Chapter 11 - Soil pH and Salinity

Soil pH - WHAT is it and WHY is it Important? pH is the concentration of hydrogen ions (H+) in the soil solution pH affects the availability of nutrients for plant uptake

Most agricultural soils Extreme pH range for most mineral soils pH Range Related to Plant Needs walnut: 6.0-8.0 Most desirable carrot: 5.5-7.0 cucumber: 5.5-7.0 spinach: 6.0-7.5 tomato: 5.5-7.5 white pine: 4.5-6.0 Very strong Strong Moderate Slight Neutral Acid Basic 3 4 5 6 7 8 9 10 11 1 2 12 13 14 Most agricultural soils Extreme pH range for most mineral soils cranberry:4.2-5.0 apple: 5.0-6.5

pH Values by Region in the US Why do we see such variation???

Causes of Acidity soil water + CO2 (gas) root growth and decaying plant litter addition by humans - commercial fertilizers Nutrients are added to the soil from several sources. The main sources of nutrients for agricultural soils are: Nutrients that are recycled to the soil from decaying plant litter, Minerals in soil that gradually dissolve and release nutrients, and Nutrients added by humans in the form of fertilizers, manure, limestone, and other materials such as sewage sludge, compost, leaves, and food processing wastes. The smoke stack shown here is another source of nutrients for soils. Interestingly a lot of sulfur used to be supplied to soils from this source, especially combustion of coal. We are now cleaning up our air and much less sulfur is falling onto our soils. We may eventually see a need for more sulfur fertilization. Lets look a little more closely at each of these nutrient sources.

Nutrient Availability Influenced by pH

Improper pH can Lead to Nutrient Deficiencies or Toxicity's normal leaf iron deficient leaf

Active vs. Reserve Acidity active acidity - H+ ions in soil solution - what is actually measured by a pH test reserve acidity - H+ and Al+++ ions adsorbed to soil colloids

pH Adjustments add a liming material to raise the pH CaCO3 + 2H+ Ca2+ + H2O + CO2 (gas) - calcitic limestone - dolomitic limestone - quicklime - slaked lime add sulfur to lower the pH 2S + 3O2 + 2H2O 2H2SO4 4H+ + 2SO42-

Benefits of Liming reduces the solubility and potential toxicity of Al and Mn supplies the essential elements Ca and/or Mg, which are generally low in very acid soils increases the availability of several essential nutrients stimulates microbial activity (such as nitrification) in the soil improves symbiotic nitrogen fixation by legumes

Saline Soils high in salts but low in Na+ high EC values (> 4 mm/cm) high pH (< 8.5) correct by leaching

Sodic Soils high in Na+ but low in other salts poor structure and drainage very high pH (> 8.5) correct by adding gypsum