ISO 7742:1988

Overview

ISO 7742:1988 - "Solid fertilizers - Reduction of samples" specifies a reproducible method for reducing a bulk sample of solid fertilizer to a smaller, representative portion for analysis or further reduction. The method is based on passing material through a rotating sample divider, collecting fractions and then rejecting or recombining selected fractions. By choosing suitable equipment the procedure is applicable to any mass above the minimum defined by particle size and particle count.

Key topics and technical requirements

• Primary principle: passage of material through a rotating sample divider; collect fractions, then reject or recombine to obtain a representative sub‑sample.
• Rotating divider design & operation:
  • Effective cutter/slot opening should be at least 3× (preferably 5×) the maximum particle size (practical minimum dimension ≈ 15 mm).
  • Typical operating frequency 60 min⁻¹, may be increased up to 360 min⁻¹ with care to avoid particle bounce bias.
  • Take at least 50–60 increments from the gross sample at each division stage.
  • Rotating dividers commonly produce 6, 8 or 10 fractions.
• Assessment of bias and precision:
  • Test divider performance using particle size distribution (most bias‑sensitive property). Use at least three sieves and compare paired analysis samples from ten gross samples to estimate division error.
  • ISO 7742 describes criteria for acceptable variance (example statistical check provided in text).
• Precautions: keep equipment clean and dry, work quickly to avoid moisture change, store reduced samples in airtight containers.
• Alternative methods (Annex A): riffle divider and coning & quartering - both are less precise and may introduce bias. Approximate relative standard deviations: rotating : riffle : coning = 1 : 1.5 : 3.5.

Practical applications

• Routine quality control and laboratory preparation of fertilizer samples for chemical and physical testing (nutrient assays, particle size, bulk density).
• Field and plant sampling workflows where a representative subsample is required before analysis.
• Use when bulk sample mass exceeds the minimum allowed by particle size/number - do not reduce below minimum mass without comminution.

Who should use this standard

• Fertilizer manufacturers and QC laboratories
• Independent testing and certification laboratories
• Regulatory and inspection bodies monitoring fertilizer quality
• Engineers and technicians designing sampling and sample‑preparation equipment

Related standards

• ISO/TR 7553:1987 - Sampling: minimum mass of increment representative of sampling unit.
• ISO 8397:1988 - Solid fertilizers and soil conditioners - Test sieving (particle size analysis).
• Prepared by ISO/TC 134 (Fertilizers and soil conditioners).