Fertilizer Mismanagement Impacts on Pasture Health

Project Overview

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


  • Agronomic: grass (misc. perennial)


  • Animal Production: pasture fertility
  • Crop Production: fertilizers, nutrient management

    Proposal abstract:

    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 from proposal:

    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.

    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.