Two plots, one soil series: reading the profile before signing a lease
Two adjacent 0.8 ha plots on the same farm near Shahjahanpur are being considered for a horticulture lease. Plot A is on the upper slope (3–4%), has 45 cm of reddish sandy loam over weathered sandstone, organic carbon 0.28%, infiltration 4.5 cm/h, and dries out within four days of a 30 mm rain. Plot B lies 400 m downslope in a natural depression, has 120 cm of dark grey clay loam, organic carbon 0.71%, infiltration 0.6 cm/h, and shows mottles and a compact layer at 35–50 cm; water stands in it for two to three days after heavy rain. Both plots have the same rainfall and the same cropping history. The lessee can afford only one shallow tube well and wants to grow banana, which is sensitive to both drought and waterlogging, with a reliable market for the fruit. The block office maps both plots in the same soil series on a 1:50,000 map.
1. The two plots are shown as one soil series on the block map. Using soil-forming factors and field evidence, explain the difference in behaviour and list the observations you would record to defend your reading.
Read the worked answer
Climate, parent material and time are effectively the same for both plots; relief is the factor that differs, and relief controls water - infiltration, runoff and internal drainage - so it drives horizon development.
- Plot A (shoulder): runoff removes the fine fraction, so the horizon sequence is thin A over a lithic contact at 45 cm. Free drainage but tiny storage: available water capacity is roughly 45 cm × 0.15 = about 6.8 cm (68 mm), which is why a 30 mm rain wets it and four dry days empty it.
- Plot B (depression): receives run-on and illuviated clay, so clay has accumulated in the subsoil and internal drainage is poor. The mottles at 35–50 cm are redox features (seasonal saturation), and the compact layer with platy structure is a barrier that perches water and deflects roots. Higher organic carbon follows from slower decomposition and sediment deposition.
Field check to record: auger to 120 cm; depth to rock or compact layer; texture by feel; structure and mottle depth/colour; root deflection; effervescence with dilute HCl; ring-infiltrometer reading; and monsoon water-table depth in a dug pit. Note too that 1:50,000 means 1 cm = 500 m with a minimum delineation near 2 mm (100 m), so a 90 m × 90 m plot cannot be separated from its neighbour - the map unit is a generalisation, not a plot-level statement.
2. Which plot would you recommend for banana, and what soil-physical interventions would you specify? Justify the choice by comparing what can and cannot be corrected.
Read the worked answer
Recommend Plot B, with drainage works.
Banana needs roughly 60–90 cm of rootable soil, steady moisture and free drainage; waterlogging for more than a day or two causes root suffocation and increases the risk of Erwinia and Panama-type problems.
- Plot A is disqualified by a permanent limitation. The rock at 40 cm is not a management problem - it caps root volume and water storage (about 68 mm) permanently. High infiltration (4.5 cm/h) plus shallow storage means summer irrigation every four to five days at 7–8 mm/day evapotranspiration, impossible with one shallow tube well, and the soil cannot be deepened economically.
- Plot B's defects are correctable. Depth is 120 cm and organic carbon is good. Specify: (i) planting on ridges or broad beds 30–40 cm high, with furrows functioning as drains; (ii) subsoiling/chiselling along the planting lines to break the 35–50 cm compact layer before planting; (iii) a graded field drain to a safe outlet so monsoon water leaves within 24–48 hours; (iv) 10–15 t/ha FYM plus a green-manure crop to stabilise structure; (v) a shallow tube well in the depression, where the water table is close, for summer supplementation.
Decision rule: prefer the site whose limitation responds to drainage and organic matter over the site whose limitation is a rock contact.
3. The lease agreement uses the block soil map to fix rent classes. Advise what field survey is actually needed, and how the two plots should be treated in the agreement.
Read the worked answer
At 1:50,000, 1 cm represents 500 m and the minimum mappable delineation is about 100 m on the ground. Each 0.8 ha plot is roughly 90 m × 90 m, smaller than one delineation, so both fall inside one polygon and the map cannot support a rent distinction. A series name also describes a range of properties, and phases (slope, depth, erosion, stoniness) are often not shown at this scale.
What is needed:
- A detailed survey at 1:5,000–1:10,000 for the two plots, with auger observations on a 25–50 m grid and at least three to five borings per plot.
- One representative profile pit per plot, described and sampled by horizon; GPS-fixed boundaries.
- Simple field measurements that translate to rent value: depth to root-limiting layer, infiltration rate, organic carbon, and monsoon water-table depth.
- A land-capability or suitability class for the intended crop (banana), and a yield/site index rather than a series name.
In the agreement, rent should be tied to measured properties of each plot - rooting depth, drainage class, infiltration and organic carbon - with the block map used only for locational context. State the depth and drainage findings as annexures so that a dispute can be settled by re-measurement, not by re-reading a 1:50,000 polygon.