Cover Image
close this book Soils, Crops and Fertilizer Use
View the document About this manual
View the document Acknowledgements
close this folder Chapter 1: Down to earth - Some Important Soil Basics
View the document What is soil, anyway?
View the document Why do soils vary so much?
View the document Topsoil vs. subsoil
View the document The mineral side of soil: sand, silt, and clay
View the document Distinguishing "tropical" soils from "temperate" soils
View the document Organic matter - a soil's best friend
View the document The role of soil microorganisms
close this folder Chapter 2: Trouble-shooting soil physical problems
View the document Getting to know the soils in your area
View the document Soil color
View the document Soil texture
View the document Soil tilth
View the document Soil water-holding capacity
View the document Soil drainage
View the document Soil depth
View the document Soil slope
close this folder Chapter 3: Basic soil conservation practices
View the document Rainfall erosion
View the document Wind erosion
close this folder Chapter 4: Seedbed preparation
View the document The what and why of tillage
View the document Common tillage equipment
View the document The abuses of tillage and how to avoid them
View the document Making the right seedbed for the crop, soil, and climate
View the document How deep should land be tilled?
View the document How fine a seedbed?
View the document Some handy seedbed skills for intensive vegetable production
close this folder Chapter 5: Watering vegetables: When? How Often? How Much?
View the document It pays to use water wisely
View the document Some common watering mistakes and their effects
View the document Factors influencing plant water needs
View the document Ok, so get to the point! how much water do plants need and how often?
View the document Some methods for improving water use efficiency
close this folder Chapter 6: Soil fertility and plant nutrition simplified
View the document Let's Make a Deal
View the document How plants grow
View the document Available vs. unavailable forms of mineral nutrients
View the document Soil negative charge and nutrient holding ability
View the document Soil pH and how it affects crops growth
View the document Important facts on the plant nutrients
close this folder Chapter 7: Evaluating a soil's fertility
View the document Soil testing
View the document Plant tissue testing
View the document Fertilizer trials
View the document Using visual "hunger signs"
close this folder Chapter 8: Using organic fertilizers and soil conditioners
View the document What are organic fertilizers?
View the document Organic vs. chemical fertilizers: which are best?
View the document Some examples of successful farming using organic fertilizers
View the document How to use organic fertilizers and soil conditioners
close this folder Chapter 9: Using chemical fertilizers
View the document What are chemical fertilizers?
View the document Are chemical fertilizers appropriate for limited-resource farmers?
View the document An introduction to chemical fertilizers
View the document Common chemical fertilizers and their characteristics
View the document The effect of fertilizers on soil pH
View the document Fertilizer salt index and "burn" potential
View the document Basic application principles for N, P, and K
View the document Fertilizer application methods explained and compared
View the document Troubleshooting faulty fertilizer practices
View the document Getting the most out of fertilizer use: crop management as an integrated system
View the document Understanding fertilizer math
close this folder Chapter 10: Fertilizer guidelines for specific crops
View the document Cereals
View the document Pulses (grain legumes)
View the document Root crops
View the document Vegetables
View the document Tropical fruit crops
View the document Tropical pastures
close this folder Chapter 11: Liming soils
View the document The purpose of liming
View the document When is liming needed?
View the document How to measure soil pH
View the document How to calculate the actual amount of lime needed
View the document How and when to lime
View the document Don't overlime!
close this folder Chapter 12: Salinity and alkalinity problems
View the document How salinity and alkalinity harm crop growth
View the document Lab diagnosis of salinity and alkalinity
close this folder Appendixes
View the document Appendix A: Useful measurements and conversions
View the document Appendix B: How to determine soil moisture content
View the document Appendix C: Spacing guide for contour ditches and other erosion barriers*
View the document Appendix D: Composition of common chemical fertilizers
View the document Appendix E: Hunger signs in common crops
View the document Appendix F: Legumes for green manuring and cover-cropping in tropical and subtropical regions
View the document Appendix G: Some sources of technical support
View the document Appendix H: A bibliography of useful references

Chapter 12: Salinity and alkalinity problems

Salinity and alkalinity problems are most likely to occur under 2 conditions:

• Irrigated soils in semi-arid and arid regions (less than 500 mm annual rainfall) where rainfall or irrigation isn't sufficient to leach accumulated salts out of the root zone. The salts are released by decomposing rock and other parent material below the subsoil and are also brought in by irrigation water and additions of chemical fertilizers and manure.

• Intrusion of salt water into low-lying areas near oceans and seas.

In humid regions, there's usually enough rainfall to flush the salts downward out of the root zone. In low-rainfall areas, irrigation may move salts downward, but they move back up again as the soil dries out between irrigations unless enough extra water is applied. The very high evaporation rates common to these drier regions aggravate this tendency. In many cases, subsurface drainage is also poor, which makes matters worse. Bringing land under irrigation may raise the water table to within a meter or so of the surface, enabling salts to move upwards by capillary action the same way kerosene travels up a lamp wick.

Saline and alkali (sodic) soils fall into 3 classes according to the amount of soluble salts and adsorbed (held by clay and humus particles) sodium they contain: (These soils are also referred to as halomorphic soils)

• SALINE SOILS: These contain enough neutral soluble salts to harm plant growth much like fertilizer burn does. The salts are mainly chlorides and sulfates of sodium, calcium and magnesium. Less than 15% of the soil's exchange capacity (see Chapter 6) is occupied by adsorbed sodium ions, and the pH is usually below 8.5. Saline soils are also called white alkali soils, because the salts tend to accumulate on the soil surface. The usual causes are lack of enough water for adequate leaching, poor drainage, or both.

• SALINE-ALKALI SOILS (Saline-Sodic Soils): These soils not only contain excessive anount of soluble salts, but also harmful amounts of adsorbed sodium (i.e. plus-charged sodium ions that adhere to the negatively-charged clay and humus particles). More than 15% of the soil's exchange capacity is occupied by sodium ions. Although sodium is strongly basic, the pH of these soils is usually below 8.5 due to the buffering influence of the the neutral soluble salts.

• NON-SALINE ALKALI SOILS (Sodic Soils): These soils contain only low levels of soluble salts but have excessive amounts of adsorbed sodium. More than 15% of the soil's exchange capacity is occupied by adsorbed sodium ions held by clay and humus particles. The pH is above 8.5 and often as high as 10, because there aren't enough soluble salts to exert a buffering effect. Sodic soils have very poor physical condition due to their high sodium content; it disperses and puddles (breaks down) soil aggregates (crumbs and clumps of soil particles), making the soil rather impervious to water. Sodic soils are also called black alkali soils, since their surfaces are often black due to the accumulation of dispersed humus brought to the surface by the upward capillary movement of water (from a high water table) and by evaporation.