Azolla, a small aquatic fern used as green manure, is especially effective in increasing soil fertility for which crop?

Introduction / Context:

This question belongs to agricultural science and biofertilisers. Azolla is a water fern that has gained attention as a natural way to enhance soil fertility and reduce chemical nitrogen fertiliser use. It is particularly associated with a major staple crop grown in flooded fields, and knowing this association is important for questions about sustainable farming practices.

Given Data / Assumptions:

• Azolla is an aquatic fern that floats on the surface of water in fields and ponds.
• It has a symbiotic relationship with nitrogen fixing cyanobacteria, often Anabaena azollae.
• The options mention cereal crops and one sugar crop commonly grown on different types of fields.
• We assume typical Asian cropping systems where paddy fields are flooded with water.

Concept / Approach:

Azolla thrives in wet, flooded conditions. It can rapidly cover the surface of standing water in paddy fields. The cyanobacteria living within Azolla leaves fix atmospheric nitrogen and enrich the fern biomass. When Azolla is incorporated into the soil as green manure, it releases nitrogen and other nutrients that benefit the subsequent crop. This method is particularly suitable for rice, which is grown under flooded conditions where Azolla can grow easily. The approach is to match Azolla habitat requirements with the crop cultivation environment.

Step-by-Step Solution:

Verification / Alternative check:

Field studies and agricultural extension literature from rice growing regions such as parts of India, China and Vietnam emphasise Azolla as a green manure in lowland paddy fields. Farmers sometimes grow Azolla before transplanting rice seedlings or between rice crops. Observed benefits include improved soil nitrogen status and reduced need for chemical nitrogen fertilisers. In contrast, there is little mention of Azolla use in maize, wheat or barley fields, which are not normally flooded, confirming rice as the main beneficiary crop.

Why Other Options Are Wrong:

Maize, wheat and barley are generally grown in upland or moderately drained soils without standing water. These conditions are unfavourable for Azolla, which needs a shallow water environment to survive. Sugarcane is grown in fields that may require ample water but are not permanently flooded like paddy fields, so Azolla mats are not typical in sugarcane cultivation either. As a result, these crops do not commonly use Azolla based green manuring in standard farming practice.

Common Pitfalls:

Some learners may choose wheat or maize because they think of general fertiliser requirements rather than the specific habitat of Azolla. Others might not realise that Azolla is aquatic and suppose it can be used like any other green manure on dry land. Remembering that Azolla floats on water and that rice is the major crop grown in flooded conditions helps avoid confusion and quickly leads to the correct answer.

Final Answer:

Azolla is especially effective in increasing soil fertility for Rice cultivation.