Laboratory Research Team

Meet the Team

Current Researchers

Soilman (Donald G. McGahan, Ph.D.) is working with scholarly investigators in more than one location. He is an Assistant Professor at Tarleton State University (TSU) in Texas and his office and research facilities at TSU are primarily on the Stephenville, TX campus. He also is a a researcher at Texas A&M AgriLife Research and has office and laboratory facilities at the Stephenville Texas A&M AgriLife Research and Extension Center. He has a faculty page at TSU. He holds Texas A&M Systemwide graduate faculty status and his faculty listing at the Texas A&M College Station Department of Soil & Crops Science is here. At the Texas A&M AgriLife Research Stephenville Center he is listed on the Faculty and Staff page. Some of his personal academic information is at

SarahShawver2014Sarah E. Shawver

Masters of Science Candidate
Thesis Submitted to Committee March 2017

Sarah holds a BS in Wildlife Ecology, a BS in Soil Science, and a Minor in Biology from University of Wisconsin Stevens Point.

Thesis Title:
Variation of Soil Microbial Communities Within a Consociation Soil Map Unit and a Vegetative Transect Through an Oak Mot

MS Thesis Abstract:
Soil microorganisms drive biogeochemical cycling. Soil microbial communities differ based on pH, texture, organic carbon, and nutrients. Soil microbial diversity and abundance decrease with depth. Ashe juniper (Juniperus ashei) is encroaching on live oak (Quercus virginiana) savannas, which may alter soil microbial communities and the ability of soil to cycle nutrients. The objectives were to determine whether soil microbial communities differ across a soil map unit and with depth, and determine whether soil microbial communities differ under the canopies of Ashe juniper and live oak.
We sampled 10 sites in a field mapped as Windthorst consociation. At each site, we dug a pit to parent material and sampled by genetic horizon, and four additional satellite surface samples were collected. In another field, we identified three transects of live oak, Ashe juniper, and grasses. At each transect, we collected four A horizon samples in four areas: under the oak canopy, under the overlapping canopies of oak and juniper, under the juniper canopy, and in the grasses.

We characterized soil pH, texture, CCE, nitrogen, phosphorous, and carbon. Microbial communities were analyzed using massively parallel DNA sequencing, and separated into OTUs based on 97% sequence similarity. We analyzed results through QIIME using Kruskal-Wallis ANOVA, Mann-Whitney U, and Spearman’s rank correlations.

Results showed homogenous soil properties across the Windthorst field. There were no differences in microbial communities within each site. Differences in four phyla and four OTUs were observed among sites. Three phyla and 21 OTUs varied with phosphorus. This suggests soil microbial communities have little variation over short distances, but have greater changes over greater distances. Prokaryotic communities likely change in response to changes in soil nutrients. Participation of many rare microbes as important nutrient cyclers will potentially be lost if soil samples are composited over a large area.

Results across the vegetative transect suggest prokaryotic community diversity differed with vegetation, while fungal community diversity did not. Trends in phyla and genera were mostly associated with vegetation, pH, and CCE, rather than nitrogen and texture. Many prokaryotic genera associated with soil properties were important nutrient cyclers. Many fungal genera associated with pH and vegetation were mycorrhizal fungi, suggesting a plant-specific relationship.

JettonFontenotJetton Fontenot

Masters of Science Candidate

MS Thesis Proposal Title:
Determining the Effects of Terracing on Soil Health Using Microbial Bioindicators

Jetton holds a BS in Widlife Management from Tarleton

Previous Researchers

AmberHardyAmber Hardy

MS  Agricultural and Natural Resource Sciences, May 2016,  Tarleton State University

Amber holds a BS in Environmental Science from University of Southern Maine

MS Thesis Title:
Phosphorus distribution with depth across three land managements receiving surface-applied dairy effluent

MS Thesis Abstract:
Livestock waste containing phosphorus (P) is added to agricultural soils as a disposal method or as fertilizer, which is a concern due to negative water quality impacts (surface water eutrophication) from P-rich agricultural runoff. Different tillage methods may impact incorporation of P applied to the soil surface. Investigating how applied P behaves in the soil helps land managers maximize P use efficiency, minimize loss, and conserve P resources. Six pits were excavated in different management sections (no-till, conventional till, and bermudagrass) of a dairy effluent application field. Each excavation’s soil morphology was described and samples were collected in depth increments using genetic soil horizons as a guide. Soil P was extracted using the Mehlich-III extraction method to determine plant-available P. Results were quantified using ICP emission spectrometry and the ascorbic acid method to evaluate the efficacy of the less demanding ascorbic acid method as an alternative to ICP. Organic matter chelated P is not measured using the ascorbic acid method. Additionally, soil organic matter was determined using two methods to evaluate the accuracy of the loss on ignition method in Paleustalfs. Another objective was to determine if the loss on ignition method of determining organic matter, coupled with the ascorbic acid method of extracted P, adequately predict ICP determined P for a first cut and predictive analysis. Results suggest that tillage management impacts the movement of P into the soil profile. Soil organic matter does affect the accuracy of the ascorbic acid method due to clay mineralogy in the argillic horizon inflating loss on ignition organic matter content.

