INSTRUMENTOS DE PROCESAMIENTO DE LA INFORMACIÓN: Cuadros: Estos nos sirvieron para ordenar nuestros datos
O: representa lo que observamos
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None of the eight trial sites showed any significant difference in biennial legume yields when S was applied alone (+S) (Table 3 .4). In contrast, application of S together with P (+SP), resulted in significant increases in accumulated legume yields at nearly all the sites, except LFLFL and HFNFL. Mackay et al. ( 1 988) reported similar results at Ballantrae, and suggested that legume growth was limited primarily by P. These researchers concluded that, S became a factor limiting further increases in legume growth only after P deficiency was alleviated.
When the amounts of S and P accumulated in the topsoil (0-75mm) of the HF farmlets were compared with the amounts accumulated in the LF farmlets (averaging all slope units), Lambert et al. ( 1 988) found that only 14% of the extra S and 47% of the extra P applied to the HF farmlets, above the amounts applied to the LF farmlets, could be accounted for. The results led these researchers to suggest that, " .. . .in a circumstance of "submaintenance" superphosphate application (eg. the LFNF and HFNF farmlets), or where an alternative phosphatic fertiliser with lower S content is used, S deficiency may be a major limiting factor in terms of legume growth, and utilisation of soil and fertiliser P" (Lambert et ai. , 1 988, pp 1 00). The lack of response to the +S treatment at the LFNF and HFNF sites in this study however, does not support this view. Rather, in terms of S required to utilise P, it is believed that the HF farmlets with higher soil P levels are more likely to be S deficient because more S would be required to utilise the higher levels of P. This is supported by the results showing that on the two sites where P alone (+P) was applied, there was a significant difference in the legume yields over the control at the LFLFM site, while the HFHFM site showed no significant difference.
Site Cont +S +N +P +SP +SN +PN +SPN LSDa - o.1 LFNFL 1 2 3 5c 1 396c - - 4439. - - 3 105b 897 LFNFM 1 544b 1 72 1 b - - 3 973. - - 3 3 3 7. 1 243 LFLFL 2 1 59b 2736ba - - 3720ba - - 4803. 2 1 99 LFLFM 2 1 00de 1 606� 1 83 1 d. 4 1 6 1 . 4467. 1 6 1 3. 3 305bc 277 1 dc 1080 HFNFL 3069, 2 3 1 0. - - 2828. - - 2287. n.s. HFNFM 1 872b 2 1 65b - - 3833. - - 2877ba 1 468
HFHFL 3205ba 3 1 47ba - - 4928a - - 1 842b 2308
HFHFM 23 1 9b 2 1 1 9b 1 464b 2 1 Mb 4026. 1 3 53b 1 803b 1 768b 1 073
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The efficiency of the +SP treatment at promoting legume growth was less affected by the application of N on top of S and P (+SPN) at the LF farmlets (Table 3 .4). In contrast, at the HF sites, application of N fertiliser dramatically reduced the effects of the applied S and P on legume growth to levels that were not significantly different from the controls. This indicates that, N2 fixation is still the dominant source of N in the LF sites, while at the HF sites, there is adequate soil inorganic N that legumes can readily substitute for fixed N (AlIos and Bartholomew, 1 959). In tandem with the higher levels of available N inhibiting the need for the sward to fix N at the HF farmlets, the grass associates would have also suppressed legume growth by shading.
3.3.3. 1 Amounts of N Fixed by Legumes
The ability of legumes to fix atmospheric dinitrogen (N2) and the eventual transfer of fixed N to promote grass growth, makes applications of S and P fertilisers to stimulate legume growth the most significant aspect of fertiliser management in grazed pastures. Indications are that some 300 to 600kg fertiliser NlhaJyr would be needed on pure grass swards to sustain the level of forage production currently attainable from well managed grass-clover associations, sustained by an annual input of 1 50 to 300kg fixed N/ha (Hoglund and Brock. , 1 979; Field and Ball, 1 982).
To investigate the amounts of N fixed by legumes, acetylene reduction measurements
were conducted at the LFLFM and the HFHFM sites, where the four extra treatments were applied to include all possible combinations of S, P, and N.
The results show the amounts of N fixed were generally higher at the LFLFM site (0.3 - 1 .0kg NlhaJday) compared to the HFHFM site (0. 1 - 0.6kg NlhaJday) (Table 3 . 5). One reason for the observed results is that at the HF farmlets, the dominant grass
associates suppress legume growth by shading, thus reducing the amounts of photosynthate available for rhizobia.
Table 3.5
Site
LFLFM HFHFM
Nitrogen fixed (kg N/haJday) using acetylene reduction assay as affected by fertiliser treatments.
CONT +S +P +N +SP +SN +PN +SPN
0.48 0.28 0.59 0.25 1 . 00 0.34 0.39 0.42
0.43 0.58 0.20 0.07 0.48 0. 1 0 0. 1 2 0. 1 6
The patterns of N fixed as affected by fertiliser treatments were similar to the patterns of legume yields, as discussed in the previous section. Like the patterns of legume yields, the +SP treatment was the most effective at fixing N2, while N fertiliser applications had detrimental effects. The effects of fertiliser applications on the amount of N fixed were similar to other studies which have already shown that N fertilisation
reduces N fixation (eg., Crush et af. 1 982; Ledgard and Saunders, 1 982; Mundy,
1 987).
An interesting feature of these results is that the positive effects of the +SP treatment were a lot more pronounced at the LFLFM site, compared to the HFHFM site. At the LFLFM site, the +SP treatment more than doubled the amount of N fixed on the unfertilised control plot. As with legume growth patterns, the negative effect of N fertilisation appears to be more pronounced at the HF site than the LF site.
When the amounts of N fixed over spring 1 990 were divided by the accumulated legume yields over the same season, the legumes on the HFHFM site appear to be more efficient at fixing N, compared to the legumes on the LFLFM site. Figure 3 . 7 shows
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that legumes on the HFHFM site fixed 0. 1 9kg N/kg DM, while the legumes on the LFLFM site fixed 0. 06kg N/kg DM. These results suggest that different species of legumes have different efficiencies for fixing N per unit mass. In a more detailed study of the botanical composition of the swards on similar sites at Ballantrae by del Pino Machado ( 1 994), the swards on a site located on the LFLF farmlet showed a total
herbage consisting of 4.2% white clover and 5 . 6% of other legume species. On a site
located on the HFHF farmlet, 8.5% of the total herbage was white clover, and no other
species of legume was found. While the legume species at the HFHFM sites were more
efficient at fixing N per unit mass, the higher legume composition at the LFLFM meant that the total amount of N fixed was higher.
E
� D) e.i
u::: Z '5 Zl c ::J!
Figure 3.7 120 100 80 60 40 20 0 0 Y = O.19 X + 15.42 .. (R-sq = 700/0) .... x x Y = 0.06 X - 9.46 (R-sq = 800/0) \1 - - - HFHFM site X -- LFLFM site 400 800 1200 1600 2000Accumulated Legume Yields (kg DMlha)
Relationships between the amounts ofN fixed during spring 1 990 and accumulated legume yields during the same season.