Vegetables and Soil Q and A

What is the best type of soil for growing vegetables?

The ideal soil is a medium loam, but vegetables will grow on a wide range of soils, from light sandy ones to heavy clays, so long as they are fertile. Vegetables are greedy and never do well on poor, starved soils. Digging in plenty of well-rotted manure or compost is the best way of making the soil fertile: aim at two bucketsful per square metre (square yard) each year. Soil bacteria break manure and compost down into humus, from which plant foods are released. Humus makes light soils more moisture-retentive, improves the drainage of heavy soils, increases the amount of air in the soil, and encourages earthworms, which play a key role in creating fertile soil.

My garden is on very sandy soil. What vegetables are best suited to such conditions?

The great advantage of sandy soil is that it is airy and well-drained, and so warms up rapidly in spring. It is excellent for early (and later) crops of carrots, ordinary radishes, beetroot, early salads, garlic, the first sowings of dwarf French beans and peas, and long-rooted vegetables such as parsnip, scorzonera, salsify, and winter radishes. Unless you can work in plenty of organic matter and are prepared to water heavily in summer, it is best to avoid brassicas, maincrop potatoes, runner beans, and plants such as celery, celeriac, fennel, and rhubarb, which require a great deal of moisture throughout their growing period.

Can you explain how soil acidity affects different vegetables?

Soil acidity is measured on the pH scale . Most British soils are slightly acid. The following vegetables prefer slightly acid soil in the range from pH 5.5 to 7.0: beans, brussels sprouts, cabbage, cucumbers, marrows, parsley, parsnip, peas, radish, swede, sweet corn, tomato, and turnip. Potatoes and rhubarb prefer more acid soils (pH 5.0 to 6.0); while asparagus, beetroot, carrot, cauliflower, celery, leeks, lettuce, onions, and spinach prefer more alkaline soils (pH 6.5 to 7.5). On peaty soils vegetables can tolerate somewhat lower levels of acidity than on normal, mineral soils. Excess acidity, the commonest problem, is remedied by liming .

We have just moved into a new house, the ‘garden’ of which is a pile of builder’s rubble. We are anxious to start growing vegetables. Should we import topsoil?

Topsoil is expensive and that offered for sale is often of poor quality and lifeless. Instead, invest in a large load of farmyard manure, spent-mushroom compost, sewage sludge, or seaweed. Spread it over your garden in the autumn in a layer 150 mm (6 in) thick. You will be surprised how it will work into the ground during the winter. In spring fork it in as much as possible, apply a base fertiliser, and plant potatoes and Jerusalem artichoke tubers to help break up the ground. For other sowings and plantings, make mini-trenches, about 150 mm (6 in) deep, filled with commercial potting compost and covered with about 50 mm (2 in) of soil. Start with lettuces, radish, cress, mustard, salad rape, spring onion, carrots, and dwarf beans: once something is growing, fertility begins to build up.

What is the difference between organic and inorganic fertilisers?

The essential difference between them is that organic fertilisers are ‘natural’ (derived from plants or animals), while almost all inorganic fertilisers are manufactured. For practical purposes fertilisers can be divided into bulky manures and concentrated fertilisers. The bulky manures are all organic. They are essential for keeping the soil in good heart, and they also release plant nutrients slowly. Most of the concentrated fertilisers are inorganic salts such as ammonium and potassium nitrate, superphosphate, or mixtures such as Growmore. They are used in much smaller quantities, release the nutrients more rapidly, but do little to improve the soil structure. The traditional concentrated organic fertilisers, such as dried blood, hoof-and-horn, and bonemeal, are expensive sources of nutrients today, but organic seaweed extracts are very useful for vegetables.

Can vegetables be grown organically and, if so, what are the advantages?

All vegetables were grown organically before the introduction of artificial fertilisers, when manure was plentiful. If you want to grow organically today you must be prepared to build up the soil fertility by very frequently working organic matter into the soil. Feed the soil, not the plants, is the organic grower’s maxim! Seaweed extracts and home-made liquid fertilisers (made by soaking bags of soot, manure, or comfrey in a rub of water) can be used to boost plant growth during the season. One of the arguments against artificial fertilisers is that they may encourage sappy, over-vigorous growth. This makes plants much more susceptible to pest and disease attack and less tolerant of adverse weather conditions.

How can I be sure my vegetables are getting enough nitrogen, phosphorus, and potassium (potash)?

Nitrogen (chemical symbol N), in particular, but also phosphorus (P) and potassium (K) are the elements vegetables require in the largest quantities . Nitrogen, which is obtained by the breakdown of humus, is the most likely to be in short supply because it is washed out of the soil in winter. Poor growth and yellowing foliage are signs of nitrogen deficiency. In soils of average or below average fertility, the natural supply of nitrogen can be supplemented by a top dressing of a balanced fertiliser when planting in spring or during the growing season. For vegetables with very high nitrogen requirements, such as potatoes, cauliflower, beetroot, spinach, brussels sprouts, leeks, and spring and summer cabbage, apply Growmore (which also contains P and K) at the rate of 270-340 g/m2 (8-10 oz/sq yd). Use half that amount for less-demanding vegetables. Adequate reserves of P and K can be maintained in the soil by working in 5 kg/m2 (10 lb/sq yd) of farmyard manure or 2.5-5 kg/m2 (5-10 lb/sq yd) of garden compost every autumn.

My father used to lime his vegetable garden religiously every autumn. Was that necessary?

Probably not: there was a tendency to over-lime in the past. Most soils need liming about every third year, if then. Signs that liming are necessary include poor growth, a ‘sour’ look, with moss and unrotting vegetation on the surface, or a problem with clubroot. If a soil test then indicates that the pH is below 6.5, apply ground limestone at the rate of 270 g/m2 (8 oz/sq yd) on sandy soil, 540 g/m2 (1 lb/sq yd) on loamy soil, and 810 g/m2 (1 ½ lb/sq yd) on clay soil.