No abstract

The weekly nitrogen (N), phosphorus (P), and potassium (K) release from 17 polymer-coated controlled-release fertilizer (CRF) formulations of Nutricote, Apex Gold, Osmocote, and a 9-month Macrocote were measured at 30.6±0.8°C and 40.0±1.5°C. Five grams of each CRF were placed at a depth of 50 mm in 280x50 mm acid washed then rinsed silica sand columns which were leached with deionized water three times each week until nutrient recovery ceased. The volume of leachate was recorded each week and subsampled for ammonium-N, nitrate-N, phosphate-P, and K analyses. Each CRF treatment was replicated three times at each temperature. Nutrient release profiles were determined. Longevities, measured as weeks to 90% nutrient recovery, were considerably shorter than the nominated release periods for all formulations. Within each CRF product group, the longevity of 9 and 12 month formulations were similar with Apex Gold 12-14 month high nitrate having the longest (38 weeks for N at 30°C) and Osmocote 8-9 month the 959 960 HUETT AND GOGEL shortest (23 weeks for N at 30°C). There were consistent trends in the nutrient release periods across all CRFs with P>K>N and with differences of around 10% in duration between nutrients. The P:N release ratio exceeded 0.10 for most CRFs during the early release period indicating an adequate P supply for most plant species. The mean reduction in longevity for Nutricote, Apex Gold, and Osmocote formulations for an increase in incubation temperature from 30°C to 40°C was 19-21 % for N, 13-14% for P, and 14-15% for K. All CRFs released nutrients unevenly with the highest rate occurring during the early part of the release period. This pattern was accentuated at 40°C and by the shorter term release formulations. The nutrient release rates of all CRFs declined steadily after their maxima.

No abstract

Nutrient losses from controlled-release fertilisers (CRFs) and an organic-based fertiliser derived from dehydrated poultry manure applied at planting were investigated under a range of irrigation conditions. The CRFs were Osmocote NPK (3–4 month) (Osm), Nutricote NPK (90-day) (Nut), and Nut+40-day, and the organic based fertiliser was Dynamic Lifter (DL). They were applied pre-planting at a standard rate equivalent to 800 g N/m3 and at double this rate to pots containing sand, composted pinebark, and hardwood sawdust medium that had received nutrient amendment during formulation. A pot containing medium without a plant was included to estimate the contribution of the medium to nutrient leaching from fertiliser treatments. In all experiments, leachate was collected weekly, the volume was recorded, and nutrient concentrations were determined. The largest losses of N, K, Ca, and Mg occurred in the first week, and of P in the second week, after potting up for both fertilised and control pots. Over a 10-week period, with the exception of the Nut+40-day treatment, P and K leaching from equivalent rates of DL exceeded (P < 0·05) Osm = Nut. The percentages of fertiliser leached (after adjusting fertiliser treatment for control) were Osm 20–38% N, 2–8% P, 12–42% K; Nut 10–43% N, 12–18% P, 22–45% K; and DL 1–14% N, 4–15% P, 78–91% K. In a laboratory experiment where each fertiliser was incubated at 35C with moist potting medium and volatilised ammonia was trapped in dilute acid, a further 33% was recovered from DL and <1% from the CRFs. The inclusion of the 40-day formulation to Nut increased (P < 0·05) the percentage of N, P, and K leached, to 52, 39, and 69%. In leachate, nitrate was the main form of N from CRFs and ammonium the main form from DL. The maximum nitrate-N concentrations (mg/L) at the standard fertiliser rate were Osm 55, Nut 56, and DL 46. These increased (P < 0·05) to 78 and 165 mg/L when the rate of Osm and Nut was doubled. A concentration of 279 mg/L was recorded with the Nut+40-day formulation. The maximum leachate ammonium-N concentrations (mg/L) at the standard fertiliser rate were Osm 64, Nut 51, and DL 125. Leachate was diluted 1 : 4 in nursery runoff water by irrigation runoff, and concentrations exceeded the 10 mg/L limit imposed by the Clean Waters Act of NSW. In a further 2 experiments, an increase in leachate volume increased (P < 0·05) nutrient leaching from pots fertilised with the 2 CRFs over 4 weeks. When the leaching fraction was increased from 0 to 56%, leaching of N, K, Ca, and Mg increased more than 4-fold. The low leaching fractions were associated with high nutrient concentrations, and at a 12% leaching fraction, maximum concentrations (mg/L) were 864 nitrate-N, 127 K, and 248 Ca. Nutrient runoff from nurseries can be reduced by adopting efficient irrigation design, by scheduling irrigation, and by minimising the use of soluble fertiliser sources.

