Updated 01 Jan 2011

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Introduction

Time spent now checking out your soil conditions may save your expensive plants later. Our guide tells you about testing and improving your soil.

The ideal garden soil drains freely in winter, retains moisture in summer, and is rich in organic matter, soil organisms and nutrients. But for many gardeners, heavy clay, compacted silt, light sand or whatever the builders left behind, is more the norm.

In this report, we show you how to assess the condition of your soil and give advice on how to maintain and improve it.

About your soil

Soil varies from place to place depending on the local geology and natural forces that have shaped the land. Knowing what lies beneath the surface will help you garden more successfully.

Your soil profile

Take a spade and dig a hole about half a metre square. Keep at least one side of the hole as straight and even as possible and dig down 50-60cm, if you can. If it's easy digging and you can go quite deep, then the soil is probably light and free draining. However, if it's tough going you've probably got a heavy soil. Or some parts might be relatively easy with the odd layer that's hard to break through (known as a "pan").

Take a breather and look at the straight side of your hole. If your soil has been undisturbed by construction or gardening you may see several layers of material (the soil profile), perhaps with different colours or textures. Closest to the surface is topsoil with one or more layers of subsoil below. There may be layers of pebbles or stones left behind by ancient rivers, or volcanic ash, clay or silt deposited by eruptions, wind or floods in days gone by. The soil profile can vary from one part of your garden to another, so checking out what's down there can give you a better idea of what to expect from your plants.

There's little you can do to change subsoil. You can, with some effort, break hard pans to improve natural drainage or install drains. But there's a lot you can do to improve the upper 20-30cm of soil, where the majority of the real growing action takes place.

What is soil?

Soil is a mixture of minerals, microrganisms, organic matter, water and air.

One way the minerals (derived from broken-down rocks) are classified is according to their size into sand, silt and clay.

Clay particles are roughly 1000 times smaller than the largest sand particles, with silt mid-way between the two. Because clay particles are so tiny, they occupy the spaces between the larger sand and silt ones.

So the more clay in a soil the fewer spaces there are to contain air and the natural downward flow of water is impeded. Even when dry it weighs more than soils with less clay, so is often called a heavy soil. Soils with a high proportion of clay (more than 40-50%) can make gardening a real challenge.

However, clay has good points too. It helps retain water and holds nutrients, unlike sand particles that allow them to leach away. A soil with around 40% sand, 40% silt and 20% clay would be the ultimate in dream gardening.

To determine the level of clay in your soil, try the Horseshoe test.

Horseshoe test

To determine if your soil has high clay content collect about a handful from 3-5cm below the surface. It should be quite moist but not soaking wet. If necessary water the ground and allow it to drain before collecting your sample.

Remove any plant material and roots. Knead the soil for several minutes, breaking down any lumps until you can squeeze it into a relatively smooth ball. You may have to add a little water if it's too dry to stick together. If you can squeeze water out easily it's too wet.

Roll the ball of soil on a piece of smooth board into a fat sausage, 25-30cm long. (pictured right)

Then try bending it gently. If it's smooth to the touch and you can make a horseshoe shape without it breaking or cracking badly, it contains a lot of clay (see image below). If it feels gritty to the touch and breaks very readily, it's probably high in sand and low in clay.

While it's useful to know the proportions of sand, silt and clay in your soil, there's not a lot you can easily do to change the situation.

But you can affect the way these particles group together into lumps and crumbs (soil condition) and this can have a dramatic effect on the success of your gardening efforts.

Find out how to test your soil condition and ways to improve your soil.

Check your soil condition

With funding and support from the Ministry for the Environment, Ministry of Agriculture and Forestry and various regional councils, Graham Shepherd, a soil scientist at Landcare Research, developed a practical way to check soil conditions.

Called Visual Soil Assessment, it was originally developed for farmers and soil conservators. But the basic principles are the same for any soil - so with permission we've adapted the technique for the home garden.

