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Seasonal and fertilizer effects on forms of soil phosphorus and potassium, partially decomposed organic debris and enzyme activities were studied over 2 years on a highly fertile yellow-brown loam (Typic Vitrandept) under grazed pasture. Fertilizer topdressing (potassic superphosphate) increased total inorganic phosphorus, NaHCO3-extractable inorganic phosphorus and NaHCO3- extractable potassium, but did not affect organic forms of phosphorus, microbial biomass potassium and phosphorus, or organic debris. Labile organic phosphorus (extracted by NaHCO3), microbial phosphorus and potassium, and organic debris accumulated over winter and declined in spring. Inter-year differences in climatic factors appeared to influence this basic pattern. Amounts of phosphorus released from labile organic and microbial phosphorus during spring were large (totalling 29 kg P ha-1 in one year) and could contribute substantially to plant P requirements. Mechanisms are proposed to explain the observed seasonal patterns in these soil characteristics. These include changes in the relative amounts of the fungal and bacterial components of the soil biomass during winter, mineralization of labile PI in spring due to increased bacterial growth and activity promoted by plant growth, and the subsequent release of P and K from the microbial biomass as a result of bacterial 'grazing' by protozoa.

The reaction of sodium fluoride solution with soils and soil minerals at constant pH has been assessed as a possible single-value characteristic in the classification of soils, particularly those containing significant amounts of poorly ordered inorganic material. A suitable method involves reaction of the soil or clay at 25 "C with 0 . 8 5~ sodium fluoride at pH 6.8, the amount of OH-released after 25 min being taken as the single-value characteristic.There was a statistically significant correlation between the amount of OHreleased and the amount of alumina extracted with cold 5 per cent sodium carbonate solution from both freely drained and poorly drained Scottish soils. The multiple correlation coefficients were improved for freely drained soils by including the dithionite-extracted ferric oxide and the silica soluble in 5 per cent sodium carbonate solution. The procedure is not suitable for surface soils, because of interference by organic matter, or for carbonate-containing soils unless carbonate is removed.Introduction SOME studies of the reaction between fluoride solutions and clay minerals have indicated disruption of the mineral structure and the formation of new compounds such as cryolite (Semmens and Meggy, 1966) or soluble fluoroaluminate complexes (Huang and Jackson, 1965), whereas others suggest that only an adsorption reaction occurs Bower

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