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Precision fertigation to improve productivity

Research & Extension

Over three years, Nigel Swarts and the team at the Tasmanian Institute of Agriculture (TIA) have investigated optimising apple orchard productivity through fertigation to get nitrogen applications just right to get the most benefit and avoid vigour and poor fruit quality. 

It is common practice in the apple industry to apply fertiliser based solely on standard off the shelf recommendations to meet the high nutrient requirements of apple production. Precision apple tree nutrition requires consideration of many factors including irrigation requirement, crop load, tree size, fruit quality specifications, the soil’s capacity to retain and supply nutrients, and minimising off-site impact. By attending to these factors and optimising nutrient inputs, fruit quality and shelf life can be improved. Our Productivity Irrigation Pests and Soils (PIPS) project aimed to:

  • Determine the influence of nutrient- and water-use efficiency on apple trees through fertigation.
  • Facilitate the development of fertigation guidelines for growers to optimise whole tree nutrition and fertiliser management.

To tackle this challenging issue we brought together a research team from within the TIAs Perennial Horticulture Centre (PHC); the Department of Economic Development, Jobs, Transport and Resources(DEDJTR), Victoria; and the New Zealand Institute of Plant and Food Research (PFR). In this report, we present the outcomes from three seasons of research trials at Lucaston Park Orchard, Lucaston, Tasmania, and the TIA, University of Tasmania. Details of fertigation and irrigation trials established at Lucaston and University of Tasmania sites. 

Trial Location Variety and rootstock Treatments
N fertigation and irrigation trial (2012-2015). Lucaston Park Orchards, southern Tasmania ‘Galaxy’ on M26 rootstock Irrigation:a) High (3.9 L/hr)b) Medium (2.3 L/hr)c) Low (1.6 L/hr)Fertigation – Nitrogen (N) supplied as Ca(NO3)2a) Control – Zero N b) Split half-  25%N Pre-harvest and 25%N Post-harvest(30 kg N/ha/annum) c) Split full-  50% N Pre-harvest and 50% N Post-harvest (60 kg N/ha/annum) d) Post-Harvest half – 50% N Post-harvest (30 kg N/ha/annum) e) Post-Harvest full 100%N Post-harvest equivalent  (60 kg N/ha/annum)
Potassium trial (2014-2015). Lucaston Park Orchards, southern Tasmania ‘Galaxy’ on M26 rootstock Potassium (K) applied pre-harvest  at 50kg K/ha supplied as Potassium nitrate (KNO3) and Potassium sulphate (K2SO4) and applied by either foliar spray or fertigation
N15 trial (2014 – 2015). TIA Horticulture Centre, UTAS Sandy Bay Campus ‘Jonogold’  on M26 rootstock Nitrogen (N) supplied as Ca(NO3)2 enriched with 5% N15a) Pre-harvest application (24g N/tree @5% N15)b) post-harvest application (24g N/tree@5% N15)c) Control (zero N15)

 

How water stress and water surplus affect nitrogen uptake

The site conditions at Lucaston Park and the significant El Niño influence for the duration of this trial meant that the study of water stress was always going to be tricky. Rainfall was evenly distributed throughout the year and tree roots were found to be accessing a high water table at this site. Water stress was difficult to impose. However, varying irrigation treatments still provided an interesting insight into the relationship between irrigation rates, nitrogen uptake and fruit quality.

Tree vigour, fruit quality, and dormant buds

Irrigation had a strong influence on tree vigour in the Lucaston trial. This was measured as both increases in trunk girth and branch length. Surplus irrigation in the high irrigation (3.9 L/hr) treatment significantly increased tree girth.

precision fertigation

Percentage increase in trunk girth between 2013 and 2015 dormancy measurements under fertigation and irrigation treatments.

