Fertilizer Mismanagement Impacts on Pasture Health

Progress report for OS20-132

Project Type: On-Farm Research
Funds awarded in 2020: $19,828.00
Projected End Date: 03/31/2022
Grant Recipient: University of Florida
Region: Southern
State: Florida
Principal Investigator:
Cheryl Mackowiak
University of Florida
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Project Information


Developing a Site-specific Nutrient Management (SSNM) technique for assessing pastures will be a powerful tool for preventing pasture decline due to poor forage fertilization. This project covers SARE focus areas 5, 6, and 7. In 2013, a UF-IFAS taskforce investigated pasture decline problems in Florida. A 2015 internal report by Dr. Lynn Sollenberger (forage agronomist) summarized that of the thirteen tested locations (all in south-central Florida), none of the pasture problems appeared related to poor grazing management. However, low soil fertility (P or K) was identified in about 70% of samples but low P (based upon IFAS recommendations) was not identified in any of the forage samples. Some observed instances of leaf fungal disease were noted, but it was reported not to be a concern. Although the sample sample size was limited, evidence suggested that plant stress due to low potassium fertility was often the cause to poor pasture health where they sampled. However, sampling was greatly limited and secondary causes, such as weather and root diseases were not well documented.

In the SSNM method, a single nutrient factor is omitted from a treatment area otherwise receiving complete fertilization. Different single plant essential nutrients can be tested at once at the plot-scale. In comparison of dose-response method, the SSNM method can more quickly determine which nutrients are most likely affecting yield and stand health within a producer’s field, under their unique conditions. We have adapted the SSNM method for on-farm pasture testing by increasing the number or treatments “omission plots” and adding replication. The SSNM technique was originally developed to test and demonstrate fertilization effects on rice and it can be a valuable tool for many on-farm commodities (Dobermann and Cassman, 2002). Applying this technique at ranches across the state (south Florida to the Panhandle) helps us to determine leading nutrient concerns using a greater representation of soil types and environmental conditions under a restricted budget. As nutrient depletion develops, the plant may also display symptoms from other stressors, such as low soil moisture and fungal diseases.

An on-farm pilot test was established in a bahiagrass pasture in 2016. By 2018, low K plots at this location had declining yields, stand density, and increased disease occurrences (Bipolaris and Take-All root rot). In comparison, omitted P plots maintained bahiagrass productivity while also negatively impacting the predominant weed species. This is an especially exciting finding, as bahiagrass was reported to have a critical tissue P value below 0.15% (Silveira et al. 2011), which is lower than for many other plant species. Fertilization management also impacts root/rhizome biomass, which is a leading method for building soil organic matter. In the pilot study, K depleted plots were losing root biomass compared to plots depleted in P or from plots receiving complete fertilization through either class AA biosolids or by mineral fertilization.

Project Objectives:

The proposed treatment plots were established in pastures at three working ranches across Florida in 2019 and another in 2016. The modified SSNM method will be used, relying on replicated “omission plots” to test and demonstrate temporal responses of bahiagrass pasture stands to individually depleted nutrients (N, P, K). We included a Class AA biosolids treatment to represent an organic, slow-release macronutrient source and viable micronutrient source, which is often overlooked in pasture and hay production operations in the southern US. Additionally, we will provide limited analytic support within the tri-state area (Florida, South Georgia, South Alabama), in terms of soil and tissue fertility and disease diagnostics of bahiagrass fields suspected of pasture decline. We will rely on extension agents to identify suspect fields, which will result in an additional 6 to 12 samples from suspect pastures to help increase what we learn at the four ranch test sites, alone. Participating extension agents and cooperators will help with bahiagrass stand assessments, review management practices, and contribute to outreach publications that will be aimed at southern ranchers. The benefit of using previously established plots is that the nutrients of greatest concern (P and K), typically take more than one growing season to become depleted and affect the forage stand. There are currently no funds supporting this effort, just as P and K deficiencies are beginning to be observed. The three pastures (and 1 hay field) were excluded from livestock via temporary fencing since the time of plot establishment, in order to minimize confounding effects of animals redistributing nutrients across treatment plots during the study.

