When thinking about soil pH, it’s easy to get confused with all of the terminology involved. Simply stated, the acidity or basicity of any solution, e.g. soil and water, is defined by its pH. Technically, pH is the negative logarithm of the ionic concentration of H+ (hydrogen) in the solution. As the hydrogen ion concentration increases, the resulting pH number decreases. The reason logarithms are used is because the concentration of H+ is actually very small, even when the soil is very acidic. For example, when the pH of a soil solution is 4.0, the actual concentration of hydrogen ions is 0.0001 moles per liter (one mole is equal to the number of hydrogen atoms in 1 gram of hydrogen).
The true meaning of the lowercase ‘p’ in pH has been purported to stand for different things throughout history. Some suggest that it stands for “power;” others claim “potential,” or even the Latin term pondus hydrogenii, potential hydrogen. In chemistry circles, the lowercase ‘p’ stands for decimal cologarithim of, and the capital ‘H’ is the chemical symbol for hydrogen.
Hard to wrap your head around, isn’t it? Luckily, you aren’t the only ones and long ago, some scientists decided to take the negative logarithm of numbers like 0.0001 moles per liter and change it to a simple number to understand: 4 on the pH scale. One numerical step in the pH scale represents a 10-fold increase or decrease in acidity. So, pH 5 is 10 times more basic than pH 4 and 100 times more basic than pH 3. Therefore, a pH of 1 is ten trillion times more acidic than a pH of 14.
Now that we have a more thorough understanding of what pH really represents, the following are ways soils can become acidic:
1) Soil parent material. Soils formed from parent material low in carbonates (both calcium and magnesium) are usually acidic, as are soils formed from sandstone and shale.
2) Climate. Soils that form under high rainfall are subject to extensive chemical leaching and weathering, which removes essential basic cations (Ca2+, Mg2+, K+, Na+), and allows acidic cations (H+ & Al3+) to occupy the empty cation exchange sites.
3) High yielding crops. Harvested plant parts take a lot of basic cations with them.
4) Acidifying fertilizers. Ammoniacal fertilizers can contribute greatly to the acidity of soil. This is due to left over H+ ions after microbes transform ammonia and ammonium into nitrate, which plants prefer for uptake, in the natural process of nitrification. Also, as plants uptake ammonium, which they will, even though they preferentially uptake nitrate, they secrete H+ ions into the soil solution to maintain a balance of chemical charges.
Fertilizers that have the highest potential for acidifying soil are: ammonium sulfate (AMS – 21-0-0) and mono-ammonium phosphate (MAP – 11-52-0), both of which are very commonly used in agriculture.
Another issue with acidic soil conditions, namely below 5.5, is that Al and Mn becomes increasingly available for plant uptake and that uptake can quickly cause toxicity within the plant, while excess Al in the soil solution will inhibit root growth and function, and also restricting uptake of certain nutrients like Ca and Mg, which further compounds problems.
With all the inputs farmers have to balance, one issue often pushed to the back burner is pH. Generally speaking, farmers might decide to apply lime every 3-5 years. What other soil amendment or farm input is treated in such fashion? Why take a reactive approach to managing pH when you can be proactive about the problem and not have to worry about a corrective measure every 3-5 years when yields start to suffer? At Calcium Products, we believe farmers should be more proactive about measuring, monitoring and correcting pH on their farms. With the advent of precision agriculture in every aspect of farming, there is no reason we should exclude lime application and pH correction from that process. Yearly applications of SuperCal 98G at rates much lower than with agricultural limestone should be part of your soil management regimen.
As acidity continues to increase, corrective measures to bring back optimal conditions for crops are harder to achieve. Act now to help the soil help your bottom line!
In the next article, I will take a closer look at how acidity works and what characteristics of soil lead to different levels of acidity, and how the current recommendations for lime-based pH correction work.