LECTURE 11

1. LECTURE 11 Introduction to some chemical properties of soils : Factors affecting plant growth (3)

2. 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 (SO42-) and bicarbonate (HCO3-).

3. 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

4. Measuring soluble salts… • Measured by electrical conductivity. • Ions in water conduct electrical current. • Units of conductance are decisiemens per meter (Ds/m).

5. 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.

6. 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.

7. Reduced permeability to water. • Dispersion = aggregate breakdown and filling of pore spaces with smaller particles. • Exchangeable sodium favours dispersion.

8. 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

9. Reclaiming salty soils… • Establish internal drainage. • Replace excess exchangeable sodium. • Leach out most of the soluble salts.

10. 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

11. Nitrogen and Phosphorus…

12. 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

13. 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

14. 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.

15. 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

16. 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

17. 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

19. Factors that influence the rate of mineralization… • Amount of soil organic matter and its N-content • Water content • Temperature • pH • Aeration • Cultivation

20. 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

21. Symptoms of Phosphorus deficiency… • Not as easy to recognize as other deficiencies • Often stunted, thin-stemmed and spindly • Foliage dark, almost bluish green

22. 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”).

23. Source of phosphorus… • The mineral apatite, which is a calcium phosphate.

25. 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