ROLL # 11041706-010
DEPARTMENT OF BOTANY
M.PHIL BOTANY FINAL SEMESTER
Plant nutrient usually exist as ions i.e.
They carry an electrostatic charge.
The positively charged nutrients are known as Cations
Negatively charged nutrients are called as Anions.
Cations. Ca2+,Mg2+,,K+,Na+, H+,Al3+
Anions: NO -,H PO -,HPO 2-,SO 2-,Cl3
From 1920s to 1940s William Albrecht did a lot of
experimenting with different ratios of nutrient cations.
Cations bound to soil in varying degree
Strongly bound to silica or Soluble in soil solution.
Between these two extremes are the exchangeable cations, which
are weakly bound to soil particles.
Soil particles carry net negative electrostatic charges as a result of
processes of soil weathering, and organic decomposition.
These sites of negative charges are most predominant in the
humus fraction of the soil, and on the edges of clay particles.
The bonding of these cations largely prevents their loss by
leaching, but is not so strong that plants cannot extract them from
The Cation exchange capacity of a soil is a
measurement of its ability to bind or hold exchangeable
cations. In other words, it is a measure of the number of
negatively-charged binding sites in the soil.
Low organic matter. Sandy
Pumice soil, Lower fertility
High fertility soil, High
It refers to Elements that are basic or Alkaline in their
reaction.e.g K,Mg, Ca & small amount of Na & Al.
Hydrogen is an element with a positive charge and acts
like a cation however soils with significant saturation
of hydrogen are acidic, or have a lower pH.
The measure is expressed as milligram equivalents per
100 grams of soil or shortened to “me”.
CEC=3.83meq/100 g soil
Total for bases=K+Mg+Ca+Na=1.43meq/100g soil
Percent Base saturation= (1.43/3.83)(100%)=37%
Exchangeable Cations can be divided into two groups.
Every CEC binding site must have a cation bound to it,
to maintain electeroneutrality.
The soil pH Will be effected by whichever cations
predominate on these exchange sites.
More base cations more alkaline soil
More acid Cations more acidic soil
It is the fraction of the negative binding sites occupied
A base saturation level of 75% means that three out of
four sites are occupied by basic cations (remaining
25% by acidic cations).
Total base saturation is determined by following
Total base saturation= Ca+Mg+K+Na
CEC also helps to characterise soils.E.g
Organic matter is the major source of Negative
electrostatic sites there is a strong correlation between
CEC values, and the amount of organic matter in the
CEC can give insight into soil quality and site
Higher CEC likely indicates more clay, poor internal
drainage, limited structure and soil compactation in
high traffic areas.
Low CEC is indicative of sandy textured soils prone to
drought that invariably needs more organic matter to
improve water holding capacity, but have open grainy
structure that resist compaction.
What we have learned
Clay and organic matter have negative charges that can
hold and release positively charged nutrients.(The
cations are adsorbed onto the surface of the clay of the
clay or humus).That static charge keeps the nutrients
from being washed away, and holds them so they are
available to plant roots and soil microorganisms.