Herself's Houston Garden

Conservation through cultivation

How soil pH affects your plants and what to do about it




I know you’ve all heard of pH and those of you with pools are probably very familiar with pH. But why do you care about your garden soil pH?

The ‘p’ in pH stands for ‘potenz’ ( potential to be ) and the ‘H’ is for ‘Hydrogen’. So pH is just a measure of the amount of hydrogen in your soil. The scale runs from 0 to 14 ( acidic to alkaline ) and you are aiming for 6-7 but your plants will be thrilled with anything between 6-8.

My soil pH is about 8.0.  I’m told the Houston area has acidic soil, most everything north and west of us has alkaline soil.

The pH is important to plants because plant nutrients are not soluble when the pH is too high or too low. 98% of the nutrients your plant needs it gets when the roots take in water. If the pH is too high or too low the nutrients in the soil do not dissolve and your plants can’t use them.

Think about being stuck on a plane. You have a bag of cookies in a sealed plastic bag and nothing sharp to slice the bag open. You have the food but no way to eat it. This is what happens to your plants when your soil’s pH is too far from center.

If your soil tests below 6.0 you’ll want to add limestone to your garden. The amount you add depends on how acidic your soil is. Limestone takes a very long time to work its way down into the soil so it may take a while to see your results.

If your soil is about 8.0 you’ll need to add sulfur to it to lower the pH. This also is a very slow process. If you add to much sulfur too quickly you’ll wipe out your plants.

pH should be checked yearly. ( Yes, I know, I’ve been gardening forever and this is my first year testing but better later then never. It just shows you can teach an old gardener new tricks. ) You can send a sample to the county for testing. Many garden supply places carry soil pH kits.

That said it is very difficult to change your soil’s pH and anything you add to your soil to change the pH is unlikely to effect the soil’s pH by much or make a long term change.

When your soil pH is 8.0 like mine you’ll find you’ll need to fertilize more frequently and add iron to the soil regularly.  I add a handful of iron once a year to each shrub, plant and tree ( about a 1/4 cup ) and fertilize when ever I see yellow leaves with green veins.  Nitrogen seems to right that, though most sources claim it to be an iron deficiency.

To raise your soil pH you’ll want to add limestone at about 1.5 pounds per 100 sq ft of sandy soil, 4.0 pounds per 100 sq ft of loamy soil, and 5.5 pounds per 100 sq ft of clay.

To lower your soil pH you’ll want to add sulphur at about 1.0 pounds per 100 sq ft for sandy soil, and 2.0 pounds per 100 sq ft of clay soil.