Treatments are as follows: 1) unfertilized control (no N, P, or K fertilizers), 2) complete fertilizer (N + P + K), 3) minus N (complete fertilizer except for N), 4) minus P (complete fertilizer except for P), 5) minus K (complete fertilizer except for K), and 6) complete fertilizer supplied as Class AA biosolids, based upon N requirement. This treatment supplies macro- and micronutrients, except for K, which will be applied as mineral K. Additionally, it will be used only with the spring application and mineral sources relied upon for the second application (to lessen high soil P accumulation rates). Treatments targeting N, P, and K will be applied at an 80 lbs acre-1 (90 kg ha-1) rate using soluble, mineral fertilizer, when soils test low or medium for nutrients other than N (UF-IFAS does not rely on a soil N analysis for determining N fertilization application recommendations).

Plots will be staged (cut to 7.5 cm stubble height) at spring “green-up”. Forages will be harvested twice per growing season (approximately June and September, depending on location), which is often the frequency used with bahiagrass hay producers and this will allow for better nutrient budget tracking, compared to more frequent harvests but limited tissue analyses. Treatments will be applied in spring and again after the first cutting. Harvested forage will be collected, wet weights recorded, then oven-dried, reweighed, and the dried forage analyzed for plant essential nutrients and crude protein (N x 6.25) by a commercial lab (Waters Agricultural Lab, Camilla GA). Soils will be sampled (composite 10 cores per plot to 15 cm depth using soil probe) at the end of each growing season and analyzed for pH, nutrient fertility (Mehlich-3 extractant), and estimated CEC, by a commercial lab (Waters Agricultural Lab). At the end of the second year, plots will also be compared for soil organic matter (Walkley Black method) by UF Plant and Soil analytic lab (Gainesville, FL). Additionally, roots + rhizomes will be sampled (one 900 cm2 x 15 cm depth sample per plot) at the end of the 2021 growing season, in order to compare treatment root mass differences, which contribute to building soil organic matter. Loose soil will be shaken from root mat and roots kept in a cooler for return to the lab, where they will be separated from soil, rinsed and then dried, in order to record dry mass. Data will be compiled and analyzed, following the final cutting of each season. The results and interpretations will be shared with participating county extension faculty and cooperators, soon thereafter. This will include a written report and personal visit or phone call to verify reports were receive and to address concerns/questions. Additionally, an advisory meeting will be scheduled in the Jan/Feb timeframe (participating agents and cooperators), in order to discuss past season results, trouble-shooting for Year 2, and potential implications for future pasture and hay field nutrient management practices, including state fertilization recommendations.


Click linked name(s) to expand
  • Roy Carter - Producer (Educator)
  • Roy Carter - Producer (Educator)
  • Libbie Schmidt - Producer
  • Paulette Tomlinson
  • Dr. Marcelo Wallau (Researcher)
  • Dr. Marcelo Wallau (Researcher)
  • Joseph Walter
  • John White - Producer


Materials and methods:

There is concern and conflict over what appropriate fertilization recommendations for (Paspalum notatum) bahiagrass pastures should be. Many of the UF-IFAS recommendations were based on decades-old data and using often unspecified varieties. Additionally, pasture decline reports are addressed individually and typically as independent events due to various causes. It is our overall goal to determine the degree fertilization practices and plant nutritional health contribute to pastures that appear to be in decline. The Omission Plot method of demonstrating and testing bahiagrass responses to gradually depleted fertility (focus on N, P and K), is a somewhat novel way to observe and track what happens to a forage stand under slowly increasing fertility stress, under real-world conditions and different soils. Additionally, in order to shorten the time it takes for a nutrient deficiency to occur, we fenced off the on-farm test plots from livestock and repeatedly removed the forage with each sampling, effectively exporting those nutrients off the treatment site.

