Which soil particles are the finest in size?

Clay particles are the smallest (finest) of all soil particles. According to the USDA classification, clay particles are less than 0.002 mm (2 micrometres) in diameter, so small that they can be seen only with a powerful microscope.

Soil particle sizes (USDA system):

• Gravel: larger than 2 mm.
• Sand: 0.05 to 2 mm (coarse, gritty particles that can be seen with the naked eye).
• Silt: 0.002 to 0.05 mm (smooth, floury when dry).
• Clay: less than 0.002 mm.

Properties of clay:

• It has a very large surface area for its volume, because the particles are so tiny and are usually plate-shaped.
• It is sticky and plastic when wet and becomes hard and cracked when dry.
• It holds a lot of water, but much of this water is held so tightly that plants cannot use it.
• It holds plant nutrients well, because clay particles carry negative charges that attract positively charged nutrient ions, such as calcium, magnesium and potassium.
• Soils rich in clay are heavy, poorly aerated and difficult to plough.

Soil texture:

• Soil texture refers to the relative proportions of sand, silt and clay in a soil.
• It is shown using the soil texture triangle, which defines classes such as sandy, loamy, silty and clayey soils.

Loam: a soil with a balanced mix of sand, silt and clay (roughly 40 per cent sand, 40 per cent silt and 20 per cent clay). It is considered the best soil for most crops, as it holds enough water and nutrients while still draining and allowing air to reach the roots.

Clay soils in India:

• Black (regur) soils of the Deccan have a high clay content.
• Heavy clay soils in river deltas and floodplains are used for growing rice, because they hold water.

Why not the other options?

• ✗Silt particles are larger than clay but smaller than sand.
• ✗Gravel consists of rock fragments larger than 2 mm, the coarsest category.
• ✗Loam is not a particle size; it is a soil texture class, a mixture of sand, silt and clay.