Soil Chemistry Part 2

SOIL CHEMISTRY SOIL – THE INTERFACE OF LIFE If texture is the most important physical property of soil, pH is the most important chemical property of soil  pH is the most important determinant of the growing capabilities of a soil sample  pH indicates the acidity or alkalinity of soil  The pH scale goes from 0-14, with 7 being neutral  The further from 7 a pH sample is, the more reactive it is; below 7 is acidic and above 7 is basic, or alkaline  For example, the acid in your stomach has a pH of 2; it could dissolve a metal razor blade because it is extremely acidic  Lye has a pH of 13 and would dissolve the skin off your finger; it is extremely alkaline.  EFFECTS OF PH Soil pH (or soil reaction) is most responsible for determining the availability of nutrients and minerals.  14 of 17 essential plant nutrients are obtained from the soil – pH determines how available these nutrients are  These nutrients are most available in a slightly acidic soil as compared to neutral or alkaline (basic)  A pH between 6 and 7 is usually most ideal for plant growth (although some plants have adapted for different pH’s)  PH SIMPLIFIED The pH scale is based on the number of hydrogen ions.  A hydrogen ion is a cation – it has a positive charge   H+ The pH number is based on the hydrogen concentration  When hydrogen cations are completely balanced by hydroxide (OH-) anions, you have a neutral, unreactive subtance (such as water)  The more imbalanced the solution, the more reactive it is.  SOIL PH  Soil pH results from the interaction of soil minerals, ions, and cation exchange  A soil with a high, alkaline pH results due to the reaction of water with calcium, magnesium, and sodium These minerals “steal” hydrogen ions, leaving hydroxide (OH-) ions behind that make the soil alkaline. These kind of soils are very common in areas with large amounts of limestone (such as Wisconsin)  This situation can also result around roadways, particularly in spring   A soil with a low, acidic pH is most often caused by acidic rain, carbon dioxide from decomposition, and acidic fertilizers EFFECT OF PH ON PLANTS  Again, most plants do well at a pH range of 6.0-7.0    The actual number of hydrogen cations does not actually affect plant growth that much.   Blueberries and evergreens are among the few that do well in acidic soils Alfalfa is one of only a few that does well in basic soils pH itself is usually does not have a direct effect Rather, other soil conditions can lessen or increase the impact of soil pH on plant growth. They are…    Nutrient availability Aluminum toxicity Soil microbes PH AND  A good example of soil pH and its impact on nutrients is phosphorus      Phosphorus is one of three key nutrients for plant growth (NPK), and is a main ingredient of ATP When soil pH falls below 5.8, phoshphorus reacts with iron to produce an insoluble iron compound.   NUTRIENTS Plants obtain their nutrients through absorbing groundwater If a nutrient is insoluble, it is unobtainable by plants When pH rises above 6.0, the reaction reverses and phosphorus dissociates from iron At a high pH, phosphorus undergoes a similar reaction with calcium Phosphorus may be present in the soil and may provide a good test result, but will be unavailable because of the pH PH AND TOXICITY If the soil pH drops below 5.5, aluminum begins to dissociate from the soil particles, especially in soils high in clay.  Normally aluminum is “held” by soil particles.  In clay-heavy soils, the clay and humus particles form a structure called a micelle.  The micelle structure has a negative charge  This negative charge attracts positively charged elements including metals such as aluminum as well as other elements such as hydrogen cations and sodium  This can help negate the impacts of road salt and other environmental assaults.  CATION EXCHANGE CAPACITY The ability of a soil to negate these often-harmful assaults on the environment is called the soil cation exchange capacity (or Soil CEC)  cation exchange capacity (CEC): ability of a soil particle to attract positively charged ions  If the pH drops too low, the strength of the chemical charge of the micelle is negated by the surrounding environmental conditions  Aluminum becomes soluble; in this case, the solubility is bad because it can form a metal toxicity when absorbed by the roots of a plant   Iron and manganese can have similar effects MICELLES AND CEC  micelles: clay and humus form a complex together known as the clay-humus micelle. A WORD ON SOIL CEC CEC can be thought of as the ability of soil to hold on to nutrients.  Clay-based soils have a very high CEC  Sand has a very low CEC  The more cation exchange a soil has the more likely the soil will have a higher fertility level.  Could CEC be too high?  Why do you suppose manure pits must be claylined?  PH AND  SOIL ORGANISMS As we discussed last week, the biodiversity of the soil is the most important biological property of the soil  Again, soil pH is the most important chemical property and texture is the most important physical property Soil organisms grow best in near-neutral soil  Acidic soil mostly inhibits the growth of organisms at the base of a soil food chain, particularly microbes and earthworms  This reduces their crucial activities, including nitrogen fixation and decomposition  SOIL ACIDIFICATION Soil Acidification is a natural process that is a part of all landscapes  Clearing land and replacing native vegetation with crops and pasture accelerates acidification  The addition of lime can raise the pH to more productive levels  The addition of sulfur can lower pH is the soil is too alkaline; ashes can also work 