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Feldspars
Silicate minerals. Feldspars are named after the German for ‘field crystals’. About half of all the Earth’s crustal rocks are made from feldspars. There are two kinds. One kind (orthoclase feldspar) is white or grey, and the other kind (plagioclase feldspar) is usually pink. You cannot see into
a feldspar; it is opaque. But the surface shines (it has a lustre like porcelain) and it breaks up into flat- faced blocks.
Feldspars are reasonably hard
(6 on Mohs’ scale of hardness). However, feldspars are easily weathered and they are the ‘weak link’ in many otherwise resistant rocks, weathering into clay minerals and allowing the rest of the rock to fall apart. Because
they are easily weathered, feldspars are found mostly in igneous and metamorphic rocks, and are not so common in sedimentary rocks (where they have usually been weathered to clay minerals).
 Feldspars – Feldspars are the most plentiful minerals in igneous rocks, often occurring as large
crystals called
phenocrysts.
Feldspars
Ferromagnesian minerals
A group of minerals that all contain a large proportion of
iron and magnesium. In general, they are very difficult to tell
apart because they are all green, dark green, or black and often occur only as small crystals. Nevertheless, they form a major part of many igneous and metamorphic rocks and affect the overall colour of the rock. These minerals weather easily, and so, while common in igneous and metamorphic rocks, they are not common in sedimentary rocks.
Fertile soil
A soil in which plants grow well. All plants need a range of ‘foods’ to grow. The ‘foods’ in a soil are called nutrients. They are mainly minerals and all are taken up from the water in the soil through the plant roots.
Plants need many nutrients, some in larger quantities than others. They need large amounts
of nitrogen, potassium and calcium. Nitrogen and potassium are released from decaying plant matter (humus), which is one reason why adding compost or manure to a soil is so vital.
 Fertile soil – Fertile soils are home to many soil organisms. The birds following this tractor simply wait for the soil to be turned over in order to get an easy meal.
Artificial fertilisers are also used to add nitrogen and potassium to the soil. Calcium comes from the weathering of the rock particles in the soil. To replace lost calcium, lime (calcium hydroxide) is added to the soil every few years. (See also: A horizon.)
Some soils are infertile because they lack just one element, such as zinc or copper. Adding special fertilisers to a soil corrects this problem.
Plants cannot usefully take up more nourishment than they need, so adding more fertiliser than necessary is not just a waste of resources, but can actually harm some plants and pollute the rivers into which the soil water drains.
(For types of fertile soil see: Black soil; Calcareous soil; Chernozem.)
Fertiliser
An artificial mixture of nutrients that plants need for growth. It
is added to water and sprayed onto soils or spread in the form of powders and pellets. Its main
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