What is Soil and what is Soil Improvement?

You already know that the soil in your garden is key to growing great plants. But exactly what soil is made of, why this matters and what is meant by soil improvement might warrant a bit more explanation to turn the frustration of plants that struggle into the success of blooming borders.

Soil Type

There are two main characteristics of soil which influence the selection of plants to grow happily:

1) Acid and Alkaline are the two ends of the pH scale and every soil sits somewhere on a line between them.  The bedrock, the layer underneath the soil, is the main determinant of the pH of your soil.  A good guideline is that soils based on sand tend to be slightly acid, silt and clay in its natural state is fairly neutral, and soils based on chalk are alkaline.  Many plants will grow in any decent border soil but some, such as Rhododendrons, need a specific level of acidity to thrive.

2) Nature supplies plants for all levels of moisture from ponds and bogs through to sand dunes and some plants are very particular about having dry or constantly damp roots.   Once again the type of soil and its look and feel is a good indicator of the water-holding capability.  Sand, gravel and chalk let water drain freely leaving the soil dry. Clay becomes sticky and inhibits drainage, keeping the soil wetter.  Silt is very fine and has no ability to cling together into crumbs.  Understanding the nature of the soil plants are growing in helps you to identify those that will do well in your garden.

Soil minerals infographicThere are several clues to the soil type without a scientific soil analysis:

  • Landscape: where is the garden situated? A garden on top of the North Downs here in Kent is highly likely to be chalky because of the underlying rock; a garden in the Weald sits over clay; either side of the Weald are ridges of sand and sandstone. Silt is mainly found in low lying areas such as the fens as the small particles are washed downstream and deposited at the mouths of rivers.
  • Look: Organic Matter is dark in colour whereas chalk, clay & sand are all pale so the colour is a general indication of the OM content, whether natural or dug in. OM is more acid so can be used to neutralise a more alkaline soil, up to a point.
  • Feel: If the soil feels gritty then it is likely to contain sand pushing the acidity up; sticky soil is usually a sign of clay that does not drain freely but hold onto nutrients and is basically neutral. Silty soil feels silky to the touch as it contains particles just a bit larger than clay, but much smaller than visible sand.
  • Finally the local vegetation itself is a real give-away. Plants such as Rhododendrons, Acers and Camellias need an acid soil to keep them healthy and the leaves green, Roses need a deep nutrient rich soil and love neutral, moisture retentive clay.

Soil Improvement

And the answer is:

Well Rotted Organic Matter

Now what was the question?

Well Rotted Organic matter increases the water retaining capacity of thin, free draining soils like sand and chalk and helps them to hold onto nutrients.  It also opens up heavy clay soils improving drainage and reducing waterlogging.  Most organic matter contains the full range of plant nutrients that are returned to the soil as it continues break down.

‘Organic Matter’ includes garden compost, leaf mould, composted manure or more or less anything that once was a plant and is now fully rotted down.  Making your own compost and/or leaf mould is a good solution, but most people cannot make enough for their whole garden at least for the first few years.

Some Councils run a green waste recycling scheme and generate garden compost for sale as a sustainable soil conditioner.  The huge heaps reach a much higher temperature through bacterial activity than your domestic compost bin will and so weed seeds are mostly destroyed.  Well rotted manure is a common, bulk source of organic matter popular where allotments adjoin stables.  It must be stacked for at least 6 months to allow the product to ‘mature’ until it is dark and crumbly and does not smell!  And most garden centres sell both composted manure and soil conditioner (such as composted green waste) by the bag.

Understanding the nature of your soil and the means of soil improvement will help you to reap the maximum rewards of all your work in the garden, whether you are growing Roses or Rhubarb.

Happy Gardening, AlisonRose 'Anniverary'

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