Compost maturity assessment — Which method(s) can be used to evaluate the degree of compost maturity and phytotoxicity risk?

Introduction / Context:
Compost maturity determines safe agronomic use. Immature compost can immobilize nitrogen or release phytotoxic compounds. Analytical and bioassay methods together provide robust evaluation of stability and maturity before field application.

Given Data / Assumptions:

• We consider non-destructive instrumentation and practical bioassays.
• Objective: assess humification, stability, and plant response.

Concept / Approach:
Infrared spectroscopy (often near-IR or mid-IR) probes functional groups and humification degree, enabling indices that correlate with maturity. Seed germination tests (e.g., cress test) directly detect phytotoxicity via germination rate and root elongation, integrating multiple chemical effects that instruments infer indirectly. Using both methods increases diagnostic confidence.

Step-by-Step Solution:

Verification / Alternative check:
Composting studies correlate IR indices and seed germination indices with respiration tests (e.g., oxygen uptake), confirming that both approaches track maturity.

Why Other Options Are Wrong:

• IR only or bioassay only: each gives partial insight; together they are stronger.
• None of the above: incorrect; both are widely used.

Common Pitfalls:
Using only pile temperature as a maturity indicator; temperature decline signals activity drop but not necessarily full stability.

Final Answer:
both (a) and (b)