This question is about the soil moisture window that gives the best tilth when ploughing, sometimes called the friable range.
- 5 to 10 per cent depletion: At this level, very little of the available water has left the soil, so the soil is still close to field capacity, too wet for clean tillage, and this option is ruled out.
- 15 to 20 per cent depletion: The soil is still wet enough here that ploughing will smear and compact it rather than break it into fine crumbs, so this is also too early.
- 25 to 50 per cent depletion: This range still sits on the wetter side of the friable window for most soils and is not the accepted optimum band.
- 50 to 60 per cent depletion: By this point, the soil has dried enough to crumble easily under the plough without smearing, yet not so much that it has turned into hard clods, so this is the recognized optimum window for tillage.
Ploughing at 50 to 60 per cent depletion of available soil moisture gives the best soil tilth with the least power and clod formation.
Let's summarize:
- Too wet a soil smears and compacts under the plough.
- Too dry a soil breaks into hard clods and needs more force.
- The 50 to 60 per cent depletion window balances both, giving the friable, workable tilth.
So the correct option is option 4.