Ashes & Soot

CECIL SOLLY’SCecil Solly - The Original Master Gardener
Ashes & Soot

Wood ashes were once one of the chief sources of that very valuable plant food, potash, for both farms and gardens.

Good quality wood ashes contain from five to seven percent potash, from 1-1/2 to two percent phosphoric acid, and from 25 to 30 percent calcium compounds. Hardwood ashes contain more potash than softwood ashes. Either kind loses much of its value if exposed to weather or rain so that its soluble chemicals are leached out.

If they are not leached or exposed to the air, wood ashes contain, in the form of oxides or carbonates, all of the mineral elements that were used to make the tree wood from which the ash was formed.

Potassium carbonate, calcium carbonate or calcium oxide are all present in comparatively large quantities in wood ash, which gives it a strong alkaline reaction and the power to neutralize acid soils. However, the value of the wood ashes as a plant food depends more on the potash content than on the lime.

Wood ashes should be applied to the soil some time in advance of planting and should be mixed well into the ground, not left on top.

Wood ashes have been used with great success in many parts of the flower garden where it not only fertilizes but greatly helps to prevent mildew and other like diseases in such plants as roses, chrysanthemums, lupine and delphiniums.

In the vegetable garden, wood ashes should be used in large quantities in every part of the soil. Every vegetable will benefit greatly – and be less likely to get disease. This is especially noticeable in tomatoes.

In view of their relatively low plant food content, wood ashes can be used in any ordinary quantities without danger of burning. An average application would be five to ten pounds per 100 square feet.

Wood ashes from a sawdust burner are much more liable to burn little plants, so this material should be used as often but more sparingly. Sawdust ash should always be sifted to take out the “lumps,” which should be discarded.


There are many uses to which soot can be put in the garden which were widely known decades ago, but which have passed out of use lately. The soot that is good to use is from wood fires. Never use oil soot in the garden.

Soot is the most valuable fertilizer for many kinds of plants. It imparts a dark color to the soil, which assists in the absorption of heat and so renders it more suitable for early crops. When applied to the soil in spring it is changed by bacteria into nitrates, in which form it is available as a plant food. Nitrates increase growth so that soot may be applied to ground on which lawns, shrubs or flowers are to be grown. These plants require large amounts of nitrates to enable them to grow quickly and healthily.

While many of the uses to which soot can be put are widely known, there are one or two points not so familiar to the average gardener. Soot, fresh from the chimney, contains about 12 percent water, 35-50 percent ash, and the remainder various volatile substances which are rich in ammonia and which form the chief fertilizing properties. The ash contains calcium, iron, magnesium, potassium and sodium, the first four of which are valuable plant foods, as they are combined with phosphoric and sulfuric acids, while silicates are also present.

If kept dry and allowed to stand for about three months, soot becomes mellowed, when it safely can be used as nitrogenous manure, in powder or liquid form.

Soot can be used to scatter along the rows of onions, carrots, turnips and radish to prevent root worms or maggots.


Cecil Solly

The Original Master Gardener

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4 Responses to Ashes & Soot

  1. Cynthia Warne 2012-04-05 at 8:28 AM #

    I’m not sure when this article was written but nothing is mentioned here about how wood ash changes the pH of soil. In relatively small quantities it can be a good source of potash for soils but in large quantities, as suggested for vegetables it can drastically change the pH of an acid or neutral pH soil to an alkaline soil which may not be a good thing. Maybe it would be a good idea to make a foot note about this for gardeners that are unaware or new to gardening. Just a suggestsion. Thanks

  2. jim 2012-12-22 at 8:48 AM #

    I just read a book about the year 1805 and it said soot and lime are used as a pesticide for apple trees.

    Know anything more about this?

    you can email me.


  3. C 2013-12-17 at 10:48 PM #

    I just watched the BBC Victorian Farm ( wherein the lady was cleaning out the flue and cooking range and applied the soot to her garden, stating that it was particularly good at preventing black spot and fungus infections on roses, not to mention helping kill some kind of infestation on another plant, with the help of water and soap (which sounded a bit like a Safer’s Soap type concoction to me). I was interested, and ended up here. I will agree with Cynthia, that I understand that very much wood ash will take soil increasingly alkaline, so it’s a good to know if you have acid or basic soil to begin with. The soot she was using was coal soot, so I don’t know how that compares to wood soot. I only know what she said, and what is stated here, but it sounds very likely that your lime/soot mix would work, esp. since lime wash is used to kill bugs overwintering in fruit tree bark.

  4. Frances Brown 2014-01-10 at 9:21 AM #

    I found this interesting site while looking for information on the acid or alkaline nature of peat [turf] ash, as we burn mixtures of wood and peat. Our soil here is very acidic, so would peat ash also be too acidic to use? Should we separate the ashes and only use wood ash to get better ph? Thanks!

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