Nutrient Recommendations

The Fate of Phosphorus in Florida Citrus Soils

Ernie NeffNutrition, soil


Multi-county citrus Extension agent Chris Oswalt discussed the fate of phosphorus in Florida citrus soils in the September Extension newsletter, Citrus from the Ridge to the Valley. Excerpts from his article follow.

The amount of phosphorus in soil is closely related to the potential availability of this phosphorus to plants … Potentially available soil forms of phosphorus are dependent on soil pH. Some is available or soluble (fertilizer forms), some will be tied up or bound in other compounds (aluminum, iron and calcium), and some could be in the sparingly soluble rock form.

In higher-pH, alkaline soils, phosphate ions react with soil calcium ions resulting in a decreased solubility of phosphorus (a process called phosphorus fixation). Due to the rapid phosphorus fixation at high soil pH, applications of soluble phosphorus in these soils will only be available to plants for a short time. In this situation, lowering the soil pH will decrease the amount of fixed phosphorus, resulting in additional previously insoluble phosphorus becoming available.

Conversely, in acidic soils with lower soil pH, phosphorus faces a similar issue, but through a different mechanism. Under lower soil pH levels, soluble phosphorus will bind with iron, aluminum and manganese ions to form hydroxy phosphates, which are insoluble and unavailable to the plant. In this situation, raising the soil pH to 6.5 will disassociate these ions, resulting in phosphorus availability.

Another source of phosphorus in the soil (not readily plant-available) is relatively insoluble phosphate rock that is predominately mined in and around the Central Florida citrus production region. Mined phosphate rock deposits include carbonate fluorapatite (francolite), quartz, dolomite, and clay materials. Francolite material is the form mined in Florida and used in the production of phosphate fertilizers. This phosphate rock is processed further using acid to form soluble phosphate fertilizers.

The availability and solubility of phosphorus are closely related to pH, and at about the 6.5 pH level, phosphorus is most readily available. When pH levels vary from this value, some soil phosphorus can become insoluble and not available for plant uptake.

In a soil test analysis, these factors can be the source of variability in determining how much of the soil phosphorus is soluble and available for citrus root uptake. Soil testing methods vary and try to determine the amount of phosphorus available in a soil sample. Actual plant response curves or extrapolated response data then try to approximate the plant-available amount of soil phosphorus.

Therefore, depending on the extraction methods, one could get different levels of extractable phosphorus. The actual amount of soil phosphorus reported by a specific soil test should be considered, along with the soil pH and soil conditions from which the sample was collected. The review of this information should be in concert with leaf tissue analysis for phosphorus. Thus, a grower can determine over time if the soil phosphorus level provides enough phosphorus to the tree or if the amount in the soil is not readily available for plant uptake. This information will allow growers to manage phosphorus fertilization better.

Source: University of Florida Institute of Food and Agricultural Sciences

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