Growers are increasingly interested in optimizing irrigation water delivery in pot and container production. For monitoring and control of the irrigation water delivery two different systems are currently in the market: Scales and moisture sensors. PPO Boomkwekerij investigated both systems for advantages and disadvantages.

Broere Beregening bv of Waddinxveen, NL has recently redeveloped the gutter scales that are commonly used in hydroponic greenhouse vegetable production for use in pot and container culture. This system continuously weighs and irrigates plants on the basis of changes in weight. PPO tested this system in 2007. The irrigation was controlled by the scales and the nutrients were supplied with Osmocote. From this test it was apparent that this system was capable of controlling irrigation water delivery. Many nursery operations (in the Netherlands) fertilize through the irrigation system to optimize nutrient levels. This makes automation of fertigation more complex. As a result a new trial was done in 2008 utilizing fertigation with soluble fertilizers.

A second system works through the use of moisture sensors. besides moisture levels, these also measure temperature and EC levels, all with one sensor. This opens up new possibilities for monitoring and control of both moisture and fertilizer levels.

Large savings in water and fertilizers

At PPO Boskoop, a trial was started with both systems in a realistic nursery setting. By tracking the amount and frequency of water delivery by day and field section, it was possible to calculate the amount of water that was used during the entire season.
While using controlled release fertilizers it was possible in this trial to reduce water use by 20 percent in comparison to fertigation. This was in part the result of eliminating the need for nutrients during rainy weather. In the treatments where the scales and the moisture sensors were controlling the irrigation frequency and quantity savings, of over 30 percent were noted. In addition, the control systems reduced nutrient use by 40 percent. All treatments had similar plant quality at the end of the growing season. Very little difference was observed between the trial treatments. Some had lower root development, some better branch development, but statistically all treatment resulted in plants.

Tracking moisture content and EC levels

From the weekly tests conducted with hand-held instruments, the observations were made that the treatments controlled by the scales or the moisture sensor did not have more stable moisture levels than the hand watered treatments. That was completely in contradiction with expectations. In part this was caused by rain, resulting in higher than desired moisture levels. And at the other end of the scale, the lower limit of the moisture levels could have been set higher. This would have resulted in more frequent irrigation and moisture levels in a narrower band.

It was also determined that it was not possible to control the EC levels in the growing medium with the EC meter. The EC level is very dependent on the moisture level of the medium. In dry media, the EC level is much higher than in well watered growing media, resulting in wrong decisions by the feeding equipment. This will require corrections to be made based on moisture levels and calibration according to the growing media used. This will be researched in a future trial.
Another point is that the irrigation computer requires sufficient inputs to utilize the readings from the scales and the moisture sensors in its decision making. In the trial the moisture level limits had to be changed periodically to ensure sufficient quantities of nutrients with the irrigation water.

Both systems have perspective

Both the moisture sensors and the scales are, according to the trials, sufficiently accurate to be used commercially. Both react to changes in moisture levels quickly and create a much better insight in water requirements and delivery for a crop. A graphing function on the control computer is advisable as this will give a much better picture of the daily water requirements than a twice per day hand reading will. It is however, difficult to translate the measurements into to real moisture levels and EC values. The scales also react to the growth of the plants, and therefore need to adjusted on a regular basis. Moisture sensors on the other hand, measure moisture levels but need to be corrected for the actual crop (plants type, growing medium and pot size). Measuring the correct EC in the growing medium moisture is less straight forward than expected, as moisture levels and EC value influence each other continuously.

To work with each of these systems it is necessary to calibrate them for each crop situation. This can be done by measuring moisture levels or weight of wet potting soil and using this as the upper limit. The lower limit can be determined by measuring moisture content or weight of dry potting soil. These limits can then be used with either the scales or the moisture sensor based system to control moisture levels.

