Choosing a Crop Under Salty Irrigation Water
A farmer in a coastal district has two fields. Field A is 2 hectares with sandy loam soil; Field B is 2 hectares with clay soil. Both fields receive irrigation water with an electrical conductivity of 4 dS/m, which is moderately saline. The farmer wants to grow either tomatoes (moderately salt-tolerant, expected yield 30 tonnes/ha, price 40 dollars/tonne) or beans (salt-sensitive, expected yield 2 tonnes/ha, price 120 dollars/tonne). Last season the farmer applied 300 mm of irrigation water to each field and noticed white crusts forming on the surface of Field A but not Field B. Rainfall is low and drainage in Field B is poor because of a compacted layer 40 cm below the surface.
1. Which field is more likely to accumulate harmful salt in the root zone, and why? Explain using the information about soil texture, drainage, and the white crusts.
Read the worked answer
Field B is more likely to accumulate harmful salt in the root zone, even though the crusts appeared on Field A. Sandy loam (Field A) has large pore spaces, so water drains through quickly and salts are leached downward below the roots, which is why salts crystallise at the surface rather than staying in the root zone. Clay (Field B) has small pores and slow drainage, and the compacted layer at 40 cm acts as a barrier that stops downward water movement. Water applied to Field B therefore sits in the root zone and evaporates, leaving dissolved salts behind. Over repeated irrigations, salt concentration in Field B's root zone rises, which can reduce water uptake and stunt salt-sensitive crops. The surface crust on Field A is a symptom of upward capillary movement and evaporation, not necessarily root-zone damage. The key judgement is that poor drainage, not surface appearance, determines salt accumulation.
2. Compare the likely gross income from tomatoes and beans on Field A, and recommend which crop the farmer should plant there. Show your reasoning, including any assumptions.
Read the worked answer
Tomatoes: 30 tonnes/ha × 40 dollars/tonne = 1,200 dollars/ha. For 2 ha: 2,400 dollars. Beans: 2 tonnes/ha × 120 dollars/tonne = 240 dollars/ha. For 2 ha: 480 dollars. Tomatoes give a much higher gross income on Field A, about five times that of beans. This assumes the stated yields are achievable under 4 dS/m irrigation water. Tomatoes are moderately salt-tolerant, so a yield of 30 tonnes/ha is plausible but may be reduced compared with non-saline conditions. Beans are salt-sensitive and may suffer more yield loss, so the 2 tonnes/ha figure could be optimistic. The recommendation is to plant tomatoes on Field A because the sandy loam drains well, reducing salt build-up, and tomatoes tolerate moderate salinity better. The farmer should also monitor soil salinity and apply extra leaching water where drainage allows.
3. The farmer considers planting beans on Field B instead of tomatoes. Using the salinity and drainage evidence, judge whether this is a sound decision and suggest one management change that would improve the outcome.
Read the worked answer
Planting beans on Field B is a poor decision. Field B has poor drainage and a compacted layer, so salts from the 4 dS/m water will accumulate in the root zone. Beans are salt-sensitive, so even moderate salinity will reduce germination, growth, and yield. The expected 2 tonnes/ha may not be reached, and income would be low. A sounder choice would be a more salt-tolerant crop, or improving drainage first. One useful management change is to break up the compacted layer by deep ripping or subsoiling to allow water and salts to move below the root zone, combined with applying extra irrigation water to leach salts. This addresses the cause of salt accumulation rather than just the symptom. The judgement is that crop choice must match both soil drainage and water quality, not just market price.