Treatment of a commercial sample of vermiculite (1–2 mm grade) in its natural (non-exfoliated) state with ammonia gas gave a product which nitrified more slowly than ammonium sulphate. The product has potential as a slow-acting fertilizer although its maximum nitrogen content is only 5–6 per cent. The product formed from exfoliated vermiculite and ammonia gas should be useful as a medium for plant growth because it combines good physical properties with a high rate of nitrogen release. re]19760730