Abstract
Cover crops have a long and significant history in Florida’s citrus industry. During the late 1800s and early 1900s, they were widely used to enhance soil quality, boost fertility, and manage pests; therefore, they served as a critical agricultural tool before the widespread adoption of synthetic fertilizers. However, during the middle of the 20th century, a decline in the use of cover crops occurred as synthetic fertilizers and chemical pest control methods became more prevalent. Despite this decline, a resurgence of interest in cover crops has occurred among Florida’s citrus growers. This renewed interest is driven by the urgent need to increase soil fertility while reducing inputs, particularly in the context of managing citrus groves affected by citrus greening [huanglongbing (HLB)], which is a devastating disease that threatens the viability of the citrus industry. Citrus greening has created a growing interest in the use of management practices that can help mitigate the increasing cost of inputs needed to manage the disease. This literature review delves into the historical use of cover crops in Florida’s citrus industry and highlights their early adoption and subsequent decline. Additionally, it examines current cover crop management practices and focuses on key components such as seed selection, planting techniques, and termination methods. Finally, this review discusses the challenges and limitations associated with integrating cover crops into modern citrus production systems.
Florida’s citrus industry plays a vital role in the state’s economy and the nation’s agricultural landscape (Tucker 2006). Despite its modest beginnings in the late 1800s, this industry has become one of the world’s largest producers of fresh grapefruit and sweet oranges by the turn of the 20th century (Hallman et al. 2022; Luckstead and Devadoss 2021). The industry now faces numerous challenges, including increased urbanization, low-fertility soils, irrigation water with high salinity, hurricanes, pest pressures, and diseases such as citrus canker and citrus greening [huanglongbing (HLB)] (Rossi et al. 2021; Singerman and Rogers 2020; Vashisth and Kadyampakeni 2020). As a result, during the past 20 years, citrus production in Florida has declined by an estimated 94% (USDA 2024). In response to these challenges and subsequent decline in production, the use of cover crops has gained renewed attention. Historically, cover crops were used in Florida’s citrus industry to improve soil fertility and aid in pest management before the widespread adoption of synthetic fertilizers and pesticides (Hume 1904). Currently, as growers seek solutions to improve soil quality and reduce chemical inputs, cover crops are once again emerging as a popular management practice.
Overview of cover crops
Cover crops can be defined as noncash crops planted in addition to a primary crop with the purpose of improving the agroecosystem (Lamichhane and Alletto 2022; Van Eerd et al. 2023). Cover crops have been used to successfully increase soil organic matter (OM), increase soil nutrient contents, improve insect and microbial biodiversity, reduce soil erosion, improve soil water-holding capacity, and help control plant pests (Quintarelli et al. 2022; Snapp et al. 2005; Treadwell 2006). As cover crops grow, they capture nutrients from the soil and atmosphere [i.e., nitrogen (N) fixation by legumes]; once ended, they decompose, thus releasing the nutrients they captured back into the soil (Fageria et al. 2005).
Before the widespread adoption of synthetic fertilizers in the middle of the 20th century, farmers relied on cover crops to supply citrus trees with adequate nutrients necessary for fruit production. The use of cover crops spans thousands of years and multiple continents. Chinese writings during both the Zhou (12th–3rd century B.C.) and Han dynasty (206–220 B.C.) described the use of noncash crop plants as green manures (Scavo et al. 2022). In the Mediterranean region, the Greek scholars Theophrastus and Xenophon (400 B.C.) described the practice of growing and ending plants, particularly legumes, to reinvigorate the soil (Semple 1928). Similarly, the Roman scholar Marcus Terentius Varro reported the positive benefits of legume use on succeeding crops during the 1st century B.C. (Scavo et al. 2022).
During the next two millennia, the use of cover crops was common in European agriculture and likely found its way to the United Sates with the first colonists in the 1600s. George Washington, arguably the most important figure in American history, reported using clover as a cover crop on his farms (Kerr 1964). The practice was widespread in United States, including Florida, throughout the 18th and 19th centuries.
