Planning your plot.

General | Allotmenteer.co.uk:

The traditional 3 plant families
If your plot is rather small - or you use only parts of it for vegetable growing - it would be adviceable to go with the traditional 3 family plan. In this plan the crop is divided into the following groups:
- Roots, Beetroot, Carrot, Chicory, Jerusalem Artichoke, Parsnip, Potato, Salsify and Scorzonera
- Brassicas, Broccoli, Brussels sprouts, Cabbage, Cauliflower, Kale, Kohl Rabi, Radish, Swede and Turnip
- Others, Aubergine, Bean, Capsicum, Celeriac, Celery, Cucumber, Leek, Lettuce, Marrow, Onion, Pea, Spinach, Sweet Corn and Tomato.
Each plant family has different needs - and the soil where you plan to grow them should be treated differently. In the following, manuring means adding manure to the top soil, and italics indicates an must have need.
Roots. Do not add manure. Do not lime. Rake in a general purpose fertilizer about 2 weeks before sowing or planting.
Brassicas. Add some manure or compost, partucularly to soil short of humus. Lime the soil, unless you are sure it’s already alkaline. Rake in a general purpose fertilizer about 2 weeks before sowing or planting.
Others. Add a liberal amount of manure or compost. Lime - but only if the soil is known to be acid. Rake in a general purpose fertilizer about 2 weeks before sowing or planting.
The 4-family system
Recently some gardeners organisations have started implementing and recommending a 4-family system. In this system, potatoes and other plants from the night shadow family of plants, and plants requiring similar treatment has been moved out into their own group.
The groups now look like this:
Potatoes, Courgettes, Marrow, Peppers, Pumpkins and Tomatoes.
Legumes, Beans, Peas and Green manure.
Brassicas, Broccoli, Brussels sprouts, Cabbage, Cauliflower, Kale, Kohl Rabi, Radish, Swede and Turnip
Roots and Onions, Beetroot, Parsnips, Carrots, Onions, Shallots, Garlic and Leeks
The recommended soil preparation for the 4 groups looks like this:
Potatoes. Add manure or compost. Do not lime. Rake in a general fertilizer about 2 weeks before sowing or planting.
Legumes. Add a liberal amount of manure or compost. Lime - but only if the soil is known to be acid. Rake in a general purpose fertilizer about 2 weeks before sowing or planting
Brassicas. Add some manure or compost, partucularly to soil short of humus. Lime the soil, unless you are sure it’s already alkaline. Rake in a general purpose fertilizer about 2 weeks before sowing or planting.
Roots and onions. Do not add manure. Do not lime. Rake in a general purpose fertilizer about 2 weeks before sowing or planting.

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