Garden Soil Basics: How To (and Why) Check Out Your Soil

4 Super- easy ways to test your garden soil

Why Test Your Soil?

It never seems like a glamorous or exciting part of planning your garden, but your soil is a critical building block both to build on and to plant in. So for the sake of 30 minutes or so of your time and armed with our easy guides below, get down and dirty for a little while.

What You Need To Know For Hard Landscaping

Anything built on soil, whether it be walls, patios, driveways, garden buildings or even shed bases needs the soil to be properly stabilised and compacted (see How To Design a Patio Part 1)..

It is much more cost efficient to remove any potential trouble-spots at this stage than have to take up a section of the final pavement and re-construct the sub-base.

Soils with a high sand or gravel content can easily be compacted to form a very strong base for construction. However, soils with high silt or clay move and depress more with forces from above. Clay soils can also hold a lot of water making them more unstable during freeze/thaw periods. Clays also shrink and contract when the soil dries out. Chalk soils can erode easily creating gaps. Any movement in the soil layer can cause your sub-base or cement layers to move and your paving to crack or become displaced. Garden buildings can crack or subside. You don’t want the next Leaning Tower of Pisa in your garden now, do you?

The good news is that if you do have clay soils, by ensuring adequate drainage or directing water away from the site so that the soils don’t get waterlogged, by excavating to and by stabilizing the soil surface with grit sand or a geotextile membrane before construction, by increasing the depth of subbase ,problems can be avoided. Prevention is almost always cheaper than cure. Trees nearby will cause the water content of your soil to change more throughout the year and require extra consideration for drought periods when severe shrinkage can occur. Cutting down a tree is not always the answer, since this can increase the soil water content and swell the ground enough to cause structural damage. Higher risk areas will need deeper subbases and foundations

Clay soils expand and contract as the water content changes throughout the seasons and so construction over these needs to take account of the instability to stop foundations and paving from cracking and moving.

Regardless of your soil type, remove all topsoil before you begin construction. Any soil containign degrading organic material will likely move.

n addition to managing drainage, compacting and stabilizing the soil before construction may have reduced settling problems.

Taken all of this information together, it is important not to make any assumptions about the soil in your garden, but to take some samples of soil from different areas and do some simple tests. First, dig out a spade's depth of soil. Do not sample the soil on the surface, but collect a handful of soil from the bottom of the spade hole. Place in a container and label where in the garden this sample came from.

Certain soils require larger foundations, or different types of foundation from others, influencing the cost of your garden build. Of course any decent landscape contractor will take this into account in their quotes, but if you plan to DIY build you need to be aware. 

Soil is the foundation of most of our work, whether it be a patio, retaining wall, or a garden. Knowing what soil type you have and how to deal with it will make your next outdoor project a success.

What You Need To Know For Plants.

Plants generally grow only in the uppermost layer of soil that contains not only particles of rock but also  decayed plant and animal remains that have been decomposed by bacteria and fungi. The organic component of soil provides the nutrients that plants need to grow and can also assist with holding more water near to plant roots. Organic material only exists in the very top layers of soil (known as the topsoil). The quality of your topsoil can vary in different parts of the garden. For example, under larger trees where all the organic material comes from one type of leaves that are slow to decompose in winter will be different from an area of spring bulbs or leafy perennials, where the softer leaves break down rapidly in spring and autumn. In many gardens, especially new-builds, the topsoil will not be original, but will have been imported from elsewhere by the builders and so may not reflect the geology of your area.  It is also very typical near buildings to find a lot more left-over construction rubble near the surface than soil!

How To Design a Stunning Patio Parts 1 and 2

The exposed subsoil will then need to be levelled and compacted. This will be covered by a layer of granular sub-base material of appropriate depth for your conditions. A firm sub-base is compacted properly by machine. Note that permeable paving (which lets rainwater drain through the patio rather than run off from it into another area) requires a different type of sub-base (known as DTp2) to non-permeable paving (DTp1).

What then goes between the prepared sub-base and your top patio layer is known as the bedding layer. Types of bedding layer vary according to your soil and environmental conditions, ty

What is Soil and Where Does it Come From?

Most of the Earth’s crust beneath our feet is made from several kilometres of solid rock, In many areas of land, the top 25 cm-1.5 metres has been transformed into soil, a mix of rock minerals and decaying plant/animal remains.

Between the soil surface and the parent rock there are usually several layers. Soil formation begins when bedrock starts to break down into smaller mineral particles when it is weathered by sun, water, wind, ice, and living creatures.

The mineral layer that forms on top mixes at the surface with organic leaf litter, animal faeces and remains and starts to attract soil bacteria, fungi and animals. This topsoil is ideal for plants with small roots to grow. It is usually a darker colour than the soil mineral layer due to the decayed organic material known as humus.

Over time the topsoil layer is exposed to more and more rain and this washes down some of the iron, aluminium and heavy clay minerals from the topsoil layer to form a new layer known as subsoil. This is usually paler than topsoil and has much less organic material and few soil organisms. Because the topsoil becomes deficient in some minerals however, strong roots of larger trees and shrubs can grow deep into the subsoil, leaving smaller, more delicate roots in the topsoil to take up water and oxygen. This whole process

Needles, twigs, leaves, stems, and roots of plants are incorporated into the soil and broken down by the different kinds of organisms that live in the soil. 

