Mike Roberts

What’s the Key to Nutrient Availability? Soil pH

Soil pH plays a huge role in nutrient availability, and it’s especially important when it comes to citrus trees affected by citrus greening, also known as HLB. HLB causes citrus trees to have smaller root systems that are weakened by the disease. Since the root systems are not able to uptake water and nutrients as well as healthy trees, optimal nutrient management and irrigation are essential to ensure tree health. This is where soil pH comes in. Soil pH affects the availability of the big three—nitrogen, phosphorus, and potassium—and more.

So What Is the ideal pH?

UF/IFAS has done extensive research on soil pH, especially in the wetted zone where irrigation from a micro-sprinkler can raise soil pH. UF/IFAS researchers maintain that the optimal soil pH range for Florida citrus production is 5.8 to 6.5. Research indicates that a pH range of 6 to 6.5 can help increase the availability of potassium, phosphorus, calcium, manganese, zinc, and iron, and that soil pH should not ever be lower than 5.

Soil pH can affect the nutrient availability of:

  • Nitrogen: There are multiple reactions that involve nitrogen that are affected by soil pH. One of the most important of these is nitrification—where ammonium is converted to nitrite and nitrate—and it occurs most rapidly at a soil Ph range of 7 to 8. The danger with nitrification is the quick leeching of applied nitrogen when it is converted to the more soluble nitrate.
  • Phosphorus: High soil pH leads to a decrease in the solubility and availability of phosphorus; this is due to phosphorus fixation where phosphorus reacts with soil calcium. If soil pH is high—above 7.2—then the soluble phosphorus will only be available to plants for a short time only. A soil pH range of around 6.5 to 7.2 will allow phosphorus to remain soluble and even release fixed phosphorus so it is available.
  • Potassium: When soil pH is high, potassium uptake is suppressed by “the occupation of the nutrient-holding sites of the soil particle surface” by calcium. In simplified terms, excessive calcium is better at occupying the exchangeable soil particle sites than potassium at high soil pH levels; lowering the soil pH will reduce the exchangeable sites of the soil particle and make potassium more available.

A Word on Soil Copper

While a lower soil pH is good for “the big three” nutrients, it can lead to copper toxicity. Higher soil pH levels, such as 6.5 to 7, cause copper to become tied up by the soil and biologically nonreactive. Lowering the soil pH releases bound copper, which can cause copper toxicity. Work with your Extension agent to find the optimal range for your grove’s soil pH.

These are all factors to keep in mind, in addition to the 4Rs of nutrient stewardship:

  • Right source
  • Right rate
  • Right time
  • Right place

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