|Constant monitoring keeps the crop on schedule.
In this article, I will look more closely at the thought processes you should consider when setting up an irrigation strategy on a daily basis. Using examples for a mature crop growing in late spring/summer, I will illustrate how measuring tools such as the Water Content Meter (WCM) and the graphics generated by the climate computer can help you make informed decisions and avoid potentially costly mistakes.
WHAT DOES A WATER CONTENT METER (WCM) MEASURE?
■ The WCM measures directly the absolute WC, EC and temperature of the rock wool slab (Figure 1). The sensor pins are placed directly in the slab about 10 centimetres from the propagation block in the direction of the drain flow. They measure the average WC and EC over the 7.5 cm slab height. The information that the WCM provides is invaluable to the grower when it comes to fine-tuning the irrigation strategy, in this way costly mistakes can be avoided.
Before going into too much detail on setting up an irrigation strategy, it is prudent to describe the key features of the graphic that is generated by the WCM. During a 24-hour period, there are three distinct phases that occur in the root zone environment as a direct response to irrigation (Figure 1).
PHASE 1: The time at which the plants start to transpire can be seen by a change in slope of the WC line shortly after sunrise. Phase 1 starts from first irrigation to the point of first drain. It is characterized by a step-like increase in substrate water content (WC). EC increases in this period as salts precipitated out of solution overnight are re-dissolved.
PHASE 2: Phase 2 is characterized by a stable WC and decreasing substrate EC as the point of drain is realized. This period normally transcends the point of highest solar radiation.
PHASE 3: Phase 3 from last irrigation in Day 1 to first irrigation in Day 2 is characterized by decreasing WC and increasing EC.
|View larger Figure 1
SETTING UP AN IRRIGATION STRATEGY
■ For simplicity, let us take typical strategies and thought processes that would be applicable for a developed crop in spring/summer conditions, i.e., one that is in growing Phase 4 or 5 of the 6-Phase model. Remember, the objectives in these phases are to realize stable conditions (WC and EC) in the rootzone environment to facilitate controlled and uniform regrowth of the crop following the first harvests and to maintain maximum production potential and fruit quality in the summer (Greenhouse Canada, June 2010).
The golden rule is always transpiration then irrigation. This helps avoid all kinds of fruit quality issues such as uneven colour, radial cracking and split fruits.
Using solely “time after sunrise” to start the irrigation day is normally fine if the outside weather conditions remain constant from day to day. However, with fluctuating weather conditions it may be too late for a bright day and too soon on a dark day, leading to instability in slab WC and EC. This can be demonstrated in Figure 2. The graphics are taken from a climate computer and depict the WC (dark blue), EC (red) and global radiation (light blue) over a six-day period.
The start time has been fixed at 08:30 hours, about two hours after sunrise. It can be seen that on the three days with “low radiation,” (Figure 2a) slab EC remains stable at approximately 3.3 mS. However, with three consecutive days of “high radiation,” EC rises up to 4 mS as timing of first drain occurs too late (Figure 2b). If the strategy (i.e., start time) is not adjusted for the bright days the end result would be that EC would continue to rise, potentially resulting in blossom end-rot (BER) and loss of revenue to the grower.
|View Larger Figure 2|
| View Larger Table 3
There is no maximum rest time activated in this period. In practice, this prevents the computer from irrigating on maximum rest setting at 07:00 hours. So on a dark day, we avoid potential fruit quality problems of uneven colour, radial cracking and split fruits.
Period 2: I selected 08:30 hours to start this period because, based on the information in Figures 2a and 2b, this was “on time” for a darker day. A maximum rest time for this period has been selected so the computer will irrigate on a “maximum rest time” trigger at 08:30 hours and thereafter every 40 minutes if a light sum start is not given.
I have selected 320 ml/m2 irrigation volume in these periods to be applied every 80 J/cm2 (4 ml/J). The thought process is simple. Assuming in this situation I have 8 l/m2 substrate volume in the greenhouse, a realized decrease WC of 10 per cent overnight (Figures 2a and 2b) equates to 800 ml/m2 loss in WC due to the process of active water uptake by the plant. In this example, let’s also assume that by 10:30 hours, 400 J/cm2 light has been accumulated, which is possible on bright days; check it on your computer. With water uptake from transpiration alone of 2 ml/J this means that I should apply an additional 800 ml/m2 (400 J/cm2 x 2 ml). So in order to bring the slab back to the same day level WC, I need to apply 800 ml/m2 to account for the loss overnight plus an additional 800 ml/m2 to account for transpiration up to 10:30 hours, i.e., 1.6 l/m2 by 400 J/cm2, the equivalent of 4 ml/J. The maximum rest time of 40 minutes in this period prevents too much water being given on a dark day.
