The acidity of soil is measured by it's pH level. pH is measured on a scale of 1 to 14 with 7 being neutral. Below 7, the soil is acidic and above 7 it's alkaline.
Measuring Soil Acidity (pH level)
Different soil types will behave differently so one vital tool for the serious gardener is a tester for acidity levels.
There are different types of test kits available. With the first type, you mix a soil sample with water then compare a colour change to a chart. The second type is an electronic meter with probes that you insert into wet soil.
Whichever kit you use, it will come with instructions and will give you a reading. You will need to take samples or tests from a number of spots to get a better general view of your soil’s acidity level.
You can also judge the acidity of the soil by the types of weeds that grow. Sorrel, creeping buttercup, nettle, dock and mare’s tail are all signs your soil is too acidic, or becoming too acidic. Reducing soil acidity will help deter some weeds but lime isn't used as a weed killer.
Each plant species is suited to different growing conditions and slightly acidic soil can help to keep down blight in potatoes for example. On the other hand, slightly alkali soil has a higher level of salt present which may not suit your particular crop. Gardeners generally tend to aim for a neutral PH of around 7.0.
Changing The Acidity Level Of The Soil
To lower soil acidity you raise the pH value by adding lime to the soil.
Lime can be applied throughout the year, but most gardeners will apply it during Winter or early Spring. Lime is insoluble in water so thoroughly mix the lime with the top soil. Once moisture is applied the lime will start to chemically react, so thorough mixing in dry conditions is very important. Don't forget that different plants thrive in different soil conditions, so make sure you know which pH level your plant needs before deciding how much or how little lime to apply.
Sprinkle the lime over the soil surface and rake it into the top couple of inches, letting it naturally work down to the root zone by spring. Do not till it deeply into the soil, it will leach down soon enough.
To lower pH and increase acidity you can add sulphate of ammonia or urea which are high nitrogen fertilizers. Adding manure will also lower pH and make the soil more acid. Adding lots of manure year after year will actually reduce soil fertility by making it too acid so the plants cannot access the nutrients.
Never mix lime and fertilizer. They will at best cancel each other out in an unpleasant, to the soil, reaction. So never lime in the same year you fertilize if you can avoid it and certainly not in the same couple of months.
Clay soils tend to become acid more quickly than sandy soils and the amount of organic matter has an effect as well. Clay soils can also be slow to react to the addition of lime as well.
The most common lime used in the garden is agricultural lime or ground limestone (calcium carbonate). It contains about 50% calcium, another essential plant nutrient . Once available only as a powder, pelletized lime is now offered as well. It can be dispensed from fertilizer spreaders and isn’t as messy to work with.
Some gardeners prefer dolomite lime, which contains magnesium as well as calcium. Magnesium is an essential nutrient, though needed in smaller quantities than calcium. Alternate the use of agricultural lime and dolomite, to get a better balance of nutrients.
Never use hydrated or slaked lime, sometime called quicklime. While this substance has many commercial uses, it is much too caustic for the garden.
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