Urea supplying 100 lb of N/acre was either broadcast on the surface or mixed with four soils (two clay-loams, two sandy-loams) and losses of ammonia were measured when the soils were maintained at 40, 50, or 60 per cent water-holding capacity (WHC) and incubated at 5" or 25' C. Urea and ammonium sulphate were similarly applied to two calcareous soils at 40 per cent WHC and 5' or 25' C. Losses varied most among soils, from 2 per cent of the N applied from an unmanured clay-loam to 13 per cent from an unmanured sandy-loam. Changing temperature and water content affected losses little on average, but their effects differed with different soils. Losses of ammonia were similar from urea and from ammonium sulphate applied to the calcareous soils. Analyses at the end of the experiments showed that: (i) nitrite tended to accumulate in the sandy and calcareous soils, accumulation being favoured by lower temperature and wetter soil; (ii) ammonia was lost at 25' C from calcareous soils until all the ammonium-N had been nitrified, from the slightly alkaline clay soils until nine-tenths had been nitrified, and from the neutral sandy soils until half had been nitrified.

Wheat in pots and in the field was subjected plied at the boot stage of growth, N increased to different watering regimes and fertilizer-N, dry matter much less than when applied at either as ammonium-N with a nitrification sowing, but increased the percent N in the inhibitor or as nitrate-N, was applied at sow-grain. Divided dressings, half at sowing and ing or during growth. More weight of leaves half during growth, increased yield and N and stems was produoed during early growth uptake by the same amount as applying all with ammonium-N than with nitrate-N, but the N at sowing. Drought during spike forthe two forms gave similar yields of grain. mation or floret development led to smaller The grain:straw ratio with ammonium-N was spikes and yields of grain, and also to a smaller than with nitrate-N, and more N was smaller grain:straw ratio; up to three-quarters retained in the straw. Both forms of N in-of the extra N taken up from fertilizer-N creased the weight per spike but not the num-remained in the straw.

Ammonium sulphate alone and treated with the nitrification inhibitor 2-chloro-6-(trichloromethyl)- pyridine, at rates equivalent to 1 and 2% of the weight of N applied, was applied to sandy- and clay-loam soils. Ammonium sulphate was given in November 1962, and soil samples, taken in spring 1963, down to 36 in. on the sandy-loam and 18 in. on the clay-loam, were analysed. When the ammonium sulphate was broadcast on the soil surface, 2% of the inhibitor, inhibited nitrification of the ammonium-N more than 1%, but when it was placed in the soil both rates were equally effective.

Soil samples from sites of field experiments testing S-fertilisers for karley, potatoes, kale and grass were used for pot experiments with ryegrass and also t o measure ' available '-N in the laboratory.Mineral-K in the fresh soil (Mineral-Nf,,h) and the increases when fresh or re-wetted airdry soils were incubated (A Mineral-Nf,,,i, or 4 Mineral-Nnir-dry) were determined. These were more highly correlated with organic C than with total N content of the soils. Among ' available '-N measurements, Mineral-xfresh and 4 h ' l i n e r a l -S a~~d r y were most correlated with one another.The yields of dry matter and N uptake by ryegrass grown in unfertilised soil, or with two rates of applied N, were significantly correlated with Mineral-Nfresll or A hlineral-Nair.dry a t each level of nitrogen, but not with 4 Minera1-NfreSh.The unfertilised yields of barley in field experiments were significantly correlated with Mineral-Nfresh, A Mineral-Xab-,jry, and total N ; responses b y barley t o 0.5 cwt. of N/acre were significantly negatively correlated with all three ' available '-N measurements and with total X. Unfertilised yields of potatoes were significantly correlated with the three ' available '-N measurements and responses t o 1.0 cwt. of N/acre were significantly negatively correlated with A Wineral-Nai,.dry and total N. Mineral-"f,.,,h and A &1incral-Nair.dry are suggested as useful measurements for advisory purposes.

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