Supercharge your soil
There’s more to garden soil than meets the eye. Alan Titchmarsh shows how to get yours into shape for borders that are bursting with healthy plants
Healthy soil means healthy plants. Follow Alan’s guide to boost yours
Spend as much on the hole as you do on the plant.’ It’s an adage we would all do well to remember from time to time – a reminder that every plant is only as good as the soil it is planted in. But soil is boring – it’s brown or grey and it just sits there. All of which may be true, but without it we gardeners would be, in common parlance, stuffed. But what exact ly is soi l? Soi l is, basically, ground up rock mixed with rotting organic matter. The nature of the rock, the size of the particles, the number of minerals that are present and the composition and amount of organic matter it contains is what makes soil so variable. As far as the gardener is concerned we are looking for something that will provide our plants with firm anchorage, nutrition, sufficient water retention and efficient drainage. Achieving that in our gardens and allotments is down to us as much as nature. A patch of earth that is disturbed by cultivation and in which we grow plants on an intensive scale needs rather more in the way of care and attention than a patch of undisturbed countryside, where the cycle of nature can achieve a balance and where humus (the natural organic content of soil) is allowed to build up without interference. Good soil is a living thing, teeming with microorganisms, from bacteria to fungi. The advent of antiseptic wipes has led to many people imagining that all bacteria are bad, but without the existence of ‘good’ bacteria we as humans would fail to survive and soil would be a poorer substance in which to grow plants. The interaction of bacteria and fungi, including mycorrhizal fungi – which have a symbiotic relationship with plant roots and are beneficial to growth – is vitally important. Many fungi are naturally present in soil, but you can help new plant s develop these benef icial relat ionships by dusting both the roots and planting hole with mycorrhizal fungi. It is widely available to gardeners now at garden centres and nurseries where it is sold in sachets and packets. The easiest to find is the brand Rootgrow (which trips off the tongue a little more easily than mycorrhizal fungi).
The importance of nutrients
Plants need three main nutrients to thrive – nitrogen, which encourages leaf and shoot growth, phosphates, which promote root development, and potash, which is necessary for f lower and fruit development. They are designated as
Good soil is a living thing, teeming with microorganisms, from bacteria to fungi
Organic fertilisers feed vital bacteria as much as the plants, and keep the soil healthier than inorganic fertilisers
N, P and K on packets of fertiliser, which show the ratio in which they are present. Aside from these, there are some vital trace elements that are needed in smaller and variable quantities for plants to be healthy: elements such as magnesium, calcium, iron, boron, zinc and the strangesounding molybdenum. The simplest way to ensure your soil has sufficient quantities of these trace elements is to regularly enrich it with ‘general’ fertiliser and wellrotted organic matter.
Acid or alkaline
Soil acidity – measured on the pH scale – will also affect plant growth. Acid soil has a pH below 7.0, alkaline soils – those containing chalk and limestone – have a pH of 7.5 or above. The leaves of rhododendrons, azaleas, pieris, camellias and other lime-haters turn yellow when the plants are grown in alkaline soil because they, as a group, naturally grow in acid soils and are unable to extract iron from those of an alkaline nature. We avoid growing them on chalky soil for that reason. If you’re growing on soil that is only slightly acid, it is a good idea to water on sequestered iron to help keep them healthy and green (sequestered simply means the iron is in a form they find easier to absorb). Regular cropping, heavy rains and constant cultivation wi l l, over time, deplete the soil of nutrients, which is why it is important to feed cultivated soil regularly – applying a general fertiliser each spring, just at the time when plants will be calling for nutrition. Soil that is depleted of nutrients will produce feeble plants that are starved of nutrition and whose growth and general health are poorer as a result.
Feeding the soil
Organic fertilisers, such as blood, fish and bone, must be broken down into an absorbable form by soil bacteria before they can be utilised by plants, and as such have the advantage of feeding the vital bacteria as much as the plants and keeping the soil healthier than inorganic fertilisers, which can be absorbed by the plants as soon as they go into solution with the moisture in the soi l. Inorganic fertilisers may feed plants but they do not feed the soil. But if plant nutrients are the ‘vitamins’ necessary for growth, then the meat and two veg – the protein and the
Worms are to be encouraged for their ability to mix in organic matter and improve soil structure
carbohydrates – come from organic matter, which improves soil structure in a way that powdered, granular or liquid fertiliser cannot. Well-rotted bulky organic matter, in the form of manure or garden compost, will bind together a thin, sandy soil, helping it to hold on to both moisture and nutrients. It will also open up a clay soil that is plastic and intractable thanks to the particles being so small that they bind together tight ly, impeding drainage and making cult ivation di f f icult . Leafmould, which is made from slowly decaying leaves, will have a similarly beneficial effect on soil structure, but adds little in terms of nutrients.
Keeping it healthy
Organic matter rots down over time and so needs to be regularly applied ( ideally annually) to keep soil healthy – bacteria and fungi thrive on its presence – but on clay soils it is also worth adding sharp gritsand, which is longer-lasting in terms of improving drainage. Worms do their bit here, too, and are to be encouraged for their ability to mix in organic matter and improve soil structure and drainage. Yes, they are a pain on the lawn where their casts cause problems, but elsewhere they do much good, so learn to live with them! There is also a host of other soil additives available to the gardener, such as volcanic rock dust, which can be used to add minerals and trace elements, and biochar – specially produced charcoal that is said to stabilise the earth’s carbon resources and improve soil’s ability to retain water and nutrients. What is indisputable is that good drainage, essential nutrients and plenty of organic matter are the ‘big three’ that will make all the difference when it comes to growing strong, healthy plants, with plenty of fruit and flowers.
Adding mycorrhizal fungi to roots boosts plant establishment and growth Collect fallen leaves to make leafmould to boost your soil’s structure
Use a pH test kit to find out if your soil is acid or alkaline