Soil Chemistry

Soil: is the top layer of the earth’s crust in which organic matter grows. There are many components which determine a soil type such as pH, nutrient level and organic content. These factors can vary depending on the type of plant or crop which grows in the soil and also on geographic location.

Soil pH

  • Soil pH is the single most important chemical property of the soil (like soil texture is to the physical properties).
  • Knowing the pH of the soil will quickly allow you to determine if the soil is suitable for plant growth and what nutrients will be most limiting.
  • Strictly speaking, hydrogen ions are protons and do not exist in the naked state in fluids; instead they react with water (H20) to form hydronium ions, such as H3O+
  • For most purposes H+ can be used to represent these hydrated protons.

Acid Base Chemistry

  • Acid-base chemistry is an important part of everyday life. The excess hydronium (H3O+) ions in acids give them interesting properties.
  • Acids can react with metals and other materials. The strong acid HCl is produced in your stomach to help digest food. In dilute concentrations, acids are responsible for the sour taste of lemons, limes, vinegar and other substances.
  • Bases are also very reactive. The strong base NaOH is used in many household cleaning agents such as oven cleaner and drain clog-remover.
  • How do we measure the concentration of an acid or base?

Measuring Acidity

  • The acidity or basicity of a solution is measured

using the pH scale. (this scale is used because of the very

small concentrations that are being measured)

  • The pH scale corresponds to the concentration of

hydronium ions in a solution.

  • If you take the exponent of the H3O+ concentration

and remove the negative sign, you have the pH of a solution.

  • For example, in pure water the concentration of hydronium ions is 1 x 10-7 M.
  • Thus, the pH of a solution of pure water is 7.
  • The pH scale ranges from 0 to 14, where 7 is considered neutral ([H3O+ ] = [OH-]),

pH

  • Acid solutions are when pH is < 7.0
  • Alkaline solutions are when pH> 7.0
  • An acid can be defined as a proton donor, a chemical that increases the concentration of hydronium ions in solution.
  • Conversely, a base is a proton acceptor, a chemical that reduces the concentration of hydronium ions in solution (and increases the concentration of hydroxide ions).

Soil pH- Measure H+ in the Soil Solution

pH: the negative log of the hydrogen ion(H+) concentration in the soil water solution.

pH = - log [ H+]

The pH scale is how we measure acidity and alkalinity of solutions.

at neutral (pH =7) the number of H+ = OH-

Remember:

at pH of 6 there are 10x more H+ ions than at a pH 7 and there are 100x more H+ ions between pH 7 & 5

Soil Cation Exchange

  • Cation Exchange: the ability of the soil to hold onto nutrients and prevent them from leaching beyond the roots.
  • Cations are positively charged ions like Ca++, Mg++, K+, NH4+,
  • The more cation exchange a soil has the more likely the soil will have a higher fertility level.
  • The interchange between a cation in solution and another cation on the surface of any negatively charged material such as “clay or organic matter”

Cation exchange influenced by:

1)Strength of adsorption:

Al+3 > Ca2+ > Mg2+ > K+ =NH4+ > Na+ >H+

held tight ------> easily replaced

2) The relative concentration of the cations in the Soil Solution

Cation Exchange Capacity (CEC):

The CEC measures the extent to which soil can hold and exchange plant nutrient “cations”. The ability of soil to hold positively charged nutrients from being leached and lost from soil is important to maintaining soil fertility. Clay and organic matter have a negative charge. They allow the soil to hold these nutrient cations due to the attraction of charges. Soils with high clay or organic matter content will have a higher CEC. Sandy soils tend to have a lower CEC.

CEC also defined as:

1)The number of cation adsorption sites per unit weight of soil

or

2)The sum total of exchangeable cations that a soil can adsorb.

CEC is expressed in milliequivalents (meq) per 100 g of oven dry soil.

Equivalent weight = molecular or atomic wt (g)

valence or charges per formula

Milliequivalent (MEQ): 1 meq wt. of CEC has 6.02 x 1020 adsorption sites

MEQ of Common Cations

Element: Na+ K+ Ca++ Mg++___

Valence: 1 1 2 2______

Eq. Wt: 23/1=23 39/1=39 40/2=20 24/2 = 12

MEQ wt: 0.023 0.039 0.02 0.012____

- Active Acidity: due to the H+ ion activity in the soil solution at any given time.

- Reserve Acidity (on sites): represented by the H+ and Al3+ that are easily exchanged by other cations (positively charged ion).

Sources of acidity in Soil

Hydrogen and Aluminum cations are responsible for soil acidity.

Exchangeable Hydrogen is the main source of H+ at pH 6 and above. Below pH 6 Aluminum is the main source of H+ due to dissociation of Al from clay minerals. Aluminum becomes more soluble at lower pH’s

Al3+ + H20 ---> Al(OH) 2+ + H+

Al(OH) 2++ H2O ---> Al(OH)2+ + H+

Al(OH)2+ + H20 ---> Al(OH)3 + H+

1.Nitrification: Ammonium to Nitrate (oxidation of NH4+)

NH4+ + 2O2 ---> NO3- + H2O + 2 H+

2.Organic Matter decomposition:

organic acids ionized :

R-COOH ---> R-COO- + H+

- Respiration:

CO2 + H2O ---> H2CO3 = H+ HCO3-

3. Acid rain

4. Uptake of basic cations by plants:Basic cations are sources of OH- to the soil solution.

Ca++, Mg++, K+, are the basic cations that are taken up by plants no longer contribute OH- to the soil solution.H+ ions are released to the soil solution.

5.Leaching of basic cations: as basic cations are removed from the soil solution by leaching they no longer contribute the OH- ions to neutralize the ever increasing amounts of H+

Ca++ + 2 H20 ---> Ca(OH)2 + 2H+

Ca++ + 2OH-

Soil Acidity and Plant Growth

- Soil acidity is a major environmental stress factor which limits the growth of most crops.

- Acid soils are widely dispersed and comprise approximately 40% of the arable land in the world.

- In addition, acid rain also accelerates the acidification of soils.

- Aluminum (Al) ion is solubilized from soils at low pH. This is a major toxic factor for plant growth for low pH soils.

Soil Buffering Capacity

This is the ability to withstand rapid pH fluctuation. The greater the buffering capacity, the greater the quantity of acid or base which must be used to alter the pH.

Soil types having low buffering capacities include sandy soils with little clay or organic matter. Soils with a higher buffering capacity would have large quantities of mineral clay and organic matter.

Therefore, a thick rich soil with a high buffering capacity would require more lime in orderto raise the pH.

Taking a Soil Sample

  • You need a pail or ice cream container, a small shovel or spade, and a sample bag.
  • Choose a few uniform spots in the field or yard and take a few small samples from each one. This will give a good representative sample of the test area.
  • The soil should be sampled to a depth of about 6 inches (15cm). You need to dig enough soil for a 500 gram sample (roughly). Try not to include too much grass.
  • Put all the soil samples in the container and mix it thoroughly.
  • Use the soil sample bags

Finally we can use these samples in performing the different tests or chemical analysis according to our purpose.