Approaching soil in garden design

Soil in a garden, regardless of its size, can be very varied. Where there are trees it is often dry, and at a low point often wet. Old gardens, which have a history of cultivation over decades, usually have fertile, workable soil. They are in contrast to the plots of Bedfordshire’s ‘new builds’ on fields of clay and which need copious quantities of organic matter to improve the soil to successfully enable young plants to establish and grow in a newly planted garden.

When garden designers take on new projects we need to try to keep the existing topsoil on site to avoid the expense of transporting it elsewhere. The landscape within the site should be designed to accommodate the existing mass of soil - for example the cut and fill of sub-soil from a slope. Where the topography or sub-soil determines, we design necessary drainage as required by the site. Topsoil requires care; if it is necessary to remove topsoil to install hard landscaping installed we need to store it correctly. The ground should also be protected from compaction by heavy machinery. Finally, the existing soil can be improved with ameliorants to achieve a suitable growing medium for the intended planting scheme.

In designing a garden we initially dig a profile pit to gain a working knowledge of the topsoil and sub-soil of the site. We identify areas of compaction and depth of pan, damp/boggy areas, look for indicator plants and note if the soil smells unpleasant. We also take a sample of the topsoil to carry out a range of soil tests to determine it’s type of soil e.g. clay, loam, silt, sand, chalk, soil pH (acidity or alkalinity) and texture. A sample can be sent for analysis to determine nutrient values and presence of pollutants. Sometimes we test a sample for the approximate percentage of organic matter and drainage materials.

A site analysis will determine which areas of the garden soil are dry and which are fertile or boggy. That, along with information about the pH of the different areas of the garden, will influence the choices of planting in the new design.

On the left, a nearly dead Buxus due to poor soil and neglect. On the right, the same plant (with new extensive new planting alongside) 5 years later, following a seaweed feed, soil improvement with farmyard manure and regular professional border maintenance.

I am saddened that house builders are rarely able to save and replace sufficient topsoil for the owners of ‘new builds’ to have anything approaching reasonable soil to grow a garden in. I have seen front and back lawns and the trees and shrubs of new gardens gradually sicken and die as a result. One solution would be to use lawns of artificial grass instead, combined with interesting hard landscaping materials/design and features with height, which provide some shade, with all the planting in containers and an automatic irrigation system. There are some better artificial plants on the market now and combined with carefully chosen garden furniture, it is possible to have a good-looking garden without relying on having the right soil to grow plants in.

Importantly, local soil composition can become influenced by the landscape around it if. For example, if the surrounding landscape is subject to a great deal of rainfall, this can lead to an excess of moisture in the soil over its saturation point. The excess rainwater finds the lowest geographical surface point in the surrounding landscape, and if for example the soil is clay (which has low permeability), and there is not a man-made ditch or a natural stream in the way, the excess rainfall can flood any garden in its way as it travels through lowest points until it reaches a ditch or stream.

Only by researching the wider landscape around the client’s site for the presence of a ditch or stream can the garden designer become aware if they need to include the facility for field runoff from the surrounding landscape in their garden design. I once had a client whose garden was 100mm local blue clay on a river terrace across the entire site! He insisted on having English country garden planting with a lawn, refusing my recommendation of raised beds as features with different textures of hard landscaping.

It is possible to improve a garden soil by using an annual organic compost mulch to protect soil from surface erosion by wind or heavy rain. This also allows water to slowly soak into the soil, provides food for the soil- dwelling organisms and air spaces for roots to grow healthily. Grit can be added to aid drainage especially for bulbs in the ground. Harvested rainwater is better for watering houseplants as it has a lower pH than tap water and is less likely to cause limescale deposits.

Earthworms in flagrante in March 2018.

Organic compost can easily be made in a shady corner of the garden by putting thin layers of alternate green and brown garden waste and uncooked kitchen waste in a heap. Then it should be covered to keep in the generated heat, and to maintain moisture. I have eight compost bins for my half acre garden; we apply our home-made compost to all the beds in the spring. The compost creates a loose top 100mm of soil which is easily weeded and is a banquet for the many birds which feed in our garden.

Soil is full of living organisms, and using garden compost will help them and your garden thrive. From worms, centipedes, and nematodes to microbes, bacteria, fungi and minute organisms involved in nutrient recycling, they make up a biological community of living organisms which tunnel through moist topsoil ingesting particles and predating on each other. Together they create a food web within the soil which keeps it healthy and productive.

This is an extract from a talk given by Susan Young Garden Design on ‘Soil in the Garden’.

About the author:

Susan Young is a qualified horticulturalist and garden designer working in Bedfordshire and surrounding counties. Established in 2002 Susan now spends the majority of her time advising clients and providing a full garden design service from initial consultation through to the hard-landscaping build and planting. She has recently won regional and national awards for a hard-landscaping design built in Bedfordshire. For more information please go to www.susanyoungdesign.co.uk