Greg Dryden, Fruition Horticulture (First seen in The Orchardist, October 2009)

Irrigation management is a key issue for pipfruit growers and becoming increasingly so, as growers look to manipulate vigour, dry matter and fruit size.  In the future growers may also need to be able to describe or justify their water footprint, much in the same way as the Carbon footprint is developing. 

This article outlines the key considerations for irrigation management and provides some insights based on my 12 seasons monitoring soil moisture and irrigation management in Nelson.

To begin a few definitions to help are:

Full point – This is the point where the soil is holding all the moisture it can. Also referred to as field capacity, it is obtained by taking readings in the middle of winter or 3-4 days after heavy rainfall

Refill point – This is the soil moisture at which tree and fruit growth will be limited

Rooting depth- The depth that the majority of root activity occurs. This is normally just over one meter for mature trees. Mature M9 trees also show regular root activity at this depth

Readily Available Water (RAW) – The difference between FULL and REFILL for the entire rooting depth

Have a Plan

The most critical component of modern irrigation management is to have a plan.  Specifically you need to consider:

            What is your readily available soil moisture?

            Do I want to provide full irrigation?

            Do I want to consider deficit irrigation (early, mid or late season)?

            What does your system deliver?

            How do I monitor and manage this?

If you are using a service provider a lot of this will be done in conjunction with them but be sure to communicate your plan with your service provider so their recommendation matches your plan!

Typical irrigation levels

I am often asked how much should I irrigate?  What are typical levels of irrigation for my crop in my district?  This is very difficult to answer as this is influenced by crop type, soil type, irrigation strategy and system capabilities.  But, as a guide, table one shows average irrigation levels for Apples on blocks where full irrigation is provided. 

Table one: Full season Irrigation (mm)

Interestingly the average Readily Available Water (RAW) for the average site in both regions is almost the same at 50mm. If your soil type has lower RAW than this, in these seasons irrigation would be more than these values and if your RAW is more, your irrigation should be less.  Of course these are just a guide as site specific rainfall, soil type, crop and your RDI strategy will influence actual irrigation but if your application was wildly different than this you need to question why.

Regulated Deficit Irrigation

Our pipfruit grower clients are now becoming increasingly interested in regulated deficit irrigation much like our grape grower clients have been practising since the 1990’s.  RDI is of significant value where water is limiting, vigour is an issue or growers are looking to influence Dry Matter (and hence Soluble Solids), flesh firmness or fruit size.  Table two shows relative positive and negative effects of applying a deficit either early, mid or late season.

Table two: Effects of deficit irrigation throughout season.

The most difficult question to answer is how much of a deficit should I impose on my trees.  To get positive benefits the deficit needs to be such that soil moisture is being held close to levels such that all readily available moisture is depleted.  Trees will still be able to extract moisture from the soil at these levels but will be under pressure to do so and hence the effects (positive and negative!) will accrue.

When working with a new pipfruit block we estimate Refill Point depending on the soil type at between 70% and 85% of Full Point.  We then put a strategy in place that will allow the grower to operate soil moisture at or just above refill point at various times during the season depending on their plan.  Refill Point can also be confirmed or checked using a pressure bomb, sap flow, tree or fruit growth measurements.  Occasionally on deep silt soil types where trees possess very deep roots refill point can be even less than 70% of full but we always prefer to err on the conservative side and impose a slightly high refill point than put trees under significant stress until detailed knowledge of the block is built up.

Over watering

One of the biggest frustrations I have as an irrigation consultant is over irrigating.  Once a soil is at Full Point there is no fathomable reason for applying additional water.  In fact our standard recommendation is to operate soil moisture just below Full Point, at such a level that growth is not restricted, but where there is some capacity for the soil to hold additional water when rainfall occurs.

Growers will often tell me they want to keep soil moisture at (or even above) Full Point in an effort to drive tree or fruit growth.  This is not going to achieve the desired result as excessive irrigation leads to leaching of precious Nitrogen and causes anaerobic soil conditions that limits root activity.  Graph one shows typical effects on soil moisture of over irrigation.  This extreme example shows a large irrigation event on January 22^nd that took soil moisture up above Full Point, active root activity did not commence again until January 29^th.

Graph one: soil moisture level (mm in 1m) following an over irrigation event.

Dwarf trees

Dwarf trees require less water than conventional MM106 or M793 plantings.  They may well require more frequent irrigation but total irrigation applied should be about 75% of conventional plantings. 

Crop use is a direct correlation to canopy size, the Pipfruit New Zealand Intensive Benchmarking exercise records the average Tree Row Volume of mature M9 blocks at 10,500m³.  This compares with an earlier study by Pratt, Maber and Manktelow (1997) showing the average TRV of New Zealand orchards was then 14,400m³.  This suggests irrigation of dwarf canopies should be about 73% of conventional canopies and compares very well with the 75% value Fruition Horticulture uses as part of our monitoring service.  It also compares well with actual measured irrigation levels. 

Fruition Horticulture’s service measures irrigation application on every block where soil moisture is monitored and on average actual application on dwarf blocks is 82% of conventional canopies irrigation as outlined above.  What is also very interesting is that on both M9 and M26 dwarf blocks we see trees actively drawing moisture from depths of greater than one meter.  Sure young dwarf trees only have very shallow root systems but these will develop if given the chance to much greater depths. 

Summary

Irrigation management is an increasingly useful tool that growers have at their disposal as a means of controlling vigour, maximising growth and improving dry matter.  Growers can expect cost savings through less pumping and reduced need for other vigour control options such as cincturing, Regalis or summer pruning.  I believe in the future growers will be taking an even more active role in their soil moisture monitoring and irrigation management. 

References

Pratt J-P, Maber J and Manktelow D. (1997). In-orchard evaluation of the Tree-Row-Volume (TRV) method for determining spray application volume rates. ENZA Contract Number LV 96 P02-12 LVL Project Number 2895.