Chemical Components of Soil, Biology tutorial

Characteristics of Rocks and Minerals:

Rocks are commonly categorized as igneous, sedimentary and metamorphic. Igneous originated from molten magma and comprise rocks as granite and biorite. Igneous rocks are composed of primary minerals (minerals which have not been altered chemically) like light - colored quartz, muscovite, and feldspars and dark-colored biotite, augite, and harnblevide. Usually, dark-colored minerals contain iron and magnesium and are more easily weathered. In view of this dark-colored igneous rocks like gabbro and basalt and more easily broken down than are granites and other light - colored igneous rocks. Sedimentary rocks are composed of cemented or compacted weathering products from older, preexisting rock. For example, quartz sand weathered from the granite rock and washed in ocean may settle on ocean floor becoming cemented in solid mass called sandstone.

Also, clays may be compacted in shale. Sedimentary rocks cover approx 75% of the earth's land surface. Metamorphic rocks are those formed because of change in form of other rocks. For instance, igneous rocks are commonly altered to form schist or gnesis in which light and dark minerals have been reoriented in bands. Also sedimentary rocks, like limestone and shale, may be metamorphosed to marble and slate respectively.


Weathering - the biochemical process which engages both destruction and synthesis. In this procedure, rocks and minerals are destroyed by physical disintegration and chemical decomposition. Without affecting the composition, physical disintegration breaks down rock in smaller rocks and finally in sand silt particles which are commonly composed of individual minerals. In addition, minerals are decomposed chemically. Releasing soluble materials and synthesizing new materials, primary minerals, like silicate clays, quartz, oxides of Fe and Al, organic complexes of Al3+, and Fe3+ and Fe3+, silicic acid, soluble materials such as; Ca2+, Mg2+, K+, Na+, Fe2+ and SO4 2- are some of the groups of minerals which remain in well - weathered soils.

i) Physical Weathering:

Physical disintegration engages change in temperature abrasion by water, wind and ice, and activities flora and fauna.


The procedure of heating and cooling during the day and might causes alternate expansion and contraction of rock minerals. As some minerals expand more than others, temperature changes set up differential stresses which finally cause rock to crack.


Water has tremendous cutting power, similarly, wind-blown dust and sand. In addition, huge moving ice masses embedded with soil and rock fragments grind down rocks in the path, resulting in physical disintegration of the rocks and minerals.

Plants and Animals:

Plant roots at times enter cracks in rocks and pry them apart, resulting in some disintegration. Burrowing animals may also assist in physical weathering of rocks.

ii) Biogeochemical Weathering:

Both physical and chemical weathering take place together for instance, physical abrasion reduces size of particles and thus increases their surface area, making them more vulnerable to rapid chemical reactions. In addition, chemical weathering is encouraged by presence of water, oxygen acids produced by microbes and plants root metabolism. There are 6 basic kinds of chemical weathering reactions, and all of these reactions are improved by presence of water.

Soil analysis:

Soil Sampling:

Because of variability in nutrient levels from spot to spot, a given field is divided in several distinct areas when taking soil samples to find out nutrient needs.

Composite Samples:

The soil auger or probe is utilized to eradicate the thin cylindrical cove of soil from at least 12 to 15 randomly scattered places inside land area to be represented. 12 to 15 subsamples are methodically mixed in the plastic bucket, and approx 0.5 L of the soil is placed in the labeled container and sent to laboratory. If soil is moist, it must be air - dried without sun or heat before packaging for routine soil tests.

Depth to Sample:

Standard depth of sampling for the plowed soil is the depth of plowed layer, approx 15 to 20cm, in forest soils approx 15 to 20cm in forest soils, approx 20-40cm while sampling the soil for nitrates a depth of 60-80cm must be sampled.

Time of Year:

Seasonal changes are frequently observed in soil test results for the given area. Time of sampling is particularly significant if year to year comparisons are to be made. The good practice is to sample each area every year or two (always at the same time of year), so that soil test levels can be tracked.

Chemical Analysis of Soil:

Different extraction solutions are used by different laboratories. For instance, buffered salt solution, like sodium or ammonrium acetate, or the mixture of dilute acids and chelating agents are extracting agents most usually used. Extractions are achieved by placing the small measured quantity of soil in the bottle with extracting agent and shaking mixture for the certain number of minutes. The complete process is generally automated so that the modern laboratory can handle hundreds of samples every day.

Interpretation of Soil Analysis Result:

The soil test values are indices of nutrient - supplying power. They don't point to actual amount of nutrient whihc will be supplied. Thus, it is best to think of soil test reports as more indicates than quantitative. Interpretation of soil test date is best achieved by experienced and technically trained personnel who completely understand to scientific principles underlying common field procedure.

Benefits of Soil Analysis:

Soil testing is the priceless tool in making fertilizer recommendations, generally when they are correlated with results of field fertilizer experiments.

In addition, soil analysis provide soil managers in decision making on sufficient provision of nutrients for plants, conservation of nutrient resources and prevention of contamination of environment with used nutrients.

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