What was once considered worthless waste—prickly pear cactus spines and edges—is proving to be a promising raw material for sustainable agriculture. A team from the National Autonomous University of Mexico (UNAM), led by researcher Bruno Chávez Vergara, has developed an organic fertilizer from this waste, with results that are noteworthy for their effectiveness and low environmental impact. The product, obtained through an anaerobic digestion process, not only provides nutrients to crops but also helps regenerate soils degraded by years of chemical use.
The research, funded between 2018 and 2020 by the Mexico City Ministry of Education, Science, Technology and Innovation (SECTEI), is part of a broader project to sustainably manage the volcanic soils of the southern Valley of Mexico. The goal is to boost food production, promote aquifer recharge, and mitigate climate change. To achieve this, the team has worked closely with the company Sustainability in Energy and Environment (SUEMA), which operates a nopal processing plant in Milpa Alta, the area that accounts for more than 90% of the nopal consumed in Mexico City.
From garbage to the field: how nopal digestate is obtained
The process begins with the waste generated when cleaning the prickly pear cactus for sale: between 15% and 20% of the cactus ends up as spines and edges. Instead of ending up in landfills or compost heaps, this material is placed in a biodigester where, in the absence of oxygen, it decomposes and produces biogas. This gas is used to generate electricity, but the remaining liquid byproduct—the digestate—is what is of interest as a fertilizer. Unlike traditional fertilizers, which are obtained through aerobic decomposition (with oxygen) and are slower, prickly pear digestate is generated in an anaerobic process that results in a rapidly absorbed liquid.
Chávez Vergara explains that this fertilizer not only supplies mineral nutrients to plants, but also feeds the beneficial bacteria and fungi in the soil. While synthetic fertilizers focus on directly nourishing the crop, neglecting microorganisms, digestate emulates what happens in nature: it provides carbon and other compounds that keep the soil microbiota alive . Furthermore, being liquid, it allows for better use of irrigation water and the nutrients are absorbed more quickly.
Results in crops: more biomass and faster maturation
The first field trials were conducted on forage oat crops in Milpa Alta, and the data are conclusive. From the first growing season, biomass increased by 20%, and the plants' maturation time was reduced by up to a month. For hillside and rain-fed farmers, who depend on rainfall and suffer from erosion problems, this progress represents a welcome relief. A shorter cycle means less risk of losses from drought or late frosts , and a higher yield per hectare.
The UNAM team has also tested the digestate in areas of Santiago Tulyehualco (Xochimilco) and Tetelco (Tláhuac), always in collaboration with SUEMA. The results have been consistent: the organic fertilizer not only improves production but also restores biological soil functions that had been damaged by decades of agrochemical use . The researchers emphasize that, unlike synthetic fertilizers, this product does not cause erosion or loss of organic matter.
Next steps: corn, carrots, and finding the optimal dose
Following their success with oats, the scientific team wants to scale the project to other staple crops in the Mexican diet, such as corn and carrots. The goal is to determine the minimum dose that is profitable for producers, so that the fertilizer can be distributed on a large scale. The idea is that rainfed farmers, who typically have fewer resources, can access an effective and inexpensive input.
For now, nopal digestate is emerging as a viable alternative to chemical fertilizers, especially in areas where mechanization is difficult and erosion is a serious problem. The partnership between UNAM, SECTEI, and SUEMA demonstrates that the circular economy can be successfully applied in the agri-food sector , transforming plant waste into a valuable resource. The coming months will be crucial to see if this technology can be implemented on a large scale, moving from the laboratory to the field, and if it can be adapted to the conditions of other countries, such as those in Europe, where the search for organic fertilizers is also a priority.
Transforming nopal waste into fertilizer not only reduces the amount of garbage generated at collection centers, but also returns nutrients to the soil and improves the profitability of farms. With a 20% increase in biomass and faster maturation, farmers see their crops respond better without the need for chemicals. And most importantly, the soil recovers, regaining its capacity to support microbial life and retain water. If this solution becomes widespread, it could mark a turning point in sustainable agriculture.