Cover crop usage has been widely adopted in agronomic systems and is gaining popularity throughout the United States. According to the United States Department of Agriculture (USDA), cover crop usage increased by 50% between 2012 and 2017 (Wallander et al. 2021), and an additional 17% increase was recorded between 2017 and 2022. Thus, cover crops were planted on 4.7% of total cropland in 2022 (USDA 2022). Many of the potential benefits of cover crops, particularly those related to soil fertility, are of interest to Florida’s citrus growers. Research conducted by Haruna and Nkongolo (2020) found that cover crops led to 12% greater soil phosphorus (P) compared with that of noncover cropped plots in a 3-year corn–soybean rotation in the Midwest. Furthermore, a meta-analysis of current cover crop research revealed that cover crops increase soil microbial abundance, activity, and diversity (Kim et al. 2020). In south Florida, Boglaienko et al. (2014) demonstrated that a buckwheat cover crop significantly increased soil OM, total soil N, and soil P after one season.
History of cover crops in Florida citrus production
In 1904, the first major work that described the use and management of cover crops specifically for citrus in Florida was published by Hume (1904). In his magnum opus, “Citrus Fruits and their Culture,” Hume described in detail the management of cover crops for citrus production. IN that work, important management considerations such as cover crop species selection, planting depth and timing, and termination methods and timing are explained. This work not only provided growers with important management information at the time of publishing but also implied that cover crops had been used before 1904 in the citrus industry, indicating its significance as a major component of citrus production in Florida.
From the early 1900s to the 1940s, growing cover crops remained a popular management method, particularly for improving soil fertility and OM levels in citrus groves. Proceedings from the Florida State Horticultural Society (FSHS) indicated that both growers and researchers studied cover crops as early as 1896 (Table 1). The early proceedings (1896–1915) described the successful improvement of soil N and OM by growers and researchers and the mowing/cultivating of a number of different plants, including Bermuda turf (Cynodon dactylon), beggarweed (Desmodium spp.), velvet bean (Mucuna pruriens), and crabgrass (Digitaria spp.) (Chilton 1909; Edwards 1912; Floyd 1914; Montgomery 1896; Pearce 1915; Sample 1915; Wakelin 1911).
Summary of Florida State Horticultural Society (FSHS) proceedings on cover crops for Florida citrus.
During the 1920s, discussions of the management and benefits of cover crops greatly increased (Table 1). In 1922, DeBusk discussed the importance of maintaining OM in citrus soils and identified growing and tilling velvet beans, beggarweed, cowpeas (Vigna unguiculata), and grasses as effective soil building methods (DeBusk 1922). Between 1924 and 1927, several proceedings specifically discussed the effectiveness of crotalaria (Crotalaria spp.) as a legume cover crop by measuring its biomass accumulation and N content (Gunn 1927; O’Byrne 1925; Stokes 1924; Stokes 1925; Wells 1926; Williams 1926). In the FSHS proceedings titled “How Can Citrus Soils be Built Up,” how a combination of manure, leguminous cover crops, and natural cover crops can be used to amend and add OM to even the most depleted soils was described (Wells 1926). Importantly, specific legumes such as crotalaria, velvet bean, cowpea, and beggarweed were identified (Wells 1926). Later, in 1929, soil chemist Barnette discussed how crotalaria grown with nonlegumes could improve soil OM and N, thus marking one of the earliest mentions of using cover crop mixtures instead of monocultures in Florida citrus production (Barnette 1929).
Discussions regarding cover crop usage in citrus production continued into the 1930s, with 15 proceedings related to the topic published between 1930 and 1935 (Table 1). As in the 1920s, many of these proceedings focused on the use of crotalaria, particularly because of its benefits to soil N and tree growth. For example, Hartt (1930) reported improved tree growth and reduced N fertilizer applications after 6 years of planting a crotalaria cover crop. Similarly, Hurlebaus (1930) reported a 50% decrease in summer N application after growing a leguminous cover crop. The general attitude of growers and researchers toward cover crops management was adequately summarized as “grow as much cover crop in the middles as possible and reduce cultivation to minimum” (Hoenshel 1932).
