[NicolasGarcia]

Popular wisdom has always considered ash as a product that benefits plants, but is it really like that? Do we benefit our organic orchards or fruit trees by spreading the ash from our chimneys? Let's know the chemical properties of ash and its possible uses in the field as natural fertilizer.
What chemical composition has the ash and what is its potential fertilizer?
First of all, we have to know that with the combustion of firewood, we consume almost all organic carbon, remaining in the resulting ash mainly Calcium, potassium, aluminum, magnesium, iron, phosphorus and manganese.

But it is calcium and potassium in the form of carbonates which we mostly find in wood ash. This made it a highly prized product as a source of potassium and calcium amendment before the chemical synthesis of fertilizers.

Another relevant aspect of wood ash in the soil is its effect on soil reaction (PH). The ash is highly basic, and causes a rapid increase in soil pH. For acidic soils this is very interesting, since it unblocks nutrients and helps to correct its acidity, bringing it closer to the most correct levels for the majority of cultivated species. But in basic soils it can bring us problems.

So, at the level of macronutrients do we have to consider ash as a source of calcium, potassium and phosphorus?

Apparently yes, but no. And in most floors of Spain we can say flat NO. The majority of soils cultivated in Spain, especially those of the Mediterranean coast, are of basic reaction, with high levels of limestone. In these soils, the contribution of ash can be harmful, as providing more carbonates can increase the blockage of nutrients such as potassium and magnesium due to the high proportion of calcium in the soil, as well as the inherent blockage of certain nutrients by raising the PH. The typical case is the Iron, which in basic soils passes to forms not assimilable by plants and causes severe chlorosis even having acceptable levels in the soil.

On the other hand, in areas with high rainfall (Cantabrian cornice, Basque Country, Galicia, etc.), the large amount of rainfall leads to more acid soils. There, if the moderate contribution of wood ash is beneficial.

Beware of ash: Only wood!

We have to be very careful with the quality of the ashes we use. Burning in the fireplace other things that are not firewood, can end up manufacturing a compound potentially very harmful to health and the environment. I mean that if in it we burn papers (from magazines, newspapers), serigraphed cardboard, remains of furniture, wood from pales etc.. The resulting ash will be a highly toxic heavy metal concentrate. Varnishes, paints, inks, etc., use, although in small quantities, heavy metals. By burning these materials heavy metals do not disappear, but accumulate in the ash. So, depending on what we burn that ash is totally inadvisable for our organic garden or orchard.

conclusion
We are aware that what we have proposed contradicts what popular wisdom proclaims, and that can always lead to controversy. We have only tried to objectively present the subject, and we think that in the vast majority of cases, the harmful effects of the ash can greatly overcome the positive effects of the latter on the ground. But like everything in life, the dose is the key, so if you use ash regularly, it is in small quantities. And if the soil of your organic garden is limy, our recommendation would be to avoid its use.