Fact: Soil pH* affects the availability of nutrients for plants.
Here is a figure that shows the relationship of soil pH with nutrient availability to plants. The thicker the color, the greater the availability of that nutrient to the plant. For the most part, plants can get the most nutrition at pH values between 6-7.
Answer: Well, one could write a book on this, a looooooong book, but this is a blog post, so let us settle with the main idea...supply and demand!
For the most part, soil has a negative charge. Negative charges attract positive charges (or cations*). Hydrogen has a positive charge...
- ...and when there is a lot of positively charged hydrogen (i.e., less basic, lower pH), there is more of it available to bind to the negatively charged soil. When more hydrogen is bonded to the soil, there is less of a chance for other positively charged nutrients to bind to the soil. Without binding to the soil, they have a chance to nourish the plant, but they are also more easily leached away (like when it rains).
- ...and when there is less hydrogen (i.e., more basic, high pH), there is less of it available to bind to the soil. When less hydrogen is bonded to the soil, there is more of a chance for other positively charged nutrients to bind to the soil instead of being allowed to nourish the plant. (Link to reference)
In case you ever found yourself in front of a classroom of middle schoolers, and all you had were test tubes, plugs, scoops, pipettes, graduated cylinders, universal indicator solution, beakers, three different soil types, a stop watch, a color chart pH scale, and nothing to talk about, I propose this lesson plan for you: Measuring pH in soil. I found this at the University of Texas Environmental Science Institute. It also has other learning resources for teachers and students.
Also, I found a webpage with great links to nutrient management teaching modules. Although they might be easier to read if you have some science background, I am still impressed with their clarity.
Soil pH: a measure of the soil’s acidity, or hydrogen (H+) concentration.
pH = -log[H+],
where [H+] = the hydrogen ion concentration.
Because of the negative sign in the definition for pH, low pH soils have more hydrogen than high pH soils.
Acidic soil: a soil with pH values <7 (high hydrogen concentration)
Alkaline soil: a soil with pH >7 (low hydrogen concentration)
Cations: We defined cations here. They are ions that carry a positive charge of electricity.
Cation Exchange Capacity: the total negative charge on soil, which is a good measure of the ability of a soil to retain and supply nutrients to a crop.