Zanda Melnalksne and Iveta Grudovska: Recovered nutrients – Farmers’ perspective

As a result of geographical, economic and political developments, the agricultural sector, especially livestock farming, has concentrated in certain regions, creating excessive nutrient accumulation in ”hot spots.” Even within the Baltic Sea region, the density of agricultural animals varies significantly across different areas, and consequently – the availability of nutrients for the crop production sector also differs. Similarly, with the development of food processing and other industries, large volumes of side products are created, in which nutrients necessary for plant growth are concentrated. Challenges are created by sludge from wastewater treatment processes and ashes from various combustion processes. These materials are geographically concentrated in or near densely populated areas. Managing by-products together with the rest of the waste stream, by disposing them in landfills or incinerating them, is not a good resource management process, and in many cases is economically disadvantageous.

At the same time, the agricultural sector is experiencing increasing challenges – limited availability of organic fertilizers and rising mineral fertilizer prices, which leads to searching for alternative fertilizer solutions. Nutrient supply is a special task for organic farmers and any farmer in regions where soils are poorer, livestock density is lower, and mineral fertilizer availability is limited. Due to all these circumstances, farmers’ interest in fertilizers based on recovered nutrients is continuously growing. However, several conditions remain very important for farmers to purchase and use these fertilizers on their farms.

First, the quality of the fertilizer material. Regardless of the raw material from which the fertilizer is derived, it must fully meet safety criteria. The main ones, clearly defined in most European countries, are the concentration of heavy metals and contamination with pathogens. There are limit values at which the material is not allowed to be used in agriculture. Two other groups of contaminants are microplastics, as well as medication and antibiotic residues. These are things that farmers do not want to see in the material.

The second important aspect for using recovered nutrient fertilizer – the material must be easy to load, transport, and spread. Farmers have specific types of equipment that allow for uniform field distribution of two specific types of materials: liquid – with a dry matter content of up to 10%, or dry, free-flowing – with a dry matter content of ~50% (min 30%). For example, if compost is being made from semi-liquid mass, sufficient filler (garden and park waste; ash; etc.) needs to be added to obtain a free-flowing product. Similarly, pure wood ash cannot be used as fertilizer in agriculture. It is granulated together with other material combinations. Also, for the biochar, it is advisable to add bran or other materials to improve physical properties.

As a third factor, the definition of recovered nutrient materials in a way that is understandable and comprehensive to farmers should be mentioned. Understanding of the product and what’s inside is important. Farmers, when preparing fertilization plans for their fields, balance nutrients by combining different types of fertilizers. However, if the fertilizer is given a nitrogen, phosphorus, or potassium content, these substances must be indicated in such a chemical form that the farmer can directly include the nutrients in their calculation formulas without any additional conversions being necessary. For example, there is a difference whether calculations for fertilizer balancing are done with phosphorus or phosphorus oxide. It is also important for farmers to know the microelements content of fertilizers, as each element plays an important role in the overall system operation, and its deficiency or overdose can cause undesirable consequences. For example, if the recovered fertilizer contains copper (Cu) or zinc (Zn) residues, it is important to know their quantity to later avoid unnecessary concentration of specific elements when using foliar fertilizer or some other complex fertilizer.

Sustainability goals, which most companies try to implement in their operations, lead to searching for solutions. For farmers, the availability of quality fertilizer is important, while for companies in other industries and society – finding solutions for sustainable processing and use of side streams is challenge. Cooperation between players from various involved sectors leads to systemic solutions that allow closing the nutrient cycle and keeping nutrients in the system. This enables sustainable resource management and prevents risks to nature, the environment, and people.