Humus soil, a dark, organic material, is primarily composed of decayed plant and animal matter, and it contains essential nutrients like carbon, nitrogen, phosphorus, and sulphur. 

To find the percentage of humus in the soil, we need to calculate the weight loss due to the burning of humus.

Initial weight loss (water removal): 28g - 24g = 4g

Weight loss due to burning of humus: 24g - 18g = 6g

Percentage of humus: \(\frac{Weight loss due to humus}{Initial weight}\times 100\)

= \(\frac{6}{28}\times 100\)

= 21.43% ≈ 22.2% (rounded to one decimal place)