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Chemical Properties of Soil
- pH
- Salinity (EC)
- Cation exchange capacity (CEC)
- Organic matter
- C:N ratio (Carbon to Nitrogen)
Soil pH
- A measure of the acidity or alkalinity of a soil.
- Neutral
= 7.0
- Acidic < 7.0
- Alkaline > 7.0
- Logarithmic
scale which means that a 1-unit drop in pH is a
10-fold increase
in acidity.
Soil pH and plant growth
- Affects availability of plant nutrients (in general,
optimal pH is between 5.5-7.5)
- Low pH soils (<6.0)
results in an increase in Al. Aluminum is toxic
to plants
- Affects availability of toxic metals
(in general,
more available in acidic soils)
- Affects the activity
of soil microorganisms, thus affecting nutrient
cycling and disease risk
Nutrient Availability
Increasing soil pH: Liming
materials (pure calcium carbonate or dolomitic lime)
will increase soil pH.
- Lime is a certified organic
product
- Slow-release product. Do not add every year.
- 15-25
lbs lime per 1000 sq ft is recommended
Wood
ashes are another product to raise soil pH. They
also are
a source
of K, Ca, and Mg. Some composts also can
increase soil pH.
Gypsum is calcium sulfate. It is not a substitute for
lime, and has little effect on soil pH. Gypsum only improves
structure in soils that have extremely high sodium
contents (rare in the NW).
Decreasing soil pH: Some plants thrive under acidic
conditions (ex. rhododendrons, blueberries, and azaleas). Elemental
sulfur is often recommended (50 lb S per 1000 sq. ft).
Ammonium and
ammonium-forming N fertilizers will also result in
a decrease in soil pH.
Soil salinity
- Potential problem in irrigated soils due to high
evaporation rates and low annual rainfall leaving salts
to accumulate.
- Salts can come from irrigation water,
fertilizers, composts, and manure.
- Salts can be
leached by slowly applying excess water.
- Three
inches removes about 50% of the soluble salts.
- Five inches removes about 90%.
Soil Salinity and Interpretation
Conductivity (mmho/cm)
|
Interpretation |
| 4 or above |
Severe accumulation of salts. May restrict growth of many vegetables
and ornamentals. |
| 2 to 4 |
Moderatre accumulation of salts. Will not restrict plant growth,
but may require more frequent irrigation. |
| less than 2 |
Low salt accumulation. Will not affect plants. |
Cation-Exchange Capacity
A cation is a positively charged ion. Most nutrients
are cations: Ca2+, Mg2+, K +, NH4 +, Zn2+, Cu2+, and
Mn2+. These cations are in the soil solution and are
in dynamic equilibrium with the cations adsorbed on
the surface of clay and organic matter. CEC is a measure
of the quantity of cations that can be adsorbed and
held by a soil.
CEC is dependent upon the amount of organic matter
and clay in soils and on the types of clay. In general,
the higher OM and clay content, the higher the CEC.
Next page: Soil organic matter
Updated
July 15, 2004
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