Variation in moisture levels

From the moisture measurements it has also become obvious that large differences can exist between individual pots. Also apparent was the correlation between the level of variation and the size of the pots and lower moisture levels. The bigger the pot the larger the variation between pots. And the dryer the potting soil the larger the variations are as well. To get accurate control the scales and/or the moisture sensors need to be placed in a location that is a good reflection of the average in a block. This is definitely not at the edge of the block. Position in relation to the sprinkler is also important. And the sensor or scale should be in or under a pot with a plant of average size. Accuracy will get better if the results from more than one measuring device are averaged for each production block. From tests in a commercial nursery, the results indicated optimum results from the use of the average measurements from three to four sensors per irrigation section. Both systems require regular checking of the measurements (once per week) to retain accuracy. That is an improvement over the current method of assessing water requirements on a daily basis (or sometimes more frequently in North America). For optimum irrigation levels it is obviously important to group crops (and ages) with similar water requirements in the same watering block.

Pieter van Dalfsen

Van Dalfsen is a researcher with PPO Boomkwekerij, (0252) 46 21 21, This e-mail address is being protected from spambots. You need JavaScript enabled to view it
This project was financed by Productschap Tuinbouw and executed in co-operation with Broere Beregening bv.

Experimental setup of gutter scales and moisture sensors

In 2008, the container production research centre of PPO Boskoop was the scene of four different research trails in the control of irrigation in container plant production.

1. With time clock and Osmocote fertilisation (control treatment)
2. With time clock and fertigation (control treatment)
3. With gutter scale system and fertigation (two repetitions)
4. With moisture sensor and fertigation (two repetitions)

In total six different production blocks were set up with each 250 plants of Viburnum tinus. A relatively light medium was used as potting soil to ensure quick drying of the pots. Treatment 1 and 2 were watered based on the observations of the technician. Treatment 3 and 4 received water according to measurements. Upper and lower limits were determined to determine under what circumstances watering would happen.

During the trials, the computer package used for the watering control (Synopta) collected a large amount of information. This included the exact times of watering, the amount of water per occurrence, the EC value of the irrigation water, the values of transmitted by the scales, the values transmitted by the moisture sensors (moisture, EC and Temperature) and meteorological data (temperature, precipitation and wind speed). All this information was measured every five minutes. Afterwards all the data were analysed. During the trails the moisture levels and EC were measured manually in at least 10 pots per block on a weekly basis.

Practical Experience

The project as executed at PPO Boskoop was also replicated at a number of commercial test facilities. From these tests it became apparent that the scale-based system is preferable for production in large pots (more than 5 litres). This is due to the fact that moisture sensors only measure a small portion of the pot that they are inserted in, whereas scales provide an average over at least five pots.

It also became apparent that not every irrigation zone in a nursery needs to be equipped with a control system. With the system, in conjunction with the experience of the grower, one zone can be the indicator for several irrigation zones. If a grower knows that a crop in zone A will thrive with 80 per cent of the water of the crop in zone B, these variables can be stored in the computer for use to control the irrigation cycles. Complete automation of the irrigation control in an entire nursery is, at the moment, realistically a step too far forward. Control from the grower remains necessary. But a scale system or moisture sensors can be an important aid. Drying out is prevented, as the grower is either given a warning signal or the control system automatically adds a watering cycle.

Pros and cons of the scale system

Positives

• Good average measurements over multiple pots
• Also suitable for larger pots (5 litres or over)

Negatives

• Measurements need to be translated to moisture levels in the pot. This has to be calibrated for each pot size and crop. Corrections are needed throughout the growing cycle to correct for crop growth.
• Precipitation temporarily increases the weight of the crop plants (water hanging on foliage and branches) and as a result influences the measurements.
• The drainage of the area under the scale has be adjusted to mimic the rest of the growing area
• In the PPO trials, the plants on the scale were consistently wetter than the surrounding plants
• In the PPO trial, the stability of the measurement from the scale was affected by temperature
• Results are difficult to compare to previous years as a result of changing crops and pot sizes.
• EC correction is not yet programmable
• Interferes with the logistics of potting and shipping practices

Pros and cons of moisture sensors

Positives

• Direct measurement of the desired parameter (moisture, EC and temperature)
• Drainage substrate of the production area does not affect sensor readings
• Stable measurements
• Wireless systems are available. Sensors are easy to move

Negatives

• Multiple sensors required per irrigation zone
• EC correction not yet programmable
• Less suited for larger pots as a result of very small measurement areas