Soilwiki

Rainwater and soil acidification science

co2_800k_Feb11
Keeling curve since 1700 (https://scripps.ucsd.edu/programs/keelingcurve/ accessed on Feb 11th, 2015)

 

As shown by the Keeling curve above, during the past 50 years, the Carbon Dioxide (CO2) concentration in the atmosphere has increased from about 310ppm to 400ppm, which is double the increase of the 250 years before that. One of the problems caused by the increasing CO2 concentration in the atmosphere is acidic soil. From the formulas below, when the CO2 in the atmosphere is high, in the high rainfall area like Haiti, preciptation would bring the CO2 down to the soil with water in the form of Carbonic Acid (H2CO3). The equilibriums are:

CO2 (gas in air)   «   CO2 (dissolved in the rainwater)

The CO2 reacts with the rainwater to produce hydrogen ions (acid)

CO2 + H2O   «   H2CO3  «   H ++   HCO3 -

When the equilibrium goes to the right, hydrogen ions are produced and causes an increase the pH value (more acidic) of the rain, and that enters the soil. The more CO2 that enters the atmostphere, the more acidic the rainwater will become. Generally, for every 10ppm increase on CO2, the rainwater pH will decrease by 0.1. That means, that since the industrial revolution in the 19th century, the rainwater pH has decresed from nearly neutral (~7) to about 5.5 today.

Since the ionization constant for the first equilibrium is greater than the one of the second equilibrium, the reaction would stay at the first equilibrium in the environment. The soil acidification would be affected by mainly two factors: the strength of the acidic rains and the buffering capacity of the soil. In Haiti, there are many agricultural zones where the soil is mostly organic and has minimum CaCO3 in it. The buffering capacity is lower than the soil that has a high CaCO3 content..

Soil pH has great influence on growing of plants. Different plants have different optimum soil pH. As in Haiti, the most profitable plants are coffee, cocoa, rice, etc. However plants like coffee and rice would grow the best at netrual pH level (6.0-7.5 for coffee, 5.5-6.5 for rice, 5.0-7.5 for cocoa). However, in Haiti, especially the agricultural zones showed in the food economy distribution figure, some have a lower soil pH level of 4.0-5.0.

cuba

Food economy distribution in Haiti (resource)

Studies has shown that acidic soil amendment by CaCO3 could greatly increase the agricultural production. Below is showing the principle of how the low pH soil is amended by CaCO3.

 H2CO3 + CaCO3   ®   Ca(HCO3)2   (Calcium Bicarbonate)

When calcium carbonate exists in the soil, it would react with rainwater to produce Calcium Bicarbonate. The Ca2+ left in the soil would also be taken up by plants. Testing the soil pH is very simple and inexpensive. Picture below shows the pH test strips particularly for soil pH testing and are able to get the result in no more than one minute.

economic_activity_haitipH Test