Since bahiagrass is our dominant perennial grass across all of Florida, we chose working ranch locations across the state that also are in close proximity to other large cattle operations, so those most affected by our research have more opportunities to visit a site near them. Florida being a large state, has some variation in weather and even climate (more subtropical as one moves down the peninsula), which translates into a greater need for locating on-farm sites across the state (Fig. 1). SARE 1-year Fig 1 The Osceola research site was established in a pasture in 2016, while the other trial sites were established in mid-2019. The six treatments are as follows: 1) Complete fertilizer (N + P + K), 2) complete fertilizer supplied as Class AA Biosolids + KCl and applied, based upon N requirement, 3) -P (complete fertilizer except for P), 4) -K (complete fertilizer except for K), 5) -N (complete fertilizer except for N), and 6) unfertilized Check. The biosolids application rate has been based on total N (not available N) through 2020. Over-fertilizing with P is a concern; however, P availability from Class AA biosolids should be less than from mineral fertilizer sources. This study will provide evidence on how available this P source is in Florida soils. Additionally, many ranchers are concerned that the current P recommendations are too low for bahiagrass and these sites will provide evidence if this is the case. Treatments requiring N, P, or K received 80 lbs acre-1 (90 kg ha-1), which ensures no target nutrient limitation and also helps test the Mehlich-3 calibration for soil test P. Soil pH (adjusted with Ag lime) and other macronutrients will be supplemented as needed, based upon IFAS recommendations, while no micronutrients (Fe, Mn, B, Zn, Cu) are routinely reapplied (biosolids contain micronutrients). Tissue testing of all plant essential nutrients (excluding Mo) is being used to track plant nutritional status within the different treatments. If any treatment approaches critical micronutrient deficiency, applications will be considered.

Research results and discussion:

Plots were to be staged (cut to 7.5 cm stubble height) at spring “green-up” but due to COVID-19 travel restrictions, this was greatly delayed (Jun for Orange and Osceola and Jul for Gulf and Columbia). Treatments were applied at this time. Harvested forage (using a push-mower with bag catcher) passed over a 4ft length was used for biomass collections (Sep for Gulf, Oct for Columbia, Orange and Osceola). Wet weights recorded, then oven-dried (60oC), reweighed, and the dried forage stored until processing (milled to pass through 2-mm mesh).

The 2020 COVID-19-related delays in travel and in-person lab activities has delayed material processing and analyses into 2021. Plant tissue will be sent for essential nutrients and crude protein (N x 6.25) analyses, to a commercial lab (Waters Agricultural Lab, Camilla GA). Soils were sampled (composite 10 cores per plot to 15 cm depth using soil probe) at the end of the growing season and are being processed (air-dried and passed through 2-mm sieve) to be analyzed for pH, nutrient fertility (Mehlich-3 extractant), and estimated CEC, by a commercial lab (Waters Agricultural Lab). All results will be compiled and shared with participating extension agents and ranchers in 2021. Additionally, the data will be used to develop written and multi-media materials for use by participating Specialists and Extension agents to send to UF-IFAS sponsored platforms (Facebook, electronic newsletters/blogs, and Youtube) in 2021 and 2022.

