The Earth Contents

The Earth - Soils - Page 15

Question 1 answer

Soils occur where the llithosphere, hydrosphere, atmosphere and biosphere meet and interact. They are vital to plant growth, acting as the medium through which nutrients are re-cycled to allow new growth. They are also an environment in their own right with huge numbers of organisms living in, and dependent upon them.

Question 2 answer

Soils are formed by the weathering of rocks, and the resulting particles are then distributed by gravity, water and ice (and in the case of the smaller particles, by wind). Loose rocks and stones gradually erode to produce a soil with even smaller particles and the different size-bands of particles become arranged in horizons. Freezing and thawing cycles, and the physical movement of the particles, increases the weathering process, until eventually there is a suitable structure to support plant life.

Question 3 answer

Clay (< 2 µm), silt (2.5 µm), sand (50 µm - 2 mm), gravel (2 mm - 2 cm) and rock (> 2 cm).

Question 4 answer

It is the growth and decay of plants, animals and micro-organisms which makes the soil a dynamic, distinctive substance. The initial plant life accelerates soil formation as the plants themselves physically and chemically modify the particles. Exudates from the plants provide substrates for microbial metabolism and, when the plants die, they contribute further organic matter to the soil. Plant roots can also be important in breaking down rocks, and organisms such as Earthworms are vital in mixing all the different components up.

Question 5 answer

Other than colour, soils can vary in depth (from a few centimetres to tens of metres), in fertility and in texture (varying from very fine, sticky coagulates to coarse-grained soils in which sands predominate).

Question 6 answer

The A-horizon is closest to the surface and contains the most organic matter. It comprises of a number of zones and is extremely biologically active. The B-horizon, also containing a couple of zones, is typically an area of accumulation of the materials transported downwards from the A-horizon. It contains less organic matter and is less biologically active. Finally, the C-horizon is a transition zone between the soil and underlying rock. It is much less altered and less biologically active than the above horizons.

Question 7 answer

Whilst the horizons are easy to detect in well-trained temperate soils, some soils in semi-arid areas or where drainage is very poor may have different characteristics. The structure of soils consequently varies from place to place depending on the interactions of the soil forming forces of climate, topography and parent materials.

Question 8 answer

Many soils, and especially those on slopes, are prone to erosion, in which materials are transported away by water and/or wind. As this happens, the organically-active layer penetrates further down into the B horizon, gradually incorporating its top layer into the A horizon. Similarly, the top of the C horizon is incorporated into the B horizon.

Question 9 answer

Soil structure is an important factor in determining the water-holding capacity of the soil and the speed at which water soaks into and through the soil. The soils capacity to support plant growth is dependent on the control of water.

Question 10 answer

Adding negatively charged anions neutralises positively charged cation particles, in particular protons (hydrogen ions). Once this happens, the neutral particles aggregate together, forming a gel. This gel maintains a good soil structure with pore spaces between.

Question 11 answer

The fertility of a soil is defined as the ability of the soil to provide plants with the essential nutrients. In general, soils with a high proportion of clays and organic matter are very fertile, due to possessing a higher capacity for nutrient store and release.

Question 12 answer

Soil texture is the main factor determining water retention. Coarse-textured soils retain less water, both physically and chemically. Physically because they have a lower surface area and less water is trapped by friction, and chemically because clays have a higher capacity to chemically bind water into colloidal structures. Slope is also important, however, as steep slopes will drain rapidly and, in contrast, flat, poorly-drained areas can easily become water-logged. This is particularly true where the soils have a high clay content, something which will inhibit air penetration to plant roots and soil organisms.

Question 13 answer

Soils are a crucial part of any ecosystem. Their characteristics are both dependent upon and affect the environment in which they are found. It is consequently true to say that soils are where the biosphere, lithosphere, hydrosphere and atmosphere meet, meaning that they are of the utmost importance in the study of the environment as a whole.