This is the content of the Soil pH page I linked to above.
How To Test Soil PH Levels
Testing your soil pH will ensure that you're getting the healthiest and biggest yield of crops from your garden. Different types of plants have different requriements for their optimal pH level. For example, tomato plants prefer a soil pH level that's between 5.5 and 7.5. Lettuce prefers a pH between 6.0 and 7.0, but won't tolerate a pH over 7. By testing soil pH in each garden bed, you'll have the opportunity to provide your garden it's best opportunity for having successful growing conditions.
Placing plants in garden soil that is to acid or alkaline can make them more vulnerable to pests and disease, can make the plant grow slowly (or not at all), and will reduce the amount of nutrients that is passed from the soil and into the vegetable or fruit and then of course into YOU when you eat it.
How To Test
Purchase a pH soil testing kit from any garden supply store (they are simple to use and inexpensive), and follow the instructions provided.
Normally, your pH soil testing kit will have a container, a testing solution, and a color chart. To conduct your soil test, the process is generally that you scoop up some garden soil and place it in the container. Add the recommended number of drops (of testing chemicals) to the container. Mix (shake) the container for the time recommended on the package's instructions. Compare the resulting color to the chart provided and pick the best match to identify your garden plot's soil pH level.
The results of your soil test will show a pH level for your garden plot, measured on a scale of 1 to 14.
1 = the highest acidity and the lowest alkalinity
14 = the highest alkalinity and the lowest acidity
A reading of 7 is considered neutral soil. Below 7 is considered acidic soil (the opposite of alkaline). Likewise, a reading of above 7 means that you have alkaline soil (the opposite of acidic).
You can also test your waters PH levels - the water in your dam, your tap, or your tanks. Whatever you water your plants with.
While at first, the difference in the results of your reading may seem insignificant, the reality is that the slightest reading change can have a big impact on your soil's productivity. On the scale, each whole number is 10 x the acidity/alkalinity of the neighboring number. For example, a reading of 6 is 10 times more acidic than a 7. A 5 is 10 times more acidic than a 6, and 100 times more acidic than a 7.
Some plants prefer acidic soil, some prefer neutral soil, and some prefer alkaline soil.
How To Change The Soils PH Levels
The absolute best additive for your soil (whether too alkaline, or too acidic) is well-rotted organic matter, like compost. Compost acts as a neutralizer in both circumstances, and provides the additional benefit of adding nutrients to your soil (for your plants, and also for the fruits and vegetables you plan to eat from them).
How To Change The Soil pH Level To A Lower Reading
If your soil test shows that the garden bed is too alkaline (the pH level is too high) for the plants you wish to grow there, you will need to add compost or other organic matter. Soils that are highly alkaline, tend to have significantly fewer nutrients available in the soils. Adding significant amounts of organic matter to your garden bed will help correct this deficiency.
How To Change The Soil pH Level To A Higher Reading
(the pH level reading is too low)
If your soil test shows that your garden bed is not alkaline enough (in other words, that your soil's pH level is too low (ie. too acidic) for the plants you wish to grow there, you will need to raise the pH level of your garden plot's soil.
Common options available for soil additives that will increase the soil's pH level (reduce acidity):
Lime for the Garden
Very finely ground oyster Shells
Apply the soil additive to your soil and rake it in to the top level of soil in your garden plot. (The majority of your plants roots will be near the top, so this is where they will be feeding most heavily from the soil).
Producing Vegetables From Seeds
What Is Permaculture?
A to Z of Vegetable and Fruit Growing
How to Test Soil pH Levels
Table Showing Preferred Soil pH Level for Vegetables
What to Harvest in Autumn