Benefits of bio-fertilizers:\nBio-fertilizers are cheaper, renewable, and non-polluting. Which aspects of soil performance do they improve when applied appropriately?

Introduction / Context:
Bio-fertilizers are preparations of beneficial microorganisms such as Rhizobium, Azotobacter, Azospirillum, and phosphate-solubilizing bacteria that enhance nutrient availability to plants. Beyond nutrient supply, sustained use can influence soil physical properties and water relations by promoting root growth and microbial biomass, which contribute to aggregation. This question consolidates these recognized benefits into a single selection task.

Given Data / Assumptions:

• Appropriate carrier-based inoculants applied to compatible crops and soils.
• Good agronomic management that allows microbes to survive (moisture, temperature, pH).
• Time scale long enough to reflect changes in soil structure.

Concept / Approach:
Primary benefits are nutritional: nitrogen fixation (symbiotic and free-living), phosphate solubilization/mobilization, and production of growth-promoting substances. Secondary benefits accrue as enhanced root biomass and microbial by-products (e.g., polysaccharides) stabilize soil aggregates, improving porosity and reducing bulk density. Over seasons, these changes can raise the soil's effective water holding capacity by increasing meso- and micropores and improving infiltration and retention dynamics.

Step-by-Step Solution:

Verification / Alternative check:
Long-term trials with legume inoculation and phosphate-solubilizing bacteria report improved soil aggregate stability indices and water infiltration, alongside yield responses due to enhanced nutrient availability.

Why Other Options Are Wrong:

• Picking only one aspect understates the multi-faceted benefits.
• “None of these” contradicts established agronomic outcomes.

Common Pitfalls:
Expecting immediate physical changes after a single application; structural and water-holding improvements emerge over time with continuous biological activity and organic matter inputs.

Final Answer:
All (a), (b) and (c).