Global Soils The Nature of Soil The Soil Water Balance Soil Development The Global Scope of Soils Global Climate Change and Agriculture
Soil is a mixture of The Nature of Soil 1) inorganic material derived from regolith weathered from bedrock; 2) organics derived from forest litter and surface vegetation), 3) Liquids (water with dissolved nutrients), and 4) atmospheric gases.
typical soil composition 25% water 5% organic 45% inorganic 25% air
The Nature of Soil Soil texture, refers to the proportions of sand, silt and clay found in the soil
The Nature of Soil the variable proportions of sand, silt and clay found in different soils facilitate the classification of soil texture classes
The Nature of Soil The negatively charged surfaces of soil colloids (mineral particles < 0.00001 mm, organic humic and fulvic acids) attract and store base ions (Ca 2+, Mg 2+, Na +, K + ) as plant nutrients
Soil color, generated by soil forming processes, is the most obvious soil property dark brown to black indicates high humus content; red or yellow indicates iron mineral abundance; white flecks or spots or a crust indicate the presence of calcium carbonate) >>> Caliche Soils of cool moist regions tend to be acidic (ph < 7), [often low base status >> hard for plants to get some nutrients] Soils of arid climates are alkaline (ph > 7) [can be high base status >> easier for plants to get some nutrients] unless it is too alkaline! Soil structure refers to the way in which soil particles are grouped together into peds
Soil water storage capacity and wilting point according to soil texture The Soil Water Balance Finer grains (clays mostly) hold more water, but don t want to share it with plants
The Soil Water Balance available water = water gain - water loss + changes in storage
The Soil Water Budget simplified soil water budget for a middle latitude, moist climate
soil horizons are distinctive horizontal layers that differ in physical and chemical composition, organic content, or structure soil horizons are often distinguished by their colour the display of horizons horizontally in a cross section is termed a soil profile Soil Development
Soil Development two types of soil horizons: organic and mineral organic horizons, designated by the capital letter O, are formed from accumulations of organic matter derived from plants and animals the upper Oi horizon contains decomposing organic matter that is recognizable as leaves or twigs the lower Oa horizon contains material that is broken down beyond recognition by eye (humus)
Soil Development two types of soil horizons: organic and mineral mineral horizons lie below the organic horizons Below the mineral horizon is the bedrock with a weathered top Figure 10.11, p. 372
the A horizon is the uppermost mineral horizon Soil Development it is rich in organic matter, consisting of numerous plant roots and down-washed humus from the organic horizons above the E horizon: clay particles and oxides of aluminum and iron are removed from the E horizon by downward-seeping water, leaving behind pure grains of sand or coarse silt Figure 10.11, p. 372
Soil Development the B horizon receives the clay particles, aluminum, and iron oxides, as well as organic matter washed down from the A and E horizons It is made dense and hard by the filling of natural spaces with clays and oxides Figure 10.11, p. 372
Soils and Climate in cold climates, decomposition of organic matter is slow and organic matter accumulates in warm climates, organic matter decomposes rapidly and soil organic matter is scarce How does this compare to their growth rates?
Global Geography of Soils
IG4e_07_02a
IG4e_07_02b
The Global Geography of Soils Group I: soils with well-developed horizons or with fully weathered minerals, resulting from long-continued adjustment to prevailing soil temperature and soil water conditions Oxisols: very old, highly weathered soils of low latitudes, with a subsurface horizon of accumulation of mineral oxides and very low base status Ultisols: soils of equatorial, tropical, and subtropical latitude zones, with a subsurface horizon of clay accumulation and low base status Vertisols: Soils of subtropical and tropical zones with high clay content and high base status (develop deep, wide cracks when dry, and the soil blocks formed by cracking move with respect to each other)
The Global Geography of Soils Group I: soils with well-developed horizons or with fully weathered minerals, resulting from long-continued adjustment to prevailing soil temperature and soil water conditions Alfisols: soils of humid and subhumid climates with a subsurface horizon of clay accumulation and high base status; range from equatorial to subarctic latitude zones Spodosols: soils of cold, moist climates, with a well-developed B horizon of illuviation and low base status Mollisols: soils of semiarid and subhumid midlatitude grasslands, with a dark, humus-rich epipedon and very high base status Aridisols: soils of dry climates, low in organic matter, and often having subsurface horizons of accumulation of carbonate minerals or soluble salts
The Global Geography of Soils Group II: soils with a large proportion of organic matter Histosols: soils with a thick upper layer very rich in organic matter
Oxisols: very old, highly weathered soils of low latitudes (tropics and equatorial) a subsurface horizon of mineral oxides and very low base status Rotten soils! How do these soils support tropical forests?
Ultisols: soils of equatorial, tropical, and subtropical latitude zones, with a subsurface horizon of clay accumulation and low base status Tough living for a plant
Vertisols: Soils of subtropical and tropical zones with high clay content and high base status (develop deep, wide cracks when dry, and the soil blocks formed by cracking move with respect to each other)
Alfisols: soils of humid and subhumid climates a subsurface horizon of clay high base status Good soil
Spodosols: soils of cold, moist climates, F. Spodosols with a well-developed B horizon of illuviation low base status A pretty crummy soil, but specialized plants survive in it.
Mollisols: soils of semiarid, subhumid midlatitude grasslands, a dark, humus-rich layer and very high base status Potentially productive
Aridisols: soils of dry climates, low in organic matter, often having subsurface accumulation of carbonate minerals (caliche) or soluble salts Need a tough plant to thrive here
Histosols: soils with a thick upper layer very rich in organic matter Just step back and watch the corn grow