Most importantly, in the 1930s, an extensive field study conducted at the University of Florida Citrus Experiment Station in Lake Alfred, FL, USA, which was reported on several times at the annual meetings of the FSHS, was published in a university of Florida Agricultural Experiment Station bulletin (Stokes et al. 1932). This bulletin provided quantitative data to support what had been previously qualitatively reported by growers. In the study, Crotalaria striata, velvet beans, beggarweed, cowpeas, and natal grass were compared as summer cover crops in a young pineapple orange grove from 1925 to 1931. Part of the grove was rotated with different cover crops, whereas clean culture was used in two plots. The average N returned to the soil comprised 108 lb from crotalaria, 39 lb from velvet beans, 29 lb from beggarweed, 34 lb from cowpeas, and 36 lb from natal grass (on a per-acre basis). Additionally, the average diameter of the tree trunks was highest in the crotalaria plots, second highest in the natal grass plots, and lowest in the clean culture plots. Fruit yields were highest in the rotated plots and crotalaria plots. Overall, Stokes et al. (1932) reported that a higher-yielding cover crop resulted in larger trees and higher fruit yields. The decomposed OM content of the soil at the end of the study was slightly less (2% decrease) than that of the virgin soil at the beginning, but the N content of the soil was maintained by the cover crops. Clean culture, however, significantly reduced the content of both decomposed OM (30% decrease) and N (27% decrease) in the soil.
Changes to the fertilizer industry between the 1920s and 1940s ushered in a new era of citrus production. The outbreak of World War I led to a massive increase in demand for nitrogenous compounds used in explosives, resulting in the industrialization of the Haber-Bosch process, which converts hydrogen (H) and N to ammonia (NH3) (Melillo 2012; Travis 2015). The subsequent expansion of synthetic N facilities in the United States continued throughout the interwar period (1918–1939) (Travis 2017). At the same time, advances in the production process of phosphates (PO43−) for fertilizers enabled the creation of a more concentrated form of this essential plant nutrient (Skinner and Bahrt 1931). After the conclusion of World War II in the middle of the 1940s, synthetic fertilizers became readily available for agricultural use (Melillo 2012). Similarly, synthetic herbicides became quite popular at the beginning of the 1960s (Hannon et al. 1967), and the citrus industry widely adopted synthetic fertilizers and herbicides. As a result, between 1950 and 1990, the use of cover crops and other organic amendments to control weeds and maintain soil quality in citrus groves nearly disappeared. This trend was evidenced by the popularity of cover crop talks at FSHS in the 1920s and 1930s and the subsequent decline after the second World War II (Fig. 1). By the year 2000, only approximately 0.1% of fertilizers came from organic sources (Obreza and Ozores-Hampton 1999).
Number of publications about cover crops, citrus, and Florida found in the proceedings of the Florida State Horticultural Society and peer-reviewed journals from 1888 to 2023.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05476-24
Renewed interest in cover crops
In the early 2000s, a noteworthy shift occurred as renewed interest in the use of cover crops in citrus groves emerged (Fig. 1). These more recent studies not only focused on improving soil characteristics but also examined the impact of cover crops on weeds and insect pests. For example, Rouse et al. (2001) described the use of perennial peanut (Arachis glabrata) as an effective way to improve soil N levels, reduce nutrient leaching, and reduce weed pressure. A report published by Lapointe (2003) examined the impact of three legume species, pigeon pea (Cajanus cajan), pinto peanut (Arachis pintoi), and rattlebox (Crotalaria pallida), on Diaprepes abbreviatus to determine whether these legumes could serve as a refuge for this citrus pest (Lapointe 2003). He found that none of the three legume species reduced the feeding damage caused by Diaprepes abbreviatus to citrus. Additionally, Lapointe determined that pigeon pea is an inappropriate cover crop for citrus because of its positive effect on larval growth and potential allelopathic effects on citrus roots. Finally, a 3-year research trial by Linares et al. (2008) found that annual plantings of cover crop mixtures containing sunn hemp (Crotalaria juncea), hairy indigo (Indigofera hirsuta), cowpea, and alyce clover (Alysicarpus vaginalis) were excellent at suppressing weeds.
