Influence of compost characteristics and compost:soil ratio on soil properties and growth of Vicia faba

Application of compost to soil can increase the chemical, physical and biological soil quality (De Clercq et al., 2016). The improved soil quality can contribute to increased crop performance and yield (Hepperly et al., 2009). However, compost quality and, therefore, the effects of compost application on soil, is highly variable depending upon feedstock and composting practices. In order to evaluate the fitness of compost for a certain use, compost can be physically, chemically and biologically characterized. One important compost characteristic is maturity, i.e. the level of completion of decomposition processes and extent of disappearance or degradation of phytotoxic organic substances, and it refers to the suitability of compost for plant production. Immature composts can cause phytotoxic effects as reduced seed germination and decreased root and plant growth (Wichuk and McCartney, 2010). Compost maturity is especially important at high compost:soil ratios, e.g. in case of superficial incorporation of compost in reduced tillage systems. Compost can be used for improvement of the structure and the nutritional status of the soil, but it should not reduce seed germination and crop emergence. It is not clear at which soil:compost ratios good plant development is possible and how compost characteristics can be used to estimate the fitness of compost as medium for plant growth. Compost maturity and fitness for plant growth increase during the composting process (Griffin and Hutchinson, 2007), but it is not clear how different composts from varying feedstocks and composting practices can be evaluated for their fitness for plant growth at high compost:soil ratios. Composts with high carbon to nitrogen ratio can cause nitrogen depletion in soils and nitrogen deficiency for plants (Wichuk and McCartney, 2010), however the use of the C/N ratio for estimation of compost stability or maturity is controversial (Griffin and Hutchinson, 2007; Vandecasteele et al., 2017). Other possible maturity measurements are, among others, humification parameters, cation exchange capacity (CEC), ammonia and nitrate and respiration tests (Wichuk and McCartney, 2010). A combination of different tests for assessing compost maturity is recommended, however, there is disagreement in literature on the best combination of tests for evaluation. Another important compost characteristic for plant fitness is salt content, commonly estimated by electrical conductivity (EC) measurement. Compost with high EC values should be used in small doses to avoid negative plant effects as plant tissue accumulation and interference with root uptake of water (Gondek et al., 2020). In this study, the influence of compost application on soil quality and garden bean yields was evaluated in relation to compost characteristics for four diverging composts at four different compost:soil ratios. It is hypothesized that compost addition increases plant performance at low compost:soil ratios, and decreases plant growth at high compost:soil ratios because of limited compost maturity depending upon the compost characteristics.