The pH of soil and water and how it affects plant growth and chemical makeup of soils and media is somewhat detailed.  In the next five newsletters, we will present different aspects about pH.  Once complete, one should have a thorough understanding of pH and how it influences plant production.

Part I – What is pH?

Part II – How is nutrient availability affected by pH

Part III – How is pH affected by fertilizers?

Part IV – How to adjust pH of soils

Part V – How to adjust pH of irrigation waters.

When it comes to plant production, whether in the soil or artificial media, the pH of growing medium and water is one of the most important chemical factors to consider aside from nutrient concentrations.  The primary factor that pH influences is nutrient availability.  If the pH is incorrect, nutrient solubility and availability to plant roots may be impaired.  

What is pH?  

The pH is the measure of acidity (H⁺) or alkalinity (OH^-) of a soil solution or irrigation water.  Chemically speaking, it is the measure of hydrogen ion (H⁺) concentration in the solution.  The lower the pH the higher the concentration of H⁺ (Figure 1).  A logarithmic scale is used (Figure 2) since the H⁺ concentration increases 10 times for every unit change on the pH scale.  For example, a pH of 1 has 100 mg of H ions for each liter of liquid while a pH of 2.0 has 10 mg of H⁺ per liter of liquid and pH 1.5 has 30.16 mg of hydrogen ions per liter of liquid.  The actual equation is pH = - log (H⁺).

Changing solution pH  

There are three ways to change the solution pH:

1)  Add H⁺ ions -  through the direct addition of acids such as hydrochloric acids (HCl), or sulfuric acids (H₂SO₄).  These elements in these compounds separate when mixed in water.  For example, when HCl is added to water (H₂O), H⁺ in HCl separates off the Cl forming ‘free’ H⁺ and ‘free’ Cl^- in the water.  The word ‘free’ means that the ion is dissolved in solution and is not connected to another element or compound.

2)  Add compounds that react with H₂O to form ‘free’ H⁺ ions.  This occurs with elements like aluminum (Al).  When Al reacts with some of the water molecules, H⁺ is released into solution as shown in the following equation:  Al + H₂O 🡪 Al(OH) + H⁺.  

3).  Remove hydrogen ions from solution - through the addition of base-forming compounds such as sodium hydroxide (NaOH) and ammonium hydroxide (NH₄OH).  These compounds react in solution in a way that reduces the amount of free H⁺ ions.  As you can see in the last part of the following equation, the ‘free’ OH^- and H⁺ react with each other to form water (H₂O).  For example, NaOH + H⁺ + H₂O 🡪 Na⁺ + OH^- + H⁺ + H₂O 🡪 Na⁺ + 2 H₂O.   

Now that the chemical properties of pH have been explained, we will discuss how nutrient availability of soils is influenced by pH (Part II).  

Figure 1.  This graph shows the pH of a solution as it relates to hydrogen ion concentration (H⁺).  The pH is on the horizontal line (x-axis) and goes from 1 to 14.  The H⁺ concentration is on the vertical line (y-axis) and goes from 1 mole/liter at the top and goes down to 1x10^-14 moles/liter at the bottom.  As you can see, it is very difficult to read the concentration of H⁺ after pH 2.  In order to read the graph, you would have to make an excessively long vertical scale or convert this to a logarithm as shown in Figure 2. 

Figure 2.  This graph shows the pH of a solution as it relates to hydrogen ion concentration (H⁺) on a logarithmic scale.  The pH is on the horizontal line (x-axis) and goes from 1 to 14.  The H⁺ concentration is on the vertical line (y-axis) and goes from 1 mole at the top and goes down to 1x10^-14 at the bottom.  The 1E-05 is just an abbreviation for .00001.  the –05, -06 just indicates the number of spaces from 1 that the decimal was moved.  For example – 05 and -06 means 5 and 6 decimal places to the left of the 1.  

Don Merhaut is an Associate Extension Specialist at UC Riverside.  He can be reached at donald.merhaut@ucr.edu.