Because of the devastating and prolonged impacts of citrus greening, cover crop usage has once again gained prominence as a key component in efforts to restore soil fertility and mitigate the adverse effects of this disease on citrus tree health. This renewed focus is underscored by the surge in research activities, with more peer-reviewed journal publications of cover crops in Florida citrus production conducted during the past 4 years than during the previous 60 years combined (Table 2). Many of the recent studies examined the impact of cover crops on soil microbial populations. Research published by Castellano-Hinojosa et al. (2022a) found that mixtures of legumes and nonlegumes increased soil carbon (C) by 25% and soil ammonium (NH4−) content by 68% and enhanced nutrient-cycling microorganisms in citrus orchards after only 1 year. Another study by Castellano-Hinojosa et al. (2022b) found that mixtures of legumes and non-legumes led to a 25% greater soil nitrate content compared with that of a nonlegume cover crop in 1 year and increased the abundance of genes associated with N fixation and nitrification. Additionally, Castellano-Hinojosa et al. (2023b) found that citrus groves treated with legume and nonlegume mixtures for 2.5 years had 55% more soil nitrate and enhanced nutrient cycling in the top 3.9 inches of soil compared with that of a grower standard row middle (perennial turf and weeds). During a 3-year trial (Castellano-Hinojosa et al. 2023a), it was found that cover crop mixtures increased soil alpha diversity as well as OM from 2.7% to 3.2% (legume/nonlegume mixture) and 2.6% to 3.2% (nonlegume mixture) compared with those of a grower standard, which experienced no significant difference in OM. However, these results were not consistent across the two locations used.
Summary of research of cover crops for Florida citrus published in peer-reviewed journals. Google scholar and Web of Science databases were used with the keywords “Cover crops,” “Florida,” and “Citrus.”
Other recent trials have examined the impact of cover crops on soil OM and nitrate levels as well as the ability to grow specific cover crop species in sandy soils. A study by Brewer et al. (2023a) found that legume and nonlegume mixtures of cover crops increased soil nitrate levels after seven seasons by 31% when compared with a control (no cover crops). Additionally, the legume and nonlegume mixture increased soil OM by 17% compared with that of the control. However, these increases were only observed in one of two locations, likely because of inconsistencies in cover crop germination. A second report by Brewer et al. (2024) found that daikon radish (Raphanus sativus) can be successfully grown without costly N fertilizer applications in sandy soil. Importantly, growers who implement cover crops in the Florida citrus industry require cover crop species that need no management inputs such as fertilizer or irrigation.
Notably, amid heightened management costs attributable to citrus greening, a cost analysis of the implementation of cover crops was conducted. The cost analysis by Chakravarty and Wade (2023) calculated the break-even price for ‘Valencia’ and non-‘Valencia’ sweet oranges based on the price per box (90 lb of fruit) and price per pound solids per box by determining the additional costs of cover crops and the short-term savings from using cover crops. They found that cover crops increased the management cost per acre by 5.73% during the first year of implementation. With the recent reductions in both citrus yield and quality during the 2020 to 2021 and 2021 to 2022 seasons primarily caused by citrus greening and hurricanes, the break-even prices for both ‘Valencia’ ($13.67/box at 193.5 boxes/acre) and non-‘Valencia’ sweet oranges ($12.97/box at 205.5 boxes/acre) were higher than the market price ($12.21/box in 2021 to 2022). Because of the current yield/quality scenario and lack of benefits of cover crops for both yield and quality in the short-term, implementation would not be profitable.
Cover crop management for Florida citrus
Effective cover crop management is essential for maximizing the benefits of integrating cover crops into Florida’s citrus groves. Management practices involve consideration of several key components, such as setting goals, selecting the appropriate cover crop seed species, planting techniques, and determining termination practices.
