How To Install Tensiometers In Horticulture Irrigation

Tensiometers are a means of measuring fluctuations of soil moisture. Acting in a similar manner to a barometer, the soil tension is measured by the mercury column. The scale measured by the mercury column is in centibars- a low reading e.g. 14-16 centibars indicating a low soil tension and a higher reading e.g. 20-25 centibars indicating high soil tension.

( Picture courtesy of Mottes Bulletin)

N .B. The achievable lower and upper readings will vary from field to filed in accordance with soil type and altitude.

Tensiometers are an important tool especially where there is the ability to irrigate accurately and efficiently and where there is some control over the environment e.g. in a greenhouse or a tunnel.

In the field it can work well when no other source of water like rain that can disturb the irrigation pattern.

With a clear understanding of the nature of soil (physical properties) , basic plant physiology coupled with accurate recording of the tensiometer, it is possible to maintain at all times a stress free root zone and this control giving a better chance of a healthy plants and higher productivity from the plant.

Aims and Objectives

Objectives

To maintain at all times a stress free environment in the entire root zone. In order to accomplish this, several factors are vital.

Aims

When watering one should always aim to achieve that;

i) There is always enough available water in the soil for plant uptake.

ii) There is not too much water in the soil as this reduces the amount of oxygen required for the basic osmosis and cation exchange on the root surface and there is a high likelihood of a conversion of Nitrates to Nitrites, which if in excess will be damaging to the plants. Thus aeration of the soil profile is vital.

It should be noted that there are several different methods of using tensiometers. The method described below has been used for many years and has been very successful on most soil types

Which horticulture crops need tensiometers?

Any crop that uses drip irrigation.

Crops like cut flower roses, tomatoes, peppers, cucumbers, strawberries etc. need the use of tensiometers as these crops are sensitive to under and or over watering.

Where in a greenhouse or tunnel should the tensiometer be installed??

While installing a tensiometer, it is important that the correct position in a greenhouse, tunnel and or the field is chosen.

The place chosen should be a representative of the whole area which the tensiometer is to monitor.

The tensiometer should not be positioned beneath a greenhouse gutter of any point that has access to free water like where there is an emitter or nozzle leak. Preferably it should be in a center bed in the center of the field.

It is important too, that since an indication of moisture levels in the root zone at trying to be obtained, that tensiometers are placed within that root zone- e.g. with speedlings or seedling which have a limited root zone- tensiometers placed in the middle of the bed will give no bearing on the soil moisture levels around the root zone. The depth of placement is also important.

Experience has shown that major root activity for most crops is at between 15 cm and 40 cm depth below the soil surface.

Thus tensiometers should be placed at 20 cm depth and 40 cm depth. Remember to place two tensiometers per station.

When should the tensiometers be placed??

In greenhouses and tunnels, the first irrigation and be by the use of misters or rose and hose to ensure that the top soil does not form a cap. But, as soon as the main drip irrigation system takes over then tensiometers will be placed in position.

In the field, once the crop has been given field capacity of water and it starts draining away, then tensiometers are put in position.

Immediately after installation, daily readings should be taken and recorded. Since management decisions on the time to irrigate are to be made with a tensiometer readings, it is important that readings are taken prior to irrigating i.e. first thing in the morning. It is also imperative that readings are taken at the same time every day. Readings should be recorded in the following manner:

N/B – It is normal for the readings to be haphazard for the first 10 days after installation, however it is still important that readings be taken from the start so that a pattern may be found as soon as possible.

General Principle

Before one can begin to rely on the tensiometer for irrigation completely, two vital points need to be ascertained: the break point and the saturation point.

The process is best defined on the graph as follows:-

Behavior Of Tensiometers

In this example, a reading of 14 is taken as the saturation point, whereby a reading of below 14 is not achievable, regardless of how much extra water is applied, the daily readings would gradually climb for example up to point B, or a reading of 22. If still no water is applied, there would suddenly be a large `jump` in the tensiometer reading, of three or more points.

In this example, from 22 to 30 in one day. Thus the break point is said to be at point B or 22. The soil water tension is increased drastically after that point, which if allowed to happen would mean that there is not enough water resulting in stress on the plants.

It is necessary to try to find this break point as soon as possible. This is done by pushing the interval between watering’s further apart and watching the tensiometer carefully. Once the break point is established, water should be applied just prior to or when that break point is reached. Enough water should be applied at this point to bring the tensiometer back down to the low point or saturation point.

Tensiometer readings

In the above example, it is clear that the beak point is in the region of 20. Thus as soon as the top tensiometer reaches 20, water should be applied.

It is also clear however that 40m3 was not enough water, since it only brought the reading down to 17 the following day, when it is known that a lower point of 15 is achievable.

Thus the water is increased in the following set to try and achieve the lowest possible point.

( Picture courtesy of Mottes Bulletin)

In this case 45m3 brought it back down to 15 centibars. It would be advisable to apply 50m3 in the following irrigation set to see if the low point drops even further. If it does not go below 15, then 45 cubic meters is obviously the correct amount of water.

The Use Of Two Tensiometers

The establishment of the break point and the saturation points should be based on the upper 20cm tensiometer, since it is early on in the life of the plant that this is done and the root depth will not be much more than 20 cm.

Thereafter, the lower tensiometer will become an important management tool. Ideally, both tensiometers should move together, which would indicate that there is even moisture of soil moisture or soil tension throughout the profile of the rooting zone.

If however, the bottom tensiometer dries out quicker than the top tensiometer, it is necessary to get the water down to the bottom of the profile and therefore an application of at least one and a half times the normal amount of water will be necessary for the bottom profile to be wetted.

If on the hand the top tensiometer is drying out quicker than the bottom tensiometer, then only half the normal application is needed.

N/B -This will only be necessary for one application before reverting back to normal watering- the tensiometers should come together a couple of days thereafter.

Conclusion

Tensiometers are an invaluable tool for precise irrigation management in horticulture, especially for sensitive crops like roses, tomatoes, and strawberries. By accurately measuring soil moisture and understanding the break point and saturation point, you can ensure your plants receive the optimal amount of water, leading to healthier growth and higher yields.

Implementing tensiometers requires careful consideration of placement and regular monitoring, but the benefits far outweigh the initial effort. With the ability to maintain a stress-free root zone and improve water efficiency, tensiometers can revolutionize your irrigation practices.

Remember, the key to successful use of tensiometers lies in consistent readings and adjustments based on those readings. As you become more familiar with this tool, you’ll find it easier to create a thriving environment for your crops.

Stay tuned for our next post, where we’ll delve deeper into advanced irrigation strategies , tensiometer maintenance and tools to further enhance your horticultural success.

Happy gardening!!!!!!!!!

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