Sensing the Invisible: Soil Moisture

Despite the funny quip that the human genome is 50% the same as that of a banana, we humans are built so vastly differently from plants that we can struggle to understand their “umwelt,” or the world as plants experience it.  And since we don’t have roots ourselves, it’s easy to forget how fundamentally important they are to a plant’s wellbeing.  

The most skilled plant growers are those who can understand how a plant responds to the world around it, then manipulate that environment to get the desired growth outcome.

But how can a person learn to speak “plant”?  Perhaps some special translation equipment can help.

Tau Research is partnering with Berryworld and SGNZ, with funding from the Horticentre Charitable Trust, with the aim to make that translation:  Make Visible what is otherwise Invisible to us—the water and nutrient content of the plant root zone.

Tau is a research company that specializes in making data visually accessible and presented usefully for decision-making.  Several months ago they came to us with the idea to test out using moisture and EC meters in strawberries, to better optimize watering and fertilizing.   We responded with an enthusiastic “yes, please.”

Moisture and EC meters aren’t new inventions, but they suggested to use affordable models coupled with a grower-friendly “dashboard” where the readings would track along in real time.  The benefit to the grower would be to see at a glance, both historically and today, what it usually takes a trained technician hours to put together as a snapshot of manual readings.  It’s almost like having an expensive greenhouse environmental sensing system with a fraction of the cost.

To test out the equipment and work out how to use it for decision-making, we offered to have the sensors installed in the annual NZBP strawberry variety trial in Auckland, where we are already taking careful measurements of the yield and berry size.  This trial is done on outdoor tabletops, but Tau’s set-up can be used in both field soil or media.

This first phase of the project is trying to define “what good looks like” for the plant establishment phase in terms of moisture content of the media as well as EC.

Below is a read-out from a soil moisture meter where plug plants were established mid April.

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Looks like heart monitor!  Prior to 22 May, those jagged bits are the day/night change in media moisture due to condensation happening during the cool nighttime hours, rather than watering events.  Water was added to the media on 15^th May when it rained 4.5mm, and on 20^th May it rained again, almost 7 mm.  On 22 May, bare root plants were put in and watered multiple times a day to aid their establishment.

Tau has added a “stress point line” which indicates the point where plants start to ration water—reduce their somata openings—to conserve water.  When plants feel a little bit of moisture stress they also grow more roots to mine water, shorten their developing internodes, and grow a tougher waxy leaf coat—all characteristics of good “strong” plants that can handle a bit of environmental stress and keep ploughing onwards.  The “Stress point line” was placed where it is after observing that, during a period of strong evapotranspiration, the moisture level in the media wasn’t going down as expected, therefore plants must have been rationing water.

Incidentally, plants with a “strong” growth habit are often termed “generative,” because another plant response to mild stress is to focus resources on reproduction, like berries.  This contrasts with a “vegetative” growth habit where the plant growth tends towards larger, floppier leaves, longer internodes, and fewer flowers.  These terms are commonly used in greenhouse production, and growers may want to steer a plant more “generatively” or more “vegetatively” depending on the growth phase and commercial goal.

The red line at the bottom represents the theoretical “wilting point line,” where a plant can’t get enough water out of the soil to prevent leaf wilt.  This severe stress risks root damage for many (but not all) plants.

A good grower is intentionally aiming to drive plants to some stress so they respond by growing strong, but not too much to cause damage. To add to the ambiguity, how much stress is “good stress” changes as the plant goes from a tiny propagule to a mature fruiting plant, as well as being influenced by the commercial goal for fruit timing.

It’s a bit like parenting a child—and I’ll keep this metaphor brief–they need to feel some stress to learn how to cope with life (like if you don’t eat your dinner, you will feel hungry before breakfast), but the level of responsibility a parent is expecting keeps changing as the kid matures, and, frustratingly, kids have different personalities so the “one size fits all” method of parenting doesn’t produce the best outcome for every kid.  So too, with growing plants.

The EC (nutrient content) of the media interacts with soil moisture and how much moisture stress plants feel as well.  More on EC next time!

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