Using green waste compost

Green waste compost, or soil conditioner, is a valuable and often cheap resource. It is produced from mixed garden and kitchen wastes on an industrial scale, and is based on a very diverse range of material including tree prunings. Its use should be encouraged as it stops material going to landfill and so helps our environment in a number of ways, including avoiding use of peat from fragile habitats. On the heavy clay soil we have on Hill Rise and in St. Ives generally, it helps to break up the clay and improve soil structure and workability.

However there are a few things which need to be known, if you are to avoid problems using green waste compost on the allotment or in the garden. In short, the rules are:

• Don't try germinating seeds directly in it
• Mix it well with soil , no more than one third green waste compost to two thirds soil, before growing plants in it

That really is all you need to know, but here is a bit more technical information.

A colleague was asked to go down to the London Eye some years ago when it had just been built, and the groundsmen reported that all the roses planted around the base of the Eye had died. When the plants were pulled up, they found all the roots had been burnt off. The reason… they had been planted in neat, green waste compost. You have been warned!

The reason neat/fresh green waste compost gives problems is that it has a very high chloride content and a high pH. This means it is a bit like growing your plants in a mixture of drain cleaner and salt. You wouldn't want to stick your hands in it would you and plants don't like having their roots in it either! The high pH is not really a problem once mixed with soil. This is because the buffering capacity of the compost is low and the clay soil is high. This is why it needs mixing well with soil.

“But I have grown fantastic carrots in it” I hear you say! Well, that may be so. Some reasons why this might be the case… the green waste compost was over a year old had been left out in the rain, so that the chloride ions have been washed out of it. Some crops can cope with salt better than others (some actually have a requirement for sodium - asparagus for example).

Unlike bought in potting compost e.g. John Innes types or grow bags, it has very little nutrient value (some P and K, but very little available N). It should only be thought of as a soil conditioner, but can be used to greatly improve the structure of our heavy clay soil.

Making your own potting compost

How to deal with green waste compost made me think, if it had been left over winter and most of the salts washed out, could it be used instead of peat to make potting compost?

I dug out one of my dad’s old gardening books and here are the recipes for making John Innes type composts. Note that when they specify using 'medium loam' they mean a light to medium soil (not heavy clay!). Some people do have this nearer the centre of town. Ideally they also recommended a loam with a reasonable level of organic matter (traditionally the suggestion was to take turves from a grassy area, to put these in a heap and leave to die down for a few months, then to shred and sieve it). Finally, they also recommended that the soil was sterilized before use - this is actually very difficult to do at home and is the reason why bought-in potting compost is such great stuff. But as long as you are not using it for germinating seeds, you can often get away without sterilizing it.

The 'recipes' in their traditional unmetricated form is as follows (note 1 bushel = approx. 35 litres)

 

John Innes Seed compost consists of…

• 2 parts by loose bulk medium loam
• 1 part by loose bulk granulated peat (or washed/aged green waste compost)
• 1 part by loose bulk coarse sand (not builders sand, unless you wash it fully first)

To each bushel of the above mixture add

• 1.5 oz superphosphate of lime (18% P2O5)
• 0.75 oz ground chalk

 

John Innes Potting compost No 1 consists of…

• 7 parts by loose bulk medium loam
• 3 part by loose bulk granulated peat (or washed green waste compost)
• 2 part by loose bulk coarse sand (not builders sand, unless you wash it fully first)

To each bushel of the above mixture add

• 3/4 oz  (three quarters of an oz ) ground chalk
• 4 oz John Innes base fertilizer (see below*)

 

John Innes Potting compost No 2 consists of…

Base material as for JI No 1

To each bushel of the mixture add

• 1.5 oz ground chalk
• 8 oz John Innes base fertilizer (see below*)

 

John Innes Potting compost No 3 consists of…

Base material as for JI No 1

To each bushel of the mixture add

• 2.25 oz ground chalk
• 12 oz John Innes base fertilizer (see below*)

 

*John Innes base fertilizer
This can be bought from a garden centre, but the original recipe was

• 2 part by weight 'hoof and horn' (13% N) now commonly called bone meal
• 2 parts superphosphate of lime (18% P2O5) now commonly called calcium superphosphate, made by mixing gypsum with calcium phosphate)
• 1 part by weight sulphate of potash (48% K)