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EFFECT OF FIRST ROTATION PHOSPHORUS APPLICATIONS ON FERTILISER REQUIREMENTS OF SECOND ROTATION RADIATA

Residual benefits of P fertiliser applied during the previous rotation were evaluated on a severely P-deficient soil in Riverhead Forest. Plots treated with 225 kg/ha P 19 years previously and their controls were subdivided at establishment of the second Pinus radiata crop. Half of each main plot received a standard establishment application of P fertiliser (17 g/seedling P) and the other half was left untreated. Five years after establishment trees in all sub-plots which had received any fertiliser had gained at least a year's growth advantage over those which had received no fertiliser. Growth of trees in sub-plots which received a single P application in either the first or second rotation was similar and slightly greater than that of trees in sub-plots which received both applications. This slight suppression was attributed to the second P application accentuating a N deficiency induced in the old + P plots by the growth response of the first rotation crop. Analysis of both foliage samples collected from old crop trees prior to felling and soil samples collected prior to re-establishment provided a good index of the residual effectiveness of the previous fertiliser application. INTRODUCTION Fertiliser trials established during the 1950s in unthrifty first rotation stands of radiata pine (Pinus radiata'D.'Don) growing on strongly weathered clay soils at Riverhead Forest, showed that the poor growth was caused by a phosphorus (P) deficiency (Weston, 1956; Conway, 1962). During the period 1958 to 1965 most of the mature first rotation stands in the North Island showing symptoms of P deficiency were aerially topdressed with superphosphate (8-10% P) at 628 kg/ha. Later trials, put in at establishment of the first of the second rotation crop on these clay soils which had received no previous fertiliser, showed that P fertiliser was required for successful re-establishment of radiata pine (Ballard, 1969). Because of the known effectiveness of large single applications of P fertilisers in supplying the P requirements of radiata pine over periods of up to 20 years during a rotation (Mead and Weston, 1968; Gentle, Humphreys and Lambert, 1965), the question arose as to what extent first rotation applications of P fertiliser would alleviate the need for fertiliser at establishment of the second crop. An opportunity to study this question came up when two pairs (control and P fertilised) of the original 1952 Riverhead trials (Weston, 1956) were felled in 1970. N.Z. J . For . Sci. 8(1): 135-45 (1978). 136 New Zealand Journal of Forestry Science Vol. 8 MATERIALS A N D METHODS First Rotation Plots In 1952 two pairs of 20 X 20 m plots were established in 25-year-old radiata pine in Compartment 3, Riverhead Forest (A99/A100 and A101/A102, hereafter referred to as Blocks 1 and 2 respectively). The soil type of the area is Waikare clay loam, a strongly leached and weathered northern yellow-brown earth formed from sandstone. One plot in each block was treated with a superphosphate compound (75% superphosphate, 15% ground Nauru phosphate and 10% ground serpentine rock) hand broadcast at a rate equivalent to 225 kg/ha P, while the other was sprayed with Zn (600 ml of 2\% ZnS0 4 solution per tree). No response was recorded to the Zn treatment and thus these plots have been considered as controls. The plots within each block were separated by a 20 m untreated buffer strip and the blocks were located some 50 m apart. These early plots were established as observational trials only and no mensurational data was collected from them until 1966. The extent of the response up to 1966 is shown by the data in Table 1 which also shows that the P application was still maintaining reasonable foliar P levels at this date. TABLE 1—First rotation plot information 14 years after the application of 225 kg/ha P Basal Foliar analysis Treatment Block Stocking area Volume N P K Ca Mg stems/ha m/ha m/ha % Control 1 890 17.8 41 0.83 0.073 0.55 0.12 0.10 2 766 9.1 15 1.42 0.068 0.41 0.22 0.14 Mean