What do you think of when you hear ‘Florida’? If you’re a Floridian farmer, citrus, sand(y soils), and Citrus Greening Disease probably come to mind. As Citrus Greening Disease, or Huanglongbing (HLB), edges up to incidences of 100%, it may seem like solutions for this devastating disease are few and far between. While HLB is not a soil-borne disease, compost applications can still help manage HLB symptoms. Compost also aids in growing a fruitful crop of Florida’s signature produce, all while building some seriously healthy and fertile soil by increasing water-holding capacity, influencing the nutrient supply and retention, buffering pH, replenishing the microbial biomass and suppressing soil-borne diseases, and managing symptoms of HLB. Let’s dig in to the five ways compost can benefit citrus crops and growers.
1. Water Holding-Capacity
Citrus-growing soils are typically fall into one of three soil classifications: entisols, which drain well, but have very poor fertility, alfisols, and spodosols, both of which drain poorly due to high water tables and while they have slightly higher fertility than entisols, it’s nothing to brag about. While alfisols and spodosols both drain poorly, they have very little water holding-capacity, as do entisols. Most citrus soils contain between 94% and 98% sand in the root zone and readily lose their already limited organic matter content, which tops out at 5% and drops as low as 1-2% after cultivation and establishment of citrus groves (Morgan & Kadyampakeni, 2020). This loss of organic matter is partially due to Florida’s high humidity and temperatures, but herbicide and pesticide usage in commercial citrus play a role in organic matter loss. Both can cause soil organic carbon to rapidly transform into carbon dioxide that’s lost to the atmosphere.
Due to the high sand content and low organic matter content of citrus growing soils, the lack of water-holding capacity is no surprise. Sand particles are larger than other more water-adsorbing soil components, like silt or clay. This larger size corresponds to an overall lesser amount of surface area per particle; clay and silt also have plenty of built-in nooks and crannies that sand particles lack, increasing their surface area and their ability to hold onto necessities like water and nutrients. In this way, compost conditions sandy soil by adding organic matter to coat sand particles and grab onto water molecules, rather than them rapidly draining away from the root zone. The slightly polar charge of water molecules, coupled with the net negative charge of organic matter, keeps water molecules where they’re needed and can reduce the frequency and duration of irrigation.
2. Nutrient Retention
In agricultural soils, nutrients are depleted from the soil due to a myriad of instances like plant uptake, pH-caused nutrient unavailability, volatilization of ammoniacal nitrogen, leaching of nutrients through the soil profile, and fertilizer runoff. Environmental effects aside, preventable nutrient loss is a frustration for all farmers. Leaching is especially common when soils are very sandy. If water is rapidly leaving the soil, it’ll more than likely be taking your fertilizer and nutrients with it. In particular, this can be an issue with trees infected with HLB, as they often show decreased root growth and a smaller root zone, causing nutrient deficiencies that can exacerbate symptoms of HLB. If the nutrients aren’t taken up by crops, and there’s not a high enough Cation or Anion Exchange Capacity to hold nutrients in soil, applied nutrients will be lost from the root zone. Why spend good money and time on fertilizers that don’t stick around in the soil when you need them?
Much like water molecules stick around for longer periods when soil organic matter is higher, ionic nutrients will stick around for far longer and be more accessible to plants when there is ample organic matter present in the soil. Organic matter has a net negative charge, meaning all the positive chemical charges found in organic matter are outnumbered by the negative charges in organic matter. These charges are integral to the capture and retention of plant-available nutrients, which must be ionic in form (have a chemical charge) and for the majority of nutrients, must also be in an inorganic, or mineralized, form. Nutrients in their organic chemical form are often unavailable for plant uptake until microbially-catalyzed mineralization occurs. Nutrients, both organic and inorganic, are susceptible to being lost to runoff and leaching. Increasing a soil’s organic matter content with compost can allow the soil to hold onto far more nutrients than soils lacking in organic matter and can boost microbial community populations, which affects the rate of nutrient catalyzation reactions and therefore, affects nutrient availability.
The charges found in soil organic matter are known collectively as the Cation Exchange Capacity (CEC) and the Anion Exchange Capacity (AEC). Each corresponds to a different charge, positive or negative, and reflects the charged, ionic nutrient they’re able to hold! If you’ve ever heard “opposites attract” it’s also true here: Cation Exchange Capacity refers to the ability of negatively charged organic matter sites to exchange different cations, or positively charged nutrients. Anion Exchange Capacity corresponds to the ability of positively charged sites to exchange different anions, or negatively charged nutrients. The more charged sites within a soil, the more nutrients that soil is able to hold onto and release as plants need them. This directly affects the money farmers spend on nutrients: when more of your nutrients are retained in the soil, you don’t need to apply as much as or as frequently!
