# On-Farm Composting of Hop Plant Green Waste--Chemical and Biological Value of Compost
## 1 Introduction  
In recent years, farms have been facing the challenges of managing and disposing of green biomass resulting from production residues. Massive amounts of hop biomass after harvest appear in a short period as a by-product of hop cones production. Hop (_Humulus lupulus_ L.) cones are crucial in the beer brewing industry because they contain resins, tannins and aromatic substances [1]. Around 2600 hop farms cover 26,500 hectares of land in the European Union (EU) and produce over 50,000 tonnes of hop cones per year [2]. Slovenia is the world's fourth largest hop growing country, accounting for over 1500 ha. Dried hop cones are the main product for the brewing industry after the harvest, whereas surplus abgewound biomass (hop leaves and stems) accounts for around 2/3 of all harvested hop biomass. Every harvest season the hop industry in Slovenia generates on average 15 tonnes of hop waste biomass (fresh matter) per hectare of harvested hop fields, resulting in 23,000 tonnes of green waste [3]. Because hop is a perennial plant, it requires twine or wire to sustain its growth. If the biomass is mixed with traditional plastic twine (which is not biodegradable nor compostable), which is a common practice in Slovenia, stringing material can be an ecologicalhazard [4]. One solution is to support hop with biodegradable twine. Hop biomass after harvest can be composted on farms using biodegradable twine, as demonstrated by the LIFE project BioTHOP [5]. With the introduction of new materials into the hop growing industry, new opportunities and demands for effective composting strategy on farms arose. Based on massive amounts of organic waste produced on hop farms, on-site composting of hop biomass needs to be considered a by-product that can be utilised as an organic fertilizer or soil amendment for hop farmers' land. This is an efficient technique to close the nutrient cycle at the point of origin. The method also meets the need for developing new substrates to reduce the use of chemical fertilizers [6; 7]. Composing is conventional low-investment technology to transform biomass into a stabilized final product with low, readily degradable, organic matter and without a phytotoxicity effect on plants [8; 9]. To avoid the phytotoxic impact, which can delay seed germination or inhibit plant growth, compost should be mature and stable before being used as a fertilizer [10]. Plant waste (hop biomass after harvest) has a nutritional composition that makes it a potential source of plant nutrients; hop biomass after harvest contains roughly 18% organic mass, 0.8% nitrogen, 0.3% potassium and 0.1% phosphorus, with the carbon-to-nitrogen ratio of 13:1 [3]. In the thermophilic phase, high microbial activity results in hygiisation of compost and degradation of input material. Temperature, thermal phase duration, moisture content, C/N ratio, oxygen concentration, pH and particle size all have an impact on the optimum composting conditions [11; 12; 13]. One of the most important variables in composting efficiency is the temperature inside compost piles [9; 14; 15]. Various microorganisms drive the composting process whose succession in community composition and population corresponds to the temperature evolution in compost [9; 16; 17]. For effective bacterial degradation of input composting material, smaller particle size is favorable due to larger surface areas where bacterial invasion occurs [11; 18]. During composting, additives like biochar are used as an adsorbent to reduce N losses with the absorption of NH\({}_{3}\)[19; 20]. Therefore, one of the objectives of the present work is to research the effects of additives (biochar and effective microorganism) on the composting process by assessing their influence on organic matter degradation, compost maturity and the quality of finished compost [21]. Compost stability and maturity are the main properties to characterize compost quality [22; 23]. The phytotoxic effect on plants is related to immature compost, while low microbial respiration indicates compost stability [22; 23].