For elements that are digested in strong reagents, the analysis units are mg/kg. These include cobalt, total manganese, sulphur and iodine. In order to simplify advice tables, it is normal to classify soil available levels of nutrients and trace elements into classes. The class is referred to as the Soil Index.

The index ranges are as follows: 0–10 is very low; 11–25 is low; 26–50 is medium; 51–100 is high; and 100+ indicates very high levels of these nutrients. When levels are low, plants will respond to addition of these nutrients.

Following are some factors that effect the organic matter level in soil:

• Climate.
• Drainage.
• Natural vegetation.
• Rotations, residues and plant nutrients.
• Temperature.
• Tillage and cropping.

Nutrient index(N.I) value is a. measure of nutrient supplying capacity of soil to plants (Singh et al., 2016)6.

about 4 percent to 6 percent

organic matter content of the soil. Most soil organic matter originates from plant tissue. Plant residues contain 60-90 percent moisture. The remaining dry matter consists of carbon (C), oxygen, hydrogen (H) and small amounts of sulphur (S), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg).

Volcanic Soil Is Soft And Easy For Plants To Break Down When plant roots come into contact with these minerals in the soil, they are able to receive the nutrients from those minerals that they need in order to grow strong, green, and lush.

However, the process of eruption is characterized by different processes which ultimately lead to the formation of new materials including rocks and soil with volcanic soils formed through this process. They are among the most fertile in the world.

Volcanic soils are ideal for vegetable production (“market gardening”) because of the high subsoil macroporosity. This encourages root vegetables including potatoes, carrots, onions, turnips and swedes, to expand without limit in a freely drained subsoil environment.

They are often very rich in nutrients and hold water well because of their volcanic ash content. These soils are called Andisols, and they are often very young, and acidic depending on which type of volcano they come from.

The soil in this region is rich because volcanic eruption deposit the necessary minerals, which are then weathered and broken down by rain. Once absorbed into the soil, they become a steady supply of nutrients for plant life.

Ashfall alone is not likely to kill mature trees, but the accumulated weight of ash can break large branches in cases of heavy ashfall (>500 mm (20 in)). Defoliation of trees may also occur, especially if there is a coarse component of ash-sized particles or larger tephra and during heavy ashfall.

The altered volcanic ashes are well-drained, yet hold water for plants, and are easily tilled. Deep volcanic loams are particularly good for pasture growth (there is a large New Zealand dairy industry), horticulture, and maize. Volcanic ash can be considered as a time-release capsule, rich in nutrients.

Older volcanic ash deposits that have been exposed to warm, humid conditions may be redder in color due to the presence of iron and aluminum oxides that formed during soil weathering processes.

Well-drained volcanic soils like the terra-roxa of Brazil which contain much potash as well as organic material are the best.