The effects of magnesium fertiliser and grass on the nutrition and growth of P. radiata planted on pumice soils in the central North Island of New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis addressed the problem of the widespread occurrence of magnesium deficiency in Pinus. radiata planted on pumice soils in the central North Island of New Zealand, the increased severity of the deficiency where trees were planted on grassed sites, and the slow increase in foliar Mg concentrations and tree growth following fertilisation of deficient trees with various sources and rates of Mg fertiliser. Plant available Mg concentrations were found to be very low in pumice soils, with soil solution concentrations often below 1 mgL-¹, and exchangeable concentrations in the upper 20 cm of the soil normally well below 1 meq 100g-¹. Published soil critical levels for a number of crops fall into the range of 0.2 to 1.0 meq100g-¹. Addition of between 100 and 400 kgha-¹ Mg (as Epsom salts, dolomite, or calcined magnesite) in a laboratory study raised soil solution concentrations by between 8 and 37 mgL-¹ and exchangeable concentrations by between 1.5 and 3.4 meq100g-¹. These levels should be adequate for tree growth. Epsom salts caused a much greater increase in solution Mg concentration than either dolomite or calcined magnesite. Changes in soil solution and exchangeable concentrations were monitored in a field experiment. Epsom salts and calcined magnesite were added at 400 kgha-¹ Mg. The Epsom salts caused a large and rapid increase in solution Mg concentrations to a soil depth of at least 45 cm. Calcined magnesite had no significant effect on solution concentrations. Both treatments increased exchangeable Mg concentrations well above 1 meq100g-¹, although the less soluble calcined magnesite was slower acting. Improvements in exchangeable Mg were still apparent after 18 months with concentrations of the order of 1.5 meq100g-¹ in the upper 20 cm, although soil solution concentrations had returned to pre- fertilisation levels. The effects of Mg deficiency on dry matter allocation patterns were investigated in a glasshouse and a field experiment. Mg deficiency caused a 25% decrease in root: shoot ratio in seedlings. In an 8 year old stand of trees, where above ground dimensions were the same, but foliar concentrations were above or below the critical level of 0.07% Mg, there was 50% less fine root length in the deficient trees when compared to the healthy. This root decline could explain the slow response of fertilised trees, either due to a smaller exploitable soil volume, or due to a need to rebuild the root system before a response is detectable above ground. Two field experiments using 400 kgha-¹ Mg as Epsom salts were established, one in a newly planted area (Halls), and the other in an 8 year old Mg deficient P. radiata stand (Kiorenui). At this high rate foliar Mg concentrations were improved to well above 0.1% within six months of Mg application. This indicated that previous slow responses were likely to have been due to use of low rates or slowly soluble sources of Mg. No growth responses were recorded in either trial, the juvenile trees were only marginally deficient by age 3, and the older trees were measured only six months after fertiliser application. Foliage analysis and tests with a total spectrum fertiliser indicated no other elements were deficient in either of the field experiments. The effect of grass on tree growth and Mg uptake was tested at Halls in the juvenile trees. Removal of grass competition caused a 30% improvement in tree biomass at age 3, 18 months after treatments were applied. Grass competition intensified the Mg deficiency in the trees and if trees were fertilised without grass control, tree growth was slightly suppressed due to stimulation of the grass. It was suggested that this was an effect on the soil moisture conditions and hence nutrient uptake rather than a direct effect of the grass in competition for Mg. The Barber-Cushman nutrient uptake model was used for a sensitivity analysis of parameters affecting Mg uptake on the newly planted site. This showed that root growth and Mg influx parameters were most important on this site, suggesting that Mg supply was not limiting growth, a conclusion supported by the fertiliser experiment results. Calculation of Mass Flow Coefficients indicated that far more Mg would be supplied to the root than would be assimilated by the tree. Calculation of Mass Flow Coefficients from published data for an age range of P. radiata up to age 12 showed that on a site with low soil solution Mg concentration (0.3-0.5 mgL-¹), once the trees were older than 3 years the proportion of Mg supplied to the root by diffusion increased until by age 5 approximately 75% would be supplied by that means. The importance of soil diffusion rates and soil moisture conditions will therefore be greater for older trees. A soil solution concentration of 1-2 mgL-¹ was calculated to be the level at which 100% of Mg would be supplied by mass flow, and this could be used for identifying potentially deficient sites, Conclusions from this study, plus other published information on the topic were synthesised into a set of rules. These will be the basis of an expert system for managing the Mg nutrition of P. radiata.