Summary: What is PH? - Power of hydrogen What's really being measured is the concentration of hydrogen (H) ions -- the more hydrogen ions there are, the more acid the thing being measured is. An anion (−) (AN-eye-ən), from the Greek word ἄνω (ánō), meaning "up", is an ion with more electrons than protons, giving it a net negative charge (since electrons are negatively charged and protons are positively charged). A cation (+) ( KAT-eye-ən), from the Greek word κατά (katá), meaning "down", is an ion with fewer electrons than protons, giving it a positive charge. Since the charge on a proton is equal in magnitude to the charge on an electron, the net charge on an ion is equal to the number of protons in the ion minus the number of electrons. Sources of Acidity Acidity in soils comes from H+ and Al3+ ions in the soil solution. While pH is the measure of H+ in solution, Al3+ is important in acid soils because between pH 4 and 6, Al3+ reacts with water (H2O) forming AlOH2+, and Al(OH)2+, releasing extra H+ ions. Every Al3+ ion can create 3 H+ ions. * Rainfall: Acid soils are most often found in areas of high rainfall. Excess rainfall leaches base cation from the soil, increasing the percentage of Al3+ and H+ relative to other cations. Additionally, rainwater has a slightly acidic pH of 5.7 due to a reaction with CO2 in the atmosphere that forms carbonic acid. * Fertilizer use: Ammonium (NH4+) fertilizers react in the soil in a process called nitrification to form nitrate (NO3-), and in the process release H+ ions. Na+, K+, Ca2+, Mg2+ and Cl cause salinity. * Plant root activity: Plants take up nutrients in the form of ions (NO3-, NH4+, Ca2+, H2PO4-, etc.), and often, they take up more cations than anions. However plants must maintain a neutral charge in their roots. In order to compensate for the extra positive charge, they will release H+ ions from the root. Some plants will also exude organic acids into the soil to acidify the zone around their roots to help solubilize metal nutrients that are insoluble at neutral pH, such as iron (Fe). * Weathering of minerals: Both primary and secondary minerals that compose soil contain Al. As these minerals weather, some components such as Mg, Ca, and K, are taken up by plants, others such as Si are leached from the soil, but due to chemical properties, Fe and Al remain in the soil profile. Highly weathered soils are often characterized by having high concentrations of Fe and Al oxides. Sources of Basicity Basic soils have a high saturation of base cations (K+, Ca2+, Mg2+ and Na+). Alkaline soils are characterized by the presence of carbonates. Too Much Acidity Plants grown in acid soils can experience a variety of symptoms including aluminium (Al), hydrogen(H), and/or manganese(Mn) toxicity, as well as potential nutrient deficiencies of calcium (Ca) and magnesium(Mg). Aluminium toxicity is the most widespread problem in acid soils. Aluminium is present in all soils, but dissolved Al3+ is toxic to plants; Al3+ is most soluble at low pH, above pH 5.2 little aluminum is in soluble form in most soils. Aluminium is not a plant nutrient, and as such, is not actively taken up by the plants, but enters plant roots passively through osmosis. Aluminium damages roots in several ways: In root tips and Aluminium interferes with the uptake of Calcium, an essential nutrient, as well as bind with phosphate and interfere with production of ATP (http://en.wikipedia.org/wiki/Adenosine_triphosphate) and DNA, both of which contain phosphate. Alminium can also restrict cell wall expansion causing roots to become stunted. Below pH 4, H+ ions themselves damage root cell membranes. In soils with high content of Manganese (Mn) containing minerals, Manganese toxicity can become a problem at pH 5.6 and below. Manganese, like aluminum becomes increasingly more soluble as pH drops, and Manganese toxicity symptoms can be seen at pH's below 5.6. Mn is an essential plant nutrient,
