LIME - WHAT DOES IT ACTUALLY DO?
"Lime" is limestone or chalk. When you put a lot of manure on the soil, it becomes more acid, or 'sour' - like moorland soil. Some plants - tomatoes, potatoes for instance - like this; others - the cabbage family, onions, among others - hate acid soil. Lime re-adjusts the balance. It also helps most plants get the goodness out of fertilisers.
We add lime when getting an area ready for lime-loving plants like cabbages and onions. Crop rotation (more of that later) means that by the time you're planting potatoes, the lime you added for the cabbages has 'worn off', washed away by rain and counterbalanced by more manure. It's a balancing act but you don't have to be exact.
There are two types of lime on the market: GROUND LIMESTONE and SLAKED LIME. Ground limestone is lovely pleasant stuff, often looking just like a bagful of these beautiful white cliffs at St Margarets Bay. Slaked Lime has been heated in a kiln then combined with water ('slaked', as in"slake your thirst"). It's slightly caustic (you don't want to get it in your eyes or breath in the dust!) and it is much stronger than ground limestone - best avoided unless you really know what you're doing. Always read the label before buying lime.
As well as using lime on the soil, you should sprinkle a layer on your compost heap every so often to help the vegetable matter rot down more quickly.