In the context of climate change, it is necessary to establish forest management by balancing more products, using less area, and minimizing environmental impacts. The use of different industrial bio-based by-products as soil conditioners in the last few decades has gain more interest, because it leads to an extended use time of these products and supports the circular economy. The aim of this study was to determine the effect of fertiliser made from cattle and pig manure biogas fermentation digestate and wood ash from two cogeneration plants, applied in different mixture ratios, to test its suitability for fertilisation of deciduous trees, using the physiological, morphological, and chemical parameters of the leaves as an indicator. We selected two poplar clones: foreign ‘OP42’ (syn. Hybrid 275) and local ‘AUCE’ annual shoot stem cuttings as planting materials. A negative control group with acidic forest mineral soil as substrate and four fertilised groups with different applied digestate and wood ash ratio mixtures to forest soil was established (ash:digestate 0:0 (Control), 1:1, 2:1, 3:1, 4:1). Mixture application improved growing conditions because all fertilised group poplars had longer growth periods and photosynthetic rates in August than the control group. Both local and foreign clones showed a good response to fertilisation in terms of leaf parameters. Poplar is a suitable culture to fertilise with bio-waste biogenic products, because of its capacity to absorb nutrients and fast response to fertilisation.

Garlic (Allium sativum L.) is a widespread crop in vegetable production. The popularity of garlic is due to its bactericidal and antioxidant properties. Digestate and wood ash are the by-products of cogeneration plants. The digestate is rich in nutrients, can provide a large part of the nutrients needed by the plant during the growing season, as well as improves the soil structure. Wood ash contains small amounts of phosphorus and potassium and is strongly alkaline (pH 8–12) due to the oxides in its composition, mostly calcium carbonate. The purpose of the study was to determine the effect of fertilizer rates of the digestate and wood ash mixtures on winter garlic productivity and harvest quality. Field trials with the winter garlic variety 'Lubaša' were established in sod clay, loamy soil. In the garlic plantation, different variants of fertilizer mixtures were used – they contained pig or cattle manure or plant residue digestate, and wood ash. The ratio of digestate and wood ash in the mixtures was 3:1; fertilizer rates for winter garlic were 15 and 30 t ha-1. In the study, the different types of fertilizers showed different effects on the yield and quality of winter garlic.

Maize (Zea mays L.) is one of the world's most productive and prevailing crops. It is widely grown as human food and animal feed, as biofuel, and as a raw material for industry. Digestate and wood ash are the byproducts of cogeneration plants. Digestate is rich in nutrients, can provide plants with many nutrients required by the plant during its growing season, as well as improves the soil structure. The aim of the study was to determine the effect of the rates of digestate and wood ash mixture fertilizer on maize productivity and crop quality. In 2020, 2021 and 2022, in field trials with maize, the variety 'Hulk', FAO250, was used; the soil – sod-calcareous, medium-heavy sandy loam. Different variants of the mixture of cattle manure digestate and wood ash were used as a fertilizer on corn plantations. The ratios of digestate and wood ash was 1:0, 1:1, 2:1, and 3:1; fertilizer rates were 15 and 30 t ha-1. In the trial, different types of fertilizers demonstrated different effects on the yield and quality of maize. The use of the new fertilizers produced on average 36.93–38.33 t ha-1 of high-quality maize green mass in three years without using mineral fertilizers.