By Brandon White
This time of year is the end of the peak nutrient demand for citrus trees. Regardless of the types of fertilizer or inputs applied in the groves, trees have been taking up nutrients while growing in full tilt during the spring season.
One of the greatest factors determining how well trees take up nutrients is soil pH. Having soil pH dialed in to the recommended range will increase nutrient uptake efficiency, thereby making better use of costly inputs and potentially decreasing the nutrient rates required for optimal tree health.
The preferred soil pH range for most plants is 5.5–6.5, but recent research suggests a slightly narrower range of 5.8–6.5 for citrus in the HLB era. Figure 1 shows 12 of the 14 essential nutrients plants take up from the soil. (The other two are not pH dependent). The wider the bar, the more available a nutrient is at that corresponding pH value; the narrower the bar, the less available it is.
Most micronutrients are more available at lower pH and less available at higher pH, while the opposite is true for macronutrients. Excessively low soil pH values for citrus can be problematic. Micronutrients are needed in smaller amounts yet are more available at these lower pH values. Toxicities can develop if plant uptake is too high. Aluminum, naturally found in high levels in most soils, can also become toxic when pH drops below 5.0.
Several factors can determine the soil pH. Many soils in South Florida tend to be high pH because of the influence of limestone bedrock, but many other soils in the state naturally have much lower pH values. Even within a grove, the soil pH can vary, so it is best to break up large groves into smaller blocks when sampling soil to check pH.
Irrigation water in Florida often has high bicarbonates and pH values, which can increase the soil pH over time. Fertilizers, particularly high ammonium sources of nitrogen, acidify the soil and their repeated use over time can drive down the soil pH. Composts are great amendments that provide many benefits to overall soil health, but many sources are high in pH (7.0+). So that needs to be considered when applying, particularly where soil pH is already high. Seasonal changes in soil and well water pH can also be observed.
MAINTAINING THE RIGHT RANGE
Maintaining soil pH within the recommended range is an important practice. From a management standpoint, raising the pH is often easier and lasts longer than lowering the pH.
Lime and dolomite are often used to raise pH and are relatively inexpensive. Composts or other additives with high pH can also increase soil pH. Large inputs of organic matter to sandy soils over time can also increase the buffering capacity of the soil and in turn help resist pH changes.
Lowering the soil pH is often achieved through acidifying irrigation water or adding elemental sulfur products to the soil. Care needs to be taken in acidifying irrigation water, and systems need to be checked regularly. Mistakes or equipment failure can cause over-acidification and quickly have negative impacts to the rootzone of the soil and impact production. When carefully managed, neutralizing bicarbonates in irrigation water by acid injection can be very effective, however.
Granular elemental sulfur products are readily available and can be used by growers to lower soil pH. These products are broken down by soil microbes that acidify the soil in the process. Because this process is dependent on microbial activity, it can take longer for this affect to take place. Growers can also use acid-forming fertilizer blends on a regular or occasional basis depending on pH trends in the grove.
Checking soil pH once a year is a recommended practice. Typically, growers will submit a soil sample to a lab for a full nutrient profile that includes pH. The best time to do this is late summer/fall before fertilizer applications need to be made and to allow time for potential adjustments in pH to take effect. Comparing soil pH year after year can be helpful to track trends over time and to aid in management decisions.
See Citrus Soil pH Management for more information.
Brandon White is a University of Florida Institute of Food and Agricultural Sciences commercial crop production and food systems Extension agent in Tavares.
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