By Tripti Vashisth
Constant supply and soil application of macronutrients and micronutrients can improve the health and productivity of HLB-affected citrus. A 20 to 50 percent higher than University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) recommended rate of micronutrients is beneficial for HLB-affected trees. However, the nutrition program should be customized for each grove, as no one size fits all.
Mineral nutrition plays an important role in metabolic and physiological processes such as growth and development of plants, fruit development and plant-defense response. Field observations have shown that there is a positive correlation between sulfur fertilization and enhanced resistance against fungal pathogens.
Use of elemental sulfur is also effective in bringing down soil pH. The recommended soil pH range for Florida citrus is 5.8 to 6.5. High soil pH limits nutrient availability, especially of micronutrients such as manganese (Mn), zinc (Zn) and iron (Fe), which are already reported to be deficient in HLB-affected leaves.
Moreover, at high pH, calcium and magnesium can form insoluble complexes in soil, rendering them unavailable to the plant. Field trials demonstrate that under high soil pH (greater than 6.5) conditions and bicarbonate concentrations greater than 100 parts per million in irrigation water, HLB-affected trees exhibit increased loss of feeder roots, severe decline, increased HLB symptoms and reduced yield.
On the other hand, HLB-affected trees have small root systems that limit their capacity for nutrient and water uptake. Therefore, the constant supply of balanced fertilizer seems to be promising for improving the growth and productivity of HLB-affected trees. As the majority of Florida growers are looking for strategies for a continuous supply of nutrients, controlled- release fertilizer (CRF) appears to be a promising tool.
The goal of a UF/IFAS three-year study was to evaluate the effect of CRF in combination with Tiger Sul micronutrient blend (at an elevated rate) on yield and performance of HLB-affected trees. This trial was conducted on bearing age (10- to 14-year-old) Valencia on Swingle in 10-acre blocks, replicated at two sites: Fort Meade (ridge) and Arcadia (southwest Florida). There was a total of 10 fertilizer combinations evaluated:
- Conventional granular fertilizer + foliar (control)
- Conventional granular fertilizer + Tiger Sul Micronutrient Mix (MM)
- CRF + foliar
- CRF + Tiger Sul Micronutrient Mix
- CRF + Tiger Sul Micronutrient Mix + Tiger Sul Mn elevated by 20 percent
- CRF + Tiger Sul Micronutrient Mix + Tiger Sul Zn elevated by 20 percent
- CRF + Tiger Sul Micronutrient Mix + Tiger Sul Fe elevated by 20 percent
- CRF + Tiger Sul Micronutrient Mix + Tiger Sul boron (B) elevated by 20 percent
- CRF + Tiger Sul Micronutrient Mix + Tiger Sul Mn and B elevated by 20 percent
- CRF + Tiger Sul Micronutrient Mix + Tiger Sul Mn and B elevated by 50 percent
The amount of nitrogen applied in treatments 1 and 2 was 180 pounds/acre; in CRF (treatments 3 through 10), it was 150 pounds/acre. Both conventional and CRF blends had the same amount of phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg). However, with reduction in the amount of nitrogen in CRF, the amount of P, K, Ca and Mg applied was reduced. The foliar micronutrients were applied based on UF/IFAS recommendation.
The base Tiger Sul Micronutrient Mix was a Mn-Zn-Fe-B: 6-6-3-1 blend and was applied at 1.5 pounds/tree. The elevated rates were calculated from the base Tiger Sul Micronutrient Mix. For example, Mn applied in treatment 2 and 4 was 12 pounds/acre whereas in treatment 5 and 9, it was 14.4 pounds /acre.
After three years of trials at both sites, many striking differences were observed. Usually, the differences observed for various parameters were not in agreement at both sites. However, it is interesting to note that for all the parameters measured, including yield, canopy volume and fruit quality, treatment 1 (the control) was always the worst performing treatment regardless of the best performing treatment and the site of experiment.
With the use of the Tiger Sul product, a gradual decrease in pH was observed. The decrease in pH was slow, and on average, the pH dropped by 0.5 or less per year. Therefore, it is advised that if soil pH adjustments are made, regular soil pH monitoring should be done to ensure that soil pH does not drop below the recommended range.
Figure 1 shows the yield in boxes per acre for the final year of study for the two sites. At the ridge site, only treatment 9 performed better than the control. At the southwest Florida site, treatments 4, 6, 7 and 10 performed better than the control, with an approximate increase of 100 to 150 boxes per acre. In addition, an improvement in fruit size and consumer preference was also observed with elevated micronutrients (treatments 9 and 10).
During the trial, both the sites had unique natural disaster issues. The Fort Meade site suffered a significant outbreak of post bloom fruit drop in 2016, and the Arcadia site was dealt a direct hit from Hurricane Irma. Therefore, we calculated the cumulative yield for both sites over the three-year trial period (Table 1). It was mind-boggling to note that the cumulative ranks for the top-performing treatment at either site was the lowest-performing treatment at the other site, although the control was ranked lowest for both sites.
IMPLICATIONS FOR GROWERS
Overall, the data suggests the continuous supply of soil-applied nutrients through CRF and 20 to 50 percent elevated levels of micronutrients in addition to soil pH acidification improved yield and fruit quality. However, these results are not consistent across sites and therefore, a nutrition program should be site specific and customized to fulfill the nutritional needs of the trees.
Regular leaf sampling can be helpful in customizing a nutrition program. Leaf nutrient concentration should be maintained at the high end of the optimal range, as per the UF/IFAS nutrient concentration recommendation for healthy citrus. For a successful nutrition program, the following tips should be considered:
- The goal of a nutrition program should be a continuous availability of all nutrients to the tree year-round.
- Right source, right rate, right time and right place are key for fertilizer application.
- A nutrition program should be a combination of soil- and foliar-applied nutrients. Do not rely solely on foliar application for any nutrient.
- Soil-applied nutrients are taken up by the plant with the water uptake; therefore, irrigation scheduling is important.
- The placement of fertilizer (right place) is critical. It should be placed in the wetted zone since the uptake of nutrients occurs in a solution form.
- Frequent leaf nutrient analysis is essential for an effective nutrition program as it helps in ensuring that the fertilizer program is meeting tree requirements.
- The focus of a nutrition program should be on leaf nutrient levels and not on the rate of nutrient applied.
- Optimal soil pH is critical for making nutrients available to the tree; the soil pH needs to be in the right range at the time of nutrient application.
- If soil pH adjustments are made periodically, the fertilizer application should be coordinated to occur after the pH adjustment.
Acknowledgment: This work is funded by the Citrus Research and Development Foundation Project 18-061C. Sincere thanks to Harrell’s and Tiger Sul for providing fertilizer for these trials. Special thanks to Matt Shook, Trey Whitehurst and Jack Zorn.
Tripti Vashisth is an assistant professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred.