PHOSPHATE TRIALS AT INVERMAY

New Zealand Journal of Agriculture, Volume 95, Issue 3, 16 September 1957, Page 263

Recent Research Work

PHOSPHATE TRIALS AT INVERMAY

THESE notes summarise the results of 5 years' investigation on the I second-class, molybdenum-deficient hill country at the Department of Agriculture's Invermay Research Station, Otago. ONE trial was designed to measure pastures responses to different forms of phosphatic fertilisers and the other to determine the optimum rate and times at which phosphates can be applied and continue, to give responses. The results obtained show that superphosphate is at least as good a phosphatic fertiliser as any of the other forms compared in these trials. Since in all trials pastures have responded . extremely well to phosphate, the soil at Invermay has proved to be ideally suited for the study of . different forms and rates'of. phosphate.

FORMS OF PHOSPHATE

Superphosphate and North African and finely ground Nauru rock phosphates were compared when applied at 3cwt. per acre. “Thermophos” (a calcium-magnesium fusion phos-

phate) and serpentine superphosphate were applied at a heavier rate so that the P 905 (phosphate) content applied was equivalent to that contained in 3cwt. of superphosphate. These different phosphates were compared both in the presence and absence of lime. . Responses: In the absence of lime (pH 5.6) all except Nauru rock phosphate gave similar yields. When lime was applied superphosphate, serpentine superphosphate, and “Thermophos” yields were increased, whereas the North . African and Nauru rock phosphate yields were depressed. In the presence of molybdenum 1 ton of lime per acre per year failed to increase responses to superphosphate, serpentine superphosphate, and “Thermophos” beyond those obtained with annual dressings of lOcwt. of lime per acre. It is interesting to note that the annual dressings of 1 ton of lime per acre and lOcwt. per acre only maintained the soil reactions established at. the outset of the. trial—pH 6.9 and 6.5 respectively. In practice pH 6.0 to 6.2 should be satisfactory provided molybdenum is used. This level could be maintained by much lower rates of liming. When the results of this trial are interpreted it should be borne in mind that the soil at Invermay has not shown any clear-cut sulphur responses. Thus on sulphur-deficient soils the results would not necessarily hold true unless some source of sulphur other than that present in superphosphate was applied. No further annual applications of phosphates to this trial are being made; the residual effects of these forms of phosphates are how being studied.

RATES OF PHOSPHATE

A trial has compared different rates of phosphate when applied in March and September and twice yearly. The forms of phosphate chosen were superphosphate, lime-reverted

superphosphate, and North African , rock phosphate. Time of application: The time of application proved to be important only during the first 2 years. It is . understandable that no great differences would occur later, when the soil phosphate reached a higher level. Superphosphate and North African rock phosphate gave their highest yields after twice-yearly application, and lime-reverted superphosphate gave its highest yields after a March application. Subsequently the yields were not affected, but the time of response depended on the time of application, for example, autumn application ' gave highest spring yields. Rate of application: In the first year 6cwt. per acre dressings of the phosphates gave appreciably higher production than 3cwt. per acre and in subsequent years slightly higher production. In other words a total of 24cwt. of phosphate per acre applied in 5 years yielded significantly more than a total of 12cwt. per acre. The residual effects of these different rates of phosphate applications are now being investigated.

CONCLUSIONS

The conclusions to be drawn from these investigations on the second-class hill country at Invermay are:—

1. On. this particular soil (not sulphur deficient) at a pH of 5.6 all forms of phosphate except Nauru rock phosphate (which was inferior) gave similar yields. Where lime was applied superphosphate, serpentine superphosphate, and “Thermophos” yields were increased, whereas the North African and Nauru rock phosphate yields were depressed. . 2. In the presence of adequate molybdenum annual lime dressings of lOcwt. per acre were as good as 1 ton per acre. Other work suggests that even lighter annual dressings would be adequate. 3. Differences in production arising from different times of 'topdressing were not important except in the first 2 years The time the response was obtained depended on the season of topdressing; for example, autumn application for early spring yields and September application for summer and autumn' yields. 4. Annual topdressings of up to 6cwt. of superphosphate gave pasture responses. This evidence suggests that on this soil type much heavier phosphate dressings than are normal could be applied.

—R. S. SCOTT