What is Soil pH?

Soil pH testing is done to see how acidic or alkaline your soil is. pH means potential Hydrogen. It measures the number of H+ ions in the soil water.

It's a logarithmic scale that goes from 0 to 14 with 0 being most acidic, 7 being neutral and 14 being most alkaline. Because it's a logarithmic scale each jump is a ten fold increase so a soil with a pH of 5 is 10 times more acidic than one with a pH of 6 and a 100 times more acidic than one with a pH of 7.

Testing your Soil

You can of course send a sample of your soil to a lab for a pH test but then you couldn't pretend to be a scientist or a CSI.

There are two home test options. The first is using a pH meter. This meter is simply plunged into the damp soil and a needle will jump to the approximate pH reading for your soil. It takes about a minute. The Luster Leaf 1818 Rapitest Mini 4-in-1 Soil Tester is a meter system that measures not only pH, but also moisture, light and fertility. I'd probably just use it for the pH. Make sure the soil is damp or you will not get a reading.

The second way is with indicator agents. I used a test kit like the Luster Leaf 1601 Rapitest Soil Test Kit.

You just fill a small test tube that comes with kit about a quarter full of soil. Then you add the capsule of the coloring agent and fill the vial with distilled water. The apsules are a bit hard to open but they work. And use distilled not tap water. This will make the test tell you soil pH instead of the pH of your water. Shake it up and in no time the water turns color - in my case a deep green showing that my soil's pH is around 8. Yikes!

Just for fun I added a few drops of vinegar and instantly the liquid turned to the yellow color for acidic soil. It's very fun to do this and your kids - even those not too crazy about science - will like the process.

What Causes Soil to become Acidic or Alkaline?

1. Parent Material - Soils are made up of ground up rock and the type of rock dictates the natural pH of the soil. Basic rock like limestone creates alkaline soil and acidic rock, with acidic rock, rock containing more silica, creating acidic soil.

2. Rainfall - Rain leaches basic elements such as calcium and magnesium from the soil. Therefore you will find areas with high rainfall generally have acidic soil while dry areas tend to have alkaline soil. In fact the work for deserts in Arabic is al khali.

3. Fertilizers - Some fertilizers tend to make the soil acidic. Ammonium urea is an example.

How Does Soil pH Affect Plants?

Most plants like a soil pH close to neutral or just a little on the acid side. A very high or very low pH can be toxic to the plants.

In Acid soils calcium (Ca), and magnesium (Mg) are less available to plants. Aluminum (Al) and manganese (Mn) may reach toxic levels. Phosphorus is tied up by iron (Fe) and aluminum (Al). Bacteria grow poorly as well.

In Alkaline soils phosphorus (P) gets tied up by Ca and Mg. Iron (Fe), zinc (Zn) and manganese (Mn) are less available. Sometimes excess salts accumulate and potatoes are subject to bacterial diseases.

Have you noticed that Phosphorus is too fussy. It's not available if the soil is acidic and it's not available when the soil is alkaline. So when is phosphorus available. Check this page on phosphorus and fungi to learn more.

How Does Compost Help?

Compost is useful to add to soils regardless of whether you have acidic and alkaline soil. How can it do this? Compost is a buffering agent.

A buffer is a substance that tends to lessen the change in pH. Compost allows the soil pH to effectively range to one point above and below the pH reading for your soil. So for example, for me with a pH of 8 adding compost gives me a range of 7 to 9 pH. As 7 is in the ideal range for plants I've solved a good part of my pH problem with the compost.

How to Raise the soil pH of Acidic Soils

As your first step try compost. It can have a marked influence on acid soils. For example an EPA study showed that water with a pH of about 2 discharged from a mine had risen to a pH of 5 by the time it passed through a compost filter.

Go slowly. It is possible to use quick acting agents to radically change your soil pH. However, you risk wiping out your soil organisms and they are critical companions to your plants. Kill them and you effectively kill your soil.

Aim for raising your pH a maximum of 1 pH point per season. Lime is normally added to raise pH. You can use Calcitic or Dolomite Lime. Calcitic Lime adds only calcium to your soil while Dolomite lime adds calcium and magnesium. These will slowly change the pH. As well it will make phosphorus more available and increase the bacterial activity with the side effect of freeing nitrogen for your crop.

The amount you add depends on your soil texture. In sandy soils use less lime and apply it more often. Follow the directions on the lime you purchase but please go slow - give soil life a chance.

How to Lower the Soil pH of Alkaline Soils

I've found that many gardening sites and books throw up their hands in defeat when faced with soil pH in the alkaline range. Personally I think it's that there are fewer people here in the dry areas where soils tend to be basic. These garden experts are not terribly motivated to solve this problem because they don't happen to have any experience of it. So they simply write soils with high pH off as impossible.

The other thing - and this concerns me - is that because lime is so routinely applied to soils in the heavily populated areas where soils are acidic you find lime for sale in garden centers where soils are alkaline. The other day I was at a big garden center in Calgary, an alkaline area, and found a huge display of bags of lime for sale. Many people from acid climes are moving here and naturally think their garden problems will be solved with lime. In fact they will be made a heck of a lot worse.

So - please do your soil tests before you try to fix your soil and then go slow in effecting changes. Give your soil biology a chance to adjust and adapt to the new conditions.

Your first line of action is compost. It does work to change the actual pH and to minimize the difficulties arising from a high pH.

To lower your soil's pH sulfur is your friend. It takes very little but it takes some time. The equation is sulfur + time + soil bacteria = a lowered pH. Again you will need less sulfur in sandy soils than in other soils so know your soil texture. Follow the directions that come with the material you purchase and don't try to change things more than one pH point per season.

Good Luck.


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