Setting goals
The first and most important determination that a grower should make when introducing cover crops to a new grove is the main goal of the cover crop. According to the USDA Sustainable Agriculture Research and Education (SARE), cover crops can provide more than 10 different services to the agroecosystem (Magdoff and Van Es 2021). These services include increasing soil N, suppressing weeds, improving soil structure, reducing soil compaction, decreasing nutrient loss, attracting beneficial insects, adding soil OM, and enhancing mycorrhizal numbers. It is important to note that one goal of cover crops may be incompatible with another. For example, it has been shown that plants with high C content such as sorghum sudangrass (Sorghum ×drummondii) and oats (Avena sativa) produce high biomass and are effective at rapidly increasing soil OM but may outcompete the cash crop for nutrients (Chapagain et al. 2020). To reduce potential negative effects of cover crops and increase the amount and types of services provided, multiple species mixtures can be used (Finney et al. 2017). Research by Finney et al. (2017) found that a four-species cover crop mixture provided both weed suppression and N retention during a 3-year field study conducted in Pennsylvania. Similarly, Finney and Kaye (2017) found that weed suppression, N retention, and aboveground plant N were all services provided simultaneously by a cover crop mixture after 2 years in Pennsylvania. In Florida, Allar et al. (2023) found both three- and five-species mixtures of cover crops grown during the summer improved soil P and potassium (K), reduced weeds, reduced root-knot nematodes, and improved pollination in organic vegetable systems. However, which service and to what extent that service is realized depends on a number of factors. These factors include cropping system compatibility, compatibility between cover crop species, compatibility with the environment, duration of the cover crops, termination method, input costs, and weediness (Chapagain et al. 2020); additionally, how these factors interact with a perennial tree crop in a subtropical climate may differ from what was previously reported by studies that were mainly conducted in temperate annual systems. Chakravarty et al. (2023) conducted a survey of Florida citrus growers to determine which benefits were among the top priorities of growers and revealed that 36% ranked nutrient retention as the top potential benefit of cover crops, 20% ranked pest control, 14% ranked soil erosion/prevention, 12% ranked weed management, 10% ranked increasing soil OM, and 8% ranked maintaining soil moisture as top potential benefits. Given the substantial interest among growers in using cover crops to enhance soil fertility, the following sections provide a detailed analysis of cover crop management methods and their direct relevance to achieving this objective.
Cover crop seed selection
The seed mixture is often the first management consideration addressed. Cover crops are often grown as mixtures comprising several plant species planted together to acquire multiple benefits. These mixtures generally contain plants from three broad categories: legumes (Leguminosae), nonlegume broadleaves (primarily Brassicacea), and grasses (Poaceae). Legumes can fix atmospheric N into plant-available forms and are thus planted to improve soil N levels, whereas nonlegume broadleaves are planted for their ability to both suppress weeds and to break up soil compaction (Ebelhar et al. 1984; Haramoto and Gallandt 2004; Mulvaney et al. 2011). Grasses are included in mixtures for their biomass and to help hold soils in place, scavenge nutrients in the soil profile, improve water infiltration (Basche et al. 2016; Shipley et al. 1992). Depending on grower goals, mixtures can be tailored with different percentages of legumes, grasses, and nonlegume broadleaves (Fig. 2).
An example of legumes, grasses, and nonlegume broadleaves used as cover crops in a Florida citrus grove. Image taken by Lorenzo Rossi at Scott Citrus Management grove in Fort Pierce, FL, USA.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05476-24
A key consideration when choosing a cover crop mixture is the C:N ratio of the plant residue. Microbes living in the soil use C and N at a ratio of between 15:1 and 24:1, which is used as a cutoff for mineralization and immobilization (Balota and Auler 2011; USDA 2011). At higher C:N ratios, plant residue becomes immobilized as microbes scavenge extra N from the soil, possibly outcompeting citrus trees for this essential nutrient in the short term (Korsaeth et al. 2002). However, this is often temporary; in the long term, N can be remobilized and become available to the citrus trees. Conversely, lower C:N ratios may result in rapid breakdown of plant material, leaving soil exposed (Cui et al. 2022). Generally, grasses such as sorghum sudangrass and millet species (e.g., ‘pearl millet’ Pennisetum glaucum; ‘browntop millet’ Urochloa ramosa; ‘japanese millet’ Echinochloa esculenta, etc.) have high C:N ratios (>30), whereas brassicas such as purple top turnips (Brassica campestris) and daikon radish have C:N ratios of approximately 20 (Kaye et al. 2019; O’Connell et al. 2015). Because of high levels of N in their leaves caused by N fixation, legumes have low C:N ratios. Common legumes used in Florida such as hairy vetch (Vicia villosa) and cowpea have C:N ratios less than 15 (Ramirez-Garcia et al. 2015). Multispecies cover crop mixtures combine the high N contents of legumes with the high C contents of grasses and brassicas, thus stabilizing the C:N ratio and allowing for more even release of N (Koudahe et al. 2022).
