Garden Compost Ingredients

Specifications / preparation of the raw materials


Garden Compost Ingredients - loam The main ingredient of a John Innes compost is the loam. It gives the compost body, and its chief function is to supply the clay and actively decomposing humus which are so essential for good plant growth.

A loam is defined as a soil in which the proportions of sand, silt and clay are well balanced, typically 7-27% clay, 28-50% silt with less than 52% sand. A medium loam contains just enough clay to be slightly greasy when smeared between the fingers without being sticky. A light loam contains a greater amount of sand, and heavy loam a greater amount of clay. A medium loam should be used for a John Innes Compost; light, heavy and chalky loams should not be used; medium loam will have a good crumb structure and is usually a little acid (pH 5.5-6.3). The loam may be obtained either from arable or pasture land and is therefore known as arable loam and turf or garden loam.

The best loam is turf loam, which is the product obtained when turves cut from a good pasture on a loam soil are stacked in a heap until the grass and some of the roots have rotted. The turves should be cut from the top 10-13cm (4-5in) of turf and soil, and measure roughly 30 x 23cm (12 x 9in) for stacking.

Standardization of the loam

The clay content is reasonably well standardized by choosing a medium or medium-heavy loam, and the humus content by choosing a turf loam as described above.

The pH value of the loam must be standardized to 6.3 (including moisture). If the soil is more acid than 6.3, ie. if its pH is below this figure, calcium carbonate must be added in the form of ground limestone or chalk. For medium-heavy loams, the approximate amount of calcium carbonate in g/m2 (oz per sq yd), for every 23cm (9in) of turves is (6.3-pH of loam) x 23. For example, if the pH of the loam is 5.8, the calculation is:

(6.3-5.8) = 0.05 x 760 = 380g /m2

(6.3-5.8) x 23 = 0.5 x 23 = 11.5oz per sq yd.

Therefore, as the loam is stacked (as detailed below), for every 23cm (9in) depth of turves 380g (11-1/2oz) of calcium carbonate should be evenly applied per m2/sq yd. For medium loams the formula should be altered to:

6.3 — pH x 594 g/m2

6.3 — pH x 18 oz/yd2

The standardization of the pH of the loam is essential as excess acidity is a common cause of poor growth in pot plants.

The stacking

It is preferable to cut the turves in late spring or early summer as at these times the grass is thick and lush. The stack, which should not be more than 1.8m (6ft) in height or width, is built up in 12cm (4-1/2in) layers (the approximate thickness of the cut turves). On top of the first layer is added a 5cm (2in) layer of strawy animal manure or half-composted straw, on the next layer the lime is spread evenly—and so on alternately, until the stack is completed. The straw layer assists in the aeration of the stack and serves as an activator by providing a ready supply of bacteria. As moisture is essential to encourage rapid rotting the turves should be thoroughly wetted with a hosepipe as stacking proceeds. Finally, the stack is covered against rain and is left undisturbed for about six months. At the end of this time the grass should have decomposed, the pH should have standardized to 6.3, and the whole thing dried out.

When the stack is being broken down, it should be cut with a spade downwards from the top to bottom so that mixing of the layers occurs to ensure a uniform material. The soil is sieved through a 1cm (3/8in) sieve and it is then ready for sterilization. Care should be taken that the soil is not unnecessarily contaminated.


This is necessary to ensure that the compost is free from harmful pests and diseases.

Only the loam requires sterilization, and it has been found that heating to 82°C (180°F) for ten minutes reduces the likelihood of high ammonia levels in the compost. The soil should be brought up to this temperature and cooled as quickly as possible.



The type of peat recommended for use in John Innes composts is known as granulated sphagnum moss peat. It should be graded so that most of the particles are of approximately 3cm On) in size. Normally, this type of peat has a pH of between 3.5 and 4.0. Other peats, including sedge peat, are not recommended, although possibly cheaper. After purchase keep it dry and clean under cover so that it cannot be contaminated by weed seeds floating about in the air. Because it is comparatively sterile, it is not necessary to sterilize the peat before use in the compost.



As with the other raw materials, the sand should be chosen with care as its structure contributes much to the drainage of the compost and to its aeration. It should be grit/sand, clean, chemically inert, and sharp with little or no clay, silt, lime (such as sea shells) or organic matter. Between 60 and 70% of the particles should be 3 — 1.5mm (1/8 – 1/16in), a typical grit/sand being sold as 3mm (1/8in) down. Builder’s sand and sea sand are usually too fine. To ease mixing, the sand should be dry. It does not require sterilization.


The food supply — John Innes base

To grow plants well the compost must contain an adequate and well-balanced food supply. John Innes base is made up of three fertilizers in the following proportions: 2 parts by weight hoof and horn, in grist (13% nitrogen) 2 parts by weight superphosphate (18% phosphoric acid) 1 part by weight sulphate of potash (48% to 50% potash) It may be made up by the amateur from the raw materials or it may be purchased ready mixed. The analysis of the complete fertilizer is 5.1% nitrogen, 7.2% phosphate and 9.7% potash. Note that no magnesium or trace elements were added as these were presumed to be adequately supplied by the loam.

31. March 2011 by Dave Pinkney
Categories: Greenhouse Gardening, Organic Gardening, Soil Cultivation | Tags: , , , | Comments Off on Garden Compost Ingredients


Get every new post delivered to your Inbox

Join other followers: