The potential of Hydrogen (pH) measures the acidity or alkalinity of a growing medium, typically soil or a soilless substrate. The scale ranges from 0 to 14, where 7 is neutral, values below 7 indicate acidity, and values above 7 indicate alkalinity. The pH of the soil solution is a master variable because it directly controls the chemical form and solubility of nutrients. For a plant to absorb a nutrient, that element must be dissolved in the soil’s water solution, and pH is the most important factor determining its solubility and availability.
Chemical Principles of pH Control
The mechanism by which pH governs nutrient availability centers on two primary chemical processes: solubility and ion exchange. When the pH deviates significantly from the neutral range, many nutrient ions cease to be water-soluble and undergo precipitation. This causes them to chemically bind with other elements, forming solid compounds that plant roots cannot absorb, effectively “locking up” the nutrient.
The concentration of hydrogen ions (H+) affects the electrical charge of nutrient ions and soil particles. In acidic conditions, high concentrations of H+ ions compete with essential positively charged nutrients, such as calcium, magnesium, and potassium, for binding sites on soil colloids. This competition displaces the essential nutrients into the soil solution, where they can be lost through leaching.
Conversely, alkaline conditions (low H+ concentration/high pH) cause many metal-based micronutrients to hydrolyze. This chemical reaction changes them into less soluble forms, such as hydroxides and carbonates, which then precipitate out of the soil solution. Small changes in pH measurement lead to substantial differences in H+ concentration and nutrient chemistry.
Availability of Macronutrients
The primary macronutrients—Nitrogen (N), Phosphorus (P), and Potassium (K)—exhibit distinct behaviors influenced by pH. Nitrogen availability is indirectly regulated by pH through its effect on soil microorganisms that facilitate nitrification (the conversion of ammonium to nitrate). The nitrifying bacteria responsible for this conversion are most active in slightly acidic to neutral soils, with activity slowing dramatically below pH 5.0.
Phosphorus is the most pH-sensitive macronutrient, having a narrow window of maximum availability, typically between pH 6.0 and 7.0. In acidic soils below 6.0, phosphorus binds tightly with soluble aluminum and iron, forming highly insoluble compounds. When the pH rises above 7.0, phosphorus reacts with calcium, leading to the formation of insoluble calcium phosphates, known as phosphorus fixation.
Potassium, Calcium, and Magnesium are generally available across a wider range than phosphorus, but extreme pH values still hinder their uptake. Potassium availability is stable between pH 5.5 and 8.0, though it can be lost to leaching in highly acidic soils. Calcium and Magnesium are commonly deficient in strongly acidic soils because they are easily displaced by high H+ or toxic aluminum ions and subsequently leached. Sulfur (often in the form of sulfate) is typically soluble across a broad range.
Availability of Micronutrients
The behavior of essential micronutrients largely contrasts with that of the macronutrients. Micronutrients such as Iron (Fe), Manganese (Mn), Zinc (Zn), and Copper (Cu) are metals that become progressively less soluble and available as the pH increases above 7.0. In alkaline soils, these positively charged metal ions combine with hydroxyl ions (OH-) to form insoluble precipitates, leading to common deficiencies like iron chlorosis.
Deficiencies in these four metals are a frequent issue in high-pH soils, even if the total amount of the element is substantial. Conversely, in highly acidic soils, these metallic micronutrients become extremely soluble, sometimes reaching toxic levels for the plant. This increased solubility can lead to issues like manganese toxicity when the pH falls below 5.5.
Molybdenum (Mo) and Boron (B) are notable exceptions. Molybdenum availability increases significantly as the pH rises into the alkaline range, often becoming deficient in acidic soils. Correcting Molybdenum deficiency is often achieved by raising the pH rather than adding more of the element. Boron is generally available across the optimal range but sees reduced uptake in highly alkaline conditions as it converts to the less available borate form.
Modifying pH for Optimal Uptake
The goal of pH management is to maintain the growing medium within a range where all necessary nutrients are simultaneously available. For most common crops, the optimal range is between pH 6.0 and 7.0. Within this slightly acidic window, the availability of phosphorus and the metal micronutrients is maximized, while the risk of aluminum toxicity is minimized.
If soil testing reveals the pH is too low (acidic), the standard practice for raising the pH is the application of liming materials, such as agricultural lime (primarily calcium carbonate). Calcitic or dolomitic lime slowly neutralizes the acidity, with the latter also supplying magnesium. The amount of lime needed depends on the soil’s buffering capacity, which is its natural resistance to pH change.
When the pH is too high (alkaline) and needs to be lowered, growers typically apply elemental sulfur or substances that generate acidity. Soil microbes gradually convert elemental sulfur into sulfuric acid, which lowers the pH. The addition of acidic organic matter or certain nitrogen fertilizers, such as ammonium sulfate, can also be used to gently drive the pH downward toward the optimal range.