Written By Avraham Penso

To most people, “agriculture” means crop farming; trees don’t typically come to mind. But agroforestry, the integration of trees and agriculture, has been practiced for thousands of years (Ferrara et al., 2023). In light of the challenges confronting modern agriculture, the benefits of agroforestry are more needed than ever.

Soil retention and quality are major challenges that threaten crop yields and the overall viability of farming. In the Corn Belt of the Midwest U.S., 24%–46% of the topsoil is estimated to have been lost since large-scale farming began, primarily from erosion due to flooding and wind (Thaler et al., 2021). Conversion of native vegetation to agriculture depletes soil nutrients, particularly nitrogen (Kopittke et al., 2017). The increasing severity and frequency of flooding has washed away phosphorus, further degrading soils while contributing to harmful algae blooms in water bodies (Zhi et al., 2024).

Planting trees among crops offers a potential solution. Trees reduce erosion and control runoff. Canopies shelter fragile topsoil from heavy rainfall, while root systems maintain soil integrity. Trees shelter both crops and soil from high winds. Tree roots help efficiently cycle nutrients (such as phosphorus) from deeper soil layers not reachable by crop roots, returning them to the surface in the form of leaf litter (Lebrazi & Benbrahim, 2022). Additionally, increasing tree cover within an agricultural system improves sustainability by sequestering greater quantities of atmospheric carbon.

Many trees in the legume family (Fabaceae) offer an additional benefit: directly enhancing soil fertility through nitrogen fixation. Nitrogen fixation is a biological process carried out by microorganisms that convert inert atmospheric nitrogen into ammonia, an essential step in making nitrogen available for plants to use. The roots of many Fabaceae contain growths known as nodules that host nitrogen-fixing rhizobia bacteria. Trees with this capability are appropriately known as fertilizer trees.

When planted alongside crops, fertilizer trees can increase yields by supplying the surrounding soil with nitrogen and potentially reduce dependence on expensive fertilizers, in addition to the other agroforestry benefits mentioned above.

Nitrogen Fixing Cover Crop Plants to Grow This Fall:

Fertilizer trees have proven their value in Sub-Saharan Africa

Desertification — when fertile land transitions to desert — is a serious challenge in sub-Saharan Africa, including Malawi and Zambia. Slowing and reversing this process is essential, as livelihoods in this region are heavily reliant on crop yields (Lebrazi & Benbrahim, 2022).

Tree legumes are a strong candidate to combat desertification. They grow readily in exhausted soils, which they help restore, and are resistant to drought and erosion. Fertilizer tree legume species including Faidherbia albida, Tephrosia vogelii and Gliricidia sepium have been increasingly adopted by farmers in Malawi and Zambia to add vegetative cover to arid land, improve soils, and boost yields of vital crops such as maize (Langford, 2009).

Zambia’s agriculture-based economy is in a precarious position. Most agricultural land is worked by small farmers who lack the financial means to implement many best practices — 69% do not use synthetic fertilizers (Garrity et al., 2010). Maize, the primary crop on which the country’s food supply depends, typically severely underperforms its yield potential and is highly sensitive to dry spells. Faidherbia intercropping with maize substantially boosts soil nitrogen and has proven to be an affordable and popular method of improving soil quality and increasing yields. Helpfully, Faidherbia sheds its leaves during the wet season when maize is grown, allowing the crop to flourish without excessive shading (Yengwe et al., 2018).

Fertilizer tree agroforestry systems in sub-Saharan Africa boost crop yields significantly. A meta-analysis of 94 studies on maize production has found 89%–318% yield increases over control plots with no added fertilizer (Sileshi et al., 2014). This result is comparable to applying synthetic fertilizer as recommended. Yields increase further when fertilizer trees and synthetic fertilizer are used in combination. Increased yields have also been noted in millet, sorghum and other crops. One study noted that the yield increase in agroforested maize was most pronounced during the 2015–2016 drought season — highlighting the system’s ability to stabilize crop yields against climatic shocks.

Applying the fertilizer tree model to the United States

Agroforestry practices are not widespread in the U.S., but experiments with fertilizer trees are ongoing. The University of Tennessee Institute of Agriculture is studying black locust (Robinia pseudoacacia) and honey locust (Gleditsia triacanthos) in alley cropping, an agroforestry system in which “alleys” of crops are separated by rows of planted trees (UTIA, 2023).

