There is a lot of advice on how to make make acidic soil both in print and on the net. You can use coffee grounds, pine needles, and sulfur to name a few. This advice has two problems. Firstly, the recommended product may not actually acidify soil. For example in Do Pine Needles Acidify Soil I show that pine needles do not make acidic soil. Coffee grounds don’t acidify soil either. The second problem is that before such advice is given it is important to know the soil types (ie soil texture) being treated. Let’s take a closer look at this.
Your soil has been made over millions of years using the rocks that were present at your location. It might have a lot of sand, or a lot of clay. It will also contain minerals based on the type of rock that was degraded to make your soil. The ability for any soil amendment to change soil acidity depends very much on the soil type you have. Let’s look at a couple of examples.
Soil that is very sandy usually does not contain much in the way of minerals. If you add a small amount of acidic material to the soil it will become acidic, at least for a short period of time. The problem with sand is that minerals and added acid leach away quickly; so the acidification of sand is a short term event – your soil will not stay acidic for long.
If your soil contains significant amounts of loam or clay, the soil could be naturally acidic or alkaline. It will contain minerals that will react with the added acid. Any acidification of the soil depends very much on the composition of these minerals. The minerals may be able to neutralize, or buffer the added acid. The importance of this buffering ability is discussed in Liming Acidic Soil. Soil testing is the only way to determine the pH buffer value.
Most of Southern Ontario is a clay loam. The base rock here is limestone and there is a lot of limestone, both as rocks and as minerals in the soil. These minerals are able to neutralize any acid that is added.
Consider this fact. Rain dissolves CO2 from the air as it falls to earth producing carbonic acid (this is not due to pollution). This rain, even without the added pollution has a pH of 5.5. This acidic rain has been falling in Ontario for millions of years and even after such a long time of ‘acidifying the soil’ our soil pH is still 7.4. How can this be? Our soil contains a lot of neutralizing minerals due to the limestone. As soon as an acidic material is added to the soil it is quickly neutralized so that it has no net effect on the soil pH.
In Northern Ontario and Quebec, the base rock is granite, not limestone. Granite is very stable and hardly reacts with acid. The soils in these areas are generally acidic and the addition of more acidic material will make the soil pH more acidic. In fact the pollution over the last 50 years has made the rain more acidic (ie pH lower than 5.5), and this has resulted in the soil in some areas becoming more acidic.
I have split Ontario into two parts, northern and southern, but in each area there are exceptions to the above statements. You can find acidic soil in the south and alkaline soil in the north.
The acidification effect of any material on soil depends very much on the soil types you have. Simply saying that “material ABC” acidifies soil is not correct. It may acidify some soils and not others.