Ash and Soot – As Plant Fertilizers

soot as a plant fertilizer

Ash and Soot

The use of oil for central heating and for the heating of greenhouses has pretty much now deprived the gardener of two useful waste products of burning coal. The use of ashes for improving and ameliorating sticky soils is often argued furiously among gardeners, but personally, I have always found many uses for the right sort of ash.

The right sort of ash for the garden is gritty and for general purposes containing lumps no greater than 1/2 inch and no smaller than 1/8 inch. A fine gritty mixture when well weathered provides an ideal medium for the rooting of all types of cuttings.

Nowadays ashes are used for the ring culture of not only tomatoes, but other plants as well.  Many corporations possess a screening plant for recovering cinders which are sold back to people. The fine screenings from these corporation plants is excellent material for topdressing grass, and is widely used by farmers. If the dust is sifted out again through 1/8 inch sieve, it is excellent for mixing with heavy clay, and can be used as generously as one sixth by volume. For example, to an area 3 ft by 3 ft  by 1-1/2 ft deep may be added around two barrow loads of gritty ash.

The whole thing about the addition of coal ashes to soil is that it must not be overdone, as apart from the chemical action on the soil the crumb structure will be ruined. Continue with the long-term practice of adding as much humus as possible in the form of compost, manure and peat. Work the soil as much as possible before planting up with permanent subjects and regard the addition of coal ash as a short-term measure to open up the clay to provide drainage and to enable air to get in.


The qualities of soot – I believe that the claims are in the main over-rated. However, although soot, as with many other things, can be reduced to a chemical formula it does not always explain why plants benefit from such simple things. Possibly one of the reasons is that the elements are contained in such minute proportions and can be assimilated by the plant both through the roots and through the leaves.

Soot, I suppose, can be regarded as one of the very first so-called bag fertilisers and was used as a source of nitrogen long before the existence of the more concentrated fertilisers such as sulphate of ammonia. The value of soot in terms of nitrogen is practically negligible as it only contains two or three per cent. But because it was so cheap, it became a source of nitrogen for the garden. Its value, though, does not end with nitrogen as it contains many mineral constituents whilst its solid matter is appreciable. Gardeners in industrial areas where there is a continual build up which acidifies the soil, have little good to say about soot except that it does confer on plants an almost complete immunity from fungal diseases. One point in its favour is that it is easily obtainable and anyone with a coal-burning fire is sure of a regular if small supply.

Soot newly swept from a chimney can be definitely harmful both to the roots and leaves of plants although up to 1/2 lb per sq metre, can be applied to vacant land during winter digging. Soot should be kept dry preferably stored in an open box in an open shed. In the soil it acts as a mild fumigant and fungicide, dusted around young seedlings it gives a certain protection from slugs and acts as a mild stimulant to growth. It can be mixed with lime so a small amount of ammonia vapour is released which benefits the plants and discourages soil pests.

Weathered soot dusted on the dew-dampened foliage of celery and parsnips discourages attacks of the leaf miners, and dusted along rows of carrots, onions and cabbages it gives some protection against the female root flies.

It is as a source of liquid manure or soot water that it is perhaps most prized. It can be used both as a spray and for occasional watering to give plant foliage a good colour and in the case of chrysanthemums and celery it makes the foliage bitter and so discourages the attention of the leaf mining fly.

To make soot water put about two shovels full of soot in a coarse-meshed sack. Suspend this in a barrel of water to obtain the clear straw or beer-coloured water. If the solution is very strong it should be diluted but the barrel which contains the  liquid should not be kept topped up or it will become almost worthless. A black syrup is not soot water and will clog the surface of the soil and do more harm than good. As a spray for foliage plants soot water is excellent, both in the greenhouse and outdoors and so, perhaps in spite of newer fertilisers and insecticides, there is still room for a certain amount of soot in the garden.

27. September 2010 by Dave Pinkney
Categories: Garden Management, Manures and Fertilisers | Tags: , | Comments Off on Ash and Soot – As Plant Fertilizers


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