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close this book Soils, Crops and Fertilizer Use
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

Why do soils vary so much?

Your host country is likely to have dozens of different kinds of soils. In fact, even a small farm often has 2 or more types that vary markedly in their management problems and yield potential. The reason is that there are 6 soil-forming factors that determine the type of soil that develops in a particular spot:

• Climate: The higher the rainfall and temperature, the more rapid and complete the weathering process is. For example, the speed of chemical weathering reactions doubles for every 10°C (18°F) rise in temperature.

• Type of Parent Material: Soils are formed from a wide variety of parent material including many types of rock, vegetation, and animal life (e.g. the soil of Pacific atolls is formed largely from coral). Rock varies a lot in its mineral makeup and other qualities. Some rocks like granite and sandstone are acid and tend to form more acidic soils than basic rocks like limestone and basalt.

• Vegetation: Soils formed under grassland differ from those formed under forest, although there are also variations within these 2 groups. For example, soils formed under pine trees tend to be more acidic than those formed under other tree species.

• Topography exerts a big influence on erosion and drainage (the relative amounts of water and air in the soil pore space). In the tropics, red and yellow soils usually form on land with some slope since they need good drainage for their genesis. Black and grey soils are most common in depressions where drainage isn't as good.

• Time: Soils change over time as they weather, a process that takes place over thousands of years. Soils vary in age a lot.

• Farmer Management: Farming practices like land clearing, tillage, and cropping influence soil development by affecting erosion, pH, and organic matter, etc.