At least three quarters of current and potential soft fruit production is in substrate, under polythene, and relies completely on irrigation for water supply. Many strawberry growers find irrigation control difficult.
Most growers use timers, commonly from the company Irritec. The number and length of the irrigations are set by the grower, and can be re-set as required. Skill, experience and regular time is required to monitor the weather, crop run-off and moisture content of the substrate and adjust the timers accordingly.
However, as many growers are not on site during the day, they cannot react to changes in weather during the course of the day, leading to conditions where plants may be over watered or under watered. On farm visits, it is common to find the peat compost in strawberry bags excessively wet or dry, as a result of day length/light levels and temperature which are constantly changing.
The time between sunrise and sunset increases from 12 hours on 18 March to more than 17 hours on June 18, then reduces to 12.5 hours on 18 September. There is normally a lag in growers’ response, so crops tend to be under watered in May and June and over watered from August onwards.
Strawberry growers are not achieving the maximum yields due to over and under irrigation. Disease is rife in over-irrigated units. Fine particles of peat accumulate at the bottom of the bags or troughs, forming an anaerobic layer which kills roots. It is not possible to quantify because no local grower has irrigation based on a system of measuring crop demand.
Fruit is small where there is under-irrigation and soft, with short shelf-life, where there is over-irrigation.
Excessive irrigation wastes both water and soluble fertiliser, which is increasingly expensive.
Most growers and potential growers have other businesses or employment and are not on-site regularly to adjust irrigation. A reliable automatic control system, responsive to crop demand, makes strawberry production more viable on a part time basis.
Control using moisture meters
Irrigation can be triggered as required if the moisture content drops below a set level, using a moisture meter linked via a controller to the solenoid valves.
As part of the Precision Water Management Project, managed by Cathal Ellis, a short investigation into the use of moisture sensors in strawberry peat bags took place during 2011.
At Greenmount, and on-farm at Mr E. Cushnahan’s in Washingbay, moisture content was recorded in two or more bags over a period of several weeks, using Delta T GP1 Dataloggers. Advised by ADAS that moisture content should be 35 percent to 55 percent, it was relatively easy to maintain this level in a small number of bags at Greenmount, (see Fig 1 below).
Fig 1. Substrate moisture, with and without control using soil moisture meters in strawberry bags at Greenmount Campus 2011(C.Ellis)
However from the on-farm measurement it became clear that control using moisture meters would be more difficult on a farm scale. Mr Cushnahan has strawberries in pots, with one dripper per pot. One pot recorded remained at a much higher moisture level than the other, even in the same part of the same tunnel.
To guarantee accuracy of irrigation within this scenario multiple moisture sensors would be required. Given the cost of moisture sensors, it would be prohibitively expensive to use the number required.
Control using evaposensors
The method of irrigation control most commonly used in strawberry tunnels in GB is based on evapo-transpiration using an evaposensor and dedicated control unit. To assist growers to calibrate and monitor the system, a mobile moisture meter provides an accurate method of management of the system. A control box records the accumulated evapotranspiration as a figure.
To calibrate the system the grower waits until the moisture content of the bag/pot/trough substrate in one tunnel is at the minimum appropriate for the crop. The figure on the control box at this point is the level set to trigger irrigation. The control box has separate channels so that different figures can be used as triggers for different tunnels or groups of tunnels.
Once set–up, the grower then checks the moisture content regularly, for example to spot where drippers may have failed.
- Delta T SM150 hand held moisture meter
- Delta T GP1 logger and SM300 moisture sensors (for data collection)
- to evaluate and demonstrate the Precision Water Management of strawberries grown in substrate, using evaposensor technology for control of irrigation.
- to quantify moisture levels in strawberry bags over a growing season with irrigation controlled using an evaposensor and compare with recommended moisture levels
- to record run off levels from strawberry bags in order to quantify loss of nutrition
- to monitor quality and uniformity of strawberries irrigated using evaposensor.
When growers were asked, in the Spring 2011 Issue of Soft Fruit News, if they would like to host an on-farm trial of irrigation control, only Mr Cushnahan volunteered. Unlike others, he is willing to open his unit for groups of growers to see a precision irrigation control system in operation.
The grower unit consisting of three tunnels was provided with an evaposensor, control panel and sequencer which allows independent irrigation of each tunnel. Initial set up was required for each tunnel. The Delta T hand held moisture meter was used to provide an accurate quantifiable value to assist in the initial set up.
The performance of the system was monitored using a Delta T GP1 data logger and SM 300 moisture sensors.
Cost of system components (all plus VAT):
- Sensor £129
- Control panel £399
- Connection to solenoids £100
- Moisture meter £351 (for grower use)
- Other equipment provided by CAFRE for duration of demonstration.
|April 2012||Purchase parts||A Stone|
|April 2012||Install control and set up control of individual tunnels||E Cushnahan, C Ellis, A Stone|
|May 2012 onwards||Monitor substrate moisture||E Cushnahan, A Stone
|May 2012 onwards||Analyse data||C Ellis|
|July 2012||Demonstration to growers||A Stone|
|2013||Modifications may be required depending on performance|
GP1 logger was set to record percent moisture at 10min intervals from beginning to the end of the season. The data was analysed and a per cent moisture chart produced.
The aim is for approximately five per cent run-off. This is not practical to calculate, since the input of water is not measurable.
Quality and uniformity data
Yield of strawberries in the second season (May-June) depends primarily on their conditions in the previous season, in 2011. Fruit quality depends more on their conditions in the current season, but depends on fertiliser recipe and pest and disease control.
- water loss due to excessive run off reduced by 20 per cent
- fertiliser costs reduced by seven per cent (as result of reducing run off)
- strawberry yield increased by five per cent
- strawberry flavour improved (comment from grower and buyer)
- grower has fully adopted the technology on his farm
- 24 strawberry growers attended a workshop at the farm on irrigation