<P> However, whereas strong positive relationships between soil temperature (5 ° C to 25 ° C) and growth have been found in trembling aspen and balsam poplar, white and other spruce species have shown little or no changes in growth with increasing soil temperature . Such insensitivity to soil low temperature may be common among a number of western and boreal conifers . </P> <P> Soil colour is often the first impression one has when viewing soil . Striking colours and contrasting patterns are especially noticeable . The Red River of the South carries sediment eroded from extensive reddish soils like Port Silt Loam in Oklahoma . The Yellow River in China carries yellow sediment from eroding loess soils . Mollisols in the Great Plains of North America are darkened and enriched by organic matter . Podsols in boreal forests have highly contrasting layers due to acidity and leaching . </P> <P> In general, color is determined by the organic matter content, drainage conditions, and degree of oxidation . Soil color, while easily discerned, has little use in predicting soil characteristics . It is of use in distinguishing boundaries within a soil profile, determining the origin of a soil's parent material, as an indication of wetness and waterlogged conditions, and as a qualitative means of measuring organic, salt and carbonate contents of soils . Color is recorded in the Munsell color system as for instance 10YR3 / 4 Dusky Red . </P> <P> Soil color is primarily influenced by soil mineralogy . Many soil colours are due to various iron minerals . The development and distribution of colour in a soil profile result from chemical and biological weathering, especially redox reactions . As the primary minerals in soil parent material weather, the elements combine into new and colourful compounds . Iron forms secondary minerals of a yellow or red colour, organic matter decomposes into black and brown compounds, and manganese, sulfur and nitrogen can form black mineral deposits . These pigments can produce various colour patterns within a soil . Aerobic conditions produce uniform or gradual colour changes, while reducing environments (anaerobic) result in rapid colour flow with complex, mottled patterns and points of colour concentration . </P>

Write a short paragraph on preparation of soil