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

How deep should land be tilled?

Most seedbed preparation methods use a plow or digging hoe to loosen the soil to a depth of 15-20 cm (i.e. the topsoil). There are 2 situations where deeper tillage may be costeffective, though not always, by any means:

• Encouraging roots to enter the subsoil by breaking up a pan (compacted layer) may enable them to tap into a valuable moisture reserve; this can make a crucial difference in a drought, especially for deep-rooted crops (e.g. maize, sorghum) grown under rainfed conditions. Loosening a pan may also improve soil drainage.

• In very hot conditions (e.g. the Sahel during the period from March through June) deep tillage may allow roots to grow deeper into cooler, more hospitable soil.

The value of deep tillage is commonly overrated for a number of reasons:

• Power and labor requirements increase greatly, especially since the subsoil tends to be more compact. With hand tools, deep tillage is seldom practical on anything but small plots.

• About 60-80% of most crops' roots are found in the topsoil, even in high-yielding fields.

• Subsoils that are poorly drained or too acid won't allow much root growth, no matter how well loosened or enriched. Also, it's not unusual for the soil below a hard pan or clay pan to be compacted and poorly drained.

• Loosening a compacted subsoil is likely to be only temporarily effective unless a soil conditioner (sand, rice hulls, etc.) is added, as well as fertilizer. This may be feasible using hand tools but is very laborious.

• On large fields, tractor-drawn subsoilers (long, narrow shanks that penetrate up to 60 cm deep) can be used to break up compacted layers (pans). Results vary from poor to good, depending on the type of pan, its wetness, and the characteristics of the soil below it. Hardpans (natural layers that remain cemented whether wet or dry) and traffic Pans (compacted layers right below normal tillage depth that are caused by machinery traffic) can usually be successfully fractured while dry. However, subsoiling claypans (natural, dense, clayey layers) often gives disappointing results for 2 reasons. First, they're often continually wet (unless the dry season is long) and aren't subject to fracture in this condition. Secondly, soil below such claypans is often compacted and poorly drained, too.