All that any compost should be is, free draining, while being capable of holding enough water for the plant.

Composts do not need to contain any fertilizer, as you should be replacing that used by the plants on a regular basis. This is covered in the section on feeding

Before going any further you need to be aware of the soil preferences of the plant you are going to use the compost on. Apart from the nutrients and trace elements in the soil, there is one other chemical aspect you need to know, is the soil Acid or Alkali (Lime). Most trees are fairly indifferent to this, however Ericaceous plants such as Azaleas and Camellias must have an acidic soil, which is usually sold at garden centers for potting Heathers. Please don't assume that the peat (or substitute) is acidic, it's likely that lime has been added to it. The acidity, or alkalinity of the soil is stated as a pH number.

What is pH

pH is an expression of the relative acidity or alkalinity of a substance, in this case the soil, it provides a guide to its overall chemical balance.

The pH scale is divided into 14 points:

    A pH of 1.0 - 6.0 is acidic
    A pH of 7.0 is neutral
    A pH of 8.0 - 14 is alkaline

The scale is Logrithmic, so a shift of one point means the level of acidity, or alkalinity has multiplied by 10, two points and the soil is 100 times more acidic, or alkaline.

    pH of the soil is an important factor in plant growth because it determines the availability of soil nutrients to trees.
    Trees will not grow well outside the range of 4.5 - 8
    The availability of nutrients differ with the pH level
    • Calcium and magnesium are much more readily available in alkaline soils

    • Iron and manganese are much more readily available in acidic soils

    • Most nutrients available in relatively neutral soils (pH 6-7.5)

Trees that have evolved to live in certain types of soil, are often starved of important nutrients or damaged by an excess of unwanted minerals.

Acid loving plants such as Azaleas, growing in an alkaline soil are often starved of important trace elements such as iron and manganese. This can be overcome to an extent by feeding the trees with sequestered iron.

Alkaline loving plants growing in acidic soil often cannot get enough calcium and magnesium or are severely damaged by amounts of dissolved aluminium or manganese.

Garden centers stock test kits that will allow you to test the pH of your soil and a neutral (pH7) reading will be OK for most plants, but Ericaceous plants will require a soil with a lower pH value.

Soil composition

There are a number of well known formulations for composts and while the formulas are tried and tested, the materials used in them often contain a lot of dust, which will be washed down to the bottom of the pot and form a solid mass, killing the roots and tree. Always sieve a compost to remove any dust before use.

Coniferous trees will benefit from grittier soil than Deciduous trees. A good general compost would be 50/25/25 Peat/Sand/Grit with 40/30/30 Peat/Sand/Grit for conifers. The sand should be Horticultural, or sieved sand as others contain a lot of fine dust.

I'd define grit as any stone that would pass through a sieve with a 4mm mesh but not through a 2mm mesh. These larger stones keep the soil open and allow air to get to the roots. The effect is reduced somewhat at the soil settles, but it helps to promote root growth after repotting.

It also increases the drainage. The last thing most plants need is to be sitting in a wet mass of soil, it's a gateway for fungal infections. Grit and Sand, it should be remembered will reduce the volume of water a given amount of soil can hold.

Here in the UK, Horticultural Grit is available in any decent garden center, It's generally crushed, sieved granite.

Grit at about actual size. This is the crushed granite mentioned above. As you can see it is fairly even in size.

Any stones of about the same size will do.

If you have purchased a bonsai from a non specialist supplier, there is a likelihood that the tree was mass produced in China, the country where bonsai (pen-jing) originated. Look closely at the compost it is almost certainly clay and you will not see grit or anything else that will open the soil and allow air to the roots. This clay will soon compact down as the tree is watered and choke the roots. The tree needs to be repotted at the earliest opportunity.

Before using any compost it's best to allow it to dry out. this will make it easier to sieve out any dust in the soil. A few days in the sun should dry it out sufficiently to remove most of the smallest particles.

Alongside you will see an illustration of a commercially available sieve set, giving a number of different sized meshes, with a large collection this gets a fair amount of use, however if you only have a few trees you might consider using a flour sieve, used in baking.

A sieve set, available from most Bonsai suppliers

Allen. C. Roffey 09:30 19/02/2003 Tuesday, July 17, 2018 13:13