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SYNOPSIS
In order to ensure the adequate survival of Eucalyptus grandis clonal hybrids on the sandy soils in the Zululand region of South Africa, seedlings are planted with large volumes of water (5 litres per planting pit). This results in a limited planting season, together with high establishment costs. A field trial was initiated with the primary objectives being to test whether tree growth and survival was affected by the volume of water added at planting, and to determine how this would be affected by the addition of a soil amended hydrogel (Stockosorb 400 K). Five levels of water (0 ml, 250 ml, 500 ml, 1000 ml and 5000 ml) were combined with five levels of hydrogel (0 g, 3 g, 6 g, 9 g and 12 g) and applied to the pit at planting in a 5 x 5 factorial treatment design. The trial was planted in summer (23 November 1998) with the tree variates of mortality, height, groundline diameter, corrected leaf surface index (LSIc ) and corrected biomass index (BIc ) being assessed at regular intervals. There was a highly significant (P>0,01) response of tree survival to the application of water, but not to the application of the hydrogel, this despite a significant rainfall event (146 mm) that occurred two days after planting. This response was quadratic (p > 0,001) with no further significant increase in survival above 500 ml of water. Both the variates LSIc and BIc indicated that growth was significantly enhanced with increasing levels of water or hydrogel. Optimum tree performance was found to occur at 1000 ml of water and 9 or 12 g of hydrogel. There was a 39,8% and 23,7% increase in the LSIc for water and hydrogel when comparing the best level against the worst, and a 24,8% and a 15,6% increase in BIc for water and hydrogel. Results from this trial indicate that increasing levels of water applied at planting had a positive impact on seedling survival. The lack of the expected response in terms of seedling survival with increasing levels of hydrogel could possibly be attributed to the significant rainfall event that fell two days after trial initiation. Increasing levels of both water and hydrogel resulted in enhanced growth. Further field trials have already been implemented under less favourable climatic and site conditions so that the full impact of the hydrogel on seasonal and un-seasonal planting can be more clearly understood.
