Q.The organic horizon is generally absent from agricultural soils because tilling, e.g., plowing, buries organic matter. Why is an organic horizon generally absent in desert soils ?

Pedogenesis is the scientific term for the process of soil formation.

Working

The term pedogenesis is derived from the Greek roots 'pedon' (soil) and 'genesis' (origin/formation), and refers to the natural process of soil formation -- the gradual transformation of parent rock material into soil through the combined, long-term action of physical, chemical and biological weathering, influenced by factors such as climate, living organisms, the underlying parent material, topography (relief), and time. This process produces the distinct soil horizons seen in a soil profile. Population refers to a group of individuals of the same species living in an area (studied under ecology, un …

Capillary water -- held in fine soil pores by surface tension against gravity -- is the form of soil water actually available for plant absorption.

Working

Soil water can be classified by how strongly it is held by soil particles:

• Gravitational water: occupies the larger soil pores (macropores) and drains downward under gravity soon after rain or irrigation; it is present only briefly and is largely unavailable because it moves out of the root zone quickly (though it does help aerate soil as it drains).
• Capillary water: held in the fine capillary pores (micropores) between soil particles by surface tension, against the pull of gravity; it is retained in the root zone at a moisture tension that plant root hairs CAN overcome by osmotic/suction force, so this is the main water source actually usable by plants for absorption and transpiration. …

Total soil water = Holard, the unavailable fraction = Echard, and the plant-available fraction = Chresard -- matching option (b).

Working

Soil water is classically partitioned into three related quantities. Holard is the total quantity of water present in the soil at any given time, encompassing both the water plants can use and the water they cannot. Of this total, a portion is held so tightly by soil particles (through adsorption, capillary forces below the wilting point, etc.) that roots cannot extract it -- this unavailable fraction is called Echard. The remaining portion, which lies between the field capacity and the permanent wilting point and can genuinely be absorbed and used by plant roots, is called **Chresar …

Capillary water, held by capillary forces in fine soil pores, is the fraction of soil water that is actually available for plant uptake.

Working

Soil water occurs in several physically distinct forms, and only one of these is physiologically available to plants:

• Gravitational water occupies the larger soil pores and drains downward under gravity soon after rain or irrigation, so it is present in the root zone only briefly and is largely unavailable for absorption.
• Capillary water is held in the fine capillary spaces between soil particles by surface tension, against the pull of gravity. It forms a thin film around soil particles and is retained in the root zone for extended periods at a tension that root hairs CAN overcome -- this is the principal source of water actually absorbed and used by plants. …

Pedogenesis is the process by which soil is formed from parent rock material over time.

Working

Pedogenesis (from Greek "pedon" = soil/ground, and "genesis" = origin/formation) refers to the natural process of soil formation. It describes how parent rock material is progressively broken down and transformed into soil through the combined, long-term action of several soil-forming factors: parent material (the underlying rock), climate (temperature and rainfall driving weathering), living organisms (plants, microbes and animals that add organic matter and mix the soil), relief/topography, and time.

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Holard denotes the total water content of soil, whether available to plants or not.

In plant-water ecology, the total water held in a soil at any time is called the holard. Of this total, only a portion can actually be absorbed by plant roots against the forces holding water to soil particles; this usable portion is called the chresard (available water), while the remainder, held too tightly by soil particles (below the permanent wilting point) for roots to extract, is called the echard (unavailable water) -- so holard = chresard + echard. Oxylophytes are plants adapted to grow in acidic, often nutrient-poor soils such as bogs and heathlands, not to ice surfaces (which would describe a chionophyte or psychrophyte- …