Liming of vegetable plots is a ritual for many vegetable growers. It normally happens most Autumns. It is not necessary to do this, unless you actually have an acid soil - or if you grow particular crops such as Brassicas
Liming once every three years is probably ok for most vegetable plots. Even then, if a ph soil test indicates a ph level of 6.5 or above, then liming is not necessary.
Heavy clay soils in particular will benefit if they are limed after rough digging over. The lime helps to break down the soil over-winter so it is easier to work in the spring.
Garden lime basically lowers the level of acidity in the soil. Too acid a soil, and many of the vital foods in the soil are locked up - unavailable for the plants to feed upon. The main exception being the Rhododendrons and Ericas, which are all classed as ericaceous plants.
The best way to determine if your soil needs liming, is to carry out a very basic test which will measure the ph level of your soil. The 'ph' level is simply a scale numbered from 0 - 14 with the lower numbers being the acid levels of the soil (or other matter) and the higher numbers are considered as being alkaline, with a ph of 7 being accepted as neutral.
Most garden plants - including vegetables - are happiest in the ph7 range. The ph7 is considered as neutral.
Brassicas prefer a slightly alkaline soil with a ph7.5 level. This also helps to prevent the dreaded clubroot disease.
Potatoes prefer an acidic soil - preferably in the range ph5.5 to ph6. A higher ph encourages scab disease of potatoes.
A soil testing kit is available at most garden centres. the ph test is the most used, accurate and convenient - quick.
The best time for adjusting the ph level of your vegetable plot - or maybe just a few areas in the plot - is in the autumn after most of the cropping has been done. The lime then has the winter in which to be absorbed in the soil, without damaging young seedlings roots if applied in the spring.
The best of the several types of lime (calcium) that are generally available, is conveniently called Garden Lime - the active ingredient being ground to a coarse, granular powder Calcium Carbonate. Ground Chalk is also available as a substitute. This is also Calcium Carbonate.
Sometimes Hydrated Lime is offered. this is what builders normally use, and it contains Calcium hydroxide, which can burn or at least cause severe eye problems. Other than that, it can be used, but the Garden Lime or Ground Chalk is preferable. Hydrated Lime can be used carefully in the growing season as it is very quick acting. Do NOT apply it touching or too near plants.