An Easy Way to Test Your Soil Type

What gardeners and landscapers refer to as soil type is really a description of the soil texture i.e. the main size of the weathered mineral particles within it. The largest particles feel rough to the touch and are known as SAND. SILT particles are medium sized and clay particles are the smallest. In practice soils usually contain mixtures of different sized particles, but for our purposes you only really need to determine the dominant type. If you are really lucky, you will have similar quantities of sand, silt and clay and have most gardeners dream- a LOAM soil.

TEST YOUR BASIC SOIL TYPE WITH THE SAUSAGE TEST

Take about a teaspoon amount of soil and try to make it into a sausage shape with your fingers (if the soil is  dry you will need to sprinkle or spray with a little water to moisten first). 

• If the soil crumbles and will not form a sausage shape it is SANDY. The sand grains will feel rough when you roll them between your fingers.

• If you can easily make it into a sausage shape it is CLAY. The more you can bend the sausage, the more clay your soil contains.

• If the sausage only bends a little before it breaks, it is likely to contain SILT. Silty soils feel smooth and a little soapy

• If the sausage bends into a half circle, but not into a full circle it is most likely to be a LOAM soil with a mix of sand, silt and clay.

Soil type is largely determined by the local geology, as it is the weathering of the underlying bedrock by sun, water, wind, ice, and living creatures that makes up soil minerals. Thus sandy soils can be found in areas of sandstone, silty soils  over quartz, chalky soils over limestone and clay soils (the most common type) occur over granite, mudstone and volcanic ash deposits. Over time, sun, water, wind, ice, and living creatures

Taken all of this information together, it is important not to make any assumptions about the soil in your garden, but to take some samples of soil from different areas and do some simple tests. First, dig out a spade's depth of soil. Do not sample the soil on the surface, but collect a handful of soil from the bottom of the spade hole. Place in a container and label where in the garden this sample came from.

Certain soils require larger foundations, or different types of foundation from others, influencing the cost of your garden build. Of course any decent landscape contractor will take this into account in their quotes, but if you plan to DIY build you need to be aware. 

 TEST YOUR SOIL DRAINAGE

In many parts of the world, gardens need to be effectively drained. Rain falling directly onto the garden, falling from roofs and from overflowing drains collects in areas of poor drainage. This can result in puddling of patios, paths and driveways if they have not been installed properly, affecting their integrity over time. Poor drainage can make slippery surfaces dangerous and can attract mossy growth that can be hard to clean. Wooden decking and some types of stone can be particularly dangerous when wet. These are not recommended for shady areas where they will take longer to dry out.

Poor drainage is also a problem for  plant beds. If water doesn't drain away sufficiently quickly from plant roots, they don't get enough oxygen to survive. This causes plant leaves to yellow or whole plants to die. Walking on muddy soil compacts it and reduces air spaces further, making the garden harder to tend.  Poor drainage in planting beds can be solved by adding compost or well-rotted manure to the soil (the Royal Horticultural Society recommends 1 barrowful per square metre). Raised beds offer another practical solution if improving the soil is undesirable or problematic. In areas of very poor drainage, bog gardens, rain gardens, ponds and various types of drainage channel or perforated plastic pipe will need to be factored into the design and budget.

Lawns, in particular,  need to be grown over sandy or other free-draining soils to prevent them from being overly muddy for much of the year. They also need to be aerated with a spiking tool every year to keep the matted roots healthy. Laying turf over compacted or poorly-draining soil will never give good results, so factor this into your budget and garden build plan.  

Clay soils are particularly prone to be poorly-draining as thick bands of clay act as solid barriers to water. Sandy soils have the opposite problem. In these, the spaces between the large sand particles allow water to drain very rapidly, so that plant roots often don't receive enough. Nutrients from decomposed organic matter also wash out very quickly from sandy soils, requiring more frequent addition of compost or manure.

If you are not sure what type of drainage solutions your garden might need, check with a reputable landscape contractor.

TEST YOUR SOIL pH WITH A DIY KIT

The pH of your soil is a measure of how acid or alkaline it is. This is important to know because many plants only thrive in a particular pH range. Don't worry, you don't need a degree in Chemistry to figure this out. Just buy an inexpensive soil pH test kit from your DIY store and follow the instructions for a result in a few minutes. 

CHECK IF YOUR SOIL TYPE IS CONSISTENT WITH THE SOILSCAPES ONLINE MAP

The natural soil types across the UK have been compiled into an online map. You can easily find yours by clicking the Search Tab and then entering your postcode. If the description for your postcode fits what you have found from your own soil tests, then you likely have the original soil from the site. However, if it is different then you most likely have one or more types of imported topsoil in your garden. This is often only added to a shallow depth, so if you want  to plant anything that will have deeper roots (such as trees or larger shrubs), then you will need to collect soil from further down and test that.   

If you are in the US, you can use the Web Soil Survey. For other countries, contact your local authority to find out what resources they might have to help you.

Once you know your soil type and pH, then you can go shopping for plants. Always take time to read the plant label or ask for help before you buy something and then your hard earned cash and time spent planting will be much more worthwhile.