Date of Award:

5-2024

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Plants, Soils, and Climate

Committee Chair(s)

Matt Yost

Committee

Matt Yost

Committee

J. Earl Creech

Committee

Grant Cardon

Abstract

There are many sources that farmers utilize to determine fertilizer needs for crops such as private and public labs, crop advisors, and fertilizer dealers. In many cases, these sources provide recommendations for a specific crop that can vary greatly, which can lead to large differences in cost. An experiment was established in 2021 with 12 sites across the state of Utah in alfalfa, small grains, and corn to test and compare fertilizer recommendations from five labs. The recommendations tested were from two public labs (Utah State University and the University of Idaho) and three commercial labs located in the Western United States. A composite soil sample was sent to multiple labs for analysis and the corresponding macronutrient and micronutrient rates recommended by each lab were applied at each site. Yield and forage quality data were collected from sites from 2021-2023 to evaluate treatment impacts. Fertilizer treatments had little to no impact at silage corn or alfalfa sites, but differences in yield and forage quality were observed at small grain forage sites.

High variability in reported soil test results for the same composite soil samples was observed from three commercial soil testing labs. Differences in soil test results are sometimes due to the accuracy of each lab's analyses, but they are also influenced by different chemical procedures being used to determine nutrient levels. Fertilizer recommendations from the five laboratories varied greatly, both for types of nutrients and rates being recommended. This is likely due to a combination of differences in soil test values (minor influence) and the fertilizer recommendation philosophies (major influence) utilized by each lab. When the recommendations were applied in field trials, higher application rates often resulted in increases in soil nutrient concentrations, but the ratio of the application rate to changes in nutrient levels varied greatly among sites and treatments. Applying higher rates to increase soil nutrient levels doesn't work for all nutrients and situations and is often not economical. The results of this study demonstrate that growers should use caution when selecting fertilizer recommendations and that there is opportunity for greater public-private coordination of fertilizer recommendations.

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