Clumber Gardener: How to understand your soil

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WHEN gardeners meet for the first time, there are likely to be two topics of conversation, the weather and the type of soil they garden on.

We can’t do a great deal to change the weather, but we can improve our garden soil. A simple soil testing kit, costing around £14, will help you assess how fertile your soil is and what you may need to do to improve it and grow better crops or flowers.

First thing to find out is how acid or alkaline your soil is. This is measured on the pH scale; pH 7 is neutral, values below are acid, above are alkaline.

The ideal soil pH is slightly acid, down to pH 6.5, which allows a wide range of plants to flourish, as plant foods in the soil are soluble and can be taken up by plant roots.

At more extreme pH levels some foods become insoluble, so roots can’t access them and growth suffers.

Ericaceous plants, such as rhododendrons and heathers, struggle if the soil is not acid; leaves usually turn yellow and growth and flowering are weak.

In the vegetable garden, potatoes are more prone to scab disease which disfigures the skin of tubers if pH is too high.

However, brassicas such as cabbages, cauliflowers and broccoli prefer a slightly alkaline soil.

A dressing of lime will raise the pH. Making soil more acid is a tougher proposition, but manures, sulphur chips and fertilisers such as sulphate of ammonia will work to lower soil pH.

A good soil test kit will also measure the levels of the three major plant foods, nitrogen, phosphorous and potassium, (referred to as N, P and K after their chemical symbol), broadly needed respectively for healthy leaves, good root growth and flower production.

These three are needed in large quantities for healthy plant growth.

If levels are low, there is a wide range of fertilisers to correct any deficiencies, including single food fertilisers, such as sulphate of potash containing potassium or superphosphate, which has phosphorous.

Most convenient to use are the compound, general, balanced or complete fertilisers, which contain all three major plant foods, and often, some of the minor and trace foods as well.

Two of the most used are growmore, and if you are organic, blood, fish and bone.

Figures are used on the pack to indicate the strength of the fertiliser. Growmore, for example, will have 7:7:7. This is the ratio of nitrogen, phosphorous and potassium. The higher the numbers, the greater the concentration of plant food per unit of weight, so the less fertiliser you will need to spread around plants or onto the soil.

Fertilisers feed your plants. You also need to feed your soil by adding materials such as well rotted manures, garden compost or leaf mould. This benefits both heavy clay soils by improving its structure, and light, sandy soils, by creating a reserve for plant foods to be held in the soil, rather than being washed out of the reach of roots.

Winter and early spring are ideal times to improve your soil. Clay soils can be dug and have manures added now; frosts will help break up the clods of soil.

On sandy soils, late winter and early spring are the best times. Soil should not be water logged or heavily frozen. Work steadily, don’t over load your spade and pace yourself; even on light soils, digging should be treated as a marathon, rather than a sprint.

There is no denying that digging is hard work, but the rewards will be some great tasting fruit and veg or fabulous flowers.

Jobs of the month

When winter aconites and snowdrops have finished flowering, lift and divide them “in the green”, that is, with leaves still showing. This is usually better than planting dry bulbs in the autumn, as plants tend to establish quicker and produce more flowers in the first year after planting.

If not already done, cut back herbaceous perennials, including ferns and grasses, and compost the leaves or fronds.

Trees, shrubs and hedging plants such as beech, hornbeam and hawthorn, are much cheaper bought as bare root plants. These can be planted now, provided soil conditions are suitable and neither waterlogged nor frozen.

Prune wisteria by further shortening shoots pruned in the summer to two or three buds from the main stem.