Prescribing the right remedy for Ultisols: How do biochar types and incubation periods synergistically increase cucumber yield?

Cucumber is a high-value horticultural crop, with fruit water content exceeding 90%, and is used for food, processing, and cosmetics, with continuously growing market demand. However, in Indonesia, cucumber cultivation is often constrained by marginal land, and problems are particularly prominent in acidic soils such as Ultisols. Ultisols account for about 25% of Indonesia’s total land area. They have acidic pH, generally low organic carbon, C/N ratio, total nitrogen, cation exchange capacity (CEC), base saturation, and exchangeable cations, and high aluminum concentrations, severely limiting nutrient availability and crop growth. Against this background, how to improve acidic Ultisols and enhance cucumber productivity has become a key issue for tapping the potential of marginal land use.

Recently, the team led by Posma Mangasi Pintaria MARBUN at Universitas Sumatera Utara, Indonesia, and collaborators published a research paper titled “Enhancing Ultisol fertility index, nutrient uptake and agronomic yield of cucumber (Cucumis sativus) through biochar types and incubation periods” in ENGINEERING Agriculture, an Engineering cluster journal of the Chinese Academy of Engineering (DOI: 10.15302/J-FASE-2026683). The study took Ultisols as the object and, at an application rate of 10 t·ha⁻¹, compared oil palm empty fruit bunch biochar (OPEFB-B), rice husk biochar (RH-B), and maize cob biochar (MC-B), combined with different pre-incubation periods of 0, 10, 20, and 30 days (three replicates per group), to investigate their effects on soil fertility and cucumber growth.

The study found clear differences in the improvement pathways of the three biochars. Maize cob biochar had the most prominent direct effect on increasing cucumber productivity, with yield reaching 47.4 t·ha⁻¹, nitrogen, phosphorus, and potassium nutrient uptake increasing by about 125%, and the harvest period advanced to 47.3 days. This is because maize cob biochar itself has higher pH, organic carbon, CEC, and nutrient levels such as nitrogen, potassium, magnesium, and calcium, which help improve nitrogen, phosphorus, and potassium availability in Ultisols and promote cucumber growth and yield formation. Oil palm empty fruit bunch biochar performed best in improving comprehensive soil fertility, with the Ultisol soil fertility index reaching up to 0.447, an increase of 38.8%. Rice husk biochar had a relatively mild improvement effect, with its main effect being an increase in soil exchangeable sodium content, while also increasing the number of cucumber leaves by 26.8%.

The pre-incubation period is an important factor regulating the effect of biochar on Ultisol improvement. Compared with no incubation, incubation to 20 days increased soil chemical indicators by 2.35%–62.5%. Under the 20-day pre-incubation treatment, cucumber yield reached 38.2 t·ha⁻¹ (this difference was not statistically significant), and plant nutrient content and nutrient uptake were generally optimal. Aboveground nitrogen, phosphorus, and potassium uptake increased and were significantly positively correlated with plant height, leaf number, leaf area, stem diameter, SPAD chlorophyll, and plant dry matter accumulation. Compared with no pre-incubation, plant nitrogen, phosphorus, and potassium nutrient uptake increased by 100%–106%, and the harvest period was advanced to 48.4 days.

In the interactive treatments of biochar type and pre-incubation period, oil palm empty fruit bunch biochar combined with 10–20 days of pre-incubation significantly increased total potassium, exchangeable magnesium, calcium, and potassium, pH, organic carbon, CEC, and total nitrogen in Ultisols; maize cob biochar under 10 days of pre-incubation gave the best soil available phosphorus; and rice husk biochar without pre-incubation gave the best improvement in soil base saturation. Oil palm empty fruit bunch biochar combined with 10–20 days of pre-incubation could raise the soil fertility grade from “low” to “medium”. However, the interaction between biochar type and pre-incubation period had no significant effect on cucumber agronomic traits, harvest traits, yield, fruit cross-section, or macronutrient nutrient uptake. The study speculated that the overall pre-incubation duration was limited, and the potential of biochar to release nutrients and supply plant growth was not fully realized.

Correlation analysis showed that cucumber yield was significantly positively correlated with soil organic carbon, CEC, base saturation, exchangeable calcium, and exchangeable potassium, with corresponding correlation coefficients (r) of 0.321, 0.342, 0.420, 0.392, and 0.487, respectively. This indicates that these soil indicators are key factors driving cucumber nutrient uptake and yield formation, and also explains the intrinsic mechanism underlying the differences in effects produced by different biochar and pre-incubation period combinations.

Based on the experimental results, the study proposed actionable agronomic recommendations: to rapidly increase cucumber yield, maize cob biochar should be preferred; if the goal is comprehensive improvement of Ultisol fertility, oil palm empty fruit bunch biochar combined with 10–20 days of pre-incubation is more advantageous. From the perspective of green and low-carbon agricultural development, converting agricultural and forestry wastes such as oil palm empty fruit bunches, rice husks, and maize cobs into biochar for field return can both help improve degraded acidic soils and develop marginal land, and provide a feasible pathway for the resource utilization of agricultural and forestry wastes, offering reference value for the sustainable management of acidic soils in tropical regions.