Thematic training material for the use of of ashes and ash-based materials as fertilizer has been published

ID401 - ASH

Ash is the burned-out solid residue powdery product after oxidative thermo-chemical processing: oxidative combustion or semi-oxidative gasification processes.

Ashes are characterized as fly ash or bottom ash or a combination thereof formed through the incineration of bio-based materials by oxidation. Co-incineration is an economically viable and widely applied waste disposal route for many nutrient-rich wastes. For nutrient recovery the mono incineration of defined biomass stream is necessary to control the input of contaminants and for concentration of the nutrient content

Ashes obtained through incineration can be post-processed with the aim to partly remove metals and metalloids, and to increase the availability of plant nutrients in the ash complexes.

Examples for ash products available on the NUTRIMAN Farmer Platform

Typical nutrient content and availability for plants

Biomass ashes: The ashes contain several plant nutrients (calcium, magnesium, potassium, phosphorus). The calcium and magnesium content are mainly in oxidic form (25 - 45% CaO, 1 - 6% MgO) So ashes have a lime effect due to this basic character. While potassium (1 – 12% K2O) and sulphur (2-7 % SO2) is almost completely available to plants, phosphorus (1 - 10% P2O5) is not in every case good plant available. Raw ashes are therefore not always suitable for direct use as fertilizer and must be processed before application. Mixing wood ash and carbonate lime fertilizer is also a suitable application form.

Ash-based materials from sewage sludge: Depending on the subsequent process the material can contain P as a Ca- or Ca/Na-Phosphate in a plant available form with a content up to 15-40 % P2O5; 1-2 % K2O, CaO and MgO.

Fields of application in agriculture:

The ash and ash-based products are usually approved and suitable for conventional farming. There are also considerations to approve these products for organic farming since alternative effective P-fertilizers are being sought here. There are no restrictions on agricultural crops. The application rate is generally based on the P requirement of the crops, considering the soil P content. In powder form, the application is more dependent on the weather conditions (wind). If ashes are mixed in with lime fertilizer, they are spread with special lime spreaders. After granulation or pelleting, the products can be applied using conventional sprinkling technology.

Benefits for farmers

The ashes contain several plant nutrients (calcium, magnesium, potassium, phosphorus). Ashes have also a lime effect due to this basic character so they can be used for balancing the pH level on acidic soil.

After appropriate preparation, ash-based P-fertilizers provide highly concentrated P-fertilizers with good plant availability and low content of impurities.

Using ashes and ash-based material is closing a material and nutrient cycle. They can represent a valuable resource as a substitute for finite minerally derived P fertilizers. In particular, the ashes obtained from sewage sludge represent a great recovery potential for phosphate. In addition to direct use, these material flows could also be used for the production of fertilizers in industry.

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