Which of the following statements with reference to Humus is/are correct? 1. It is a dark coloured amorphous substance. 2. Being colloidal in nature, it serves as a reservoir of nutrients. Select the answer using the code given below:

Correct — C, Both 1 and 2. Humus is what is left when soil organisms have finished breaking down dead plant and animal matter: a dark brown to black material that has lost every trace of the cell structure it came from, which is exactly what 'amorphous' means here, so statement 1 holds. Statement 2 states the reason humus matters agriculturally. Its molecules are colloidal — particles small enough to stay dispersed through the soil solution — and they carry a large number of negatively charged sites. Positively charged plant nutrients such as calcium, magnesium, potassium and ammonium are held on those sites instead of being washed down out of reach, and are released back to roots by ion exchange as the plant draws them off. That is what makes humus a store, or reservoir, of nutrients rather than merely a source of them, and it is why a humus-rich soil stays fertile through a rainy season that would leach a humus-poor one bare.

• (a)1 only — Accepts the description but rejects the function. The colloidal, negatively charged nature of humus is the standard explanation for the high cation-exchange capacity of organic-rich soils.
• (b)2 only — Rejects the description, which is accurate. Humus is dark brown to black and amorphous — it has no cellular structure left.
• (d)Neither 1 nor 2 — Both statements are correct; there is nothing in either that soil science disputes.

Soil has four components — mineral particles, humus, water and air — and humus is the organic one. It forms by humification, the microbial breakdown of dead vegetation and animal remains into a stable dark residue. How much of it a soil holds depends heavily on temperature: bacterial action is slow in cold climates, so organic matter accumulates and can build up into peat in sub-arctic and tundra soils, while in humid tropical and equatorial climates bacteria oxidise dead vegetation so fast that very little humus is left.

Two-statement items on humus almost always pair a description with a function, and the function half is where marks are won. Fix the mechanism rather than the phrase: humus is colloidal and negatively charged, plant nutrients that matter are mostly positively charged, opposite charges hold, and holding is what a reservoir does. The same idea explains a set of connected facts you can be asked separately — why humus improves water retention, why it improves soil structure, and why laterite soils, whose humus is destroyed quickly by bacteria in the heat, are poor in nitrogen and organic matter and need manuring before they will crop well.

• Humus is the dark brown to black, amorphous organic fraction of soil, formed by the decomposition of dead plant and animal material.
• It is a negatively charged colloid, and those charged sites raise the cation-exchange capacity of the soil, holding nutrient cations against leaching.
• Humus can hold a large fraction of its own weight in moisture, so it also improves a soil's resistance to drought.
• In cold climates slow bacterial action lets humus accumulate, and peat develops in sub-arctic and tundra soils; in humid tropical climates intense bacterial action leaves humus content very low.
• Laterite soils are poor in humus and nitrogen because bacteria that thrive at high temperature remove the organic matter fast.

• Reading 'amorphous' as 'unstable'; it refers to the loss of cellular structure, and humus is in fact the stable end product of decomposition.
• Assuming tropical soils must be rich in humus because tropical vegetation is lush; rapid bacterial oxidation leaves them poor in it.
• Confusing humus with raw organic litter; humus is what remains after decomposition, not the leaf fall itself.

As a two-statement item pairing a description of humus with its role in fertility, or as a question on which soil type is richest or poorest in organic matter.

• practice — not a real PYQ

  The high cation-exchange capacity of an organic-rich soil is chiefly explained by which one of the following?

  • (a)The presence of coarse sand particles
  • (b)The negatively charged colloidal humus
  • (c)A high content of soluble chlorides
  • (d)Rapid percolation of rain water

  Answer(b) The negatively charged colloidal humus — its charged sites bind nutrient cations and release them to plant roots by ion exchange.

• practice — not a real PYQ

  Layers of peat develop in sub-arctic and tundra soils mainly because of which one of the following?

  • (a)Heavy leaching by tropical rain
  • (b)Low bacterial activity leaving organic matter undecomposed
  • (c)High evaporation concentrating salts at the surface
  • (d)Intense weathering of the parent rock

  Answer(b) Low bacterial activity leaving organic matter undecomposed — cold slows decay, so dead vegetation accumulates instead of being oxidised.