The test we describe below is designed for parts of the garden that have not been cultivated or cropped for several years (such as lawn), so it's ideal for checking out soil condition before breaking in new ground.

Allow around 15 minutes for the test. For best results the soil should be moist, neither soaking wet nor bone dry.

You'll need:

• A spade
• A plastic basin
• A flat piece of wood (such as ply or particle board) to place in the bottom of the basin
• A large plastic bag (around 500 x 700mm)
• This report and a pen.

The drop shatter test

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Dig out a 20cm cube of soil with the spade. (Most spades are 18-20cm wide.) Many soils will come out as a single lump, especially if slightly moist. If yours doesn't cling together at all, dig out enough to have the equivalent of a 20cm cube.

Drop the soil from a height of one metre (about waist height) onto the wood in the bottom of the basin so it shatters into pieces. Drop large clods again once or twice. Don't drop any piece more than three times. If it breaks into small pieces with the first or second drop, move on to the next stage. If roots are holding the soil together, pull it apart along any large cracks. Avoid crushing any pieces smaller than they break into naturally.

Spread the plastic bag flat on the ground beside the basin and transfer the soil onto it. Grade the fragments as you go so the largest clods are at one end and the finest at the other.

Soil structure and consistence

Compare your sample with the photos below and put the appropriate score on the Your soil score column of the score card .

Soil porosity

Now take a slice of soil from the side of the hole created by taking your original 20cm cube, break it in half and look at the exposed soil. Alternatively, take large clods from your plastic bag.

Compare what you see to the photos below. Mark the score on your card.

Soil colour

Examine the colour of a handful or slice of your soil. Using the photos below as a guide, record the score.

Soil mottles

Look at the side of the hole or at the largest clods on your plastic bag and compare them to the photos below and record the score.

Earthworms

Sort through the soil on the plastic bag and count the number of earthworms you can find in five minutes. Make sure to look carefully through the soil that was closest to the surface.

Fewer than 10 earthworms scores 0, 10-20 scores 1, more than 20 scores 2.

Record the score on your card.

What now?

Use the soil score card to calculate your total score. This will give you a good indication of the quality of your soil.

Then see Improving your soil for advice on how to improve and maintain your soil quality.

Soil score card

Print this score card and use it to record the scores from the soil tests.

Then calculate your total score to find out what condition it's really in.

Visual indicator of soil quality	Your soil score	Weighting*	Ranking
Soil structure and consistence		x3
Soil porosity		x3
Soil colour		x2
Number and colour of soil mottles		x2
Earthworm count		x3
Total score			______

* Some factors are more important than others so a weighting system is used. Multiply your visual score by the weighting to get the ranking. Then add up the rankings to get the total score.

What your score means

If your total score is...

• less than 10 Your soil is in poor condition
  
  
• 10-20 Your soil is in moderate condition
  
  
• 20+ Your soil is in good condition.

See Improving your soil for advice on how to improve and maintain your soil quality.

Improving your soil

If your soil scored 20+ in the visual soil assessment, don't be complacent as it can deteriorate if you don't look after it. The following advice on improving soil also applies to maintaining it's good condition.

Improving heavy soil (high clay content)

