Digging Deeper: Evaluating the Impact of Customized Fertilizer Recommendations on Smallholder Farmer Investment, Yields, and Learning in Tanzania

Last registered on November 21, 2025

Pre-Trial

General Information

Title

RCT ID

AEARCTR-0017252

Initial registration date

November 17, 2025

Initial registration date is when the trial was registered.

It corresponds to when the registration was submitted to the Registry to be reviewed for publication.

First published

November 21, 2025, 8:02 AM EST

First published corresponds to when the trial was first made public on the Registry after being reviewed.

Locations

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Primary Investigator

Name

Hope Michelson

Affiliation

UIUC

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Other Primary Investigator(s)

PI Name

Sarah Janzen

PI Affiliation

UIUC

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PI Name

Christopher Magomba

PI Affiliation

Sokoine University

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PI Name

Annemie Maertens

PI Affiliation

Sussex University

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PI Name

Anna Fairbairn

PI Affiliation

UIUC

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Additional Trial Information

Status

In development

Start date

2026-01-05

End date

2029-01-01

Keywords

Agriculture, Behavior

Additional Keywords

Information, Extension, Soils

JEL code(s)

Secondary IDs

Prior work

This trial does not extend or rely on any prior RCTs.

Abstract

Soil nutrient limitations vary widely across plots and are often unobservable to farmers, yet they are crucial for the effective use of fertilizers, hybrid seed, and irrigation. This study evaluates whether rapid, affordable soil testing can improve farmer decisions and outcomes through better information. In collaboration with OCP Tanzania, we will implement a cluster randomized controlled trial across 270 villages during the 2025–2026 agricultural season to assess the effects of providing soil test results at the plot and village level. In all treatment villages, both randomly selected and lead farmers will have their plots tested. While tested farmers receive their own results, untested farmers will receive either the national fertilizer recommendations or aggregated village-level results. This design allows us to estimate the direct effects of receiving a soil test, as well as spillover effects on untested farmers. We further analyze heterogeneity to inform targeting strategies and explore whether converting individual-level soil information into village-level recommendations generates broader public value. Geo-referenced soil test data will be used to characterize local variability and improve future recommendation systems.

External Link(s)

Registration Citation

Citation

Fairbairn, Anna et al. 2025. "Digging Deeper: Evaluating the Impact of Customized Fertilizer Recommendations on Smallholder Farmer Investment, Yields, and Learning in Tanzania." AEA RCT Registry. November 21. https://doi.org/10.1257/rct.17252-1.0

Sponsors & Partners

Experimental Details

Interventions

Intervention(s)

Intervention Start Date

2026-03-02

Intervention End Date

2026-10-30

Primary Outcomes

Primary Outcomes (end points)

farmer beliefs, crop yields, farmer investment, farmer learning, spillovers, profitability

Primary Outcomes (explanation)

Secondary Outcomes

Secondary Outcomes (end points)

Secondary Outcomes (explanation)

Experimental Design

The proposed study is a clustered randomized control trial and takes place in 270 villages in the Njombe, Singida, and Songwe regions of Tanzania. Of these, 180 villages will be randomly assigned to treatment, and 90 villages will be randomly assigned to a pure control. In each treatment village, 15 lead farmers (as determined by OCP in consultation with village leaders) and 10 randomly selected farmers from the general population will receive a free soil test. For plots receiving these tests, the test results will be shared directly with individual farmers, including plot-specific input recommendations based on these soil test results.

Experimental Design Details

Not available

Randomization Method

randomization done in office by a computer

Randomization Unit

Treatment status: village; lead and non-lead survey respondents: individual

Was the treatment clustered?

Yes

Experiment Characteristics

Sample size: planned number of clusters

270 total villages

Sample size: planned number of observations

In each village, we will survey a stratified random sample of 12 farmers. These samples will be stratified by lead farmers (4), and non-lead farmers (8). In treatment villages, all lead farmers in our sample will receive a free soil test. Among the non-lead farmers in the same villages, half will be assigned to receive soil tests, while the other half (“spillover farmers”) will not receive soil tests for any of their plots. Our total sample size is 3,126 smallholder farming households.

Sample size (or number of clusters) by treatment arms

180 villages will be randomly assigned to one of the two treatment arms (individual recommendations only and individual plus village-level aggregation recommendations), and 90 villages will be randomly assigned to a pure control

Minimum detectable effect size for main outcomes (accounting for sample design and clustering)

Supporting Documents and Materials

IRB

Institutional Review Boards (IRBs)

IRB Name

University of Illinois Office for Protection of Research Subjects Institutional Review Board

IRB Approval Date

2025-08-28

IRB Approval Number

IRB25-0709

IRB Name

Sokoine University Ethical Clearance

IRB Approval Date

2025-09-01

IRB Approval Number

N/A

Analysis Plan