Phosphorus deficiency

The soil of the Lees Valley was also phosphorus deficient (Tables 4.1, and 4.5). The root growth parameters of sown legumes were more responsive to applied P, than the shoot DM. The phosphorus effect on lucerne shoot DM was not significant in Pot Experiments 2 or 3. However, the root volume and root surface area of lucerne plants was increased significantly by adding 250 mg P/kg soil (Table 4.6 and Figure 4.27). In Pot Experiment 3, the root DM of lucerne was affected by the three way interaction effect of lime, phosphorus and nitrogen treatments (Figure 4.25). The highest root DM was achieved by adding 2 t lime/ha + 250 mg P/kg soil + rhizobia inoculant. This was due to decreased levels of Al and the nitrogen provided by rhizobia, and also the beneficial effect of P on nitrogen fixation (Israel, 1987) by stimulating host plant growth (Robson et al., 1981). In contrast, the response to P rates by lucerne and Caucasian clover showed significant differences in terms of total shoot yield, root dry matter (Figure 4.12), and the number of nodules (Figure 4.13) in Pot Experiment 1. Lucerne DM was also higher than Caucasian clover with phosphorus rates over 60 mg P/kg soil (Figure 4.11). The interaction effect of species × P rates on total shoot yield indicated (Figure 4.11) higher production for lucerne than Caucasian clover in response to P rates. However, the yield response to lime was greater than phosphorus in the aluminium-saturated soil of the Lees Valley. This result agrees with Mugwira and Haque (1993) who showed that in acid soils with toxic aluminium levels, yield response to lime was greater than phosphorus and vice versa. There was no significant difference in total shoot yield of Caucasian clover in response to added P rates over 100 mg/kg soil (Figure 4.11). ). This could be due to a lower phosphorus requirements for Caucasian clover than lucerne.

The availability of added phosphorus was directly affected by soil pH. At a soil pH of 5.4, Olsen P was 36 mg/l and increased to 51 mg/l at pH of 5.5. This suggests that the added P was partially fixed by Al when the pH was 5.4. This result agrees with previous reports indicating most of the phosphate fixed was bound to Al in low pH soils (Haynes, 1982; Hsu, 1964; Saunders, 1965; Wells and Saunders, 1960; Williams, 1981). Adding P to the Lees Valley soil in PVC tubes (Figure 4.22) decreased 1.6 mg Al/kg soil (pH 5.4). When the soil pH was elevated to 5.5 the added 250 mg P/Kg soil decreased 1 mg of available

Al per kg soil compared with the no-P treatment. The application of half a tonne of lime with P, reduced Al to 1.8 mg/l (Figure 4.22), elevated the soil pH, and more P became available (51 mg/kg). This available P then started to be consumed by the increasing number of lucerne plants in P.V.C tubes as the soil pH elevated by lime rates. Therefore, the reduction of available phosphorus above soil pH of 6.0 (Figure 4.21), could be due to a combination of higher soil pH effect that fixed P again (by forming calcium phosphate), and P uptake by plants. There was no effect of lime rates on P availability in the original soil where no P fertiliser was applied (Figure 4.21).

4.4.4

Conclusion

The combined results from these pot experiments suggest the main limiting factor for lucerne production and persistence in the Lees Valley was nitrogen deficiency. This constraint resulted from the inefficient nitrogen fixation of lucerne plants due to Al toxicity. The toxic levels of aluminium suppressed the root growth, and caused nodulation failure. The suppressed fine root growth of the sensitive lucerne limited nodule formation and nutrient uptake. In contrast, the nature of root architecture of Caucasian clover plants allowed them to expand their rhizomes in shallower depth (0- 100 mm) where the soil pH was elevated to 6.0 by lime application. The shallow and variable top-soil depth of the Lees Valley soil profile were also considered in Chapter 3. In contrast with the previous report (Moir and Moot, 2010), the shallow top soil at the Lees Valley site is not considered a critical issue in the rooting of lucerne plants. However, if this variable top-soil depth in the alluvial fan of the Lees Valley is correlated to variable soil pH and exchangeable Al, then this would affect nutrient availability and plant growth. From a practical perspective, if the soil can be amended by the addition of 2 t lime/ha, incorporated down to at least 30 cm, lucerne may be a viable option. However, if surface applied lime is the only option, Caucasian clover would be recommended ahead of lucerne.

Results from these pot experiments need to be tested in the field. However, it was not possible to go back to the Lees Valley for another field experiment because the farm changed ownership. Therefore, two other experimental sites were selected to continue the research. The next two chapters will describe those field experiments.

Results in pot experiment 3, showed both lucerne and Caucasian clover were negatively affected by Al levels higher than 3 mg/kg soil. To confirm these, a third species for high country regions was added to this project. Russell lupin has been shown to be suitable feed for sheep in high country of New Zealand (Section 2.3.3), and to be persistent in high Al low pH soils.