The organic phosphorus contents of a range of soils were determined by the extraction method of Mehta et al. (1954) and by the ignition method of Legg and Black (1955) and the results compared with those obtained by an extraction method using Na, EDTA and caustic soda. The two extraction methods compared well, especially on soils over pH 5. Lower values were obtained by the ignition method on the more acid soils. The Na, EDTA method caused less hydrolysis of added glucose-I-phosphate than did either of the other methods. In combination with caustic soda Na, EDTA was less effective than Na, EDTA.
1. Between 1955 and 1961 nineteen experiments were carried out on Majestic potatoes in various parts of England comparing muriate and sulphate of potash at three rates of application, in the range 75–336 lb./acre of K2O. Most of the experiments were on light soils.2. Total yield of tubers was on average the same from both sources at all rates of application, and this similarity was not affected by environmental conditions.3. Muriate of potash consistently gave a higher yield of large tubers than sulphate, with an average difference of 0·6 ton/acre. Conversely, muriate gave a lower yield of medium tubers than sulphate, again with a mean difference of 0·6 ton/acre.4. Muriate of potash significantly depressed tuber dry-matter content in eleven of the experiments, particularly at the higher rates, while sulphate had this effect in three experiments. Muriate gave a small increment in two experiments, at low rates only, while sulphate had this effect in five, most markedly at low rates. Sulphate gave a higher dry-matter content than muriate in most of the experiments. The mean difference was 1·0% at 300 and 336 lb./acre of K2O and 0·4 to 0·5% at lower rates.5. Sulphate of potash gave a more floury texture to the boiled tubers than muriate in three experiments, but otherwise no difference in texture was detected between the sources.6. The degree of off-flavour was not appreciably affected by rate or source of applied potash.7. After-cooking blackening was more marked at low than at high rates of applied potash in seven of the nineteen experiments. Muriate and sulphate of potash had similar effects.8. Farmyard manure ploughed down in winter was tested in three experiments and did not influence to any important extent the relative effects of muriate and sulphate of potash on tuber yield and quality.9. Application of the two sources on the flat before ridging and in the open drills was compared in the same three experiments, but did not alter the relative effects of the sources on yield or dry-matter content.
The potassium requirements of potatoes are described, followed by a consideration of the relationship between potassium rate and source and the carbohydrate metabolism of the potato.Potassium nutrition and potato quality (texture and after-cooking colour) are discussed, and the subject of potato quality is summarised with reference t o the tests carried out a t Levington.
SPRAY residues of the fungicide captan, N-trichloromethylthio-4-cyclohexene-1 : 2-dicarboxyimide, are generally determined by methods based on the formation of a colour with resorcinol.192 Burchfield and SchechtmanS have described a simpler method, based on the reaction of captan with pyridine and tetraethylammonium hydroxide to form a coloured complex, and have applied this technique to the determination of captan in soil. We have found their method to be suitable for determining captan in spray residues, as it is not susceptible to interference from plant extracts or other spray chemicals likely to be present on the surface of the plant. Certain refinements in technique are also suggested. METHOD REAGENTS-Benzene-Fractionate before use. Pyridine-Fractionate in presence of potassium hydroxide before use. Tetraethylammonium hydroxide solution-Dilute the commercially available 25 per cent. aqueous solution to 0.073M (check by titration against standard acid), add charcoal, set aside for 15 minutes, and filter through a Whatman No. 50 filter-paper. Repeat this procedure until the solution has a negligible optical-density blank value at 430 mp.Pyridznetetraethylummonium hydroxide solution-Add 3 volumes of the 0.073 M tetraethylammonium hydroxide to 7 volumes of the pyridine.Cuptan-Recrystallise from benzene until the product has a constant melting-point (173' t o 174' C).Prepare this solution freshly before use. PROCEDURE-Transfer a suitable aliquot of a solution of captan in benzene to a round-bottomed flask. Place the flask on a water bath, and evaporate the benzene in a stream of clean dry air. Just before evaporation is complete, remove the flask from the water bath, and allow the final droplet 20 ml of redistilled analytical-reagent grade acetone to between 2 and 3 ml of the concentrated benzene extract and then setting aside at 5" C overnight. The wax was separated by filtration, and the solution was evaporated as before. However, the results in Table I show that slight losses occurred when this procedure was used. TABLE I RECOVERY OF CAPTAN FROM PLANT EXTRACTS Amount of Amount of CLg rg Apple fruits . . .. 25 25, 24.5 50 49, 51 Apple leaves . . .. 25 25, 24 50 48.5, 49 Strawberry fruits .. 50 47, 46 Broad-bean leaves . . 25 25, 25.5 50 48, 49 Potato leaves . . .. 50 50,48 Sample captan added, captan found, Recovery,
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