THE TIMING OF THE FIRST DRAIN
■ We can also set targets for when we would like to see first drain. This is normally two to three hours after the first irrigation is applied.
Period 3: Drain is required to stabilize and refresh EC to the required day level. It is important that in spring and summer this is achieved around 400 J/cm2 or 600 W/m2 (Table 6). It is for this reason that I have timed Period 3 to coincide with my expectation of first drain. From now into the afternoon, it is important that EC remains under control when radiation is at its highest and stable between consecutive days.
| View Larger Table 6
WHAT HAPPENS IF DRAIN OCCURS TOO LATE?
■ If drain occurs too late on a sunny day in this growing phase, EC will continue to rise. In the example shown in Figure 3, it can be seen that although the first irrigation is given around 08:00 hours (+2.0 hours sunrise), EC is not reduced until 12:00 hrs. The result is that EC increases significantly over 24 hours. To correct this situation, the grower should apply larger irrigation volumes in the morning. Remember, if everything is working correctly, the time of drain and EC refreshment in the substrate should coincide.
| View Larger Figure 3
From drain at 10:30 hours, the goal now is to apply irrigation in line with transpiration. The general rule of thumb is 3 ml/J, 2 ml for transpiration and 1 ml for drain, assuming 30 per cent drain over 24 hours. For this reason I have elected to supply 210 ml/m2 every 70 J/cm2 (Table 6). However, with low outside humidity in combination with high temperatures this figure can be higher (3.5-4 ml/J). Generally, the irrigation volumes in this period of the day should be smaller but more frequent than in the morning. This prevents “false drain,” a phrase I use to describe the situation where slab WC falls and EC rises as a result of high drain per cent per cycle. Figure 4 illustrates an example in practice.
| View Larger Figure 4
Period 4: Period 4 (Table 6) starts at 15:50 hours. By now EC should be stable and the substrate nutrition refreshed, and with outside radiation decreasing, I have elected to increase the joule sum to 95 J/cm2 applying the same volumes of water just to keep the 24-hour drain per cent from being too high. Again, remember this is just another tool you can use; it is not absolutely necessary.
■ A simplistic approach would be to stop the irrigation one to two hours before sunset. Using solely “time before sunset” to stop the irrigation day is normally fine if the outside weather conditions remain constant from day to day. However, with fluctuating weather conditions, especially in spring, it may be too soon for a bright day leading to loss in fruit weight or too late on a dark day leading to a degradation of root quality. An optimized stop time using the climate computer is demonstrated in Figure 5 using the settings in Period 5 and 6 (Table 8).
| View Larger Figure 5
Period 5: Period 5 starts at 17:30 hours Table 8). In the computer settings, I have again de-selected the maximum rest time and told the computer that it should only irrigate if two start conditions are achieved, i.e., it must be over 200 W/m2 outside radiation and 85 J/cm2 must have been accumulated since the last irrigation for a start to be given. This avoids irrigating too late in the day. The outcome of this strategy can be seen in Figure 5. Radiation is very variable for each of the three days, but with the adjusted stop time in relation to plant activity, the decrease in WC overnight remains constant. This helps maintain the generative/vegetative balance in the crop, fruit size and root quality.
| View Larger Table 8
Period 6: Period 6 starts at 21:00 hours. Maximum and minimum rest times have been selected for 24 hours; in practice this means that no irrigation will be given until Period 1 or Period 2 the following day. Remember, night irrigation sessions are the exception rather than norm. Night sessions should only be given in respect to plant activity more then likely coinciding with the continued use of pad and fan or fogging systems to cool the greenhouse overnight.
■ This article has highlighted the thought process for optimum root zone steering over a 24-hour period in line with the 6-Phase model. It highlights the need for a substrate that can re-saturate quickly and at the same time offers good EC replacement and refreshment.
There are many ways to set up the ideal approach in the climate computer – the previous tables are only intended as examples. Whatever approach you take, make standardized graphics (WC, EC global radiation) on the climate computer. These will provide you with the right management information on which you can base your decisions. Focus in over one or two days to see the detail, focus out over seven or 10 days to see the trends. Look for the key triggers in the decision-making process should changes be required:
- Transpiration then irrigation.
- Drain by 400 J/cm2 or 600 W/m2.
- First drain of the day in line with EC refreshment.
- EC refreshed and stable in line with global radiation during peak solar hours.
- Applied irrigation in line with radiation sum 3 ml/J in the afternoon to ensure WC remains stable and EC under control.
- Stop in relation to plant activity for a stable decrease in WC overnight. ■
Andrew Lee works for Grodan BV as Business Support Manager for North America and Export Markets. He is a PhD graduate from the University of London, England.