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Title: A study of the 'availability' to plants of different forms of phosphate
Author: El-Reweiny, F. M.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1966
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Three pot experiments were carried out during the seasons 1963-64 to investigate the "availability" to plants of some Na~ condensed phosphates in comparison with Na-orthophosphate. Two of the experiments were carried out in a wire cage open to atmospheric conditions, and the third was conducted in a greenhouse. Ryegrass was the test plant in all the experiments. Different soils, including a calcareous soil, were used. The effect of N on both soil- and fertilizer-P uptake was studied in the greenhouse experiment using ³²P-labelled fertilizers. Hydrolysis of condensed-phosphates in the soil was also studied in soil-sampling pots with no grass on them. Different rates of P (0, 60, 120 and 180 lb. P₂O₅/acre) were used in all experiments. P-fertilizers were thoroughly mixed with the soil-sand mixture in two experiments, and banded 2.5 in. below the seed-level in the third experiment. All P-treatments were done just before seeding. Mg was applied as a basal dressing during the early stages of growth in all experiments. K was applied as a basal dressing in the two cage experiments, while in the greenhouse experiment, it was applied at similar rate to N, at the beginning of the experiment and after each cut of grass. Nitrogen was applied as NH₄NO₃ at rates of 40 and 80 lb. N/acre per cut in the two cage experiments, and 40, 80, 120, 160 and 200 lb. N per acre per cut, in the greenhouse experiment. The results obtained and conclusions reached may be summarised as follows:- 1. In the first cage experiment (1963) condensed-phosphates were as effective as the orthophosphate in terms of yield production, percentage P in the dry matter, and P-uptake. This was not affected "by the rate of N or by the soil. 2. The residual effect of all forms of P was very similar in the second season (1964) of the first cage-experiment. 3. In the second cage experiment, where P-fertilizers were handed, the metaphosphates were superior to the orthophosphate only in terms of percentage P of the dry matter and P-uptake, but not in terms of dry matter-production. 4. In the greenhouse experiment, the two metaphosphates (trimetaphosphate and metaphosphate glass) were slightly, hut significantly, inferior to the orthophosphate only in the first cut of grass. This was found in terms of yield-production or of P-uptake, hut not in terms of percentage P of the dry matter. 5. The metaphosphate glass tended to he slightly more effective than the ring-structured trimetaphosphate. The former prohahly hydrolyses more rapidly in the soil. 6. The similar effectiveness of both polyphosphates and orthophosphates, in increasing yields or P-uptakes by grass, is thought to he a result of fast hydrolysis of the polyphosphates. The rate of hydrolysis seems to he sufficient to meet plant-requirements of PO₄ under normal soil-conditions, in the cage experiments, where the rate of growth is relatively slow. In the greenhouse, where the rate of growth is very rapid, the rate of hydrolysis of metaphosphates may not "be sufficiently fast to cope with plant-requirements of PO₄ particularly in the early stages of growth. 7. The slightly greater P-contents of dry matter produced "by the metaphosphates, compared with those produced "by the orthophosphate, in the second cage experiment, were explained "by gradual hydrolysis of the metaphosphates in the soil, since the three forms produced similar yield-increases. This "behaviour was similar on both acid and calcareous soils. 8. In the greenhouse experiment, the metaphosphates were equivalent to the orthophosphate, after the first cut, in terms of yield-production and total P-uptake, "but they produced slightly smaller uptakes of labelled-P than the orthophosphate. This was explained by a slower isotopic exchange, in the soil, between applied-metaphosphate and soil-P. It is also suggested that equilibrium between soil-P and applied ³²P-metaphosphates may not be attained before their hydrolysis in the soil. 9. The possibility of direct absorption of polyphosphates by plants was discussed, but needs more study. 10. The rate of hydrolysis of the metaphosphates in the soil is extremely rapid. It was also found that the hydrolysis was faster on a neutral soil than on an acid soil. This was probably due to greater microbiological activity, therefore greater enzymic hydrolysis, on the neutral- than on the acid soil. 11. The effects of applied-P, as well as increasing its rate of application, were very pronounced in increasing yields, P-uptakes, and P-contents of the grass. The responses to applied-P were increasing up to the highest rate, 180 lb. P₂O₅/acre, in most cases. The mean-apparent recoveries as per cent of applied-P were: 10.6 in 3 cuts of the cage experiment I (1963); 9.9 in 3 cuts as the residual effect in the second season of the cage experiment I; 35.4 in 8 outs of the greenhouse experiment II (1964); and 14.1 in 3 outs of the cage experiment III (1964). Factors that affected the per cent apparent recovery were; soil, rate of N, rate of P and rate of growth. 12. Where labelled-P was used, P-applications increased the uptake of soil-P. This effect was greatest early in the season and decreased with time. Possible explanations given were increased rooting system, slow isotopic exchange, and decreasing availability of applied-P with time. 13. Increasing rate of applied-N greatly increased yields as well as P-uptakes, particularly where P was applied. N considerably decreased percentage P of the dry matter which was probably a result of a dilution effect brought about by great yield-responses to N. In the greenhouse experiment, there was no considerable difference between yields produced by the highest two rates of N (160 and 200 lb, N/acre/cut). 14. N x P interaction was very pronounced in most cases. The increasing response to applied-P with increased rate of N was explained by effects of N in promoting growth, increasing the quantity of N-compounds in the plant, increasing the root-capacity for P-absorption, and speeding up P-metabolism. 15. 'A'-values, calculated from ³²-measurements, were not considerably affected "by rate of applied-N. Increasing rate of applied-P increased the 'A'-values in the 1st cut while decreasing them in the 4th cut of grass. For totals of 2, 3 or all 4 cuts of grass the 'A'-values were not affected by P-rate. The metaphosphates produced greater 'A'-values than did the orthophosphate. 16. In the greenhouse, sub-irrigation can be troublesome, particularly with high rates of fertilizer-application. Surface irrigation may be necessary to avoid salt-accumulation on the soil-surface. 17. If metaphosphates can be produced at^cost comparable to that of orthophosphates, they can well be a good substitute as a source of P for plants. Having the advantage, over the orthophosphates, of being of higher P-contents, they can also be more useful than orthophosphates in manufacturing concentrated compound-fertilizers.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available