Like the Faidherbia in Africa, honey locust and black locust fix nitrogen for crops nearby. Unlike Faidherbia, they won’t drop their leaves as the crops underneath grow. Instead, their purpose is to provide the crops with partial shade in extreme heat and serve as a buffer from severe weather events. The goal is therefore not just to increase yields, but also to improve the farm’s long-term resilience.

Integrating alley cropping into U.S. farming systems at a broad scale may not be simple. A recent survey of Tennessee fruit and vegetable farmers found only 22% would be willing to adopt a hypothetical black locust alley-cropping system (Velandia et al., 2025). Farmers expressed concerns ranging from added costs to increased labor and the choice of black locust, a competitive and thorny species. But farmers who had encountered challenges with heat stress or low organic material in soil were more willing to try something new. A larger body of evidence supporting the integration of fertilizer trees in U.S. farming systems may help cultivate greater enthusiasm.

In summary

Fertilizer trees offer a unique opportunity to address multiple challenges that have made agricultural systems worldwide more fragile and less productive than they could be. Realizing the full potential of modern agriculture will depend on resilient farming systems like agroforestry that help farmers get the most from every seed they plant.

Written By Avraham Penso, Yale University - School of the Environment Master of Forestry Student

If you are interested in winning a $5000 scholarship towards higher education in an agriculture related field, check out our scholarship page for more information. Our scholarship program started back in 2016 in honor of our founder Demetrios Agathangelides. Demetrios immigrated to the United States from Greece and attended Utah State University, graduating with a degree in Plant Science. With a love of seeds and an appreciation for education he continued to teach in seminars and local talk shows. Today we honor him by awarding a scholarship to a deserving student in the agricultural sciences.

References:

Ferrara, V., Sala, G., Ingemark, D., La Mantia, T. (2023). The green granary of the Empire? Insights into olive agroforestry in Sicily (Italy) from the Roman past and the present. Italian Journal of Agronomy 18(1). https://doi.org/10.4081/ija.2023.2184

Thaler, E.A., Larsen, I.J., Yu, Q. (2021). The extent of soil loss across the US Corn Belt. https://doi.org/10.1073/pnas.1922375118

Kopittke, P.M., Dalal, R.C., Finn, D., Menzies, N.W. (2017). Global changes in soil stocks of carbon, nitrogen, phosphorus, and sulphur as influenced by long-term agricultural production. Global Change Biology, 23(6): 2509-2519. https://doi.org/10.1111/gcb.13513

Zhi, W., Baniekci, H., Liu, J., Boyer, E., Shen, C., Shenk, G., Liu, X., Li, L. (2024). Increasing phosphorus loss despite widespread concentration decline in US rivers. https://doi.org/10.1073/pnas.2402028121

Lebrazi, S., Benbrahim, K.F. (2022). Potential of tree legumes in agroforestry systems and soil conservation. In book: Advances in Legumes for Sustainable Intensification (461-482). https://doi.org/10.1016/B978-0-323-85797-0.00004-5

Langford, Kate (July 8, 2009). “Turning the tide on farm productivity in Africa: an agroforestry solution.” World Agroforestry Centre. Archived from the original on June 20, 2010. Retrieved June 29, 2025.

Garrity, D.P., Akinnifesi, F.K., Ajayi, O.C., Weldesemayat, S.G., Mowo, J.G., Kalinganire, M.L., Bayala, J. (2010). Evergreen Agriculture: a robust approach to sustainable food security in Africa. Food Security, 2(3): 197-214. https://doi.org/10.1007/s12571-010-0070-7

Yengwe J., Amalia O., Lungu O.I., De Neve S. (2018). Quantifying nutrient deposition and yield levels of maize (Zea mays) under Faidherbia albida agroforestry system in Zambia. European Journal of Agronomy, 99: 148-155. https://doi.org/10.1016/j.eja.2018.07.004

Sileshi G.W., Mafongoya P., Akinnifesi F.K., Phiri E. (2014). Agroforestry: Fertilizer Trees. In book: Encyclopedia of Agriculture and Food Systems (222-234). https://doi.org/10.1016/B978-0-444-52512-3.00022-X

UTIA (April 4, 2023). “UTIA Analyzes Strategic Cropping System to Promote Climate Change Resiliency.” https://utianews.tennessee.edu/utia-analyzes-strategic-cropping-system-to-promote-climate-change-resiliency/

Velandia M., Trejo-Pech C., Butler D., Chen L., Wszelaki A., DeLong K.L, Schexnayder S., Hasan H. (2025). HortTechnology, 35(2): 166-176. https://www.doi.org/10.21273/HORTTECH05582-24