This question was popular among the candidates. In 4(a-b), majority of the candidates were unable to mention the biotic factors that affect agricultural production and  state the negative effects of continuous cropping and overgrazing on the soil.  However in 4(c), most of them were able to give the advantages and disadvantages of surface drainage.
The expected answers include:
4.  (a)   Biotic factors that affect agricultural production
              -    Predators
              -    Parasites
              -    Soil organisms(any named soil organism)
              -    Pests
              -    Pathogens/diseases( any named pathogen/disease)
              -    Weeds

    (b)    Negative effects of farming practices on the soil
            Continuous cropping
             -   Frequent crop removal depletes the soil of nutrients
             -   It encourages multiplication of soil pathogens that reduce soil quality
-     It exposes the soil if crop residues are removed, leading to erosion of nutrients
-    Frequent cultivation may lead to excessive oxidation and loss of organic matter
-    It encourages multiplication of pests

    (ii)    Overgrazing

•  Exposes  the soil to agents of erosion such as wind and water
• Trampling by livestock leads to destruction of soil structure/soil compaction

     which reduces the rate of water percolation and infiltration

• Leads to loss of soil fertility
• Exposes the soil to the direct effects of solar radiation