Investigating Impacts of Increased Fertilizer Use in Africa

By Kate Tully

The rains came late this year in Kenya. I was there for several months in the winter and spring to conduct research for a post-doctoral fellowship with the Tropical Agriculture and Rural Environment Program. Specifically, this research examines the consequences of increases in fertilizer use on soil fertility, maize yields, nitrogen gas emissions and nitrogen leaching losses. The research relied on a range of instruments and techniques, including a weather station, lysimeters, and handheld nitrate meters. Altogether, these data will yield rich information on an under-researched set of circumstances—the effects of increased fertilizer use in a degraded environment, such as is found in Nyanza Province, Kenya.

All of the preparations had been made—together with a small team, I had built a weather station, installed the instruments and prepared the fields for planting. We were ready in time—but the weather wasn’t. We found ourselves, like the rest of the farmers in the region, holding our breath and staring up at a clear blue sky, stating hakuna mvua (no rains). At first we had kale, chard, and local vegetables to eat, but as the drought wore on, even the upscale grocery stores had just a few yellow bunches of kale. But, finally, when the rains hit in early April, they came with such a force that Nyanza Province received a quarter of its mean annual rainfall in just three weeks. The maize germinated in less than three days. The photo above shows what the fields looked like a month after planting.

The Alliance for a Green Revolution in Africa has set a goal of increasing fertilizer use five-fold in sub-Saharan Africa, which will inevitably have environmental consequences. Increased mineral fertilizer use has led to higher yields, which improves food security and soil fertility. However, environmental consequences of nutrient pollution from agriculture may include the degradation of downstream water quality, the eutrophication of water bodies and elevated concentrations of nitrous oxide, a powerful greenhouse gas. Though most areas where such contamination has occurred have been at rates of fertilizer application almost twice that the African Green Revolution is hoping to achieve, we know little of the consequences from Africa.

Throughout the long rains, we tracked nitrogen in soils, soil water (water that passes through the soil column), and gas emissions to examine the consequences of increased fertilizer use in this environment. To do so, I installed lysimeters—a device that measures evapotranspiration—to collect soil water from different depths and then used a combination of methods for analyzing nitrate in these samples. Combined with data collected by the weather station, I calculated the amount of nitrogen that is lost through the downward movement of water in the soil from each fertilizer treatment.

The fields seen above were fertilized with different levels of nitrogen. The field on the left received zero fertilizer and the one on the right received 200 kg N/ha/yr. You can see a difference not only in height but also in color. After the harvest (end of August), we will be able to compile complete nitrogen balances for these fields and describe where nitrogen is stored and how it is lost. Thus, we can evaluate the costs and benefits of increased fertilizer use in sub-Saharan Africa.

We have recently received a Cross-Cutting Initiatives Grant to expand this work in Mbola, Tanzania. That research will duplicate what we are doing in Kenya, but will expand our research by also looking at the use of organic fertilizer. We will integrate field data into hydrology, ecosystem, crop and economic models to evaluate the balance of effects of agricultural intensification on food security, poverty rates and the environment in Tanzania.

Kate Tully is an Earth Institute Fellow at Columbia University. Her research examines the effects of increases in mineral fertilizer use on nitrate leaching and soil fertility in subsistence farming systems. For more information on the TropAg Program and Kate Tully’s research, please see tropag.ei.columbia.edu.