Most importantly, growers are limited to what species of cover crop they can grow based on location and season. Often, even slight changes in location can greatly influence what can and cannot be grown. Additionally, certain species of cover crops that thrive in the hot and rainy weather of Florida during summer may not thrive or germinate in the cool and dry winter season. Therefore, growers may use different cover crop seed mixtures when planting during different seasons. Although cover crop usage in citrus groves is growing in popularity, there are no specific species recommendations for citrus. Most of the literature that examines cover crops species for citrus in Florida is either severely outdated (Hume 1904) or was conducted in the context of choosing the best forage for cattle (Kretschmer 1958, 1964, 1966; Kretschmer and Hayslip 1964). Although lists of potential cover crops for citrus in Florida have been compiled, these compilations tend to be relatively small and are based on what thrives well across the entire state rather than within the citrus agroecosystem specifically (Rezazadeh 2022).
Seed planting: Methods and timing
Seed planting methods are a crucial component of cover crop management. Generally, seeds are planted either by a broadcast seeder or by seed drill (Haramoto 2019) (Fig. 3A, 3B). Broadcast seeders are relatively inexpensive; however, germination rates are lower and require 50% to 100% higher seeding rates compared with those of seed drills (Ball 1986; Brennan and Leap 2014). Additionally, broadcast seeding often requires disking to gain enough soil to seed contact for germination (Fisher et al. 2011). This disking can result in more rapid breakdown of soil OM, result in poor soil physical characteristics, and potentially damage citrus tree roots (Ding et al. 2002; Havlin et al. 1990). Seed drills have been shown to increase germination rates by improving soil–seed contact, allow for no-till planting, and improve biomass production (Fisher et al. 2011; St Aime et al. 2022). Many modern seed drills can handle diverse seed sizes in cover crop mixtures, thus allowing for all seeds to be planted within a single pass. The major drawback to seed drills is cost; for example, some box drills cost $49,000 (Kientzy et al. 2023a, 2023b).
Management of cover crops in a Florida citrus grove. (A) Cover crop planting using a seed drill. (B) Cover crop planting using broadcast seeder. (C) A stand of recently planted cover crops. (D) Cover crop termination after mowing. Images taken at the Scott Citrus Management commercial grove by Lukas Hallman in Fort Pierce, FL, USA.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05476-24
The timing of planting can greatly influence germination rates and, thus, the effectiveness of the cover crop planting (Fig. 3C). In Florida citrus production systems, cover crops are generally planted twice per year (Chakravarty and Wade 2023). A warm season cover crop is usually planted between May and June, and a cool season cover crop is planted between October and December (Campbell and Treadwell 2021). Because growers do not irrigate cover crops, rain within a relatively short period after planting is essential for germination. Seeding should be conducted within a few days of rain; otherwise, germination may be low. This can be particularly challenging for larger operations that may not have the ability to seed all their acreages at the optimal time, thus leading to inconsistent germination.
Cover crop termination
Termination of cover crops can be performed using several methods and at different times of the season. Some growers opt to let the cover crops grow for their entire life span and “self-end.” This is a much cheaper option for the grower because less fuel and labor hours are used. Additionally, if the cover crops are allowed to self-seed, then the growers could potentially build a seed bank of cover crops in the soil. Cover crop termination via mowing, herbicide, disking, and crimping are also popular methods (Alonso-Ayuso et al. 2020) (Fig. 3D). These methods have higher associated costs; however, they allow the grower to control when the cover crops are ended. Additionally, different methods have been shown to impact the cash crop yield. Research by Bavougian et al. (2019) found that termination via tillage led to 57% to 78% higher maize yield compared with that of no-till crimping termination. However, research by Kichler et al. (2023) found that ending cover crops via mowing and crimping led to higher collard green (Brassica oleracea) yield compared with that of mowing/incorporation of residues. In citrus production systems, it is essential to minimize disturbance to the trees. Therefore, termination methods such as disking, which can damage tree roots in the row middles, may not be suitable for mature citrus groves. Disking, however, can be used on newly planted groves before the tree roots reach the central portion of the row.
Termination timing greatly influences the effectiveness of cover crops. If termination occurs too early in the season, then biomass accumulation will be low, resulting in less nutrient release to the soil, whereas late-season termination can maximize soil benefits (Balkcom et al. 2015; Denton et al. 2023). However, competition between cover crops and cash crops has been shown to increase with later cover crop termination (Alonso-Ayuso et al. 2014). Nutrient concentrations of cover crops also fluctuate throughout the season depending on plant growth stage, which should be considered when determining the termination date. For example, if legumes flower, then a large portion of N will be partitioned to seeds instead of biomass. Furthermore, as legumes stop actively growing, N fixation symbiosis stops (Serrantonio 2008). Additionally, organic residues rapidly break down in the warm and humid weather of Florida. Research by Nyabami et al. (2023) found that residues released more than 60% of N within the first month of decomposition. Therefore, growers should factor this into their decision-making to maximize potential benefits for citrus trees.
