Soils and Crop Growth

Question

Step-by-step solution

1. Step 1

   Mineral particles — broken-down rock fragments of varying size: clay (very fine), silt (medium), and sand (coarse). They form the physical skeleton of soil and provide anchorage for roots. The mix determines texture, drainage, and nutrient-holding capacity.

2. Step 2

   Organic matter (humus) — decomposed plant and animal material broken down by bacteria and fungi. Humus improves soil structure, retains moisture, and releases essential nutrients (nitrogen, phosphorus, potassium) slowly as it decomposes further.

3. Step 3

   Water — held in pore spaces between particles. Plants absorb water through roots for photosynthesis, nutrient transport, and cell support (turgor). Without sufficient water, plants wilt and die.

4. Step 4

   Air (gases) — oxygen in pore spaces is essential for root respiration and for the aerobic bacteria that decompose organic matter. Waterlogged soils have poor air content, causing anaerobic conditions that harm roots and promote denitrification (loss of nitrogen).

Question

Step-by-step solution

1. Step 1

   Good drainage but moisture retention (loam texture) — loam soils have a mixture of sand, silt and clay that drains excess water (preventing waterlogging and root rot) while retaining enough moisture for uptake between rainfall events.

2. Step 2

   Neutral to slightly acidic pH (6–7) — most crops grow best in this pH range. Acidic soils (pH < 6) lock up phosphorus and cause aluminium toxicity; alkaline soils (pH > 7) lock up iron and manganese. Farmers add lime (calcium carbonate) to raise pH or sulfur to lower it.

3. Step 3

   High humus content — organic matter improves soil structure, binds particles into aggregates (crumb structure), retains moisture, and slowly releases nutrients. High humus content increases soil biodiversity (earthworms, bacteria) which aid further decomposition.

4. Step 4

   Good aeration (pore spaces) — adequate air spaces allow roots to respire aerobically. Compacted soils (e.g. from heavy machinery) have few pore spaces, restricting root growth and causing waterlogging.

Question

Step-by-step solution

1. Step 1

   Adding organic matter (manure or compost) — animal manure or decomposed plant material is mixed into soil. It improves structure, adds nutrients (especially nitrogen), increases water retention and stimulates soil organisms. Limitation: requires large quantities; labour-intensive; organic nitrogen releases slowly (may not meet peak crop demand); manure can introduce pathogens.

2. Step 2

   Adding inorganic (artificial) fertilisers — NPK fertilisers (nitrogen, phosphorus, potassium) are applied to replace nutrients removed by harvesting. Benefit: fast-acting, precise nutrient ratios, increased yields. Limitation: expensive; can leach into waterways causing eutrophication; overuse damages soil structure and reduces earthworm populations over time.

3. Step 3

   Liming — adding lime (calcium carbonate, CaCO₃) to acidic soils to raise pH towards neutral. Benefit: unlocks phosphorus and micronutrients; improves structure in clay soils. Limitation: needs careful monitoring — overliming makes soil too alkaline; requires repeat application as rain leaches lime.

Question

Step-by-step solution

1. Step 1

   Soil changes causing yield plateau: (1) Soil structure degradation: heavy machinery and continuous flooding (paddy conditions) compact the soil, reducing pore space for air and water. Compacted soils restrict root penetration, reducing nutrient and water uptake even when nutrients are present. (2) Decline in organic matter: intensive monoculture with straw removal (rather than incorporation) reduces organic matter inputs. Humus content falls, reducing water retention, soil biodiversity (earthworms, bacteria) and the natural nutrient cycling that previously supplemented fertiliser inputs. (3) Nutrient imbalance and soil acidification: continuous application of ammonium-based nitrogen fertilisers acidifies the soil over time (ammonium oxidation releases H⁺ ions). As pH falls below 6, phosphorus becomes less available despite being present in the soil — a phenomenon called phosphorus fixation. Micronutrient deficiencies (zinc, iron) also emerge. (4) Loss of soil biodiversity: the microbial community responsible for nitrogen fixation, decomposition and phosphorus mobilisation declines under continuous monoculture and synthetic fertiliser use.

2. Step 2

   Long-term consequences of continuing: (1) Environmental — eutrophication: excess nitrogen and phosphorus continue leaching into rivers and coastal areas, causing algal blooms that deplete dissolved oxygen and kill aquatic life, threatening fisheries that many communities depend on. (2) Environmental — soil salinity: in irrigated paddies, if drainage is poor, evaporation concentrates salts in the topsoil over decades. Salinised soils are toxic to most crops — the land may eventually become unproductive. (3) Economic — diminishing returns: farmers spend more on fertiliser while yields stagnate or decline. If input costs exceed revenue, farms become economically unviable, leading to rural poverty and abandonment of land. (4) Food security — regional risk: if multiple farms in the region suffer yield decline simultaneously, regional food production falls, increasing food prices and vulnerability to famine during drought years.

