Cover Image
close this bookSoils, Crops and Fertilizer Use: A Field Manual for Development Workers (Peace Corps, 1986, 338 p.)
View the document(introduction...)
View the documentAbout this manual
View the documentAcknowledgements
close this folderChapter 1: Down to earth - Some Important Soil Basics
View the documentWhat is soil, anyway?
View the documentWhy do soils vary so much?
View the documentTopsoil vs. subsoil
View the documentThe mineral side of soil: sand, silt, and clay
View the documentDistinguishing ''tropical'' soils from ''temperate'' soils
View the documentOrganic matter - a soil's best friend
View the documentThe role of soil microorganisms
close this folderChapter 2: Trouble-shooting soil physical problems
View the document(introduction...)
View the documentGetting to know the soils in your area
View the documentSoil color
View the documentSoil texture
View the documentSoil tilth
View the documentSoil water-holding capacity
View the documentSoil drainage
View the documentSoil depth
View the documentSoil slope
close this folderChapter 3: Basic soil conservation practices
View the document(introduction...)
View the documentRainfall erosion
View the documentWind erosion
close this folderChapter 4: Seedbed preparation
View the document(introduction...)
View the documentThe what and why of tillage
View the documentCommon tillage equipment
View the documentThe abuses of tillage and how to avoid them
View the documentMaking the right seedbed for the crop, soil, and climate
View the documentHow deep should land be tilled?
View the documentHow fine a seedbed?
View the documentSome handy seedbed skills for intensive vegetable production
close this folderChapter 5: Watering vegetables: When? How Often? How Much?
View the document(introduction...)
View the documentIt pays to use water wisely
View the documentSome common watering mistakes and their effects
View the documentFactors influencing plant water needs
View the documentOk, so get to the point! how much water do plants need and how often?
View the documentSome methods for improving water use efficiency
close this folderChapter 6: Soil fertility and plant nutrition simplified
View the document(introduction...)
View the documentLet's Make a Deal
View the documentHow plants grow
View the documentAvailable vs. unavailable forms of mineral nutrients
View the documentSoil negative charge and nutrient holding ability
View the documentSoil pH and how it affects crops growth
View the documentImportant facts on the plant nutrients
close this folderChapter 7: Evaluating a soil's fertility
View the document(introduction...)
View the documentSoil testing
View the documentPlant tissue testing
View the documentFertilizer trials
View the documentUsing visual ''hunger signs''
close this folderChapter 8: Using organic fertilizers and soil conditioners
View the documentWhat are organic fertilizers?
View the documentOrganic vs. chemical fertilizers: which are best?
View the documentSome examples of successful farming using organic fertilizers
View the documentHow to use organic fertilizers and soil conditioners
close this folderChapter 9: Using chemical fertilizers
View the document(introduction...)
View the documentWhat are chemical fertilizers?
View the documentAre chemical fertilizers appropriate for limited-resource farmers?
View the documentAn introduction to chemical fertilizers
View the documentCommon chemical fertilizers and their characteristics
View the documentThe effect of fertilizers on soil pH
View the documentFertilizer salt index and ''burn'' potential
View the documentBasic application principles for N, P, and K
View the documentFertilizer application methods explained and compared
View the documentTroubleshooting faulty fertilizer practices
View the documentGetting the most out of fertilizer use: crop management as an integrated system
View the documentUnderstanding fertilizer math
close this folderChapter 10: Fertilizer guidelines for specific crops
View the document(introduction...)
View the documentCereals
View the documentPulses (grain legumes)
View the documentRoot crops
View the documentVegetables
View the documentTropical fruit crops
View the documentTropical pastures
close this folderChapter 11: Liming soils
View the document(introduction...)
View the documentThe purpose of liming
View the documentWhen is liming needed?
View the documentHow to measure soil pH
View the documentHow to calculate the actual amount of lime needed
View the documentHow and when to lime
View the documentDon't overlime!
close this folderChapter 12: Salinity and alkalinity problems
View the document(introduction...)
View the documentHow salinity and alkalinity harm crop growth
View the documentLab diagnosis of salinity and alkalinity
close this folderAppendixes
View the documentAppendix A: Useful measurements and conversions
View the documentAppendix B: How to determine soil moisture content
View the documentAppendix C: Spacing guide for contour ditches and other erosion barriers*
View the documentAppendix D: Composition of common chemical fertilizers
View the documentAppendix E: Hunger signs in common crops
View the documentAppendix F: Legumes for green manuring and cover-cropping in tropical and subtropical regions
View the documentAppendix G: Some sources of technical support
View the documentAppendix H: A bibliography of useful references

(introduction...)

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.