Gardening > Soil Ph

Soil Ph

Soil Ph Kit

Soil ph kit
© Jack Schiffer

If you have just taken over a new garden or if you are having problems with certain plants it is as well to check whether your soil is acid or alkaline. Some plants such as rhododendrons, azaleas and lupins need an acid soil whereas syringa, philadelphus and forsythia prefer an alkaline soil. Most plants, including most vegetables, prefer a slightly acid soil but some will not tolerate acid at all. It saves time and disappointment if you find out the exact ph value of the soil in your garden. If you then take steps to try to alter the ph the test needs to be done on a yearly basis.

There are several kits on the market for home testing the ph of soil. They are fairly straightforward to use and can be relied upon to be accurate. It is the lime content of a soil that determines the ph value. This is measured in units from 1 to 14. 7 is neutral. Readings above 7 are alkaline and below 7 they are acid.

Generally speaking it is better to choose plants to suit your soil rather than trying to alter the soil to suit the plants. The vegetable plot is a different matter however because most vegetables need a ph of 6.5 in order to do well.

An over acid soil can be made more alkaline by the addition of lime. This is available in several forms: slaked lime, hydrated lime, ground limestone and calcified seaweed.

Different types of soil require different amounts of lime. It is important not to apply lime immediately after manuring as this can cause the release of valuable nitrogen into the atmosphere. The addition of too much lime can result in nutritional deficiencies and lime should be added to the soil 2-3 weeks before planting and sowing.

You will not however see immediate dramatic changes and the best method of application is to give the garden small but regular dressings of lime. It is not a good idea to put on a large amount all at once as this can damage plants.

It is easier to make an acid soil alkaline than to make an alkaline soil acid. If you really want to raise acid-loving plants and your soil is alkaline it is best to create an acid raised-bed for them by using a raised-bed system filled with a peat-based soil mixture and sharp sand.

The ph of the soil in the vegetable garden can be lowered i.e. made more acid by creating raised-beds and digging in plenty of organic matter. In practice there are very few gardens so alkaline that vegetables cannot be grown but alkaline soil presents problems associated with nutritional deficiencies because a chalky (alkaline) soil drains very rapidly and the draining water takes nutrients away with it. Using a raised-bed bypasses this problem because by raising the level at which the crops are grown the alkaline water running out of the surrounding chalky soil is unable to drain into the higher growing area.

To raise the ph of soil by 1 unit (per 100 square yards) you will need to add:

for sandy soil: approx. 2lbs lime
for medium loam: approx. 6lbs lime
for heavy clay: approx. 8lbs lime