Citrus groves, like other crops, require certain nutrients at certain times for the optimal crop production. Nitrogen, phosphorus, and potassium, as always, are the most common nutrients applied. In the case of some nutrient deficiencies, foliar applications are preferred, but a comprehensive nutrient management plan should focus primarily on soil-added nutrients. Correctly finished compost can supply a percentage of nutrients, NPK, other macro- and micronutrients, like a slow-release. Compost is often more affordable than synthetic chemical fertilizers and has the added benefit of a robust microbial community included to assist in nutrient conversion of organic to inorganic forms. Because compost can contribute to the overall nutrient supply, it’s important to see a nutrient analysis of the compost supply to subtract these nutrients from the comprehensive chemical fertilizer plan. As we’ve learned, compost will increase the soil’s ability to hold onto these chemical nutrients and over time, will reduce the amounts needed for application.
Just like synthetic chemical fertilizers, it’s important to apply compost amendments at the right time and in the right place. Compost should be incorporated when new saplings are planted so young and maturing trees can benefit from this healthier soil. Compost can be applied in bands around mature trees, similar to a mulch. Seasonal timing is important, as well. Apply compost when the rainy season has finished and avoid laying down new compost just prior to the rainy season’s beginning. Compost applications will work best when applied mid- to late September through April. The Southwest Florida Research and Education Center in Immokalee showed an increase in caliper and canopy size measuring 39% and 45%, respectively, in citrus saplings planted in soils amended with yard waste-based compost over two years and two months of growth time (Ozores-Hampton, et al. 2015). At the Florida Research Center for Sustainable Agriculture in Vero Beach, researchers measured the effects of compost applied to already-planted citrus trees and found that two applications of yard waste-based compost over a time frame of 1.5 years showed an increase of 30.2% and 27.8% in caliper and canopy measurements over compost-less trees (Ozores-Hampton, et al. 2015).
3. pH Buffering
While most Florida citrus growing soils are acidic in pH, the irrigation water may be alkaline due to high amounts of bicarbonates found in Florida waters. For both healthy trees and HLB-infected trees, a soil pH of 6.0-6.5 promotes optimal root growth, including feeding roots, and nutrient availability in soils (Vashisth et al. 2019). Organic matter aids in retention of nutrients, but what about maintaining an optimal pH for nutrient availability? Yep, compost does that, too!
Most nutrients, including the all-important NPK, are more or less available to plants at a certain pH. In general, most nutrients are at peak availability at a pH around 6.0-7.0. Managing the correct soil pH can ensure proper nutrients are present in plant-available forms. For instance, plant usable nitrogen source ammonium (NH4) is the predominant form of ammoniacal N when soils are below a pH of 7.0. Ammonium is present as a solid-phase in soils, while ammonia (NH3), is the dominate form of ammoniacal N in alkaline soils (pH >7.0 and exists in a gaseous state that volatilizes out of the soil at high pH. Phosphorus is also most available in slightly acidic to neutral soils. At a higher pH, phosphorus tends to bond to calcium, rendering both unavailable to plants. Most micronutrients become highly unavailable at a higher pH, as well. Since Florida’s growth season is so long, a constant and available supply of these nutrients is best for tree production and health.
Most Florida irrigation water is high in dissolved bicarbonates, which can lead to an increasing soil pH over time, causing loss of available nutrients, a decline in tree health, and reduce citrus yields (Vashisth et al. 2019). Graham and Morgan (2015) found bicarbonate stress reduced the uptake of calcium, magnesium, potassium, and iron and can partially be attributed to a reduction in fibrous root density cause by bicarbonate stress. This is a similar issue found in trees infected with HLB; root density is lost, nutrients are present, but are taken up more slowly due to the reduction in roots.
Soils high in organic matter are much more effective at resisting pH change, meaning they have a high pH buffering capacity while sandy soils change pH fairly easily. Similar to how compost additions increase nutrient availability, adding compost to a poorly buffered soil can increase the surface area and charged surfaces for hydrogen ions to reside on. This inherently increases the soil buffering capacity as these hydrogen ions are able to release to counter any alkalinizing agents. Compost additions to soils being prepared for production or compost added to soils already in production can help combat the high bicarbonates in irrigation water from the ground up.
4. Microbial Biomass: Disease Suppression
Microbial communities in soil are recognized more and more as an integral part of the plant-soil relationship. While some microbes are pathogenic in nature, most are present to do good things in the soil. Microbial communities can affect nutrient availability, nutrient mineralization (aka the transformation of organic nutrients to inorganic, plant available nutrients), combat pathogenic microbes within the rhizospheric zone (the area where soil microbes reside near plant roots) and can even symbiotically produce nutrients, given the right host plant. Common practices like tilling, herbicide and pesticide usage, and monoculture can severely disrupt the soil microbiome. Herbicide and pesticide usage in particular can slow the return of soil organic carbon through decomposition of fallen citrus leaves. While some use of pesticides and herbicides is unavoidable, as it is in control of the HLB vector psyllid, herbicide and pesticide use should be used in a targeted fashion, rather than blanket applications. Microbiome disruption can lead to an increase in the presence of pathogenic microbes, poor root health, and a decrease in nutrient uptake by plant roots. This is particularly concerning for trees infected with HLB, as a weakened root system can cause the tree to lose yield, branches, and eventually die off.
