Tag: biology

Soil water is often referred to as soil moisture. Soil is the amount of water present in the interparticle spaces in the soil and is available to plants. When soil moisture gets depleted, enough water is not available for plants. So they have to apply more pressure in obtaining water from around the soil particles. 

Soil pH is important because it influences several soil factors affecting plant growth, such as (1) soil bacteria, (2) nutrient leaching, (3) nutrient availability, (4) toxic elements and (5) soil structure. Bacterial activity that releases nitrogen from organic matter and certain fertilizers is particularly affected by soil pH, because bacteria operate best in the pH range of 5.5 to 7.0. Plant nutrients leach out of soils with a pH below 5.0 much more rapidly than from soils with values between 5.0 and 7.5. Plant nutrients are generally most available to plants in the pH range 5.5 to 6.5. For most plants, the optimum pH range is from 5.5 to 7.0.

Soil particles vary greatly in size, and soil scientists classify soil particles into sand, silt, and clay. Starting with the finest, clay particles are smaller than 0.002 mm in diameter. Some clay particles are so small, that ordinary microscopes do not show them. Silt particles are from 0.002 to 0.05 mm in diameter. Sand ranges from 0.05 to 2.0 mm. Particles larger than 2.0 mm are called as gravel or stones. Most soils contain a mixture of sand, silt and clay in different proportions.

A physiologically dry soil is a soil in which the concentration of oxygen is very less and that of salt is very high. Normal plants cannot grow in such soil and only halophytes are found in them. The Sunderbans in West Bengal are rich in physiologically dry soil. Thus the correct answer is option C.

• The process of absorption of water by the soil is called percolation. It is different for different soil types and depends upon the soil compositions. It is calculated by the formula i.e., percolation rate = amount of water/percolation time.

In the steppes and deserts, evaporation greatly exceeds precipitation. The products of rock weathering are brought by intermittent flowing streams during a flood and left behind on the basin floor. This soil has clay, silt and sand are the dissolved mineral salts and is called halomorphic soil. Thus the correct answer is option B. 

• The topsoil is darker because it contains dead organic material in many stages of decomposition. 
• This degradation produces a substance called humus that is composed of carbon-rich compounds and imparts a dark color. Thus, the correct answer is option D.

Humus is organic matter containing nutrients from the soil. It is nothing but dead and decaying organic matter where saprophytes convert complex organic material into simple nutrients. These nutrients are then available to the plants through humus.

Black cotton soil is rich in clay, which has very small sized particles. These small sized particles obstruct water movement and hence, this soil has high water holding capacity and low percolation.