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Agglomerate
rainfall is heavy enough to wash the nutrients away as they are released from the rock by weathering
(as is the case in moorlands
and in tropical rain forests). (See also: Eluviation; Leach, leaching; Moder; Mor; Podzol.)
Agglomerate
A rock made from the compacted particles thrown out by a volcano (for example, tuff).
acidic and will not support crops. A deep A horizon is usually
best for crop growth. The deepest A horizons occur in soils called chernozems. They are the soils of the prairies. The A horizons of chernozems are black and may be a metre or more thick. This is why, with sufficient irrigation, they are among the world’s most productive soils. (See also: Horizon.)
Alfisol
A soil type found naturally in
cool, humid, regions with broad- leaved forests. It is also called
a brown soil or brown earth.
 A horizon – The A horizon, or topsoil, can be identified as the part that contains
most of the fibrous plant roots as shown by this grass clump. The dark colour of the soil indicates that it is fertile.
It is the second most abundant soil in the world, accounting for about 15% of the world soils.
Alfisols have good fertility (see: Fertile soil) and are rich in
clays, and so hold moisture well. At the same time, they have
a large amount of humus, and this, combined with the ground- breaking activity of the tree roots, helps keep the soil well aerated and drained.
The clays in these soils are important because their surfaces trap nutrients released by weathering or carried down from the surface by rainwater passing through rotting humus. They are the soils on which most of the world’s barley, corn and wheat are grown.
Alkaline rock
A type of igneous rock containing less than half silica and normally dominated by dark-coloured minerals (for example, gabbro).
Alkaline soil
A soil with a pH of more
than 7. Alkaline soils are rich
in nutrients. There are two reasons for this. Firstly, they
may have been formed on very alkaline rocks such as chalk and limestone. Typical alkaline soils of this kind are called rendzinas. Alternatively, the nutrients are kept in the soil because the rainfall is too low to wash them away. This is the case in chestnut soils, which develop on the less humid prairies and Great Plains
of North America. However, if the rainfall is very low, then some harmful substances, such as salt, are not washed out of the soil,
and they will keep plants from growing. This occurs in the soils of the dry prairies. (See also: Salinisation.)
 Agglomerate – An agglomerate with many angular fragments of different sizes fused together.
A horizon
The term used by soil scientists for the upper part of the soil
that contains a mixture of finely ground rocks and minerals (such as sand and clay) and organic matter (such as humus). It is a more technical way of referring to the topsoil.
The A horizon gives a good indication of what is going on in
a soil. If the A horizon is a dark
red or brown and merges gradually into the B horizon (subsoil) below it, then the soil is usually fertile and good for crops. If the A horizon contains well-marked layers, and especially if the upper layer is thin and grey in colour, then the soil is
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