rainfall in that area ... 

Thank you zinabu for mentioning three basic causes out of four. The soil genesis and leaching of bases is fourth one.  

Yes, you are right  Dr Zinabu and Dr Nazir ...

There are four major reasons for soils to become acidic: rainfall and leaching, acidic parent material, organic matter decay, and harvest of high-yielding crops. Wet climates have a greater potential for acidic soils. In time, excessive rainfall leaches the soil profile's basic elements (calcium, magnesium, sodium, and potassium) that prevent soil acidity. Soils that develop from weathered granite are likely to be more acidic than those developed from shale or limestone. Organic matter decay produces hydrogen ions (H+), which are responsible for acidity (an ion is a positively or negatively charged element). Like that from rainfall, acidic soil development from decaying organic matter is insignificant in the short term. Harvest of high-yielding crops plays the most significant role in increasing soil acidity. During growth, crops absorb basic elements such as calcium, magnesium, and potassium to satisfy their nutritional requirements. As crop yields increase, more of these limelike nutrients are removed from the field. Compared to the leaf and stem portions of the plant, grain contains minute amounts of these basic nutrients. Therefore, harvesting high-yielding forages such as bermudagrass and alfalfa affects soil acidity more than harvesting grain does.( Source : Jeff Ball 1999, Soils and Crops)..

Another PDF enclosed for further reference ..

You are quite right Dr. Kumar.