The Pro Farmer

Irrigation- how should i know how much to irrigate

Every new farmer/gardner knows that lack of irrigation, will end up in the wilting of the plants and their death. The plant needs constant supply of water in order to live. The water are constantly arriving with high pressure (carrying minerals) from the soil through the roots, up the stem,(in the transport tissue 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 deficiandy, 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. Too frequent watering can cause rot in the root area and prevents the passage of oxygen to the roots. The plant uses its roots, among other things, to absorb the oxygen it needs for normal growth. 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 that some demage had already been done, and though the plants are not dead, their potencial is reduced and their fruit production will decline. Even one day without enough water can cause fruits demage,specialy 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 built 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 irrometer is pushed into the soil to the area where the roots are at. Usually the roots of vegetables are between 0-30cm (0-12 inches) deep, so 15cm (6 inches) is the right depth to stick the irrometer tip. Also, not right under the main stem, but 10cm (4 inches) to the side, is the right place to mesure the moisture.

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

Mobile irrometer are good for small scale gardens, flower 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 its wet. Also when it shows “moist”, you should irrigate until it shows wet. Only if you check and it shows “wet”, you should do nothing, dont add water, and wait more time, (whether its 1- 3 days or more) until checking again. This mobile moisture meter can not stay in the soil and must be removed after the check, because the probe might rust.

Permanent mechanical irrometer is more professional and is suitable for big gardens, or small, medium farms. The irrometer 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 irrometer is placed near. This way you dont have to measure 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 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 a 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 is rising and displayed by the gauge. Standard gauge displays the vacuum power in Centibars or Milibars . Following rain or irrigation the tension gets lower. By following the results continuously the user can get the right decision when and how much to irrigate for best results.  

The irrometer has to be tightly fit into the ground. There should be adequate contact between the soil and the ceramic tip.

As it was mentioned before, most of the vegetable’s roots are between 0-30cm depth, so in order to get a better picture of the moisture in these depths, most of the farmers place two irrometers, 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. IT means that when you see the clock shows 17cb, its time to open the water and irrigate. Crossing 17cb doesnt mean that the plants will wilt, not at all, infact they may not wilt even at 50cb. Its only means that when crossing 17cb the plants start to feel stress and this can express in less yield.

Transmitter irrometers 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 everyday to look at the irrometer.

When growing hydroponics like in pots, measure the quantity of water that is geting 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. The quantity of the runoff shoud be at least 20% from the quantity of the irrigation water. For example : if the pot is getting 1 liter of water per day, the runoff water should be 200ml per day. The water of the runoff are not wasted ,they are necessary for the washing of the media and to Prevent salt buildup.

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