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

The mineral side of soil: sand, silt, and clay

The mineral part of soil is composed of varying amounts of sand, silt, and clay. Their characteristics have a big influence on soil behavior and management needs.


• Of the 3 kinds of mineral particles, sand is the largest in size; about 50 sand particles laid side by side would equal 1 centimeter (125 per inch).

• Sand is mainly quartz (silicon dioxide) and contains few plant nutrients.

• Moderate amounts improve soil drainage, aeration, and filth (workability).


• It consists mainly of ground up sand particles (quartz), which are often coated with clay.

• It contains few nutrients in itself except those that might be in the clay coating.

• Silt particles are too small to help improve drainage and aeration.


Clay particles are the smallest of the 3 (about 4000 of them laid side by side would equal 1 cm). Farmers know that clay has a big influence on soil behavior. High clay content usually makes for harder plowing, more compaction, and poorer drainage, but it does assure good water-holding capacity. Aside from this, clays have 3 other important features:

• Source of plant nutrients: Unlike sand and silt, clays are aluminum-silicate minerals that also have varying amounts of plant nutrients such as potassium, calcium, magnesium, and iron, etc. A good part of a soil's native fertility can come from its clay portion.

• Clays have a negative charge: This makes them act like tiny magnets to attract and hold those plant nutrients that have a positive (+) charge like potassium (K+), calcium (Ca++), magnesium (Mg++), and the ammonium form of nitrogen (NH.+). This helps greatly to keep these nutrients from being carried downward beyond the root zone by rainfall or irrigation. (The term leaching is used to describe this type of loss.)

• Tremendous surface area: Each clay particle is really a laminated structure consisting of tiny plates. This lattice arrangement plus small particle size gives clays an amazing amount of surface area for attracting and holding positively-charged nutrients. In fact, one cubic centimeter of clay particles contains about 1-3 square meters of surface area.

All Clay isn't the Same

There are several different types of clay, and most soils contain at least two. Understanding some basics about clay types will help you interpret the soils in your work area. It's important to understand the difference between temperate clays and tropical clays and why both types are found in the tropics.

• Temperate clays: These are 2:1 silicate clays such as montmorillinite and illite that dominate the clay portion of most temperate zone soils but may also be found in the tropics. The 2:1 figure refers to the ratio of silicate to aluminum plates in a clay particle's laminated structure. Soils with a good amount of these temperate clays are very sticky and plastic when wet; some kinds such as montmorillinite shrink and swell readily, forming large cracks upon drying out. They also have a relatively high negative charge (good for holding positively-charged nutrients>.

• Tropical clays: These are 1:1 silicate clays, such as kaolinite, and hydrous oxide clays of iron and aluminum that often make up most of the clay portion of old, well drained soils in the tropics and subtropics, mainly in areas with at least 6 months of rainfall. These clays have lost lots of silicate due to centuries of weathering and leaching. Unlike the 2:1 clays, these "tropical" clays are much less sticky and plastic and are easier to work with, even when clay content is high. However, they usually have much less negative charge and lower natural fertility than temperate clays. Soils whose clay portion is largely "tropical" can usually be identified by their red or yellow colors.