Challenges to cover crop implementation for Florida citrus
Although much research supports the benefits of using cover crops, the challenges related to the implementation of cover crops into an existing conventional citrus management system may reduce the number of growers willing to adopt this method. The additional cost of cover crops is the major limitation to adoption. Research by Chakravarty et al. (2023) estimated that the cost of cover crops is $107.30 per acre ($265.03 per hectare), equivalent to an increase of 5.73% per acre for the first year of implementation, which reduces operational profitability in the short term. The primary costs of cover crops are the seed, machinery, labor, and fuel. Although growers may be able to recapture some of the costs associated with cover crops by reducing mowing and, thus, labor/fuel cost, this option may not be available to all growers. Although increases in N have been observed in the short term, no research has explored the impact and cost of reducing fertilizer applications. Finally, Castellano-Hinojosa et al. (2023a) found that after 3 consecutive years of cover crop growth, the yield of citrus greening-affected trees was not improved. Therefore, without yield improvements in the short term to offset the cost of cover crop implementation, less growers are likely to implement the practice.
In addition to the cost of cover crop implementation, the logistics associated with introducing cover crops into an existing growing operation may be a challenge. Most growers currently use broadcast seeders or seed drills to plant cover crops, which must be moved. Although it may be relatively easy to accomplish by smaller operations, many growers manage multiple properties that may not be congruent with each other. The time and effort needed to move equipment must be considered. When coupled with the small planting windows, the time required to move equipment—often to multiple locations—and seed cover crops could result in late planting and, thus, poor germination. Growers must also plan cover crop seeding around citrus harvest operations as well as pesticide and fertilizer applications, which may occur at approximately the same time.
Competition between citrus trees and cover crops is another potential challenge that may reduce the number of growers who are willing to implement cover crops. Generally, cover crops are used in annual agricultural systems, not in perennial tree systems (Castellano-Hinojosa and Strauss 2020). When used in an annual system, the cover crops are planted during the fallow season, when no cash crop is in the field, thus eliminating possible competition. In a perennial fruit tree system like citrus, cover crops are in the field at the same time as the cash crop, potentially competing for water and nutrients (Giacalone et al. 2021). If competition does occur, then the health of citrus greening-affected trees may be reduced further. Additionally, competition could result in increased nutrient and irrigation applications, further decreasing profitability.
Planting cover crops with the goal of improving soil fertility is a long-term strategy that may take more years to realize benefits. Although small improvements in soil fertility have been observed in the short term, long-term positive impacts on yield are likely (≥7 years) (Boquet et al. 2004; Olson et al. 2014). This potentially long timeframe is a large challenge for citrus growers, particularly in the age of citrus greening. The gap between the investment of money and time in implementing cover crops and the time it takes to realize benefits may be too long for citrus growers who are already burdened by increased management costs. Although cover crops are not expected to cure citrus greening, the rationale behind their implementation is to mitigate the increasing cost of inputs needed to manage the disease. Finally, the absence of updated practical cover crop management guidelines for growers poses a major challenge to widespread cover crop implementation. The last practical guide to managing cover crops in Florida citrus groves was published more than 100 years ago (Hume 1904).
Conclusions and future opportunities
With the massive changes in the citrus industry and advancements in agricultural technology over the past century, there is a pressing need for new research focused on cover crop management. Although recent studies have addressed various aspects of cover crop management in Florida, they often fall short of providing comprehensive guidance regarding optimizing cover crop management practices. Consequently, citrus growers are left without critical information, such as the best cover crop species for diverse mixtures, optimal termination timings to maximize nutrient benefits, and the specific impacts of cover crops on the health of citrus trees affected by citrus greening. This research gap presents numerous opportunities for scientific field trials, including species selection, termination timing, effects on soil and tree health, integrated pest management, and economic viability. These research opportunities aim to refine cover crop management practices, thus making them more effective and accessible for citrus growers. By addressing these gaps, it may be possible to develop practical guidelines and solutions to enhance the resilience of Florida’s citrus industry in the era of citrus greening.
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