Researchers from the Instituto Valenciano de Investigaciones Agrarias (IVIA) in Moncada, Valencia, led by corresponding author Isabel Rodríguez-Carretero, spent two consecutive growing seasons testing composts in a commercial orchard of adult Tango mandarins grafted onto a hybrid rootstock and irrigated by traditional surface flooding. The team compared two industrially produced composts: one made from pruning residues and sewage sludge at a 1:3 fresh-weight ratio, the facility’s usual recipe, and a second made from rice straw and sewage sludge at a 1:8 ratio, using the rice straw as a locally available bulking agent. Composts were surface-applied each June within the tree canopy projection at two rates, 10 and 20 tonnes per hectare, alongside unfertilised-with-compost control plots, in a randomized complete block design with three replicates.

The chemical characterisation of the two composts revealed a striking difference. The rice straw–sewage sludge compost contained significantly higher total nitrogen, largely as organic nitrogen, and a lower carbon-to-nitrogen ratio of 10.3 compared with 13.8 for the pruning-residue version. Most notably, its phosphorus content, expressed as P2O5, was more than double that of the conventional compost and exceeded average values typically reported for livestock manures. The researchers suggest this makes the rice straw compost a promising phosphorus-rich organic fertiliser, a significant finding at a time when phosphate rock reserves are finite and Europe depends heavily on imported fertiliser raw materials.

Both composts met Spanish legal requirements for fertilising products, including minimum organic matter thresholds and maximum carbon-to-nitrogen ratios. One caveat emerged: the rice straw compost slightly exceeded the 500 milligrams per kilogram zinc limit for Class B classification under the older RD 506/2013 regulation, though it complied with the stricter thresholds of the more recent RD 1051/2022, which governs sustainable soil nutrition. The compost’s slightly acidic pH of 6.71, unusual for sewage sludge-based products, may actually be an advantage in the alkaline, carbonate-rich soils of eastern Spain, where it can enhance the solubility and availability of nutrients that are otherwise locked up by calcium carbonates.

In the field, the effects on soil chemistry unfolded over time. Soil organic matter and organic nitrogen rose significantly in the first season at both application rates, and in the second season at the double dose. Available phosphorus increased significantly in both seasons regardless of dose, pushing soil levels from normal or high into the high to very high categories used in citrus nutritional diagnostics. Available potassium rose significantly only at the double rate, by 17 percent in the first season and 29 percent in the second. Interestingly, exchangeable sodium decreased in composted plots in the second season, while soil pH dropped significantly under the double dose, a cumulative effect the authors attribute to repeated organic matter inputs and the release of organic acids during decomposition.

Salinity was the one indicator demanding caution. Electrical conductivity rose by 21 percent in the first season and 105 percent in the second under the double application rate, reflecting the salt load carried by successive compost additions. The values nonetheless remained below both the 4 dS per metre threshold that defines a saline soil and the 1.7 dS per metre level at which citrus yields begin to decline. The authors recommend that long-term management might involve reduced annual rates or biennial applications, combined with irrigation practices that promote salt leaching through the improved soil permeability that organic matter confers.

Soil biology responded quickly but transiently. In the first season, microbial biomass carbon jumped 65 percent under the double compost dose, and dehydrogenase activity, a standard proxy for overall microbial metabolic activity, increased under both rates. By the second season, however, no significant differences among treatments remained. The researchers suggest the first-year compost, richer in readily oxidisable organic carbon, offered microorganisms a more accessible substrate, and note that biological indicators are highly sensitive to soil moisture, temperature, and sampling timing. They call for future studies with multiple sampling dates across crop phenological stages to disentangle these dynamics.

Heavy metal concentrations in the soil, including copper, zinc, nickel, lead, cadmium, and chromium, stayed within permissible limits under both older and current Spanish regulations throughout the trial, although the double compost dose raised total soil zinc by roughly 17 to 18 percent in both seasons. Foliar nutrient analysis told a similarly reassuring story: macronutrient concentrations remained within optimal ranges in all treatments, with the sole exception of potassium in the second season. Compost significantly increased leaf zinc only in the first season. Encouragingly, because soil and foliar phosphorus were already elevated after the first compost application, the team cut mineral phosphorus fertilisation by 10 percent in the second season, and leaf phosphorus stayed optimal, evidence that compost-derived phosphorus can partially replace mineral P inputs under comparable conditions.

Perhaps the most commercially significant result is what did not change. Yield, fruit weight, diameter, peel thickness, colour index, total soluble solids, titratable acidity, and maturity index showed no agronomically relevant differences between composted and control trees in either season. All fruit met EU marketing standards for mandarins, with diameters above 45 millimetres, juice content above 33 percent, and maturity indices exceeding 7.5. Because mineral fertilisation met crop requirements across all plots, the soil fertility gains from composting simply had no yield gap to fill. The authors caution that their findings come from a flood-irrigated orchard, a system still used on roughly 13 percent of Spanish citrus acreage, and that responses under modern drip irrigation may differ. Still, the study demonstrates that industrial-scale composting of rice straw and sewage sludge is a technically viable, regulation-compliant route to closing nutrient loops in Mediterranean agriculture, turning two disposal headaches into a soil-building resource while keeping the region’s mandarins just as sweet.

Subject of Research: Effects of rice straw–sewage sludge compost on soil fertility, tree nutrition, and fruit quality in Mediterranean citrus orchards

Article Title: Field application of rice straw–sewage sludge compost in Mediterranean citrus orchards: effects on soil properties, nutrient status and fruit quality

Article References: Rodríguez-Carretero, I., Canet, R., Pérez-Piqueres, A., & Quiñones, A. (2026). Field application of rice straw–sewage sludge compost in Mediterranean citrus orchards: effects on soil properties, nutrient status and fruit quality. SOIL, 12(2), 841-853. https://doi.org/10.5194/soil-12-841-2026

Image Credits: AI Generated

DOI: 10.5194/soil-12-841-2026

Keywords: compost, rice straw, sewage sludge, citrus, soil fertility, circular economy, phosphorus, soil organic matter, microbial biomass, Mediterranean agriculture, mandarin, organic fertiliser