GrahamLItkeGraham R. Litke

MS Agricultural and Natural Resource Sciences, May 2015, Tarleton State University

Graham holds a BS in Computer Information System from Tarleton State University

MS Thesis Title:
Perceptions of Women in Agronomy Careers

MS Thesis Abstract:
There are an insufficient number of university students, specifically women, graduating with an education in agronomy to fill work force demands.  This need, driven partially by population growth, is increasing due to growing rates of industrialization and consequential environmental issues.  Agronomy pays special attention to the supply and demand of resources from the environment.  Though there is an apparent regression in students choosing an education in agronomy, there is a need for their skill set.  This study hypothesized that urban and rural women have different perceptions that influence them towards agronomy careers.  To quantify these perceptions, a survey was issued to women at the 2014 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America Joint Annual Meetings in Long Beach, CA, USA.  Rural and urban women had significantly different (p < 0.05) perceptions about their birthplace environment’s influence on career choice, proving the hypothesis.  Rural women were more influenced by this setting than their urban counterparts, which could prove to be a major issue if urban encroachment progresses. This study defines the rural urban birthplace population break at 25,000 for women in agronomy careers. This population break knowledge should be helpful for revisions of marketing, recruitment, and retainment programs. Other trends presented are helpful because together they disclose potential future investigations into agronomy women’s perceptions, their decision-making processes, and what influences their career choice.

Where is Graham Litke Now?

Working the family ranch and a technician at Skinnys Repair Shop.

JonathanRuizJonathan Ruiz

MS Agriculture 2012 Tarleton State University

BS Horticulture Tarleton State University

Thesis Title:
Assessing Micronutrient Availability In A North Central Texas Vineyard

MS Thesis Abstract:
Micronutrients are found in small concentrations in plants yet have an essential role in plant growth, development, economic yields, and pathogen resistance. Micronutrient availability from soil can be below amounts sufficient for optimal growth. Sixty locations were sampled by genetic horizons across a 5 acre vineyard in North Central Texas from soils mapped as Windthorst (Fine, mixed, active thermic Udic Paleustalf) in support of other research into micronutrient impact on grapevine health. Plant available (labile) Cu, Zn, Mn, and Fe contents determined in DTPA (diethylenetriaminepentaacetic acid) soil extractions by inductively coupled plasma flame spectroscopy to determine if: the availability varies across the landscape. If these micronutrients varied across this vineyard we hypothesized that the variability would be correlated with landscape position. All surface soils were enriched compared to subsurface soil (P < 0.05). The micronutrients Zn and Mn were positively correlated with organic matter in surface soils, but Cu and Fe were not. In support of our hypothesis only Mn had a relationship to landscape where the summit and backslope landscape positions were significantly different from toeslopes (p < 0.05), but not between the summit and backslope. The plant available Cu, Zn, and Fe were not significantly different across the landscape positions and did not support the hypothesis.

Where is Jonathan Ruiz Now?

As of June 2014 Jonathan is a Firefighter Emergency Medical Technician at Rendon Fire Department (Dallas/Fort Worth metroplex) Texas, USA.

AlanDeubler(Larry) Alan Deubler

MS Agriculture 2010 Tarleton State University

BS Agronomy and Range Management Tarleton State University

Thesis Title:
Efficacy of Soil Extracting Solutions for Assessing Potential Phosphorus Loss from a Dairy Waste Application Field Soil in North Central Texas

MS Thesis Abstract:
Elevated phosphorus (P) from manure application field (MAF) soils transported to surface waters is a concern throughout the world due to its role in eutrophication.  Our goal was to determine why the common labile extractants Olsen, DTPA, and Mehlich-III produce different (P) values given the same soil sample. Ten sites within a dairy MAF were sampled by genetic horizons from fine, mixed, Thermic, Udic Paleustalf (Windthorst) map units.  In addition to determining plant labile P, soluble and total P were determined.   All P pools decreased with depth, but concentrations among the labile P were variable.  Mehlich-III extracted more plant labile P at neutral and above to slightly alkaline pH.  Mehlich-III also dissolved more carbonates.  Therefore, assessment of the fraction of labile P that Mehlich-III extracts in excess of Olsen and DTPA may warrant further characterization of the carbonate associated P to assess its role in plant nutrition and environmental quality regulation.

 Where is Alan Deubler Now?

As of June 2017, Alan is a Soil Scientist at the USDA-NRCS at Stephenville Texas (USA) Service Center.