Fertilizer Mismanagement Impacts on Pasture Health

A Site-specific Nutrient Management (SSNM) approach was used to help assess potential relationships between soil fertility imbalances on bahiagrass stand productivity, health, and longevity. With changing and often more hostile growing conditions, maintaining pasture health will likely become more challenging in the southeast U.S. To better understand producer awareness of potential problems, thirty-three Florida cattle producers completed a survey during 2020 that addressed potential pasture issues from their perspectives. In terms of impacts over the previous three years, over a third had often been impacted by excessive rainfall, compared to only 12% who had often been impacted by excessive drought conditions. Again, nearly 25% often had trouble establishing new pastures and over half often had weed pressure challenges. Weed species often proliferate in weak pastures. Approximately 25% recognized challenges with maintaining adequate soil fertility, while a third conveyed that soil fertility never or infrequently impacted their pastures or hay fields. More revealing was that among those who had observed pasture decline in the previous five years, just under half used soil fertility reports and less than 10% relied on tissue analysis as tools to address suspected pasture decline. When asked what training topics they wanted more information on, 42% included soil health and/or fertility and 51% ranked soil sampling as being a highly important tool for maintaining pasture health. Although the polling pool was small, a couple of trends seem apparent: 1) only half of sampled producers consider soil maintenance and management highly important to managing pasture health, and 2) and less than half who had pasture decline considered soil fertility to as a primary contributor to decline.

In 2020, 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).

Expanding our SSNM demonstration from Osceola County to an additional three counties across Florida, we provided additional opportunities for getting the word out that long-term fertility trials were taking place on-farm. Pasture decline is typically, a chronic and slowly progressing situation that can often go unnoticed until something(s) occurs that triggers a system crash. When it comes to poor soil fertility (or nutrient imbalances), expression in forages may not be obvious, particularly when there is no positive control (an area without any soil nutrient limitations) to compare against. These past two years (2020 and 2021) provided initial visual and compositional information of three major essential nutrients, N, P, and K regarding their deletion from normal soil fertility management.

Of the four trial sites reported here, three trials (Orange, Columbia, and Gulf counties) were initiated in 2019, while the fourth (Osceola County) was initiated in 2016. To better compare across locations, the results are presented as years 1, 2, …n, rather than dates, to better compare Osceola County results at the same treatment age as the others, even though growing conditions (weather/climate) can change from year to year. Annual bahiagrass production interacted with year, location, and treatment, as might be expected. However, by the end of Year 3, plots receiving complete fertilizer as either mineral or as class AA biosolids performed similarly and plots that did not receive P fertilizer performed equally well. In comparison, plots that did not receive K fertilizer had at least 20% less production, while plots that had not received N fertilizer had at least 40% less forage. In comparison, plots that were not fertilized (check plots), lost at least 50% production.  Table 1 provides production values by location and year after initiation. Identifying similar temporal responses under different soil types and conditions helps with educating producers on managing their soil fertility and the potential impacts they might encounter. It should be noted that Gulf County, with a somewhat heavier soil type was also associated with greater annual bahiagrass production than the sandier soil sites (all other sites), while the remaining three were similar, even though the Orange County site had relatively high initial soil fertility (Table 2) due to a previous, multi-year receivership of Class B biosolids that ended several years ago. The Osceola site can be used as a model regarding the potential timing of impacts through lack of balanced fertilization longer term. For example, eliminating P fertilizer had no impact on yield over three years at any of the on-farm sites, but even after six years, forage production remained comparable to the complete mineral fertilization management at Osceola County.

Soil fertility is often the preferred method for determining bahiagrass nutritional requirements and fertilizer needs. Initial soil fertility varied among on-farm sites, where P ranged from 19 to 817 mg kg^-1 and K ranged from 13 to 22 mg kg^-1 (Table 3). As with many southeastern U.S. states, Florida does not recommend testing for available soil N. Based upon initial soil fertility, Osceola County had an initially low soil test P and all counties had low initial soil test K. Regarding soil test P, there was a time x location interaction, when after three years, soil P fertility remained statistically unchanged at all but the Osceola site, which declined from 19 to 12 mg P kg^-1. However, there was no treatment impact on soil P fertility within the first three years. For soil K, there was no location by treatment interactions. Across locations, the minus K treatment had similarly low soil K to the Check plots, while minus N and minus P treatments had greater soil K over time (Table 4). It was expected that K fertility would be greater under the minus N treatment, since there was less forage to pull K out of the soil, thereby allowing it to accumulate. The complete, biosolids and minus P treatments had similarly high soil K fertility compared to the depleted treatment but relative to IFAS recommendations soil K fertility remained low (< 35 mg kg^-1) in all but the minus P and minus N treatments. In Florida pastures, it may be economically challenging to meet the IFAS medium soil K rating.

