Wednesday, March 13, 2013

Wood Ash: The Organic Soil Amendment

If your home is equipped with a wood-burning stove or fireplace, you will be pleased to learn that once the fire burns out, there is a good use for the wood ash in permaculture and organic gardening. Wood ash should never be considered as waste, but rather as a useful organic resource. As long as you follow a few simple rules, it can be used to benefit your garden in three ways: as a fertilizer, a soil pH amendment and a pest repellent.

As a fertilizer: Wood ash is a good source of potassium, phosphorus, magnesium and aluminum. It also typically contains less than 10% potash, 1% phosphate, and trace amounts of micro-nutrients such as iron, manganese, boron, copper and zinc. It replaces many of the macro- and micro-nutrients removed from the soil during plant growth and harvesting. When the soil pH is maintained at a proper level, productivity would be enhanced by using wood ash as a soil amendment. The exact chemical make-up of the ash varies according to the wood type. For example, hardwood ash contains a higher level of potassium than softwood ash. If you were to compare it to a commercial fertilizer, wood ash would probably gauge about 0-1-3 (N-P-K). 

As a soil amendment: Wood ash contains a good percentage of calcium carbonate, an ingredient in garden lime. If your soil is very acidic (with a PH of 5.5 of lower), wood ash can improve the pH and thus create a remedy for excessively acidic soils.  Adding wood ash raises the pH, which can be beneficial to some plants but can interfere with the growth of others. It should be avoided around acid-loving plants like rhododendrons, blueberries or potatoes, which would probably get scab disease if the pH were too high. On the other hand, raspberries flourish if given a good mulching plus the occasional addition of wood ash. 

Before getting started, it’s always best to get a soil test done. Applications should be limited to a level that maintains the soil pH for the plant you intend to grow. If your yard or garden soil has a pH of 7 or higher, you should avoid using the wood ash amendment.

Ash can be mixed directly into the compost pile or worked into each layer of compost as the pile builds up. Wood ash can be incorporated directly into the soil in the springtime and lightly worked into the top layer around plants that benefit most from a boost of potassium, calcium, and pH.  

As a pest repellent: Wood ash helps to eliminate soil invertebrate like slugs and snails. You can also place a little ash around the base of your plants to discourage surface feeding pests. Once ash gets wet, it loses its deterring properties. Continual use of ash, in this way could increase the soil pH too much or accumulate high salt levels harmful to plants – so easy does it.

Common uses for a bucket of ash are:
  • Sprinkle some ash on your lawn. Applied lightly and followed by a good watering, ash will benefit the grass and it fosters the growth of clover in the lawn. If you’re accustomed to using limestone, apply twice as much ash as lime for the same effect.
  • Use the ash to make a tea for tomatoes. Put five pounds of wood ash in a permeable cloth or burlap bag; tie it shut and lower it into a 50-gallon garbage can that is filled with water, as if it were a giant tea bag. Let it sit for about four days; then dip the tea out with a watering can and pour a cupful around your tomato plants once a week, as soon as they begin to flower. Most crops can use a potassium boost, but especially tomatoes.
  • Spread ashes around the base of hardwood trees, returning this valuable product to its source. Apple trees, in particular, love this treatment. A bit of ash from an occasional fire makes a good end-of-winter gift for a favorite tree.
Wood ash is not a miracle fertilizer, so you should be aware of the following.
  • Do not apply ash to newly germinated seeds because it contains too many salts for seedlings.
  • Do not add ash with nitrogen fertilizers, such as: ammonium sulfate, urea, or ammonium nitrate. These fertilizers produce ammonia gas when placed in contact with high pH materials such as wood ash.
As you can see, wood ash application can supply additional nutrients, but could also cause plant damage if misused. Using wood ash is just another way we can use the organic matter around us to supply organic amendments to our garden soil and improve this valuable resource.