3. Step 3

   Recommendations: To break the yield plateau, farmers should: (1) Test soil pH and apply lime to correct acidity (unlocking fixed phosphorus). (2) Introduce organic matter through green manures (nitrogen-fixing legumes grown between rice cycles) and incorporating rice straw. (3) Practise integrated soil fertility management — combining reduced artificial fertiliser with organic amendments. (4) Reduce continuous flooding where possible (alternate wetting and drying) to improve soil aeration and reduce compaction.

Question

Step-by-step solution

1. Step 1

   Three ways organic farming maintains/improves soil health: (1) Organic matter additions: organic farmers use compost, green manures and animal manure instead of synthetic fertilisers. These continuously add organic matter, building humus levels, improving soil structure and water retention. Conventional farming with synthetic fertilisers alone does not restore humus. (2) No synthetic pesticides: organic farms avoid herbicides and insecticides, maintaining soil biodiversity including earthworms, beetles and microbial communities. Earthworm activity improves soil aeration and drainage (bioturbation). (3) Crop rotation: organic farms must rotate crops to manage pests and nutrients without chemicals. Rotation with nitrogen-fixing legumes (beans, clover) naturally restores soil nitrogen; different root depths reduce compaction; rotating root and leafy crops balances nutrient draw-down.

2. Step 2

   Evaluation — Can organic farming replace conventional farming?

   Arguments FOR: Long-term studies show organic soils have higher humus, better water retention and more resilience to drought — qualities that become more valuable as climate change increases rainfall variability. Organic farming eliminates eutrophication from synthetic fertiliser leaching, protecting water quality. It preserves biodiversity (pollinators, natural pest predators), providing ecosystem services that conventional farming destroys.

   Arguments AGAINST: Organic farming yields are typically 20–25% lower than conventional per hectare. To produce the same total food output organically, ~25% more land area would need to be farmed — land that would have to come from natural habitats. The global population of 8+ billion, set to reach 9.7 billion by 2050, may not be able to be fed exclusively by organic production. Organic produce is more labour-intensive and expensive, making it unaffordable for low-income populations. In LICs with limited organic inputs (manure, compost), the transition from synthetic fertilisers could trigger immediate yield collapses and food insecurity.

3. Step 3

   Conclusion: Organic farming is the better system for soil health and long-term sustainability, but replacing conventional farming entirely is not currently realistic without advances in organic yield improvement and major changes in land use and diet (less meat, less waste). A hybrid approach — using organic soil management principles (rotations, organic matter) within a reduced-input conventional system — is the most practical path to sustainable intensification.

Question

Step-by-step solution

1. Step 1

   Field B has better soil quality. Justification using each measurement: (1) pH: Field B (pH 6.8) is within the optimal 6–7 range for most crops. Field A (pH 5.2) is too acidic — at this pH, phosphorus becomes chemically bound to aluminium and iron compounds (phosphorus fixation) and becomes unavailable to plants despite being present; aluminium itself becomes soluble and toxic to roots. (2) Humus content: Field B (4.5%) has significantly higher organic matter, which improves soil structure, water retention, nutrient release through decomposition, and soil biodiversity. Field A (2%) is low — crops will have poor moisture retention, weaker nutrient supply, and less structural stability. (3) Bulk density: Field A's bulk density of 1.8 g/cm³ indicates severe compaction — the soil particles are tightly packed with very little pore space. Healthy agricultural soils have bulk density 1.1–1.4 g/cm³. Roots struggle to penetrate compact soil; water infiltration is poor, increasing runoff and erosion risk. Field B at 1.2 g/cm³ is within the ideal range.

2. Step 2

   Two management actions for Field A:

   (1) Apply lime (calcium carbonate) to raise pH from 5.2 towards 6.5: Lime neutralises soil acids by reacting with H⁺ ions. Raising pH to 6–7 will release phosphorus from aluminium/iron complexes, making it available for plant uptake. It will also reduce aluminium toxicity. The application rate should be calculated from a soil test; repeat application may be needed after 3–5 years as rainfall gradually re-acidifies the soil.

   (2) Add organic matter (compost or green manure) combined with deep tillage to address compaction and low humus: Adding 5–10 tonnes/hectare of compost will increase humus content, improving soil structure (aggregation), which will reduce bulk density over time. Deep tillage (subsoiling) can mechanically break the compacted layer to immediately improve drainage and root penetration — though this is a short-term fix and organic matter additions are needed for lasting improvement.