LECTURE 11

Introduction to some chemical properties of soils :Factors affecting plant growth (3)

Soil Salinity…

Definition: “The amount of soluble salts in a soil, expressed in terms of conductivity of the saturation extract.”

Most soluble salts are composed of the cations sodium (Na+), calcium (Ca+) and magnesium (Mg2+); and the anions chloride (Cl-), sulphate (SO4

2-) and bicarbonate (HCO3

-).

Once again, it’s all about balance! Amount of salts, plant requirements, amount of water etc. A buildup of salts can be natural

e.g. fossil deposits of salts laid down in the past (saline seeps).

Arid areas where salts are not leached out. Buildup can be induced by irrigation

Measuring soluble salts…

Measured by electrical conductivity. Ions in water conduct electrical current.

Units of conductance are decisiemens per meter (Ds/m).

Effects of excessive salt concentration…

Reduces plant growth by the osmotic effect. Plants have to expend more energy to extract water. Sometimes, when it is very dry, plants can die as water is

pulled from them (exosmosis).

Can slow or inhibit seed germination. Biggest effect when plants are young, less

when they are mature. Depends on type and purpose of crop.

When corn is being produced for grain, salt levels must be kept lower than when it is being produced for forage.

Barley and cotton have high salt tolerances, and high salt concentration affects vegetative growth more than yield.

Rice grain yields are reduced before vegetative growth is affected.

Reduced permeability to water. Dispersion = aggregate breakdown and filling of pore

spaces with smaller particles. Exchangeable sodium favours dispersion.

Classification of salted soils…

Electrical conductivity. Exchangeable sodium percentage (ESP)

– Proportion of exchangeable cations that are sodium

ESP = (Exchangeable Na/Total exchangeable cations) x 100

Sodium adsorption ratio (SAR)– Easier to calculate than ESP.

SAR = [Na+]/([Ca2+ ]+[Mg2+])½

Reclaiming salty soils…

Establish internal drainage. Replace excess exchangeable sodium. Leach out most of the soluble salts.

Elements essential for plant growth…

MACRONUTRIENTS: Carbon (C) Hydrogen (H) Oxygen (O) Nitrogen (N) Phosphorus (P) Potassium (K) Calcium (Ca) Magnesium (Mg) Sulphur (S)

Obtained from air and water

Primary fertilizer nutrients, obtained from soil and fertilizers

Secondary macronutrients

Nitrogen and Phosphorus…

Nitrogen…

Roles of Nitrogen in plants:– Major part of all amino acids (building blocks of all

proteins)– Nucleic acids (hereditary control)– Chlorophyll (essential in photosynthesis)– Essential for carbohydrate use– Stimulates root growth and development– Stimulates uptake of other nutrients

Different plants need different amounts…

Deficiency of nitrogen can cause: – Pale yellow-green colour (chlorosis)– Stunted appearance– Thin, spindly stems

Oversupply of nitrogen can cause:– Enlarged but weak cells in plant stems– Top-heavy plants can fall over– Delayed maturity– Susceptibility to disease– Low crop quality

Forms of Nitrogen

Nitrogen occurs in various forms (e.g. N2 gas, ammonium compounds, organic compounds).

Plants take up nitrogen mainly in the form of nitrates (NO3

-) and NH4+ ions.

Various processes are required to make Nitrogen available to plants in the soil in the correct form.

Processes by which nitrogen moves from one form to another…

Mineralization Microbial conversion of organic N to mineral N

Nitrification Oxidation of ammonium-N to nitrite and nitrate by specific

micro-organisms

Immobilization Conversion of mineral-N to organic N Occurs when micro-organisms cannot satisfy their N-needs

from the organic materials on which they feed

Volatilization Loss of ammonia gas (NH3) from the soil Occurs under alkaline conditions

Denitrification Loss of nitrogen and nitrous oxide gas from the soil under

anaerobic conditions Nitrate and nitrite are reduced to these gases by micro-

organisms

Nitrogen fixation Conversion of N2 gas in the soil into NH4+ by specialized

groups of micro-organisms The NH4+ is then assimilated as organic N Ammonium ions can be adsorbed by negatively charged

surfaces of clay and humus, but are sometimes entrapped within cavities in the crystal structure of certain clays

Nitrate leaching Process by which nitrate is lost from the soil in drainage

waters Nitrate is not adsorbed onto soil particles unless they are

positively charged

Net mineralization vs. net immobilization

Factors that influence the rate of mineralization…

Amount of soil organic matter and its N-content Water content Temperature pH Aeration Cultivation

Phosphorus…

Roles:– Vital in early plant growth (constituent of nucleic

acid in which genetic patterns are encoded– Plays roles in cell division, stimulation of early root

growth, hastening plant maturity, energy transformations within the cells

– Fruiting and seed production

Symptoms of Phosphorus deficiency…

Not as easy to recognize as other deficiencies Often stunted, thin-stemmed and spindly Foliage dark, almost bluish green

The phosphorus problem…

Soil phosphates have VERY low solubility Plants need about 1/5 to 1/10 as much

phosphorus as they do nitrogen and potassium, but the concentration of phosphates in solution is only about 1/20 or less of these 2 nutrients

When soluble forms of phosphorus are added to soils, they react to form highly insoluble compounds (i.e. they become “fixed”).

Source of phosphorus…

The mineral apatite, which is a calcium phosphate.

See Brady & Weil (1999) pg 549 for diagram. Ways in which phosphorus is lost:

Plant removal Erosion Surface runoff water

Ways in which phosphorus is added: From the atmosphere (sorbed onto dust particles) Fertilizers