The Sigma Probe in Action in Substrate Strawberries

Are we missing a critical ingredient with traditional monitoring of "water in" and "water out"?

Traditionally growers of strawberries in substrate, whether peat, coir, rockwool, perlite or whatever, keep a close watch on their liquid feed programme by monitoring the EC and pH of the feed solution and the drainage water. These are often referred to as: "EC In" and "EC Out", and "pH In" and "pH Out".

There are many ways of doing this. One method is to put one or two spare drippers into a jug for the "In" measurements and to put a trough under two or three bags to collect the drainage for the "Out" measurements. Using inexpensive hand-held meters, EC and pH are recorded daily. and, at the same time, the volume of the samples is used to calculate the ratio of "water in" to "water out".

Irrigation consultant, Rupert Knowles, has long suspected that readings from the drainage water were not giving a true picture of what is happening at the roots of the plants. With the very high AFP (Air-Filled Porosity) composts used for strawberries, ""water out" is mostly "water in" that has run straight through," he says.

But this year, with the new Sigma Probe from Delta-T

which measures pore-water conductivity, growers have been able to "see" into the root zone and measure what the plant is experiencing. How the Probe works The SigmaProbe is a completely new sensor, which has been developed at the IMAG-DLO Institute in The Netherlands. For the first time it is possible to sample the conductivity of the pore water, directly within the substrate. Traditionally this measurement has been made by the time-consuming and error-prone method of extracting pore water from the substrate by suction, before measuring it with a standard conductivity meter. In contrast, the SigmaProbe is simply pushed into the peat (or other growing medium) and then read directly using either a portable PC or Psion Workabout. Using the Psion to collect data and then exporting the file to view in an Excel spreadsheet is the preferred option.

Compost performance.

This Spring, it soon became apparent when using the probe on strawberries in bags that there were large variations in pore-water conductivity both between bags and within individual bags. "Initially I thought that this variation would rule out the Sigma Probe as a useful management tool for growers", says Rupert Knowles, "but then I realised it was a highly significant factor that we had been missing when striving for consistent flavour and higher yields in the crop". Using the probe, it is important to decide what you want to find out. This will determine the sampling method. This example is of data from a glasshouse strawberry crop:

Note on units: performance is quoted in mS.m-1 because it is an S.I. preferred unit, and gives reasonable values. 100mS.m-1 = 1dS.m-1 = 1mS.cm-1. Strawberry growers usually think in mS.cm-1 , so these figures need to be divided by 100.

Using the probe, it is important to decide what you want to find out. This will determine the sampling method.

In this example data from David Dunn's glasshouse strawberries at Ardleigh, near Colchester, all the readings were taken by inserting the probe 100mm into the side of the bag at the mid-point between drippers. This would be expected to give maximum variation from sample to sample and maximum deviation from the EC of the "water in". Measurements taken in compost directly under drippers were generally found to be the same as "water in" measurements and were not recorded. (To save time, the probe can also used to measure the EC of the "water in" and "water out" containers). The glasshouse was sampled on a grid. It shows high readings (orange- red) in rows 3 - 10. This area was a trial of different manufacturers' bags. The readings taken in March reflected the uniformity of spread of feed solution and the flushing of the compost. It emphasised the benefit of premium quality compost. In some composts, the EC in the centre of the bag was alarmingly high and when readings were taken from the same area one month later, the differences between bags had increased.

After these results, David Dunn used a student to sample the whole of the glasshouse (200 readings per hour was feasible depending on walking distances.)

Three points emerged:

1. EC's were higher in the houses with older irrigation systems.

2. In spite of adjusting rates of run-off and frequencies and timings of irrigations, it was very difficult to reduce the EC in these areas.

3. There were no significant differences down the length of the drip pipe. Other growers have used the probe to monitor strawberries in Ward Wing troughs and in bags under Spanish tunnels. With troughs, the readings were taken by inserting the probe up into the compost through the central drainage hole.

Outside the problem was the opposite: very low readings were recorded after rainfall before the tunnels were clad. It took several days to correct and readings were particularly low directly under the plant as leaves caught the rain and channelled it to the crown and through the planting hole.

Management tool.

Many growers will wish to carry out trials like those above to improve consistency in flavour and yield. However some growers have used the Probe purely as a convenient management tool adding a new parameter recording both average EC and the variation in EC. It is meaningless to have a target for the former without taking into account the latter. If the target EC for the fruiting period is 1.5mS.cm-1 but it varies between 0.8mS.cm-1 and 2.4mS.cm-1 , this is highly unsatisfactory. The grower must fine-tune the irrigation system to bring the variation into acceptable limits.

For management purposes:

· The grower has to decide the size of sample based on how much time he is prepared to budget.

· The grower and his adviser must decide the acceptable limits for variation between samples.

· The job of taking readings must be given to a responsible person who will do it on a regular basis - in many cases a student.

· The point of sampling must be standardised. In some cases it was found convenient to mark and always sample through the same hole in a particular bag or trough. In this way upward or downward trends in conductivity were easily noticed.

· Once the grower has become confident, it is no longer necessary to record "water out" but the probe can be used to measure and record "water in".

Future uses:

This has been the first season that a simple measure of pore water conductivity has become available to growers. Growers and their advisers have yet to recognise the full potential of this exciting new tool. From experience in the glasshouse crop, the effect of conductivity on flavour, size and fruit quality is known. Growers will find out how to manipulate irrigation and nutrition to improve consistency for the market. Will they also be able to manipulate soil conductivity to their advantage given a simple tool to measure it?

Contact Peter White for more advice.

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