By Davie Kadyampakeni, Alisheikh Atta, Jonas Pereira de Souza Junior, Christopher Vincent and Tripti Vashisth
Typically, citrus water requirements are low in the winter and spring due to cooler temperatures but peak in summer and early fall due to rising temperatures. Thus, improved irrigation management is critical in these times of the year for improved production. Because most citrus trees in Florida are planted in soils with low nutrient- and water-holding capacity, growers need to pay close attention to the supply of both water and nutrients.
In this article, we describe some of the tools for irrigation and nutrient management to improve citrus tree productivity, especially for trees impacted by citrus greening.
IRRIGATION SCHEDULING TOOLS
There are several tools for deciding when to irrigate and how much to irrigate. One example is the Smartirrigation app (Citrus Microsprinkler Irrigation Scheduler), using the Florida Automated Weather Network to schedule irrigation, available for android and iOS. This app allows the grower to specify tree row spacing, emitter discharge rate, soil type and tree rooting depth, among other things, and provides a tentative irrigation plan for two weeks.
Typically, the Smartirrigation app will advise delaying irrigation for about 2 days if a ½-inch or more of rain occurs. However, if there is no rain, the app will provide how much water to apply and for what duration. As a guide for trees impacted by citrus greening, daily irrigation is recommended, since diseased trees tend to have shallow and weak root systems with limited water- and nutrient-uptake capacity.
Soil moisture sensors can also help manage irrigation based on actual conditions in the grove. There are many sensors commercially available, but the ones ideally suited for Florida’s sandy soils include the capacitance probes, time domain reflectometry sensors and time domain transmissometry sensors. These sensors require little or no calibration, need minimal maintenance and can send data to the cloud, iPhone or computer platform. They are very easy to handle and are good at showing when the soil moisture is below the optimal threshold. The optimal threshold is called field capacity, which for sandy soils is around 8% to 10%. Growers should always aim to keep the moisture slightly close to the field capacity to make sure water is optimal for efficient uptake of water and nutrients.
NUTRIENT RECOMMENDATIONS
Nutrient management requires consideration on the timing, placement, rate and source. Fertilizer management decisions should be guided by soil and leaf analysis (Table 1) to avoid applying too much or too little fertilizer.
In terms of timing, the goal is to spread out the release of fertilizer to the soil to reduce fertilizer losses to leaching, while also making the majority of the fertilizer available to the tree during the time that it can make the most use of it (spring and early summer). If using conventional granular fertilizers (which last about three months), growers are encouraged to apply fertilizers a minimum of four splits per year, usually around February, March, May and September/October. The more splits the better because this will minimize the leaching risk.
Blends that contain nitrogen, phosphorus, potassium and a little bit of calcium, magnesium, sulfur and polymer-coated micronutrients such as manganese, iron, boron and zinc are favored and can improve tree productivity. Controlled-release fertilizers (CRF) take longer to break down and can last between six to 12 months. They can be applied in about two to three splits per year. For trees affected by citrus greening, CRF products that contain polymer-coated micronutrients are also encouraged.
In recent years, some growers have switched to liquid fertigation systems. If using liquid fertigation, weekly, bi-weekly or monthly fertigation practices to ‘spoon-feed’ the tree’s weak root systems from February to October are recommended. However, in the summer months of June, July, August and September, the use of CRF to minimize nutrient leaching due to heavy rains is recommended.
With respect to placement, apply the fertilizer in the active root zone, around 1 to 2 feet from the tree and within the wetted area of the microjets to allow better uptake since this is where most roots are located. However, some nutrients are applied to the foliage, such as micronutrients and selected macronutrients (e.g., potassium and calcium, mainly where the soil pH is high, like in the Indian River region). The ideal scenario is to apply the foliar fertilizers as a solution to correcting short-term nutrient deficiencies. Foliar micronutrients can typically be applied three to four times per year. Foliar potassium applications are considered ideal during the fruit phase to improve fruit size and juice quality.
OTHER CONSIDERATIONS
Water quality is another important parameter to consider, especially when using fertigation. Some groves have wells that are high in bicarbonates, which can result in elevated soil pH. Test the water pH regularly to make sure it is between 5.8 and 6.5. This will keep most nutrients available in the root zone. If the water pH is greater than 7, regularly acidify the water with dilute sulfuric acid, elemental sulfur or another acidifying source until the pH is in the desirable range.
Soil pH needs to stay around 5.8 to 6.5 because this is where most trees impacted by citrus greening tend to acquire the most water and nutrients (Table 2). Very low pH will make most nutrients unavailable (e.g., calcium and phosphorus) or toxic (e.g., copper, manganese, zinc, etc.) and may result in root injury. Very high pH can result in precipitation of some nutrients, especially phosphorus and calcium.
Finally, soil amendments should also be considered for predominantly sandy soils. Florida sandy soils contain less than 3% soil organic matter and very low silt and clay fractions (<5%). Additions of compost or other carbon-containing compounds, such as biochar, can help improve water- and nutrient-holding capacity. These soil amendments tend to contain organic carbon, which can stimulate more biological activities in the soil, resulting in better nutrient transformations and cycling to the benefit of long-term grove productivity.
CONCLUSION
In conclusion, use of irrigation apps and soil moisture sensors is important for guiding water management decisions in citrus groves. Frequent (possibly daily) irrigation is encouraged. On the other hand, nutrient management decisions should always be guided by leaf and soil testing to make sure there is adequate nutrient supply for better tree performance.
Davie Kadyampakeni, Christopher Vincent and Tripti Vashisth are associate professors, and Alisheikh Atta and Jonas Pereira de Souza Junior are postdoctoral research associates, all at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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