Soil amendments with composted organic materials are recommended to increase soil organic matter (SOM) and promote soil fertility. Growing areas of hedged olive groves in the southern Iberia peninsula generate huge amounts of olive leaves, and their potential as an organic soil amendment is not fully studied. An experimental field trial in a hedged olive grove (“Cobrançosa”) was set up near Portalegre, Portugal, to test a compost of olive leaves plus sheep manure (with a ratio of 2:1) when applied in a row at the soil’s surface. Nominal rates of zero, 2.5, and 5.0 kg m−2 (T0, T1, and T2, respectively) were applied in a complete randomized block setup (three treatments, three replicas, and nine plots), and soil properties of layers between 0–5, 5–15, and 15–30 cm were annually monitored. More expressive results occurred in the soil layer 0–5 cm, and with the dosage T2. After one year, there were significant increases in the total N, carbon of the particulate organic matter, permanganate oxidizable carbon (POX-C), extractable phosphorus, and zinc. After two years, there was 16% more soil organic carbon (SOC), an absolute increase of 0.5 in pHKCl, 1.9 times more extractable phosphorus, and ten times more zinc. The soil’s C-stock in the 0–30 cm layer, after two years of T1 and T2 dosages, was 0.11 and 0.35 kg m−2 (~3 and ~9%, respectively), which was higher than with T0. POX-C was the most sensitive SOM-related indicator, showing increases of up to 30 cm deep after one year. This compost improved soil fertility but should be monitored over longer periods of time.
