Wood ash and charcoal, both derivatives of wood, each boast centuries-old reputations for improving garden and farmland soil. Yet each have their drawbacks. Reliable availability -- at least as a free resource -- is one obvious disadvantage. It's also more difficult to standardize amounts needed because both charcoal and wood ash can vary greatly in terms of their effectiveness, depending on factors such as the trees from which the material originated and whether they've been exposed to rainfall. Finally, in some parts of the country, you may find that adding either to your soil may be too much of a good thing.
Benefits of Wood Ash and Charcoal
Wood ash and charcoal are used in similar ways. They are rich in potash, which adds potassium to the soil. One of the three key nutrients in both organic and synthetic fertilizers such as 10-10-10, potassium promotes plant vigor by allowing plants to turn other nutrients into fuel for growth and health. It also controls the rate at which plants take in water. In addition, wood ash and charcoal are alkaline materials, so they help neutralize acidic soils. Finally, some people use the tree-derived materials to act as carbon additions to the compost pile, which requires layers of both carbon and nitrogen sources.
To add potassium to your garden without drastically changing the pH level, use a source other than charcoal or wood ash.Wood ash and carbon are useful in regions with acidic soil, but if your pH is already high based on a soil test, it would be unwise to raise it even higher. Alternative potassium-rich amendments include granite meal, greensand, and kelp meal, as well as an organic blend usually sold as "sul-po-mag" because it also contains sulfur and magnesium. All come labeled with recommended amounts of application. You'll generally need about five pounds greensand or granite meal per 100 square feet of garden, for example, while one pound is about right for sul-po-mag and kelp meal.
When soil becomes too acidic or too alkaline, most plants can't take in the nutrients they need. Wood ash and charcoal are useful for raising the pH of acidic soils. If you don't have access to these materials, or if you're not sure how much to use because the potency level varies, consider alternatives such as limestone. Limestone is a traditional neutralizer of acidic soil. In general, use about five pounds of limestone for every one point you need to raise the pH level per 100 square feet. Peat moss is a second alternative, but can be bulkier and more expensive to buy than limestone.
Because wood ash and charcoal are high in potassium and alkaline properties, they can be poor choices to use for a carbon material in the compost pile. Instead, use more neutral carbon sources, which also tend to have more of the bulk needed in compost. Leaves and hay are the two most common choices. Sawdust also adds bulky carbon. Because it tends to be on the acidic side, it can be helpful as a compost ingredient in alkaline regions, but a poor choice in acidic areas.