• Add organic matter regularly. Fork compost into the top 15-20cm of the vegetable garden and annual beds each time you plant. Mulch garden beds in spring and top up over summer. Home-made compost, mushroom compost and bagged compost from a garden centre are all suitable. If using sawdust or wood chips (both from untreated wood), sprinkle fertiliser containing nitrogen on the soil first to avoid depleting soil nitrogen reserves as the woody material decomposes.
• Use bulky manures such as sheep pellets or animal manures. It's a good idea to mix them in the soil as they can get hard and crusty on the surface if they dry out, and some types smell.
• Grow green manure crops such as ryegrass, oats, mustard, barley or lupins. Sow in autumn, cut them down in early spring and dig in.
• Drain the soil if waterlogging is a problem. Also, prevent run-off going into the garden.
• Stay off the soil when it's soaking wet. Don't dig or cultivate when very wet or very dry.
• Start a new patch: In cold areas dig the soil in autumn, leaving it in rough chunks for frost and rain to break down. In warm areas dig in spring or early summer, break it down with a fork and mix in organic matter. From then on, in all areas, try to cover bare soil with mulch to protect it from the pounding of heavy rain that can cause a hard surface crust later when it dries out.
• Apply lime or gypsum. See Sweet and sour.
• Alternatively, avoid planting in the soil altogether.
  Build raised beds up to 50cm high for trees and shrubs, 15-30cm for annuals and vegetables (see our article on Growing vegetables for more on raised beds). Fill with enough bought-in good-quality topsoil, a garden mix such as Living Earth or Dalton's Garden Mix, or a topsoil/garden mix combination. You can plant directly into this. Earthworms and soil micro-organisms will eventually combine it with the upper layer of soil, and plant roots will grow down into it. This is an expensive solution but allows you to establish a garden relatively quickly. Some garden mixes dry out quickly, so need regular watering in summer. A layer of newspaper under the mix helps kill off existing weeds.
• Mix coarse sand or pumice into the top 15cm or so of soil.

Improving light soil (high sand content)

• Again add lots of organic matter, both mixed in and as a mulch, to conserve moisture - sandy soils dry out rapidly.
• Where practicable add silt and clay by mixing with bought-in topsoil.
• Use bulky animal manures (as already mentioned in the section for improving heavy soils).
• Nutrients wash out of sandy soils readily so you may need to feed them to get good growth. Use fertilisers of organic origin (they release their nutrients relatively slowly) or slow-release types. Or apply soluble fertilisers little and often. But be aware that constant use of highly soluble fertilisers may discourage soil organism and biological activity.
• Grow green manure crops but make sure to dig them in while there's still plenty of moisture to aid decomposition.
• Don't raise garden beds - the soil will just dry out more quickly.

Sweet and sour

Another important soil feature is its pH.

pH is a measure of acidity or alkalinity. The pH scale ranges from 0 to 14, with 7 being neutral. Above 7 is alkaline ("sweet"), below 7 is acidic ("sour").

The majority of plants thrive best when the soil is slightly acidic, pH 5.8-6.3. Some, such as rhododendrons and azaleas, prefer it more acidic (around pH 5-5.5); others such as dianthus and some members of the cabbage family (brassicas) prefer it more alkaline (pH 7-7.5) and don't do well in acidic conditions.

Soil pH affects the availability of essential nutrients to the plants. As pH moves either above or below 5.5-7.5, many plants begin to suffer nutrient deficiencies that can severely affect growth.

Raising ph

You can raise pH by adding lime to the soil, but don't overdo it. The amount you need to apply to raise the pH depends on the existing pH, the soil type, the type of lime used and what pH you want to end up with. Unless you've had an accurate pH test done, we recommend caution. Liming every year out of habit can raise the pH too high. Don't lime unnecessarily and even then follow packet recommendations or advice based on a reliable soil test.

Results of our tests in the past suggest many pH testing kits and meters don't give accurate readings. Soil-testing labs can give much more reliable results along with some advice, usually for around $40-50. Look under Soil Testing in the Yellow Pages of larger centres.

Lime also has the added beneficial effect of encouraging clay particles to combine together into very small groups, which helps improve the structure of heavy soil. It comes in different forms but the two easiest types to use are garden lime (calcium carbonate) and the more expensive dolomite (calcium and magnesium carbonates combined). They're best applied in autumn to have an effect by the following spring. You can sprinkle lime on the surface and leave it to wash in over time, or fork in - either way works well.

Gypsum (calcium sulphate) is also useful in helping to condition a heavy soil. It has much the same effect on clay as lime but does not raise pH, and for this reason is often an ingredient in products promoted as "clay-breakers".

Lowering pH

You can lower pH by adding acidic organic matter such as peat, mixing finely ground sulphur (often sold as flowers of sulphur) into the soil, or using acidic fertilisers such as those specifically formulated for azaleas and rhododendrons.