How do you collect a sample of the root zone solution?

In my answers I frequently refer to the root zone solution. That is, the solution around the plant roots from which the plant takes up water and nutrients, as explained in my previous two columns. I stress that it is this root zone solution, which is the basis for good hydroponic management. Many growers obsess about their feed solution, but this is only a means to managing the root zone. I am often asked how to collect a root zone solution sample from a dripper-fed container of medium, so this is my practical answer to that question.

Answer by RICK DONNAN

Conditions within a container of a growing medium
The sun is the major driver for transpiration, which is the evaporation of water from the plant leaves in order to keep them cool. Transpiration accounts for about 90% of the water uptake by the plant. Because of high radiation levels during the day there will be more uptake of water than nutrient ions. This will result in a rise in the strength of the solution within the medium as it makes its way from dripper to outlet (which typically takes 1 to 2 days for mature plants).

Solution strength is measured by electrical conductivity (EC). For a mature crop of tomatoes on an average summer day, with a run-off of 30%, I would expect a dripper input EC of 2.6 mS/cm to rise about 3.7mS/cm in the run-off. If not kept under control and the percentage run-off falls, its EC can rise much higher leading to a loss of yield from problems such as blossom end rot.

Good hydroponic management is based around measuring daily what goes in from the dripper and out as the run-off by collecting samples of both.

Sucking a solution sample direct from a medium
I have often mentioned that basically the only medium, which it is possible to syringe a sample, is rockwool. When trying to suck a sample from other media the syringe invariably gets clogged. Even with rockwool there will be a distribution pattern within the rockwool slab, so that the analysis of the syringed sample will vary significantly from place to place. Consequently, to get a good composite sample you would need to collect samples from twenty different places in a slab, which is unworkable.

For that reason the most practical sample to take is that of the run-off solution from the medium. While biased, this is the most consistent sample that can be taken. It has other advantages in that it can be done for all media. It is also the basis behind the root zone solution recommendations published by the Dutch research stations, as used by many growers and laboratories around the world.

Analysing a sample of solid medium
Whatever medium is used in whatever type of container, if fed by a dripper and with an outlet at or near the bottom, there will be a distribution pattern within the medium. The pattern is not really predictable other than in general terms. It is influenced by the medium characteristics, the height, width and depth of the medium, the location of the dripper/s and outlet/s, the flow rate of the drippers, the amount of feed solution given in an irrigation cycle, etc. It will also vary from bag to bag.

It would be possible to get a good sample by taking a whole bag. Even for a research project this is difficult, but for the daily analysis of EC and pH it would be totally unworkable.

Squeezing a small sample of medium
While in India recently with a colleague, I met a different method of sampling the medium. This was to take a handful of cocopeat from a bag and squeeze out some solution to measure pH and EC. This would only be feasible with a medium with high water-holding capacity like cocopeat. I have serious misgivings about this technique.

As described above, there will always be a distribution pattern within a bag of medium. The most obvious place to take a sample without causing too much damage to the plant roots is from the upper part of the medium. Also, if there is drying towards the outside edge of the bag, which is common, then the most ‘squeezable’ sample is that under the dripper. Any sample taken from near the dripper can be expected to have the same analysis as the feed from the dripper.

As an example, while in India we visited a farm and in the early afternoon inspected a mature tomato crop in cocopeat. There was no sign of run off yet (normally first run off occurs about 11am) and the plants looked stressed. We arranged to have a run off sample collected, expecting a substantial EC rise, but the manager reported that its EC was the same as the dripper feed, which was quite surprising. With later questioning we were told that because there had been no run off yet, he had ‘squeezed’ a sample from the medium.

This example is evidence of the common misconception that the solution in the medium and the run-off will always be the same as the feed solution coming from the dripper. In fact, what is in the medium, as indicated by the run-off solution, will always have a different nutrient balance, usually a different EC and possibly a different pH compared to the feed.

Setting up a sampling station
It is best to set up more than one test station per greenhouse and they need to be located in an unbiased typical position within the crop. That is, not under a gutter or at the end of a row or in a side row.

The collecting of a sample of the feed input is easy. All that is needed is an extra dripper and a collecting bottle large enough to hold the full day’s feed. Other points to watch: the dripper should be at the same level as those into the bags, especially if it is micro-tube (less important for pressure compensated drippers – the best type to have); it needs to be placed out of direct sunlight.

Sampling the run-off (drain) solution
Collecting from a single bag is OK, but from a number of bags is better. Collecting from an entire row or the total greenhouse is better still, but requires the use of meters to measure flow volumes and sub-sampling equipment.

Your sampling container also must be large enough to hold the total volume of run-off for the day, and must be protected from direct sunlight. Avoid having a large surface area of exposed sample because of the resultant evaporation, especially if exposed to direct sunlight. Often growers place their bags in hanging gutters or on supports such as polystyrene boxes, enabling a collecting tray to drain the run-off solution into a bottle underneath. Otherwise, if bags are directly on the ground, you may benefit from digging a small recess to hold the sample bottle.

There are many types of commercial sample collecting systems available, usually giving direct measurements of volume, pH and EC, and often linked into the irrigation control computer. However, manual sampling is quite OK and often growers with automated systems will also have a manual station they use to check the computer and to give them the basis for setting the irrigation parameters in the computer.

Sampling schedule
It is important that collecting and measuring the samples be done daily and at the same time each day. What time that is done is up to the grower, however there are advantages in doing it relatively late in the day when the bulk of the run-off has been collected.

For both feed and drain samples the pH and EC are measured and recorded and then the volume. From the volumes the percentage run-off can be calculated. Both bottles are emptied and put back to be ready for the next 24-hour cycle. Check that the bottles are clean.
Measurement records should be kept in a diary along with relevant comments, such as weather conditions, any changes in irrigation strategy, etc. The records are not only a history, which can be useful later, but form the basis for your irrigation and fertiliser strategies – RD  Ω

PH&G April 2017 / Issue 178


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