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Nitrogen management in red blush pears

Research & Extension

Research has commenced at the Horticulture Centre of Excellence on nitrogen management in red-blushed pears.

To obtain optimal yield and fruit quality, nitrogen must be managed to meet the crop’s requirement based on the available nitrogen in the root zone and tree demand. Current nitrogen fertiliser recommendations for newly planted pear orchards vary by more than ten-fold irrespective of the soil nitrogen status. These recommendations exceed tree nitrogen uptake and this most likely leads to nitrogen leaching.

Building on the basics

Repacking soil into a drainage flux meter prior to installation at 0.9m depth to collect and measure drainage and nitrogen below the root zone in the pear field laboratory at Tatura.

Existing guidelines for nitrogen management in mature pear orchards were developed for the cultivars Williams’ Bon Chrétien and Packham’s Triumph grown using traditional orchard systems. This project aims to deliver guidelines for nitrogen fertiliser management to consistently maximise fruit quality in new red-blushed pear cultivars and minimise below root zone leaching of nitrogen.

Our research will focus on developing a cost effective method of assessing leaf nitrogen using remote sensing; understanding the effects of microjet and drip irrigation on nitrogen efficiency; and identifying relationships between nitrogen application, leaf nitrogen and nitrogen leaching.

The research will be conducted as part of the Productivity Irrigation Pests and Soils (PIPS) program at the pear field laboratory, Horticulture Centre of Excellence, which is part of Victoria’s Department of Economic Development, Jobs, Transport and Resources (DEDJTR).

Remote sensing

Exploring remote sensing of nitrogen status in apple was undertaken in the fertigation project in the previous PIPS program. Results suggested that the canopy chlorophyll content index may have potential to detect the nitrogen status of apples. The study included some satellite data for pear but concluded that satellite data could not discriminate between the tree and the understorey nitrogen status.

In the present study we are examining high resolution imagery using an unmanned aerial vehicle equipped with multi-spectral and thermal cameras. Observations are being made in a nitrogen experiment in a mature red-blushed pear orchard. The treatments range from zero to high application rates of nitrogen. Initial results will be published in a forthcoming article.

Drip and microjet irrigation

A comparison of the effects of drip and microjet irrigation on young tree growth and precocity has been undertaken in the pear field laboratory since the trees were planted in 2012. Considerable debate on nitrogen use efficiency in drip and microjet orchards emerged during this experiment. A small project in PIPS2 has taken the opportunity to investigate nitrogen losses below the root-zone by installing drainage flux meters in the drip and microjet treatments.

Drainage flux meters were installed close to the irrigation emitter and in the mid-row in spring 2015. The top of the drainage flux meters was positioned at 0.9m depth by drilling a hole to 2m, sliding the drainage flux meter into the hole, and repacking soil to create similar texture and bulk density as undisturbed soil.

The drainage flux meter has a sump at its base to collect water and soluble nutrients that can be sucked out through a tube running to the soil surface. Drainage and nitrate concentration were measured every two weeks during the season. The data will be combined with leaf, fruit and wood nitrogen concentration data to develop grower recommendations for nitrogen fertiliser and irrigation management to avoid below root zone losses of nitrogen.

In the previous PIPS project, drainage flux meters were installed in a commercial block of microjet irrigated Cripps Pink apple. Drainage and nitrogen leaching were measured over a three-year period. Results showed that the total amount of drainage collected in the sumps over the three-year period was equivalent to over 1,000 mm and this was dominated by rainfall during late autumn, winter and early spring, and periods of irrigation that exceeded crop water requirement.

Nitrate collected in the sumps over the three-year period in the tree-line and in the mid-row was 4.9 and 1.9gms, respectively. Such nitrate amounts appear to be small but when scaled up to represent the orchard, the losses of nitrogen were substantial (around 65kg N/ha/yr) and could not be attributed to the annual applications of nitrogen fertiliser.

The cultivar ANP-0118 (marketed as LanyaTM) is being used in the experiment at Tatura. Future research hopes to study the effects of nitrogen on colour development and fruit firmness to provide the Asian market with crisp, sweet and attractive red-blushed pears.

Other sources of nitrogen are from mineralisation in the soil and perhaps historical applications of ammonium based fertiliser. Either way, these results suggest that rainfall contributes substantially to leaching of nitrogen but care also needs to be taken to match irrigation to crop water requirement of the trees.

Drainage and nitrogen leaching are being investigated in micro jet (left) and drip (right) irrigation treatments at Tatura.

Nitrogen and fruit colour

The focus of our research on nitrogen use efficiency extends beyond field observations of nitrogen status and nitrogen leaching into the effect of nitrogen on fruit quality, particularly red colour development.

There is very little published on the effects of nitrogen on red colour development in pears. Most published research on the effects of nitrogen on fruit colour has been on apples where high nitrogen application reduced red colour and increased the green background due to a decrease in both chlorophyll degradation and anthocyanin synthesis in the skin.

Red colour development in the new red-blushed pear cultivars occurs very early in the season and then continues throughout the season as the fruit grows. Shading can significantly reduce the red colour but colour can also bleach later in the season when the fruit is exposed to direct solar radiation.

High levels of nitrogen in spring will cause excessive vegetative growth and this will shade the fruit and reduce colour. In contrast, shading late in the season will prevent loss of colour from bleaching.

The direct impact of nitrogen on red colour development in pears is unknown. Preliminary studies will be undertaken in this project to identify gaps in knowledge and future research emphasis in nitrogen management to maximise quality of red-blushed pears.

Acknowledgement

This project is funded through the PIPS program by Horticulture Innovation Australia Ltd using the apple and pear industry levy from growers, matching funds from the Australian Government and co-investment from DEDJTR.

About the authors

Ian Goodwin and Lexie McClymont are horticultural researchers and David Cornwall is a horticulture technical officer at DEDJTR Tatura. Eileen Perry is a specialist in remote sensing based at DEDJTR Bendigo. Steve Green is a research scientist and modeller at Plant and Food Research New Zealand.

 

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