Understanding The Soil

The soil type greatly influences the range of plants which can be grown. Most rhododendrons, for example, cannot tolerate chalky soil, while clematis absolutely thrive on it. The basic soil types are listed below, and all can be made to produce good crops or ornamental displays, given suitable soil management and improvement.


The ideal soil is loam and it is the one most gardeners would opt for if they had the choice. It is very fertile and easy to cultivate. Although there are plants which prefer sandy or chalky soil conditions, there are very few plants which fail to thrive in loam. All other factors being equal, it offers the widest range of growing potential, for both ornamental and edible plants.

Loam is made up of particles of sand, clay and decayed organic matter (humus), as well as air, moisture, plant nutrients and micro-organisms. It is free-draining enough not to become waterlogged, unlike clay, although it is moisture-retentive enough not to dry out in summer, unlike sand.


Although clay soils are usually fertile, they are notoriously difficult to cultivate. They have a tendency to become iron hard when dry in summer and, in winter or after a heavy rainfall, lack of natural drainage leads to sticky, waterlogged conditions.

Clay soils are very slow to dry out and late to warm up in spring.

Never walk on clay soil when it is wet or, worse still, attempt to dig it or push a wheelbarrow over it. The individual particles of clay are minute, and when compressed, the tiny pockets of air which normally keep the soil ‘sweet’ disappear. Technically, the beneficial, aerobic bacteria are replaced by harmful, anaerobic bacteria and the soil turns ‘sour’.

Providing drainage, in the form of land drains and soakaways, improves clay soils, as does digging in well-rotted organic matter and sand. Peat will also help to improve the condition.


Compost is the converting of pest- and disease-free vegetable remains, including kitchen waste, and lawn clippings, into a useful substitute for farmyard manure. Perennial weeds should not be used in composting, as the seeds may survive intact. Evergreen leaves should also be avoided as they are slow to break down, and any animal remains, such as bones and fat, will attract rodents.

To make a heap, stack up 15 cm layers of moist vegetable remains. Alternate layers should be covered first with an activator, such as sulphate of ammonia, at 20 g/m2 and then with ground limestone at 160 g/m2. Proprietary activators are available and should be used according to the manufacturer’s instructions.

Wood ash (but never coal ash) can be added to the heap in thin layers, and provides a valuable source of potash. In dry spells, water the heap to speed decay. Do not over- water, though, as waterlogged compost will putrify rather than rot down.

Oxygen is also necessary for decomposition and compost heaps should be allowed to settle of their own accord, not forcibly compressed.


Usually free draining and early to warm up in spring, sandy soil is rather more demanding than loam. It needs regular applications of bulky organic matter together with quick-acting chemical fertilizers to get the best results. This is because, whenever it rains, the nutrient content gets washed down, or leached, to the lower layers of subsoil and is inaccessible to plants’ roots.

A related problem with sandy soils is that of drying out in summer. This can be partially remedied by digging in bulky organic matter which helps to retain moisture. Artificial irrigation is usually necessary, and mulching is also beneficial, provided the soil is thoroughly soaked before the mulch is applied.


Chalk and limestone soils are usually pale; the humus-containing topsoil level is often shallow, with the pure chalk or limestone very close to the surface. They are free draining and tend to dry out in summer, although stickiness is a problem after heavy rain.

Because of its alkaline nature, certain elements, such as magnesium, iron and potassium, become ‘locked’ in the soil and unavailable to the plants. Liberal quantities of manures and fertilizers are necessary, year after year, as nutrients are quickly leached out of chalk soils.

It is best to stick to plants which thrive in chalk soils, such as many alpines, rather than suffer defeat with known lime-haters.


Peat soils are dark, spongy and contain large amounts of partially-decomposed vegetable matter. They usually occur in areas of high rainfall and humidity, so drying out is not a problem. However, they tend to have high water-tables, and special provision for drainage is usually necessary.

Acidity and lack of nutrients are additional problems with peat soils, but when regularly limed and dressed with fertilizers, excellent results can be had.

Peat, as purchased in bags at garden centres, has no nutrient value at all but is excellent for improving the soil conditions of sandy, chalky and clay soils. Always wet peat thoroughly before using.


Mulching, or top dressing, is the practice of covering the surface of the soil, usually to prevent evaporation. Additionally, mulching helps keep down weeds and, in hot weather, can keep the soil


Soils act as reservoirs for nutrients and moisture, and the deeper a plant’s roots can travel, the better the plant’s growth will be. During autumn or winter, when the soil is neither frozen nor waterlogged, prepare new beds by digging to at least a spade’s depth.

For plain, or single, digging, excavate a trench a spade deep and a similar width, heaping the soil to one side. Fill this trench with soil from a second, repeating the process until the last trench dug is filled with the soil from the first.

To break up compacted subsoil and improve aeration and drainage still further, dig a trench one spade deep and 60 cm (2 ft) wide. Fork over the bottom of the trench and work in well-rotted compost or manure. Fill with the soil dug from a second trench, and so on.