Bearing in mind the continuously cold weather and at the risk of boring the knowledgeable people amongst you, I am going to start covering some very basic concepts of gardening which are fundamental to understanding and practice and may stimulate the horticulturally challenged.

This month we will look at compost. No, not the stuff you make in your bin that is ‘composting’. What you produce there is a soil improver, because you cannot control its make up or nutrient value it is not suitable for raising seeds or plants. Composted garden material is almost 100{c8c3b3d140ed11cb7662417ff7b2dc686ffa9c2daf0848ac14f76e68f36d0c20} organic matter and is high in nitrates. As a soil improver it breaks open the soil, relieves compaction, increases drainage, assists nutrient retention and improves rooting. Its value is priceless but it is not a growing medium in its own right.

Compost is the stuff in brightly coloured plastic bags that greet you at the garden centre. Different sizes, different functions and a selection that bamboozles you when you just wanted to fill a few pots with plants. So which compost to buy and why? It depends on two things primarily. Firstly, for which plants you are choosing it and secondly what purpose do you wish it to fulfil? The packaging should give you that information.

Seed compost is very fine with the correct amounts of added nutrient for raising seeds but it is insufficient for a plant once it becomes a size for potting on. (Many of the multi purpose are now fine enough and strong enough not to bother with seed compost). At that stage multi- purposecompost can be used but as these are generally peat based they need to be monitored carefully or the plants can quickly become waterlogged or if too dry the medium is often difficult to re-wet. There is great debate about the continued extraction of peat and the decimation of the peat bogs and all sorts of alternatives are available and undergoing scientific and on the job testing. Many multi-purpose composts now incorporate well composted garden waste (from blue bag collections). Other medium available include coir compost from coconut husks but these of course have to travel many miles. Bark based composts are becoming my favourite as they are actually a combination of our garden wastes and forest floor detritus available in abundance in this country. These are carefully formulated by manufacturers with the absent nutrients added for general garden purpose.

As to the future of peat; well there is a lobby which says gardeners use so little of it in comparison with the power stations that it isn’t worth worrying about. But peat bogs only renew at a rate of 5mm per year so any saving has to be considered valuable, particularly as much of the peat now comes from Russia, so more air or sea miles to consider.

So whichever you choose, a compost labelled ‘multi purpose’ is good for potting on or for bedding out in pots for a season. If plants are left outside in it too long it will quickly become waterlogged and any nutrients will leach away quickly. Always buy it from a supplier that has kept it undercover as bags left outside in the open will be quickly leached of nutrient and be pretty much worthless to your plants. If the bag feels wet it will also be too heavy to carry safely.

Any of these multi purpose varieties will contain lime, so if you are looking to plant acid loving plants an ericaceous compost should be used. There is no lime in this compost and nutrients have been added for acid loving plants such as azaleas, camellias, rhododendrons etc. But beware, a multi purpose ericaceous will leach nutrients also and is for short lived usage as explained above.

Now to the mysterious world of ‘John Innes’ compost. Mr Innes was a nineteenth century property and land dealer in the City of London. On his death in 1904 he bequeathed his fortune and estate to the improvement of horticulture by experiment and research. Directly he had nothing to do with compost, it was his money that enabled the research. Before the introduction of JI composts gardeners generally used a different compost for each species of plant. Usually the soil was not sterilised or heat pasteurised and so very often plants suffered from soil born insect and disease attack. The plant foods being added to the soil were unbalanced and so growth was variable in the extreme. In the 1930s two scientists at the John Innes Horticultural Institute, William Lawrence and John Newell, set out to formulate composts that would give consistently reliable results. After six years of experiment they determined the physical properties and nutrition necessary in composts to achieve optimum rates of plant growth and they developed methods of heat sterilising whilst not affecting the plant. These plant composts revolutionised the ways composts were produced and also the ways in which plants are grown in pots. Naturally the plant nutrients have been updated to gain the benefits of improving technology but the formulae were passed on to manufacturers (now the JI Manufacturers Association) and the Institute never sold or made money from actually producing the composts.

John Innes composts are soil based (not peat based) and the soil is carefully selected loam which is made up of sand, silt and clay (40-40-20) to which is added moss peat, coarse sand, grit and fertilisers. It is screened and sterilised and mixed in the appropriate proportions of the ingredients to achieve optimum air and water holding capacity and nutrient holding for different types of plants. Thus today one should think of John Innes as a formula, not a person.

The formula are mixed and numbered:
John Innes Seed Compost – for sowing almost any seed with nutrient for early development.
John Innes No 1 – for pricking out and potting on young plants.
John Innes No 2 – for general potting of house plants and plants in medium sized pots.
John Innes No 3 – a richer mixture for final planting in pots and generally used for larger specimens.
John Innes ericaceous – for lime haters and acid lovers.
The level of nutrient increases as the number goes up.
The fertilisers within we will examine at another time but these composts are a natural growing media for plants.

Humus and clay are a buffer to over or under feeding.
The combination of loam, peat and sand (rather than just peat in multi) provide a good balance between the amount of water held and the air capacity when the plant has drained.
The compost is easier to re-wet if it dries out.
Loam will retain higher levels of nutrient which would be dangerous in peat based composts
It lasts a lot longer than soil-less/peat based compost.

Finally, if January 2009 was the coldest for twenty years and January 2010 is vying for the record and February is still traditionally the coldest month, how low can it go? Got any echium still alive?