Tanzania field trial finds soil testing and subsidies can increase fertilizer use and maize yields
Tanzania field trials
Photo by Aurélie Harou

URBANA, Ill. – The right mineral fertilizers applied appropriately can alleviate nutrient deficiencies in soils and increase crop yields, but most small-scale farmers in Sub-Saharan Africa do not have their soils tested to reveal these deficiencies.

A Tanzania field study shows targeted fertilizer recommendations from low-cost, on-site soil tests paired with subsidies to purchase the recommended fertilizer can increase maize yields on small-scale farms. The soil tests also showed that a change in basic fertilizer recommendations from the government of Tanzania could address an important soil deficiency in Morogoro, where the study took place.

“The result stands out among similar studies because of the magnitude of the yield change – nearly 30% - and its statistical significance,” says Hope Michelson, associate professor of agricultural and consumer economics at the University of Illinois and a principal investigator on the research.

Michelson worked with a multidisciplinary team of soil scientists, agronomists, and agricultural economists from U of I, Sokoine University in Tanzania, University of Florida, and McGill University. The team conducted a randomized controlled trial (RCT) in Tanzania over three years to test the impacts of plot-level soil tests and targeted recommendations. The research was supported by USAID through the Feed the Future Innovation Lab for Markets, Risk & Resilience at UC Davis.

Morogoro is an area of good agricultural potential but low maize yields. About 40% of households in the area have incomes below the poverty line. The 1,007 farmer plots sampled in the study were all rainfed with no irrigation. When the study began, less than one percent of farmers in the sample used any chemical fertilizers on their maize crops.

For the trials, farmers from 50 villages were randomly assigned to one of four groups. One group received fertilizer recommendations as well as a voucher to purchase fertilizer and other agricultural inputs. One group received only the voucher, one group received only the recommendations, and the final group served as a control group that received no intervention.

The initial idea behind the study was to test whether plot-level soil information on its own could be transformative for farmers. However, only farmers who received both the targeted fertilizer recommendations and the subsidy increased their use of chemical fertilizers. They were also the only group of farmers whose yields increased, the researchers say.

The recommendations together with a subsidy increased farmers' profits by an average of 37,000 Tanzanian shillings (about $16) on a one-acre farm, even if farmers pay the full costs of soil tests. The increase is equivalent to about seven days of work at the median daily wage.

“We see behavioral change in terms of fertilizer purchasing and application increases in response to providing vouchers and information, and that behavioral change results in higher yields,” Michelson says.

The soil testing showed nearly every plot included in the study was deficient in sulfur, which proved critical for achieving significant maize yield increases. However, sulfur is not included in current regional or national fertilizer recommendations from the government of Tanzania.

“The correct fertilizers are not even available to people in many of the rural areas,” explains Cheryl Palm, a research professor in agricultural and biological engineering at the University of Florida and lead principal investigator on the study. “The fertilizers they do have are in general not the ones that are needed to get the best crop response.”

Palm says an important context for efforts to increase small-scale farmers’ use of chemical fertilizers is that it isn’t going to destroy the environment or poison the soil.

“The amounts of fertilizer being used by farmers in our study are quite small,” Palm says, “so the chances of leaching and other types of environmental contamination are also very small. At their current application rates, greenhouse gases and leaching do not increase at all.”

According to World Bank data, farmers in Tanzania applied an average of 15.9 kg/ha of chemical fertilizers in 2018, compared to 128.8 kg/ha applied by farmers in the United States. At the same time, that number in Tanzania is a substantial increase from below 1 kg/ha in 2001.

“These are not intractable problems,” Palm says. “There was just a long time without updated research and agricultural extension that are needed to increase agricultural productivity. A lot of effort is being put into it now.”

The Department of Agricultural and Consumer Economics is in the College of Agricultural, Consumer and Environmental Sciences, University of Illinois.

The paper, “The joint effects of information and financing constraints on technology adoption: Evidence from a field experiment in rural Tanzania,” the Journal of Development Economics. [https://doi.org/10.1016/j.jdeveco.2021.102707]. Authors include Aurélie P. Harou, Malgosia Madajewicz, Hope Michelson, Cheryl A. Palm, Nyambilila Amurie, Christopher Magomba, Johnson M. Semoka, Kevin Tschirhart and Ray Weil.