Date of Award:

5-2026

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Plants, Soils, and Climate

Committee Chair(s)

Matt Yost

Committee

Matt Yost

Committee

J. Earl Creech

Committee

Grant Cardon

Committee

Jeanette Norton

Committee

Ryan Larsen

Abstract

Fertilizers improve crop yield and quality by supplying crops with essential nutrients, but can also be a significant input cost. Determining what nutrients and rates to apply for optimal crop growth can be difficult, leading many growers to use external sources for these decisions. Land-grant universities are a common source for fertilizer recommendations, but often based on decades-old research and should be evaluated periodically to verify effectiveness and calibrated to current soil analysis methods. Current Utah State University (USU) guidelines for sulfur (S) and zinc (Zn) were tested across Utah in on-farm trials in 2021-2025. Zinc applications of 5 or 10 lb Zn ac-1 did not impact crop productivity at 9 sites with Zn soil test values ranging from 0.6 to 3.0 ppm in one year. Sulfur fertilizer rates of 7.5, 15, and 30 lb S ac-1 applied annually and evaluated for 1-4 years had no impact on crop yield or forage quality at 9 sites with initial S soil test values ranging from 3.7 to 10 ppm. Soil samples collected by plot each year indicated that S application increased soil S levels at 2 of 7 sites sampled, but changes were not consistent with rates or across sites. Utah has a broad range of soil challenges, with some soils having a combination of restricting factors that limit crop productivity. These “combination” soils can be difficult and there is currently little guidance for their management. Several common soil amendments (elemental sulfur, gypsum, composted steer manure, and humate) were applied on restricted soils on eight farms across Utah in 2022-2024 and evaluated for 1-3 years using changes in soil characteristics, crop impacts, and application costs. Soil amendments had no impact on crop yield or quality, but occasionally impacted soil characteristics. The results of these studies demonstrate S fertilizer guidelines should be updated and that other factors besides soil S should be considered, Zn critical values should potentially be adjusted, and fertilizer applications are unnecessary unless deficiencies are confirmed using tissue tests, and more research on soil amendment sources and rates on challenging soils is needed to guide their use.

Download

Share

COinS

Copyright for this work is retained by the student. If you have any questions regarding the inclusion of this work in the Digital Commons, please email us at .