Understanding your soil


Your soil is one of the most important aspects of successful gardening and yet it can also be the most confusing as unfamiliar words get thrown around and experts talk about problems you never knew you had!  Let see if we can demystify it for you to help you determine if yours needs improving.

Soil Ingredients
Soil comprises a complex mix of different ingredients including mineral particles, organic matter, moisture, air and living things.  It’s the proportion of the following key ingredients which determine whether it needs improving:

Clay – fine mineral particles that act like magnets for holding onto nutrients.Sand – large mineral particles which are weak at holding nutrients.
Humus – thoroughly composted organic material which holds moisture and nutrients.

Common Soil Types

Clay Soil
Here the clay component is dominant.  Clay soils contain loads of nutrients however drainage is poor as the small particles compact together.

Sandy Soils
Obviously sand is the major component.  Sandy soils have excellent drainage but dry out too quickly.  They are usually very poor soils as nutrients are easily flushed through.

Loamy soils have a better balance of sand and clay but also have a high component of humus and other organic matter.  It combines the best attributes of all ingredients, minimising their flaws.  This is the soil everyone wants!

Determining what you’ve got
Take half a fist of soil, moisten it and roll into a sausage shape.  If it refuses to hold this shape, feels gritty and just falls apart then you have sandy soil.  If it’s pretty easy to make the shape and feels silky when squeezed then you’ve got clay.  Loam sits somewhere in the middle and is often a rich brown colour.

Making Improvements
Regardless of soil type the common mantra is to add organic material. The large particles improve drainage in clay but at the same hold moisture and nutrients needed by sandy soils.  Unfortunately you need a LOT of organic material added over years to make lasting improvements.  Here’s a faster approach to improving the soil structure:

  • Clay Soils – add sand and gypsum + organic matter
  • Sandy Soils – add clay + organic matter

These simple actions address the structural problem with your soil and the results are permanent as neither will breakdown and ‘disappear’ like organic matter does.

You can also improve drainage with raised beds in clay, use thick mulch in sand to conserve moisture and choose plants which thrive in your soil type.

Soil pH
This measures how acid or alkaline the soil is and runs from 0 (extreme acid) to 14 (extreme alkaline). 7 is neutral. Most plants prefer neutral to slightly acid soils (pH of 6-7).  Outside of this range they begin to suffer.  Nutrients that are physically in the soil become ‘locked up’ and plants can’t absorb them resulting in the poor growth.  Never fertilise an underperforming plant without first testing the pH.

pH Test Kits, available from garden retailers, are quick and easy to use. Test in several spots around your garden as pH can vary even over small areas.

For acid readings below 6.5 apply dolomite or lime (known as “sweetening” the soil). These liquids are faster acting than the traditional powdered versions.

For readings above 7.5 apply powdered sulphur.  Organic material will also help but is not as powerful. There is no liquid alternative to powdered sulphur which takes two years to finish working so it is slow process.

Some plants prefer extreme soil pH like azaleas which like very acid conditions.  pH Test Kits normally contain a list of these plants.

Salty Soils
Salts can easily build to toxic levels in soils.  Common reasons are:

  • Greywater use which is often high in salts
  • Swimming pool splash
  • Use of artificial fertilisers
  • Rising local salinity issues

Flush salts through the soil by using gypsum 1-2 times a year for the first three points.  If your region has overall salinity issues then choose salt tolerant plants.

Nutrient Deficiencies
Soils can be deficient in a particular nutrient with plants showing strange growth.  This is not common when the pH is correct and different organic nutrients are applied regularly like compost, manures, mulches and seaweeds.  Often yellow stunted plants are simply hungry or are reacting to external factors like cold weather.   A few applications of OCP eco-seaweed and OCP eco-aminogro will quickly improve things if plants are hungry.

Trace elements can correct true deficiencies but be careful when self diagnosing as they can be tricky to get right. Over applying a specific nutrient can create toxicity issues.  Instead consider having your soil professionally tested. Soil laboratories can provide reports on nutrient levels and advise which are lacking.  Many will let you simply post samples in so it's pretty simple.