From waste to fertilizer The impact of rose-waste compost on cut rose cultivation in Kenya

The impact of compost amendment on crop yield and quality in cut rose cultivation under large-scale commercial conditions was examined near Lake Naivasha, Kenya. The experiment, conducted between August 2022 and February 2024, involved over 7500 rose plants. Growers often hesitate to incorporate compost due to limited knowledge about its long-term effects on yield and quality, highlighting the importance of this research. Three mature rose-waste-based composts were incorporated into the soil prior to seedling planting: (i) pure rose waste compost, (ii) compost made of 80 % rose waste and 20 % tomato waste, and (iii) compost made of 90 % rose waste and 10 % mature compost. Standard fertigation was applied to all treatments, except for one additional treatment where fertigation was halved to evaluate compost as a fertilizer substitute. Crop performance was assessed through daily harvest records, cumulative yield analyses, and economic profitability based on auction prices. Soil and plant quality parameters were assessed every 3–6 months. Compost amendment combined with full fertigation increased the number of harvested stems by 3.2–4.7 % over 18 months. This resulted in an economical yield increase of approximately €22,000 per hectare ($9600 per acre) for the first 18 months of cultivation. The halved fertigation treatment yielded a comparable harvest to the control with fertigation only. Compost amendment improved soil organic matter content with ca. 30 % throughout the experiment and enhanced short-term nutrient availability, while facilitating long-term nutrient release without compromising vase life, stem length, or bud size. In conclusion, rose-waste compost significantly enhanced soil quality by e.g. increasing nutrient availability, which in turn enhanced crop performance during the growth phase of cut roses, with sustained differences thereafter. These findings underline the potential of compost amendment to sustainably close the cycle of resources within Kenyan rose cultivation, while boosting crop yields and improving soil health.