Generations of allotmenteers have grown a healthy loam on our site - of which I am only too happy to take advantage!   What makes a good soil?

Soil acidity/alkilinity

The more calcium in the soil the more alkiline it is, measured on the pH scale of 0 to 14.   Simple kits will allow you to test your own soil.

• above 7 shows you have an alkiline soil
• 7 is neutral, (neither acid nor alkiline)
• and below 7 shows an acid soil.

Allotment crops prefer different levels of alkilinity.

• Potatoes - pH level 5.6, more acid than for many vegetables;
• Beans and Peas - pH level 6.5;
• Brassicas - pH level 6.5 to 7;
• Onions and Roots - pH level 6 to 6.5;
• Soft Fruits - pH level 6 to 6.5.

Adding compost and manure over the years gradually makes the soil more acid.   The soil becomes "sour".   To keep your soil "sweet" and well-balanced, after the potato crop (which likes acidity) you will probably find you need to add lime to reduce the acidity to 6.5.   Add it for the beans, and you'll get the benefit of the bean-fixed nitrogen and the lime for following brassicas.

300gm lime per square metre will raise the pH by about 0.5 (half that on sandy soils, 50% more on clay soils).   It's not so much a nutrient in its own right, so much as an element which makes available other essential nutrients to plants - OR prevents plants accessing nutrients if there's too much or too little.

Lime comes in different forms - fast-acting hydrated lime (which doesn't last well in free-draining soils); ground limestone (cheaper, works more slowly, and lasts longer).   The packets of lime from garden centres contain a mixture of the two.

Three words of caution -

• it's better to raise the pH level slowly - add several small doses of lime rather than one large one.   It's a bit like adding salt to soup - easier to add more as necessary, than to take it out when you've put in too much!
• do NOT raise the pH above 7!
• do NOT add lime within two months before/after adding fertiliser.

Key plant nutrients

• Nitrogen - for green leaves and stems;
• Phosphorus - for healthy root growth, and forming healthy seeds and fruits;
• Potassium - for overall growth, photosynthesis, and disease resistance.