Tree vigour was also influenced by nitrogen treatments. Current season (pre-harvest) nitrogen application rather than the total nitrogen applied over a season increased branch length, an indication of greater tree vigour. This was supported by the N15 pot trial where the majority of the current season’s nitrogen supply was found to be present in the canopy. Flower buds sampled at dormancy received the benefit of both a pre- and post-harvest nitrogen application. However, the result wasn’t as clear for vegetative buds. We understand and predict that increased nitrogen content in the buds at dormancy facilitates a healthy start to the following growing season.

precision fertigation

Total nitrogen (%) of flower and vegetative buds at dormancy in 2014 under fertigation treatments. Error bars denote standard error and letters indicate significant differences between treatments.

Irrigation supply was found to have no effect on the nitrogen content of woody tissue, buds, fruit or leaves. The ready supply of water from rainfall and ground water meant that a true deficit irrigation treatment was not possible. Despite this, irrigation was shown to significantly increase fruit size under the highest treatment. The low irrigation treatment produced the smallest fruit with the greatest fruit firmness and total soluble solids. This indicates that fruit size is likely to be most affected by a relatively small reduction in water supply, while nitrogen uptake is less responsive.

How nitrogen application timing and rates affect yield and quality

The nitrogen application rate had a strong effect on tree nitrogen uptake and fruit quality for Galaxy apple trees at Lucaston. Applying nitrogen at the highest rate in the current season always had the strongest influence on leaf nitrogen content and fruit quality, although the results were not always significant. For example, leaf nitrogen in 2015, under the highest pre-harvest treatment, was consistently higher than other treatments, matched only later in the season by the greatest post-harvest nitrogen treatment. As you would expect, the response in leaf nitrogen content to applied nitrogen was most pronounced in the month after application.

precision fertigation

Total nitrogen (%) in bourse leaves of apple trees under fertigation treatments during the 2014/15 season. Fertigation periods are coloured transparent bars.

At harvest, on average, fruit nitrogen was greatest with current-season nitrogen supply; however this result was only significant in the final season of the trial. These results are important as they had an influence on fruit quality outcomes. At commercial harvest, fruit colour indicated that high current season nitrogen delayed ripening. Fruit red colour was reduced and more green background colour was observed under high current-season nitrogen supply. Strong correlations irrespective of treatment between fruit nitrogen (%) and fruit colour further highlighted its influence. A strong correlation between fruit nitrogen (%) and firmness demonstrated the detrimental effect of high levels of pre-harvest nitrogen. These results were not surprising given the sink strength of fruit demonstrated by the N15 trial, where over 30 per cent of current season nitrogen supply was present in fruit.

precision fertigation

Correlations between total nitrogen content in fruit and fruit quality parameters of background colour and firmness.

Other nutrients We measured the nutrient content (calcium, potassium and magnesium) of fruit at the final harvest when treatment effects were expected to have had their greatest cumulative effect. Nitrogen fertigation treatments affected the ratio of nitrogen to each of these elements. Fruit nutrient ratios of N:K and N:Ca increased in a similar pattern to nitrogen supply. High nitrogen is associated with an increase in tree vigour. The bigger canopy of the high nitrogen treatment trees may explain the lower concentration of these nutrients in fruit as they are directed to the more rapidly transpiring new leaves. The greater fruit size achieved with the high nitrogen treatment could also have contributed to the higher ratio of nitrogen to these nutrients in fruit due to a dilution effect. Indeed, the highest levels (% dry matter) of potassium (K), calcium (Ca) and magnesium (Mg) were found in the smallest control fruit. This treatment effect was not repeated in the leaf nutrient content. Interestingly, three seasons of nitrogen supply as Calcium Nitrate (Ca(NO3)2), did not lead to an increase in calcium in the fruit or leaves. This may be due to the long legacy of Ca(NO3)2 applications at the site as seen in the very high calcium level in the leaves (2.5%). Potassium treatments did little to increase levels of potassium in the fruit, however leaf potassium levels one week post application were increased by foliar potassium treatments. Although not significant, there was a trend for decreased nitrogen, calcium and magnesium content in harvested fruit under potassium treatments and therefore increasing ratios N:K, K:Ca and K:Mg. No significant differences were found for all fruit quality parameters with exception to Total Soluble Solids after 10 weeks in storage where the foliar treatments performed superior to fertigated treatments. No nutrient deficiencies were observed in the orchard after three seasons of fertigation and irrigation treatments, however some preference for nitrogen and potassium nutrient uptake was observed following nitrogen and potassium fertilisation.

