The pH (hydrogen ion concentration) of soil affects the form and availability of nutrients and the ability of roots to absorb nutrients and water. Nutrient deficiency or toxicity symptoms commonly develop when plants grow in soil with unfavorable pH. The activity of beneficial soil microorganisms (e.g., nitrifying bacteria) also depends partly on pH.
A scale from 0 to 14 is used to express pH. A pH of 7.0 is neutral. Lower numbers represent increasingly acidic conditions. Higher numbers indicate increasingly alkaline (basic) conditions. Because a negative logarithmic scale is used, ten times as many positively charged hydrogen ions are available at a pH 6 than at pH 7. For optimal nutrient availability for plant growth, a good range of soil pH for most gardens and landscapes is 6.0 to 7.5.
High pH is common in California soils. Because soil pH affects nutrient availability to roots, the primary symptoms of adverse soil pH are similar to those that can occur from nutrient deficiencies or excesses (toxicities). High pH causes interveinal chlorosis and bleaching, pale mottling, and blotchy or marginal necrosis of new growth. Damage is primarily due to reduced availability of minerals, especially iron, manganese, and zinc, so any of the symptoms of those deficiencies may occur in high-pH soils.
If soil pH is below about 5.5, new foliage becomes chlorotic, distorted, and possibly necrotic. Plant growth slows. In severe cases affected roots can become discolored, short, and stubby. Symptoms result primarily from aluminum toxicity and deficiencies of calcium and magnesium. Copper and manganese toxicity and phosphorus deficiency symptoms may also occur. Acidic soils occur mostly in conifer forests and regions with high average rainfall. Low pH is also common in exposed subsoils in the Coastal Range westward to the Pacific Ocean, mostly due to soil cuts and grading during development.
Determine if soil pH is not favorable to plant growth using knowledge of local soil conditions and the existing plant species’ susceptibility to unfavorable pH. Especially test the pH of soil from the root zone. If sodic soils (soils high in sodium) are suspected, obtain a value for sodium adsorption ratio (SAR) and a separate test of calcium carbonate (percent lime), as discussed below in "buffering capacity."
Where soil falls outside the desirable range of pH 6 to 7.5, grow species tolerant of the pH at that site. To learn what landscape plants grow in acidic soils (e.g., in California’s North Coast) versus alkaline and sodic soils (many arid locations) consult resources such as Abiotic Disorders of Landscape Plants and The New Sunset Western Garden Book.
To raise pH in the few landscape locations where pH is too low, mix appropriate amounts of finely ground limestone into soil before planting (soil tests can provide specific rate recommendations) or fertilize with calcium nitrate. To lower pH, mix elemental sulfur, iron sulfate, and organic matter into topsoil. Apply mulch and when nitrogen application is warranted, use acidifying fertilizers such as ammonium sulfate. Follow the other management practices described under "Solutions" in Iron Deficiency and Nutrient and Mineral Excesses, Salinity, or Salt Toxicity.
Buffering capacity. The ability of soil to resist changes in pH is called buffering capacity. Soils’ response to acidifying amendments is usually estimated by measuring calcium carbonate (percent free lime). The calcium carbonate measurement indicates the amount of acidifying amendments or fertilizers needed to lower the pH. When calcium carbonate is low, the pH of soil more readily changes after the application of certain fertilizers or other amendments (e.g., sulfur) that make the soil more acidic. If calcium carbonate is high, the pH tends to stay high. If both pH and calcium carbonate are high in soil, it is difficult to lower soil pH sufficiently to grow certain plants. Only species more tolerant of high pH may grow there.
Acidic soils with high buffering capacity can resist the effects of adding lime or limestone to raise pH. Raising pH is not usually needed for California soils, but may be required for plants where soil pH is naturally low or has been lowered by agricultural practices or development.
For more information see Abiotic Disorders of Landscape Plants and Water Quality: Its Effects on Ornamental Plants .
Adapted from the publications above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Marginal leaf necrosis of avocado irrigated with alkaline water.
Chlorosis of chrysanthemum irrigated with alkaline water (left) compared with healthy leaves.
Chlorotic sweet gum leaves due to iron deficiency caused by alkaline soil.
Dieback of Melaleuca myrtaceae from iron deficiency caused by alkaline soil.
Chlorotic and necrotic Catalina ironwood leaves caused by alkaline soil.