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 role of soil microorganisms

The soil is a thriving biological laboratory, and a teaspoonful easily contains a billion microorganisms such as fungi and bacteria. Some cause plant diseases, but most are beneficial to agriculture. Some examples:

• Humus production: Many kinds of soil bacteria and fungi decompose organic matter into crumbly humus that does all those great things for the soil. The compounds produced by decomposition are also beneficial.

• Release of plant nutrients from organic matter: Most of the nitrogen, phosphorus, and sulfur in fresh plant residues is tied up in the unavailable organic form which plants can't use. Soil microbes change these tied-up nutrients into available inorganic (mineral) forms which plants can use.

• Mycorrhizae are a kind of mushroom fungi commonly found in most soils and infest the roots of many plants and trees. They cause no harm but actually enhance the host's uptake of plant nutrients, especially phosphorus (P); they also improve water uptake, lessen the toxicity of salinity or excess aluminum, and stimulate the growth of other beneficial microbes like rhizobia. They may even secrete growth-promoting hormones. In return, the plant provides the fungi with simple sugars for food. It's believed that mycorrhizae play a particularly important role in aiding the P uptake in some crops like sweetpotatoes and cassava (manioc) which seem to tolerate soils with low levels of available P. In the case of sterilized field or greenhouse soils that lack the fungi, considerable savings in phosphorus fertilizer have sometimes been obtained by innoculating them with a mycorrhizae culture, notably in the case of citrus nurseries. Topsoil from a disease-free, actively growing organic garden is likely to contain an especially good population of the fungi, and a few shovelfuls can be transferred to a new plot to encourage development. (However, mycorrhizae do not colonize the roots of beets, spinach, chard, and brassica [crucifer] family plants such as cabbage, broccoli, radish, turnip, and pak choy.)

• Nitrogen fixation by rhizobia: Several kinds of bacteria "fix" (capture) nitrogen from the air and convert it to a form that plants can use. The most important type are rhizobia bacteria (of the genus Rhizobium) that live in small nodules on the roots of legumes. (Legumes are plants that produce their seed in pods such as beans, peas, and peanuts.) The rhizobia have a symbiotic (mutually beneficial) relationship with legumes. The bacteria live off sugars provided by the plant and supply their host with nitrogen. Some legumes such as cowpeas, peanuts, mungbeans, soybeans, and pasture legumes like clovers receive all the N they need from the rhizobia if the right strain is present.

• Other kinds of N fixation:

•• Blue-green algae (cyanobacteria) inhabit flooded rice soils and fix N. Free-living types (i.e. those requiring no host) fix modest amounts of N, and farmers in Egypt, India, and Burma purposely inoculate their rice paddies with these algae.

•• The Azolla plant is a low-growin8, aquatic fern which harbors a type of N-fixing, bluegreen algae (Anahaena azollae) in its leaves. Azolla has been used as a green manure and also intercropped (grown in combination) with flooded rice for centuries in China and Viet Nam and can supply considerable N to the rice plants. (For more information on Azolla, refer to the section on rice in Chapter 10.)

•• Azotobacter are free-living, N-fixing bacteria commonly found in unflooded soils of warm areas.

•• Casuarinas are pine-like trees used for firewood, soil stabilization, and windbreaks in warm climates. Although not a legume, they do fix N, thanks to an association with an Actinomycete bacteria of the genus Frankia.