Ash produced by the burning of tree biomass appears to be a suitable fertilizer for radiata pine plantations, according to a piece of research carried out by Neiker-Tecnalia, the Basque Institute for Agricultural Research and Development. The study highlights the heterogeneity of the ash characteristics. The ash that underwent tests in this study did not display any toxicity in the biotests carried out. Chemically, it was characterised as having a high, but not corrosive pH, an adequate magnesium content for fertilizer purposes, and a quantity of heavy metals at concentrations below those present in nature.
Energy production through the combustion of biomass is regarded as clean energy because it is carbon neutral. It does, however generate large amounts of ash which have to be properly dealt with. Therein lies the interest in seeking a way of taking advantage of this product and in closing its production cycle.
The tests carried out by Neiker-Tecnalia highlight the fact that the ash from biomass-fuelled boilers is a heterogeneous product whose composition depends, among other factors, not only on the raw material used, but on the combustion process. According to the legislation in force on the characterisation of toxic or hazardous waste, the ash tested did not display toxic levels in biotests carried out, although it did display a certain level of toxicity for bacteria of the Microbacterium and Citrobacter genera. Its non-corrosive, base pH, its low concentration in heavy metals, and its low leaching of these elements in column tests in the laboratory confirmed its harmlessness when applied in the field where it showed its fertilizing potential for P and Mg.
What is equally of interest is the fact that the levels of non-essential heavy metals analysed (cadmium and lead) were found to be within the normal ranges that allow the optimum development of living organisms. Another favourable characteristic of the ash tested is that its particle size ranges between 2 mm and 250 microns. The powdery part (<250 microns) is very small, which facilitates its application in the field by personal involved in applying it.
A large proportion of the nutrients of the system is extracted with the harvesting, opening up, thinning out and pruning of the trees in intensively managed forest plantations. So the application of tree biomass ash is emerging as an alternative way of putting these nutrients back and correcting soil nutrient levels.
Another potential benefit and which is being studied at Neiker-Tecnalia may be the improvement in the physical properties of the soil. The ash may increase the porosity of the soil, which means that soil aeration is improved and consequently this leads to greater water permeability. What is more, the soil can increase its water holding capacity, which would lower the risk of surface run-off.
It is essential to continue with the study of this trial since forestry systems offer great inertia. Likewise, it is important to set up others in which different kinds of ash in different agricultural systems are tested to find out their potential for being applied as fertilizer on a major scale.
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