The influence of fertigation on nitrogen storage and remobilisation

Nitrogen uptake versus remobilisation

This trial showed that total current season nitrogen uptake did not vary significantly between applications made pre- and post-harvest. Despite similar total uptake, the current-season nitrogen distribution throughout the tree was significantly different.

precision fertigation

Distribution at dormancy of the relative proportions of recovered N15 of pre- and post-harvest application over nine separate organs.

Pre-harvest nitrogen accumulated predominantly in the canopy with over half of the pre-harvest nitrogen applied present in buds and fruit. In contrast, less than a quarter of post-harvest nitrogen was found in the canopy.  This is believed to be a result of the sink strength of developing fruit and leaves in the pre-harvest period. There was little difference between trunk N15 content of pre- and post-harvest nitrogen treatments, however, N15 partitioning towards the trunk was found to increase approaching dormancy indicating its importance as a storage region. Post-harvest nitrogen application directed more current-season nitrogen to the below ground region compared with trees receiving pre-harvest application. Given that we could only assess the current season’s uptake, we were unable to determine the influence that remobilised nitrogen had on current season growth.

Nitrogen storage

Results show a trend for greater allocation of current season nitrogen to storage after receiving the post-harvest nitrogen treatment. This has potential to increase nitrogen availability for the following season’s early spring growth. This is not surprising because the pre-harvest treatment diverted a greater proportion of its nitrogen to fruit (35%), which are removed from the system. Yet the difference in the quantity of nitrogen stored between the treatments wasn’t that stark. This is due to the highly efficient withdrawal (100%) of current season nitrogen from the leaves back into storage organs.

Key points

  • High rates of irrigation increase tree vigour and fruit size with a corresponding decrease in fruit firmness.
  • High rates of nitrogen pre-harvest increase tree vigour with a large proportion of this nitrogen being directed to the canopy. Pre-harvest nitrogen application also increased fruit nitrogen content. There was a corresponding reduction in the proportion of calcium, potassium and magnesium relative to nitrogen in the fruit. This has potential to negatively affect fruit post-harvest quality. High nitrogen content of the fruit was associated with delayed ripening, reduced fruit colour and decreased fruit firmness.
  • The distribution of nitrogen within the tree was strongly influenced by timing of nitrogen application. A greater proportion of nitrogen was directed to the canopy from pre-harvest nitrogen application whereas post-harvest nitrogen was directed to storage.

Recommendations

  • The total nitrogen supply needs to be matched to the site/soil conditions with consideration to tree and fruit responses to historic fertiliser regimes, which requires accurate records of fertiliser management and crop response.
  • Pre-harvest nitrogen supply should occur no earlier than four weeks after full bloom and uptake efficiency (avoiding leaching) will be optimised through providing weekly applications.
  • The remaining balance of total nitrogen supply should be provided post-harvest but this may not be ideal for later cropping varieties in some regions.

Future research

Discussion between the PIPS’ Precision Fertigation team and an industry panel comprising growers and advisors during the course of the project has identified key knowledge gaps that should be addressed in the next PIPS2:

  • What are the sources, temporal patterns, and relative contributions to nitrogen supply of plant-available nitrogen in the orchard?
  • When is the peak nitrogen demand by the tree and how much is provided by internal tree nitrogen?
  • A decision support tool for advisors and growers to assist with fertigation and irrigation management in all major apple-growing regions.

Acknowledgements

Many thanks to everyone who has contributed to this work including TIA staff Garth Oliver, Justin Direen, Marcus Hardie, Sally Bound, Michele Buntain and Dugald Close. Special thanks to TIA honours students Matthew Morris and James Ridges for their projects which added significant value to the project. This project was funded through Horticulture Innovation Australia Limited using the apple and pear industry levy funds from growers and funds from the Australian Government.

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