Soil Properties Related to Soil Fertility
A. Physical Properties
Relative amounts of sand, silt and Clay
influences porosity, permeability, ease of tillage and nutrient
retention. Sand - 0.05-2.00 mm contributes to aeration, drainage and
tilth - little to soil fertility Silt - 0.002-0.05 mm - Also contribute
little to soil fertility. Clay - < 0.002 mm - Negative charge, large
surface area, retains nutrients, poor physical properties Soil
Textural Classes - a grouping based on the relative proportions of
sand, silt and clay. Ex. sandy loam clay loam
2. Soil Color
direct effect on plant growth but is an indicator of properties that
1. Black - High in organic
matter drainage may be poor
0 - 4 %
2. Bright red and yellow colors well drained.
3. Gray colors - poorly drained reduced iron.
of soil particles into aggregates. Size shape and stability affect
porosity and permeability, resistance to crushing and good filth. Can
be destroyed by tillage compaction and erosion.
4. Soil porosity
is total pore space - 30-60% by volume. Permeability - refers to ease
with which water and air move through the soil. Depends on texture
structure and organic matter content.
Moisture holding capacity
by soil texture and organic matter water is held in films around soil
particles attraction is strong between water and clay sized particles
therefore the more clay present, the more water a soil will hold.
- all pore space is filled with water
capacity - moisture content when a soil is holding all the water it
can against the force of gravity.
point - the percentage of soil moisture at which plants wilt
d. Plant available water =
depth affects the volume of soil available for roots to take up water
and nutrients. Can be restricted by bed rock, pans, low pH, high water
runoff; amount of erosion and management practices
B. Soil Chemical Properties
colloids make up the chemically active portion of the soil.
oxides of iron and aluminum
have net negative charges. The amount of negative charge is called the
CEC (cation exchange capacity). Positively charged cations are adsorbed
on the surface. Cation exchange is simply the reversible process by
which cations are exchanged between solid and liquid phases.
quantity of cations held by 100 g (oven dry) of soil. Expressed as meq/lOOg. Meq = 1/1000 of an equivalent weight.
weight is equal to weight of an element that will replace 1.008 g of H.
or 8 g of oxygen.
eq. wt. = atomic weight
wt Ca 40 valence 2
eq wt = 40 = 20
meq wt = .020
general order of strength of adsorption of the common soil cations to
cation exchange sites on the surfaces of colloids is called the
> Ca+2> Mg+2 > K+ = NH4+
The position of H+ is not clear. It is a
highly hydrated monovalent cation that should be near Na+
degree of hydration,and
concentration affect the strength of adsorption.
Charge (valence of the ion)
The higher charge, the greater the attraction to exchange sites
Degree of hydration
The hydrated size of an ion is the size of the ion plus its water
shell. The larger the hydrated size of an ion, the less attraction to
Effective concentration of the ion
cation exchange capacity of the whole soil is affected by:
2. kind of clay
3. organic matter
oxides of iron and Al
CEC (cmol Kg-1)
Adapted From Sparks 1995. Envornmental Chemistry
of Soils. Academic Press.
is affected by pH (pH dependent CEC). Lower when acid (3-4). Highest
when alkaline (8-9) due to ionization of OH on clay edges, hydrous
oxides, and organic matter.
d. % Base
saturation - Proportion of the total CEC occupied by basic cations. A
high base saturation is desirable
BS = total bases X 100
1. The OH groups
associated with the surface of iron and aluminum oxides and hydroxides
and with 1:1 clays can be a source of positive charges under acid
reacts with soluble iron, aluminum and manganese and hydrous oxides of
these minerals sulfate is held moderately
and chloride held very little and is leached by soil water. The pH of
most productive soils is too high for development of anion exchange
capacities. Anions with the exception of phosphate and to a lesser
degree sulfate, will not be retained.
C. Soil organic matter
Factors affecting soil organic matter content.
- high temperatures favor decomposition of plant residue. Less organic
matter in warm climates
Precipitation - O.M. is less where low soil moisture limits plant
drained soils - organic matter accumulates when oxygen supplies are too
low for microbial activity. Plant residues are not decomposed so they
accumulate. Example - swamps, marshes, blacklands
of NC and organic soils of Florida. When these soils are drained the OM
decomposes causing subsidence.
d. Texture -
Finer textures accumulate more organic matter.
Harvesting plant material returns less to soil than the natural
Tillage increases rate of decomposition
Good fertility and crop rotation help maintain organic matter.
Maintenance of organic matter in cropped soils.
Attempts to increase organic matter levels beyond those which occur
naturally in a given climate are usually ineffective.
State University research applied 30 tons manure/acre annually for 9
years to a loamy sand and raised O.M content from 2.63 to 2.83%.
pounds acre-furrow slice x .01 to raise OM 1% = 20,000 pounds of humus
10 tons of organic matter are added most decomposes leaving only a
small amount of humus.
erosion and producing high crop yields are essential in maintaining
soil organic matter.
root systems aid in conditioning soils.
matter decomposition prevents accumulation of plant residue,
mineralization of plant residue provides source of nutrients for
of Fe and Mn prevents toxicity
c. N fixation
Blue-green algae legumes
improve structure and aeration
slugs, Plant diseases,Nematodes
,Competition for nutrients with the crop, plant. ex.
High C/N residue will reduce available soil N due to utilization of N
E. Movement of
nutrients from soil to roots.
Root interception - Growth of roots throughout the soil mass.
Possibility of contact exchange - Cation on root surface exchanges for
one on colloid. Dependent on root growth. Only about 1% of nutrients
are taken up due to root interception.
Diffusion movement is response to a concentration gradient within the
soil solution. Important in P and K uptake.
Mass flow - movement of water with dissolved ions. Important in
supplying N, Ca, Mg to plants