Soils provide the nutrient sources to the plant. However, soil fertility values may not always reflect the plant nutritional status. Since plant tissue nutrient concentrations can shift during the season, mid-summer (during active growth) tissue sampling may better reflect plant nutritional status. For this discussion, only data following the first of the seasonal harvests were further compared over three years of sampling for tissue content. Whole (above-ground) plants were sampled, rather than the most fully expanded leaf tissue to simplify the process and provide data for a nutrient removal budget. Across locations and time, the minus N treatment had among the lowest tissue N values but remained near or slightly above 1% N (~6.25% crude protein). In this case, crude supplementation would be needed to support beef cattle grazing. Bahiagrass has been noted to have developed associative N₂ fixation and this may have helped maintain the plant N status even while production declined. Interestingly, the minus K treatment maintained greater tissue N than any of the other treatments (Table 5). Even so, tissue N values never exceeded 2% (~ 12.5% crude protein).

Across locations and time, the minus P plots had significantly lower tissue P than all other treatments, even the check plots that received no fertilizer (Table 6). The recommended critical low tissue P for bahiagrass was set at 0.15% for Florida. By Year 6, bahiagrass at the Osceola County location would sometimes test below 0.15% but not consistently and yields did not seem negatively impacted, even at this low content. As with N, the minus K plots had among the highest tissue P values and Class AA biosolids had equally high tissue P. Biosolids are a good source of slow-release, plant-available P and these data reflect that. Additionally, under biosolids management, soil P fertility was comparable to other treatments, as mentioned previously. The soil P test did not correlate strongly with tissue P and further calibration between tissue and soil test P may benefit Florida producers.

Tissue K values were excessively low (0.56%) in the minus K plots, even when averaged across locations and years (Table 7). None of the treatments resulted in high tissue K values. Combined with the relatively low soil K fertility, applying K as one-time application per season should be reconsidered. In this study, K fertilization was split-applied (half in spring and half after first harvest). In sandy Florida soils, split K applications should be considered (apply with N fertilizer), whenever soil test results show low soil K.

The budgeting of fertilizer nutrients or amount removed by the production system via the forage biomass, provides additional insight on potential demand and losses to the environment (Table 8). For example, the complete mineral fertilizer, biosolids and minus P treatments had similar amounts of N removed through their biomass. In comparison, the minus K treatment had significantly less N removed from the soil compared to the complete mineral fertilizer of the minus P treatment, due to lower production rates. It was most pronounced at the Columbia County farm, where nearly one third less N was taken up. With high rainfall events noted by many surveyed ranchers, there is an increased risk of nitrate leaching loss from the unused fertilizer. It is interesting to note that N removal from the Orange County site was similar for all fertilized treatments. This is likely due to the high inherent fertility (past Class B biosolids applications) prior to the start of this study. The check treatment (no fertilizer applied) response appeared to be reflective of initial soil properties, where the finer textured soils (Gulf County) accumulated 61 kg ha^-1), compared to Columbia County at 25 kg ha^-1).

As mentioned previously, the three-year comparison of eliminating P from the fertilizer had little to no impact on production but it had impacted the amount of P that was removed from the land. The greatest impact was at Gulf County, with a 36% decline in removal by the bahiagrass. In comparison, there was no loss in P uptake at the Orange County farm since inherent soil P concentrations were high from the start of the study. Based on an average removal rate of 20 kg P ha^-1 (or 46 kg P₂O₅ equivalents) per annum, it may take decades of hay removal before soil P concentrations would become deficient. If continued to be managed as pasture, it might never become deficient. Meanwhile, the sandy Florida soils are prone to nutrient leaching, including P.

Eliminating K fertilizer had a large impact on forage production within a couple of years of deleting it from the fertilization management. Within three years, K removal dropped 60 to almost 80%, depending upon the location. Removing P from the fertilizer did not limit K uptake or removal, while N depleted systems removed only about one half the amount they could have if they received ample N fertilization (Table 8). Low N limited forage production but there was little sign of stand thinning. Soil P deficiency may often take many years to reach under most Florida pastures, due to P from excreta. In comparison, eliminating K fertilization for a short time (one or two years) while continuing to apply moderate to high rates of N fertilizer, impacts can lower forage production directly. However, in the case of Argentine bahiagrass, Take-All root rot and various foliar fungal diseases, such as Bipolaris, can hasten a stand’s demise (Figure 3) (Figure 4). In this case, fertilizer imbalance (fertilizer N:K too high) can lead to a train wreck and it might be one contributor to pasture decline. In situations where fertilizer prices are limiting a farmer’s fertilization options and production can be put on hold, consider prioritizing the most productive fields for a balanced fertilizer application (based upon soil fertility report) and greatly limit or avoid N applications to other fields, particularly if other major soil nutrients are deficient and are not affordable. As demonstrated by the data, bahiagrass that does not receive N fertilizer will tend to maintain adequate nutrient balance over multiple years. Additionally, tissue analysis through the collection of above-ground clippings during the active growing season can provide greater insight into potential soil fertility deficiencies. This tool should be used in combination with a soil report rather than as a substitute.

We will continue with a longer-term assessment of these treatments. At the Osceola farm, the lack of K applications had decimated plots by Year 6. In 2022, those treatment plots were split to determine if returning to normal K fertilization practices will result in stand recovery. Additionally, more time is required to assess when soil P deficiencies may begin to manifest in bahiagrass plots. The impact of chronic, fertilization imbalances and interactions with the environment likely affect other soil properties, including soil carbon stocks. These sites may help us elucidate the potentially larger impacts poor fertilization management can have on the larger, agroecological system.