The Pro Farmer

Optimizing Plant Growth with a Soil Moisture Sensor

Every new farmer/gardener knows that lack of irrigation, will end up in the wilting of the plants and their death.

The plant needs a constant supply of water in order to live.

Water is constantly arriving with high pressure (carrying minerals) from the soil through the roots, up the stem, (in the transport tissue pipe called xylem) to the leaves and fruits and then from the leaves, evaporates to the air (a process called: transpiration).

Lack of water will drop down the pressure of the water, resulting in wilting.

Too much time in this situation of water deficiency, will lead to dehydration of the plant and its death.

On the other hand, less growers know, that over irrigation is also dangerous to the plants, and can easily lead to their death.

Watering too frequently can cause rot in the root area and prevent oxygen supply to the roots.

The plant uses its roots, among other things, to absorb the oxygen it needs for normal growth.

Roots also need a certain amount of oxygen to perform respiration, which is a process that takes place in all cell types in which organic compounds decompose and energy for the plant’s growth is formed.

For instance, one of the reasons why plants growing in a hydroponic system manage to reach a very impressive growth rate is the oxygen-rich root environment that the plant enjoys.

The hydroponic growing media will usually be particularly airy, allowing a high oxygen presence.

Furthermore, too much water will fill the soil with mildew or mold, the roots will rot, and the plant will fade and begin to wither.

Too much irrigation is also a waste of water which is an expensive resource/commodity.

In both cases, even if you notice the plants are wilting, and you manage to restore them to their normal condition, it may happen that some damage had already been done, and though the plants are not dead, their potential is reduced and their fruit production will decline.

Even one day without enough water can cause fruit damage, especially when they are in the early stages of development.

The way of getting a lot of tasty fruits is to make your plants prosper, that means enough water, fertilizer and protection from pests.

The irrometer/tensiometer/soil Hygrometer Sensor/soil moisture sensor/soil moisture meter, is a simple but effective device for assessing the availability of water to a plant by measuring the water tension in the soil.

The device helps the farmer decide when and how much to water.

The tensiometer is working on the principle that the drier the soil, the greater its water suction power, that is, the water tension in the soil increases.

The tip of the moisture meter is pushed into the soil to the area where the roots are at.

Usually, the roots of vegetables are present between 0-30cm (0-12 inches) deep, so 15cm (6 inches) is the right depth to stick the moisture meter tip.

The right place to measure the moisture is not right under the main stem, but 10cm (4 inches) to the side.

There are three kinds of moisture meter: mobile, permanent and transmitters.

Mobile soil moisture meter

These are not so reliable, good for pots, raised beds etc.

You simply stick the soil moisture meter in the planting media and it shows you on a scale, if it is dry/moist/wet.

If the scale shows “dry”, of course you need to irrigate until it’s wet.

When it shows “moist”, you should irrigate until it shows wet.

Only if you check and it shows “wet”, you should do nothing, don’t add water, and wait more time, (whether its 1- 3 days or more) until checking again.

This mobile moisture meter cannot stay in the soil and must be removed after the check, because the probe might rust.

Permanent mechanical moisture meter

This is more professional, commercial farmers use, and is suitable for home gardens, and medium farms.

The moisture meter is placed in a place that represents the whole plot or field.

It means that all the plants in the field/greenhouse are the same and under the same conditions as the one that the moisture meter is placed nearby.

We will add or cut water quantity to the plot/ field according to one place measurement.

This way you don’t have to check each plant.

The tensiometer consists of a PVC tube, with a transparent part (the control window) at the top and a porous ceramic cap at the bottom.

Water can move from the soil to the tube, or from the tube to the soil, through the ceramic cap.

A clock shaped mechanical vacuum gauge is screwed on the top, with a rubber seal that prevents vacuum from being lost from the device.

In order to fill up the tube with water the gauge is removed through its adaptor and installed manually and firmly after refill.

The tensiometer is installed into the root zone to create good contact with the ceramic tip and the soil water.

When the soil is drying the water tension gets bigger and vacuum power in the pipe rises and is displayed by the gauge.

Standard gauge displays the vacuum power in Centibars or Millibars.

Following rain or irrigation the tension gets lower.

By following the results continuously, the user can make the right decision when and how much to irrigate for best results.  

The moisture meter has to be tightly fit into the ground.

There should be adequate contact between the soil and the ceramic tip.

As I mentioned before, most of the vegetable’s roots are situated between 0-30 cm depth, so in order to get a better picture of the moisture in these depths, most of the farmers place two moisture meters, one at 10cm and the other at 30cm of depth.

The operating range is 0 -100 centibars, when it shows 0 it means that the soil is at full capacity of water, that is to say very wet, and 100cb is very dry.

In order to give the best conditions to the plants and make them prosper, it is recommended not to cross 17cb.

This means that when the clock shows 17cb, it’s time to open the water and irrigate. Crossing 17cb doesn’t mean that the plants will wilt, not at all, infect they may not wilt even at 70cb.

It only means that when crossing 17cb the plants start to feel stress and this can express in less yield.

Transmitter moisture meter

These are the same as the mechanical ones, but they can transmit the information to phone or online.

This is best for medium/big farm managers that are not at the field every day to look at the moisture meter.

Measuring runoff water

Soil moisture meter does not work when growing in other planting medium except soil.

In Perlite, Coconut Coir, Vermiculite, Rockwool, Peat moss, or sand the moisture meter cannot work simply because it doesn’t have a tight hold.

So, in these cases we will use a different method.

When growing in hydroponics, as pots, measure the quantity of the water that is getting into the pot from the drips, in the time of the irrigation.

After the irrigation, measure the quantity of the runoff.

The runoff is the excess liquid that comes out of the drain holes of the pot during irrigation and after.

The quantity of the runoff should be at least 10%-30% of the quantity of the irrigation water.

For example: if the pot is getting 1 liter (1/4 gallon) of water per day, the runoff water should be 200ml (6.4 Fl oz) per day.

The water of the runoff is not wasted, they are necessary for the washing of the planting media and to Prevent salt buildup.

Put one of the pots on a stand and measure the runoff water every day or every few days.

According to the measure you can decide to raise or to lower the water quantities.

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