The fall harvests demonstrated that a complete mineral fertilizer resulted in similar herbage yields for three of the four locations (Fig. 2).SARE 1-year Fig 2 The Columbia site produced less forage and it is suspected that this is likely due to the inherent lower moisture at this location. Additionally, lower surface soil wettability might play a factor at this location. We will follow-up by testing surface soil infiltration at all locations. It is interesting to note that Columbia and Gulf share the same soil series (Blanton sand), so there may be other factors at play, as well. Plots not receiving phosphorus (P) fertilizer performed as well as the complete fertilizer treatment for at least half of the locations and in fact, there is a trend of improved yields at Osceola, which also has among the lowest soil P fertility values. Forage tissue concentrations and soil fertility data will be shared in the final report. As one would expect, removal of N from the fertilizer depressed production, but this was also the case for potassium (K), where yields were dropped at most locations (Fig. 2). Bipolaris (a fungal, leafspot disease) was sometimes prevalent on leaves in the low K plots, particularly at Osceola. Leafspot disease is often associated with low K fertility in bermudagrass and many cultivated grass crops. Although it infects bahiagrass, it is not as often reported. Extension participants have been sharing these observations with their rancher clientele. The class AA biosolids treatment performed as well as the complete mineral fertilizer at most locations, which suggests that the organic N was mineralizing quickly enough for adequate plant uptake by the second season (2020). The tissue results will help to verify if that was the case. The Check treatment provides the worst-case situation, where no fertility was returned to the plots. Osceola, which is the oldest site (2016) and the driest (Columbia) had Check plots with the least amount of biomass. Bahiagrass is noted for its relatively large rhizomes that likely provide some water and nutrient storage during water and nutrient deficits. It will be interesting to discover if the belowground (roots and rhizomes) biomass mimic the aboveground in terms of yield and health, when it is sampled in fall 2021. Ranchers have expressed concern that although aboveground yields may not be affected by current fertilizer recommendations, depletion of some nutrients might be negatively impacting roots and rhizomes. If that is happening, the plant might more easily succumb to various stresses, such as drought, heat, and cold, thereby contributing to pasture decline.

Participation Summary
4 Farmers participating in research

Educational & Outreach Activities

48 Consultations
4 Curricula, factsheets or educational tools
4 On-farm demonstrations
3 Published press articles, newsletters
3 Webinars / talks / presentations
1 Other educational activities: Created producer survey on pasture decline

Participation Summary:

40 Farmers
6 Ag professionals participated
Education/outreach description:

With the advent of COVID-19, all in-person activities were prohibited in 2020. Three pasture health articles were published (1 as an electronic newsletter and two in the Florida Cattlemen Magazine. Jung-Chen Liu (MS student, Mackowiak advisor) presented a poster at ASA-CSSA-SSSA on bahiagrass subsoil fertility when grown as a monoculture or as  a mixture with rhizoma peanut (Arachis glabrata), under low fertilizer management. In this case, the ability of rhizoma peanut (legume) to fix N2 provided N fertility to the bahiagrass system and increased surface soil reactive N content. Her poster won 2nd place in Division C-06, Forage and Grazinglands. Additionally, two video trainings on bahiagrass fertilization and management were published on YouTube and can be accessed from the UF-IFAS, Panhandle Ag Enews website. Together, these two videos have received over 10,000 views.

In 2021, we expect to publish more video trainings, including diagnosing and addressing nutrient deficiency symptoms and related diseases in pastures. The project team will meet in April to begin developing the 2021 outreach activities, including field day tours and online training events. At the time of this submission, in-person activities were partially constrained but with expectation that more group events will be allowed over time, we hope to hold multiple outdoor activities. However, we also will continue developing online materials and publications, as well.

Learning Outcomes

8 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation

Project Outcomes

5 Farmers changed or adopted a practice
2 New working collaborations
Project outcomes:

The clear responses in bahiagrass performance due to improved fertilizer management has gained much attention by extension agents and producers. This is exemplified at this time with extension outreach efforts to promote adequate potassium fertilization in pastures. The project's extension team has had discussions with at least 40 clientele describing the early results of this on-farm research. If the data trends continue, we can show that by managing fertilization, ranchers can increase yields by over 30% and likely save on fertilizer applications that they often do not require, such as phosphorus. Since bahiagrass covers much of the southern coastal plain, savings in fertilizer costs and pasture stand health may be in the $millions over the next several years.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.