Peer-reviewed publication · Soil & plants · In the living system
A humic product raised maize yield only on eroded upland soil in drier seasons.
Across whole fields this humic product did not raise maize yield in any of four seasons; gains appeared only on eroded upland soil, mostly in the driest year.
Olk et al., 12 May 2021. Frontiers in Plant Science 12:672078.
Scope, material and route
- Studied
- Maize
- Design
- Field-long, eight-row strip plots in four replications, each treatment randomised by strip within a replication; yields read along soil-type transects within the same strips
- Setting
- Two rainfed production fields managed by one farmer in Story County, Iowa: Ames in 2012, 2014 and 2016, and Kelley in 2013
- Material
- Micronized leonardite in an acidic aqueous suspension of about 28 percent solids
- Route
- Broadcast spray
- Exposure
- 2.5 L/ha at the V4 stage, or 2 L/ha before emergence plus 1 L/ha at V4; 2.3 and 4.7 L/ha at V4 in 2016. Study exposure, not application guidance.
Study design
Whole field
Two fields on one farm: Ames in 2012, 2014 and 2016; Kelley in 2013.
4 site-years
- Grain yieldNot significant in any of the four site‑years
Upland soil
- 2012, the driest seasonPlus 11 percent (P = 0.10) and plus 19 percent (P = 0.01) over an 8.41 t/ha control
- 2013 and 2014, split applicationPlus 7 percent (P = 0.08) and plus 8 percent (P = 0.05)
Lowland soil
- 2013, a wet seasonCombine yield down 6 percent (P = 0.18)
More study details
- Research question
- Does the product change maize yield, and does the response depend on soil and weather?
- Comparator
- Unamended control strips
- Statistics
- Mixed models, with significance set by the authors at P < 0.10
- Outcomes measured
- Combine yield, yield components, leaf area, grain composition, plant and grain nutrient concentrations, and soil properties
Findings
Researchers applied a micronized leonardite product to rainfed maize in replicated, field-long strips on two Iowa farms across four site-years, then read yields by soil type along each strip. Across whole fields, no year showed a significant yield effect. On eroded upland soil in the driest season, yields rose 11 and 19 percent over an 8.41 tonne per hectare control, and smaller upland gains of 7 and 8 percent appeared in two other years. A seasonally wet lowland showed a small, non-significant fall in combine yield. Nutrient concentrations did not respond consistently. The authors note that a field design cannot identify the mechanism.
| Outcome | Comparison or result | Interpretation |
|---|---|---|
| Field-wide grain yield | Not significant in any of the four site-years | No general yield effect |
| Upland soil, 2012 (driest season) | Plus 11 percent (P = 0.10) and plus 19 percent (P = 0.01) over an 8.41 t/ha control | Gains confined to the stressed upland soil |
| Upland soil, 2013 and 2014 | Plus 7 percent (P = 0.08) and plus 8 percent (P = 0.05) for the split application | Smaller gains in the same landscape position |
| Lowland soil, 2013 (wet season) | Combine yield down 6 percent (P = 0.18) | Not significant; the direction was unfavourable |
| Nutrient concentrations | No consistent response | The authors read this as evidence against a nutrient-uptake mechanism |
Interpretation and limits
What this record supports
For this product at these rates, a conditional yield response on eroded upland soil in drier seasons, and no general field-wide effect.
What remains uncertain
- Weather. The relationship with dry seasons is the authors' interpretation; a treatment by year interaction was not tested.
- Threshold. Significance was set at P < 0.10 rather than the more common 0.05.
- Transfer. The results apply to this product, these rates and high-yielding central Iowa fields.
Which part of the framework it informs
Environmental conditions
The physical, chemical and management context.
This record informs this area.Microbial activity
What microbial communities do within that context.
Observable function
What can be observed or measured in the host or wider system.
This record informs this area.
Source, funding and review
- Source type
- Peer-reviewed publication
- Associated products
- This record does not assess an associated product.
- Funding and relationships
- A Trust Fund Cooperative Agreement between the American Colloid Company and the USDA Agricultural Research Service, with USDA-ARS base funds. Two authors were employed by Minerals Technologies, Inc. The paper states that the company agreed to publication and was not involved in the study design.
- Review status
- Prepared as an editorial summary. External scientific review has not been completed.
- What was read
- Full text read on the publisher's page.
- Original title
- Maize Growth and Grain Yield Responses to a Micronized Humic Product Across Soil Types and Annual Weather Patterns in Central Iowa, United States
- Authors
- Olk DC, Dinnes DL, Scoresby JR, Darlington JW, Hurburgh CR, Rippke GR
- Publication
- Frontiers in Plant Science 12:672078, 12 May 2021
- DOI
- 10.3389/fpls.2021.672078
- Summary prepared by
- Carbon Biome Project
- Date added
- 19 October 2026
- Cite this summary
- Carbon Biome Project. A humic product raised maize yield only on eroded upland soil in drier seasons. Evidence library record, added 19 October 2026. carbonbiomeproject.com/evidence/olk-2021-micronized-humic-product-maize-iowa/
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