This study investigates the effectiveness of phytoremediation for diesel-contaminated soil using rye grass and fescue grass in both controlled and natural conditions. The research examined plant performance across various soil compositions (100% topsoil, 50:50 soil-sand mixture, and 25:75 soil-sand mixture) with differing diesel oil concentrations (10, 15, and 20 ml/kg) and NPK fertiliser treatments (5, 10, and 15 g/kg). The experiment was carried out in two phases: an initial 7-day period under controlled laboratory conditions, followed by a 3-month period in natural outdoor conditions (July to October) to simulate real-world applications. During the outdoor phase, plants were exposed to typical Scottish weather, with average temperatures ranging from 17°C (July-August) to 13°C (October), and atmospheric pressure fluctuating between 980 and 1040 hPa. Initial experiments demonstrated that while both grass species exhibited resilience in contaminated environments, optimal growth occurred in 100% topsoil and 50:50 soil-sand mixtures, with minimal to no growth in 75% sand compositions. Oil concentration after phytoremediation was measured using UV spectrophotometry at a 306 nm wavelength and converted using the calibration curve equation Y = 0.5731 x (where Y is absorbance and x is oil concentration percentage), revealing varying effectiveness across soil compositions. Results indicated significantly lower remaining oil concentrations in 25% soil mixtures compared to 100% soil samples, primarily due to higher soil permeability in sand-mixed samples. For both grass types, the remaining oil concentration ranged from approximately 0.2% to 1.2% wt/wt across different initial contamination levels, with both rye and fescue grass demonstrating comparable remediation capabilities. The addition of NPK fertiliser showed promising results in enhancing plant growth and potential remediation capabilities. This research contributes to an understanding of the practical applications of phytoremediation, particularly focusing on the role of soil composition and nutrient supplementation in plant survival and diesel concentration reduction under both controlled and natural conditions.