Soil microbial communities are necessary to catalyze nutrient release reactions for organic nutrients, like those found in compost. Compost does contain a low level of nutrients, so it’s important to request nutrient analysis results from your compost supplier to factor this nutrient supply into crop fertilization rates and schedules. Soil tests are also useful here to check nutrient levels over years of compost use to avoid unnecessary over-application of fertilizers. As compost is added to soil, the microbial community grows. These microbes are essential for the transformation of organic nitrogen and phosphorus into inorganic, plant available nutrients. Microbes will break bonds between organic molecules, like urea or phytate aromatic rings, which release the now inorganic molecules of nitrogen and phosphorus as ammonium and orthophosphate ions. Research summarized by Litvaney and Ozores-Hampton show that over ten years growing commercial citrus with compost at an application rate of 3 tons/acre split into three applications indicates mineralization of compost-held nutrients is complete around 90 days after application. This research was conducted with mature compost with a carbon: nitrogen ratio of less than 25:1. Because these nutrients are released at a much slower pace than synthetic fertilizers, citrus trees are able to make much more use of these nutrients and take up more over time. Compared to application of compost, synthetic fertilizers lose about half of their nutrients to runoff (Litvaney and Ozores-Hampton, 2002) and leaching nearly immediately because trees cannot take up all the fertilizer applied at once.
In addition to creating plant-available nutrients, microbial communities will work to suppress pathogenic microbes that cause plant diseases. Compost helps to manage phytophthora root rot, greasy spot disease, and can even help with HLB. Widmer et al determined that citrus nursery seedlings grown in compost reduced the occurrence of Phytophthora root rot by 90%. When applied to mature citrus trees, compost continues to combat root rot through improving the soil structure and helping prevent standing water conditions that can lead to occurrences of root rot. This can also help diagnosing HLB-infected trees. If one condition can be ruled out due to good soil management, a correct HLB diagnosis can be made much earlier and treatment can begin. Litvaney and Ozores-Hampton (2002), showed that regular application of compost decreased the use of fungicides to control greasy spot disease caused by Mycosphaerella citri due to hastened microbial decomposition of infected citrus leaves, which prevented fungal spores from spreading in citrus leaf litter. In regards to HLB management, trees infected with HLB have shown weakened root systems, leading to a slowdown in nutrient uptake, leaving trees malnourished and more susceptible to citrus production-disrupting symptoms. Ginnan et al, in research published in 2020, determined that trees infected with HLB showed a native microbiome that had shifted from symbiotic and mutualistic relationships to a microbiome dominated by pathogenic and saprophytic microbes. Regular applications of compost can replenish positive microbial communities and reduce the abundance of pathogenic microbes in the biome of HLB-infected citrus.
5. HLB/Citrus Greening Symptom Management
While there is no cure for HLB, several improvements in HLB symptom management and preventative measures to decrease spread of the disease. Li et al found that a combination of compost (applied 60 cm from the base of trees), Phi-N (a potassium phosphite and urea fertilizer), and plant defense-elicitors (B-aminobutyric acid and ascorbic acid) slowed HLB progression and reduced the disease severity in mildly diseased plants by 18%.
Citrus trees infected with HLB can lose up to 80% their root mass. While the remaining roots don’t have a diminished nutrient uptake capacity, the reduction in root mass and surface area affect a tree’s ability to acquire the necessary nutrients for growth and fruit production. To prevent this, multiple small applications of chemical fertilizers would be necessary to ensure the trees are receiving enough nutrients without losing most to runoff or leaching, which is a largely unsustainable practice for cost, time, and equipment use. Applying around trees will enhance the soil’s ability to retain nutrients for longer periods of time, allowing chemical and organic nutrients alike a longer period of availability for plant uptake.
Compost can help in a roundabout way with early diagnosis of HLB by providing a natural level of disease suppression for diseases with similar beginning symptoms, like Phytophthora root rot. While working with Florida citrus farmers, Graham and Morgan (2015) found that groves that had been excessively limed with Dolomitic lime and that were irrigated with high bicarbonate-content water showed the most severe HLB symptoms highest rates of fruit drop, like off-color foliage, thinning canopies due to leaf drop, and twig dieback.
Compost helps to adjust and buffer soil pH, effectively allowing the soil to function as a lower-pH soil. Because HLB-infected trees are so susceptible to nutrient deficiencies due to the loss of root density, it’s important that all nutrients remain in an available form and remain held in the soil for the slow-release uptake HLB-infected citrus needs to avoid loss in production.
SUMMARY
Compost, by nature, is full of organic carbon and bursting with microbial life! Applying compost can aid in returning organic matter to the soil, rehabbing the depleted microbial community, and conditioning soil to increase the water-holding capacity and nutrient retention, and buffering the pH to keep nutrients available. This all comes together to help commercial citrus growers lower their irrigation water consumption and reduce the amount and times needed to apply fertilizers each year, saving time and money.