Topic 4




Soil Properties Related to Soil Fertility

A. Physical Properties

1. Soil Texture

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

Little direct effect on plant growth but is an indicator of properties that do.

1. Black - High in organic matter drainage may be poor
    0 - 4 %
    Lighter Darker
2. Bright red and yellow colors well drained.
    Oxidized iron
3. Gray colors - poorly drained reduced iron.

3. Soil structure

Arrangement 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 and permeability

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.

5. Moisture holding capacity

Affected 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.

Soil Water Terminology

a. saturation - all pore space is filled with water
b. field capacity - moisture content when a soil is holding all the water it can against the force of gravity.
c. wilting point - the percentage of soil moisture at which plants wilt permanently

d. Plant available water = FC-WP

6. Soil depth

Rooting 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 table

7. Soil slope

Determines runoff; amount of erosion and management practices

B. Soil Chemical Properties

1. Soil colloids

Soil colloids make up the chemically active portion of the soil.

a. clay
b. humus
c. oxides of iron and aluminum

Colloids 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.

2. Cation exchange capacity.

The quantity of cations held by 100 g (oven dry) of soil. Expressed as meq/lOOg. Meq = 1/1000 of an equivalent weight.

a. equivalent 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


atomic wt Ca 40 valence 2

eq wt = 40 = 20

meq wt = .020

The general order of strength of adsorption of the common soil cations to cation exchange sites on the surfaces of colloids is called the lyotropic series.

           Al +3 > Ca+2> Mg+2 > K+ = NH4+ > Na+

The position of H+ is not clear. It is a highly hydrated monovalent cation that should be near Na+

Charge, 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 exchange sites

Effective concentration of the ion

b. The cation exchange capacity of the whole soil is affected by:

1. clay content
2. kind of clay
3. organic matter
4. pH
5. oxides of iron and Al

Colloid Type

CEC (cmol Kg-1)







Vermiculite (Trioctahedral)


Vermiculite (Dioctahedral)








Adapted From Sparks 1995. Envornmental Chemistry of Soils. Academic Press.


c. CEC 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.

Alkaline conditions

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
            total CEC

3. Anion Exchange

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 conditions.

Phosphorus reacts with soluble iron, aluminum and manganese and hydrous oxides of these minerals sulfate is held moderately

Nitrate 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

1. Factors affecting soil organic matter content.

a. Temperature - high temperatures favor decomposition of plant residue. Less organic matter in warm climates

b. Precipitation - O.M. is less where low soil moisture limits plant growth

c. Poorly 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.

e. Cropping

1. Harvesting plant material returns less to soil than the natural condition

2. Tillage increases rate of decomposition

3. Good fertility and crop rotation help maintain organic matter.

2. Maintenance of organic matter in cropped soils.

1. Attempts to increase organic matter levels beyond those which occur naturally in a given climate are usually ineffective.

Michigan 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%.

2,000,000 pounds acre-furrow slice x .01 to raise OM 1% = 20,000 pounds of humus per acre

If 10 tons of organic matter are added most decomposes leaving only a small amount of humus.

Reducing erosion and producing high crop yields are essential in maintaining soil organic matter.

Residual root systems aid in conditioning soils.

D. Soil Organisms

1. Beneficial effects

a. Organic matter decomposition prevents accumulation of plant residue, mineralization of plant residue provides source of nutrients for plants.

b. Inorganic chemical transformations

NH4+---------nitrification------------- NO3-

oxidation of Fe and Mn prevents toxicity

c. N fixation Blue-green algae legumes

d. Physical properties

Earthworms improve structure and aeration

2. Detrimental Effects

Snails, 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 by decomposers.

E. Movement of nutrients from soil to roots.

1. 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.

2. Diffusion movement is response to a concentration gradient within the soil solution. Important in P and K uptake.

3. Mass flow - movement of water with dissolved ions. Important in supplying N, Ca, Mg to plants


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