Biochar Cuts Guano Fertilizer Needs by Half for Peruvian Smallholders

In a controlled field experiment conducted in Lurín, Peru, the study found that combining biochar with half the standard dose of guano de isla maintained lettuce yields comparable to those achieved with the full dose of guano alone. This finding suggests a practical method to conserve a declining natural resource while supporting the productivity of smallholder farmers in the Global South.

Guano de isla, derived from seabird droppings, has historically been one of the most valuable natural fertilizers due to its exceptionally high nutrient concentration. In Peru, this resource is reserved primarily for economically vulnerable smallholder farmers who depend on subsidized access through national agricultural programs. However, the long-term availability of guano is under severe threat. Seabird populations along the Peruvian coast have experienced a dramatic decline, driven by H5N1 avian influenza outbreaks and climatic disturbances such as a strong El Niño event. Seabird numbers fell from approximately four million in December 2022 to around 650,000 by March 2025, creating an urgent need to improve the efficiency of the remaining guano supplies.

The study was designed to test the hypothesis that biochar could enhance the nutrient retention and use efficiency of guano de isla, allowing for reduced application rates without compromising crop yield or farm profitability. The experiment took place in sandy soil in Lurín, Lima, between January and February 2025. Sandy soils were selected because they represent a highly vulnerable agronomic environment, characterized by low organic matter content, low cation exchange capacity, and high hydraulic conductivity. These conditions typically promote rapid nutrient leaching and reduce fertilizer use efficiency, making them a stringent test for the effectiveness of biochar amendments.

The experimental design utilized a randomized complete block layout with ten replicates per treatment. Eight distinct fertilization treatments were evaluated, including a control group with no fertilizer, inorganic NPK fertilizer, full-rate guano de isla, biochar alone, a microbiological soil amendment called Biomax, and various combinations of these inputs. The key treatment for the study was the combination of biochar and guano de isla, where the guano application rate was reduced to 200 kg per hectare, which is 50% of the standard 400 kg per hectare rate. Each experimental replicate contained six lettuce plants, and irrigation was applied every four days. Harvest occurred in the fifth week, with the foliar portion of the plants collected and weighed to determine yield.

The results indicated that the highest average lettuce weights were recorded in the full-rate guano de isla treatment and the biochar plus guano de isla treatment. Statistical analysis revealed no significant difference between these two groups, meaning that the combination treatment achieved statistically equivalent yields to the full-rate guano treatment while using only half the amount of the organic fertilizer. This represents a 40% increase in yield compared to the unfertilized control group. The microbiological amendment, Biomax, showed no discernible effect on lettuce fresh weight, and treatments including Biomax were subsequently excluded from the detailed economic analysis due to their lack of significant impact on yield.

Beyond yield, the study assessed the economic viability of the different fertilization strategies using the value/cost ratio, or VCR. A VCR below 1 indicates a financial loss, while a VCR greater than 2 indicates that the investment is fully recovered with at least a 100% return. Under conventional market prices, all fertilization treatments resulted in VCR values below 1, indicating that they were not economically viable for farmers selling in standard markets. This finding highlights the economic challenges faced by smallholder farmers who rely on conventional market prices for their produce.

However, the economic landscape changed significantly when organic market prices were considered. Under organic market conditions, the guano de isla, biochar plus guano de isla, and biochar alone treatments all surpassed the profitability threshold of 2. This indicates that the investment in these organic amendments becomes profitable when farmers can access premium organic markets. The most compelling economic scenario involved the biochar plus guano de isla treatment combined with carbon credits. In this scenario, the production cost of biochar was offset by carbon credit sales, resulting in a maximum VCR of 12.9. This exceptionally high return demonstrates the potential for organic farming systems supported by carbon markets to create strong economic incentives for adopting sustainable practices.

The mechanism behind these results lies in the physical and chemical properties of biochar. Biochar is a highly porous material with a large specific surface area and variable surface charge. These characteristics contribute to substantial increases in soil cation exchange capacity, improving the retention of nutrient cations such as ammonium, potassium, and calcium that are prone to leaching in sandy soils. Additionally, biochar surfaces contain functional groups capable of adsorbing organic and inorganic nitrogen forms, moderating nutrient release dynamics and reducing losses through leaching and volatilization. By enhancing soil structure and water-holding capacity, biochar also mitigates drought stress and facilitates sustained nutrient uptake by plant roots.

The study highlights the importance of market access in the adoption of sustainable farming practices. While the agronomic benefits of biochar and guano de isla are evident, their economic viability is heavily dependent on the ability of farmers to sell their produce in organic markets or access carbon credit mechanisms. Under conventional market prices, the lack of economic incentive may hinder the adoption of these practices by smallholder farmers. However, the combination of organic price premiums and carbon credits for biochar production offers a pathway to profitability that aligns with environmental goals.

The implications of these findings extend beyond individual farm profitability to broader issues of food security and resource conservation in the Global South. As guano supplies dwindle due to ecological pressures on seabird populations, strategies to enhance the use efficiency of this resource become critical. By integrating biochar into standard fertilization practices, farmers can increase resilience and sustainability, mitigating the risk of projected guano shortages. This approach safeguards agricultural productivity and food security for vulnerable communities that depend on organic fertilization strategies.

The production of biochar also offers additional benefits for smallholder farmers in low- and middle-income countries. The process facilitates the conversion of biomass residues into a high-quality substrate using cost-effective and straightforward methodologies that produce minimal emissions. This makes biochar an ideal solution for regions where technical expertise and financial resources may be constrained. Low-technology biochar production systems have been identified as potentially powerful nature-based climate solutions in developing regions, further enhancing the appeal of this strategy for sustainable agriculture.

Despite the promising results, the study acknowledges the need for further research to evaluate the long-term effects of this fertilization strategy on soil health and its efficacy in other crops commonly cultivated by smallholder farmers. The initial benefits of biochar can be moderate, but long-term application is expected to improve soil structure and create a more favorable environment for root development and efficient nutrient absorption. Increased benefits from the co-application of guano de isla and biochar over time are a realistic expectation, potentially leading to sustained or increased yields. Future studies should explore the durability of these benefits and their applicability to diverse agricultural systems in the Global South.

Subject of Research: Enhancing nutrient use efficiency of guano de isla with biochar in sandy soils for smallholder agriculture

Article Title: Biochar enhances the use efficiency of guano de isla and could boost profitability and food security for smallholders in the global south

Article References: Espejo, S. E. L., De La Cruz, R. C. C., de Mola León, U. L., Carruitero, J. L. M., Zavalaga, C., & Ladd, B. (2026). Biochar enhances the use efficiency of guano de isla and could boost profitability and food security for smallholders in the global south. Discover Life, 56(1), Article 22. https://doi.org/10.1007/s11084-026-09742-8

Image Credits: AI Generated

DOI: 10.1007/s11084-026-09742-8

Keywords: biochar, guano de isla, smallholder farmers, nutrient use efficiency, sandy soils, organic farming, food security, Peru, fertilizer efficiency, carbon credits, enhances, efficiency