Agro/Hort 100 Intro to Plant Science Soils
Soil Morphogenesis Physical Properties of Soil Chemical Properties of Soil Soil Organisms

Soil Morphogenesis

Soils start to form by weathering of parent rock. Over time the weathered material differentiates into horizons or layers. At maturity, there are well defined soil horizons. This weathering can occur by chemical or physical processes.

O Horizon	leaf litter decaying organic matter		if present several cm
A Horizon	zone of leaching intensive weathering dark color organic matter		up to 25 cm
B Horizon	zone of accumulation moderate weathering lighter color		between 30 cm and 1 m
C Horizon	zone of parent material slight weathering		below 1 m
parent rock	no weathering

What factors affect soil morphogenesis or development?

1. Climate
2. Topography
3. Time

1. Climate influences soil morphogenesis at two levels: directly via temperature and precipitation and indirectly by controlling plant type and density.

• Direct effects of climate on soil morphogenesis:
  1. high rainfall will increase chemical weathering, leach soils of minerals and increase acidity of soil
  2. low rainfall will decrease chemical weathering, increase salt content of soil, and increase alkalinity of soil
  3. higher temperatures increase chemical reaction rates
• Indirect effects of climate on soil morphogenesis:
  1. climates supporting plant growth increase the rate of soil maturation. increased plant growth increases acidity of soil, increasing chemical weathering; increased plant growth increases organic fraction of soil, decreasing leaching of minerals from soil.

Tropical regions typically have very mature shallow soils; arid regions typically have immature soils.

2. Topography

• steep slopes increase erosion rates, less water percolation, slower soil maturation
• flat terrain if high rainfall will also be a problem, leached soil, or water-logged soil

Ideal topography for soil maturation: gentle slopes with lots of vegetation will result in deep water percolation and well defined soil profile (horizons)

3. Time

Chemical and physical weathering even under ideal conditions take hundreds to thousands of years to develop mature soils.

Soil Properties That Change Over Time
Properties that change over minutes or hours	Properties that change over months or years	Properties that change over hundreds and thousands of years
temperature moisture content composition of air in soil pores	soil pH soil color soil structure soil organic matter content soil fertility microorganisms density	kinds of minerals particle size distribution horizon formation

Table from GLOBE Teacher's Guide 1996

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