Making Sense of Moisture Sensors

This year, we decided to grow some of the plants in a more commercial set-up, in 9L strawberry troughs (500mm long), fertigated using a somewhat more sophisticated irrigation timer and a dosatron.  We installed two sets of moisture meters from Tau Research which are tracking moisture and EC in the little 9L bins and the big 70L bins, and sending the data to Tau’s website where we can see it in real time as well as see the history.  The project is supported by the Horticentre Charitable Trust and Tau Research, with help from Berryworld.  It’s been fascinating, and we’re learning lots.

Step one was to plant the trial, and there the learnings began.

We have historically used a large volume plant, 600ml, which is the same grade we produce as motherstock for the runner growers.  It turns out that it’s quite hard to stuff four such big transplants into a pre-filled 9L trough.

Step two was to figure out an appropriate watering/fertilizing regime.

For this, we had the help of the sensors, which were giving us constant data about the root zone.  The sensors use the fact that wetter media can hold more electricity to estimate the water content.  The resulting read-out is simply a line that moves up and down as the media is wetted or dried.  We still had to define “what good looks like”—how much we wanted that line to go down before we watered.

We decided to try and follow a common strategy in crop irrigation—allow the media to dry to a mild stress point before bringing it back up to “field capacity,” or the point where the media can’t soak up any more water and instead it starts to drain out the bottom.

Of course, that still means we have to decide what constitutes a mild stress point, and again we turned to a standard irrigation theory that says the mild stress point is half way between “field capacity” and the crop wilting point.  Find the saturation point by watering heavily, then find the wilting point by drying out a trough until the plants literally wilt, then pick the middle between the two off the graph.  Thankfully, for the plant’s sake, this is only required once, and just on the one bin that houses the soil sensor.

Below, you can see the moisture sensor graph when we started out by watering the trough to “container capacity,” when the media freshly watered and wet as it’ll get but the dripping out the bottom has stopped.  That’s where our moisture line hits the black line.  Second, we allowed the trough to dry until plants just started to wilt, which established our “wilting point,” or the red line on the graph.  The green line is the theoretical point half way between fully wet and wilting.

Moisture meter record for Lincoln variety trial black troughs, from mid October to early December

We did this process with our small 9L troughs, which naturally are easier to dry out than the 70L bins.  This let us establish an approximate “green line” on our moisture graph which represents a mild stress point, hopefully one that helps keep the plants “generative” (making flower initials, not too bushy) but doesn’t compromise fruit size.

This is where lesson number 1 became apparent—Don’t over compact the media.

We discovered that when we water with the drippers, the water tends to run through the troughs and drain out the bottom quite quickly, before the water moves sideways and saturates the whole media….not ideal when it comes to water or nutrient efficiency.  This also means that once we dip below our “green line,” in late November we struggled to bring the moisture up with our drippers, even when we have the irrigation set to water for 2 minutes at 9:00, 9:15, 9:30, 9:45 and 10:00.

An experienced grower chuckled and advised us to “puff up the bags,” or loosen the media.  In the process of planting big transplants into a small trough we had over-compacted the media.

“Puffing up” is easier said than done, given that the troughs are rigid and the roots now fill the whole container.  Next year we’ll use smaller plugs and be careful not to over-compact the soil.  This year, we will require overhead hand waterings periodically to bring the moisture to our “field capacity” target.

Learning to “speak sensor”

We didn’t want to blindly believe what the sensor was telling us about root zone moisture, so as we were trying to “learn sensor-speak,” we translated several times by digging around by hand, looking and feeling the media moisture.

Simply speaking, if a handful of media squeezed in a fist shows water between the fingers, irrigation is not required.  If the firmly squeezed media doesn’t exude a drip of water, then it’s time to irrigate.  Here’s what that looks like, in practice.

Yes, there is nuance to consider, such as time of day.  When checking in the evening or before rain I’d be comfortable leaving the media relatively dry over night, while that same check would see me applying irrigation in the morning of a hot summer’s day.

The other consideration is the penalty is for over-drying or over-watering the media.

You can see on figure 1 that we have dipped below the “green line” multiple times as we’re struggling to work out our irrigation schedule for these bins.  Being that this is a variety trial, we are measuring the yield and berry size on 4-plant-plots.  That means we can match up plots in the big bins (plenty of moisture) with the same varieties in the little black bins (with some moisture stress).  While the impact on fruit size so far has ranged widely (from no difference to 40% smaller), over 14 side by side comparisons we have realized an average 13% decrease in fruit size.  We think that’s our penalty for water stress.

The penalty for over-watering is often a weak root system, sometimes even diseased with fungal pathogens.

Do you have an handy way you chose when to irrigate? Email me! I’m interested to know how growers are making this choice.

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