The management of mature macadamia orchards has evolved largely through the need to control inter-row crowding to maintain machinery access. The current study was undertaken to identify physiological constraints to production and priorities for future research and development. The two components of the study were a preliminary field study to determine the impact of current pruning and hedging management strategies on canopy photosynthetic performance and a literature review to identify physiological issues affecting orchard productivity. The field photosynthesis study demonstrated that emerging flush leaves have a negative light saturation net assimilation rate (Amax) that increases to that of mature leaves (Amax 8–10 µmol CO2/m2.s) over 28 days. Leaf age has no effect on Amax of light-adapted leaves. Shade-adapted macadamia leaves cannot attain the photosynthetic capacity of light-adapted leaves. This means that a late hedging strategy to remove around 1 m of canopy from the side of trees to improve orchard access reduces the photosynthetic capacity of the orchard until sufficient flushing occurs to restore the canopy. The literature review focussed on light interception and distribution, photosynthesis, carbohydrate and nitrogen cycling, environmental response, flowering and fruit set, nut abscission, canopy management, and nutrition and disease control. Light interception modelling work, which has been widely adopted for deciduous temperate fruit crops, has immediate application to macadamia production and can explain many of the yield responses recorded by macadamias to canopy management. Macadamia yields appear to increase up to about 96% light interception. Variation in light distribution within canopies that affects the yield and quality of temperate crops is also a present problem with macadamias. It leads to uneven distribution of leaf and fruit throughout the canopy and to a heavily shaded void developing in the middle of trees. The relationship between irradiance, leaf photosynthetic capacity and longevity, flushing, flowering, and fruit set is poorly understood. No information is available on the training of young trees to improve light distribution and canopy photosynthetic efficiency. The current practice of light annual hedging of mature macadamias appears to restrict the production of fruiting wood. Earlier nut abscission can be achieved by the use of ethephon, which offers greater flexibility in the timing of hedging. Research work is required on the effects of timing, frequency and severity of hedging on fruiting wood production, flowering, and fruit set of macadamias. Information on the need to supplement mechanical hedging with manual pruning to improve light distribution throughout canopies is also required. The cyclical and highly variable nature of macadamia yields needs to be further analysed to determine whether a consistent pattern exists and whether environmental factors are an influence. A simple measure of the storage levels of the major assimilates, carbohydrates and nitrogen compounds, is unlikely to predict cyclical yield patterns because both are continually cycled within the tree and it appears that, from comprehensive studies on another evergreen fruit crop, avocado, the major supply of carbohydrate during the fruit-filling stage is from current photosynthesis. Several studies have failed to demonstrate irrigation responses by field-grown macadamias. The cyclical and variable yield of macadamias demonstrates that yield responses from field experiments cannot be expected in less than 5 years, even assuming adequate plot size, buffering, and replication. On the drier sites with light-textured soils in south-eastern Queensland, yield responses could be expected. Following the success of temperate fruit tree crop breeding programs and trends with other evergreen tree crops, macadamia breeding needs to focus on dwarfing clonal rootstocks that provide uniformity, and vigour control to improve cropping efficiency, to reduce canopy management costs and to minimise the reduction in yield following tree topping.