Special Series: In the rootzone #7
November 2, 2011 By Andrew Lee
Understanding the effect of early stop time and volume/radiation on EC behaviour
|Figure 1.0: Effect of early stop time on substrate EC. Example illustrates a cluster tomato crop, but whichever crop you grow, the principles for EC control with an adjusted stop time are the same.
During the spring and summer months, light levels begin to increase rapidly as do the outside temperatures.
Most likely, ventilators will be opened, subject to restriction with adequate P-band settings and outside weather influences to prevent cold air entering the greenhouse. Consequently, water uptake by the crop will increase but the weather can also fluctuate massively at this time of year. However, maintaining a stable substrate EC is not difficult so long as correct settings are implemented. The standard graphic that depicts WC, EC and radiation is a useful tool to use.
In order to maintain the correct generative/vegetative balance in the crop as the first harvests are taken and the fruit load is stabilized, it is important to maintain a stable rootzone environment – most importantly a stable substrate EC. As such, irrigation must now be matched to the light levels and enough drain has to be achieved over 24 hours, especially on bright days to level EC and balance the substrate nutrition.
During this time of year, almost all growers tend to become nervous of the substrate EC. In some circumstances, a first reaction may be to decrease the dripping EC as soon as the days become brighter, while keeping the stop time fixed so as not to give too much water on the darker days.
Figure 1.0 illustrates two irrigation strategies. The start time is fixed for both as is the dripping EC at 3.2mS. However, in this trial researchers have structured one strategy to stop at a fixed time setting of 14:30 hrs (light blue WC curve), allowing the other to continue irrigating beyond this time but based only on light intensity setting (dark blue WC curve). The result, in the latter case, is a stable substrate EC (orange line), as sufficient irrigation is supplied in line with crop activity.
|Figure 2.0: Optimal start and stop times and irrigation gift maintain stable rootzone conditions in variable weather. Example illustrates a cluster tomato crop, but whichever crop you grow, the principles for EC control with an adjusted stop time are the same.
Focus now on the stop time to help you control substrate EC, particularly on brighter days, as at 14:30 hrs the light intensity can still be high.
It is best practice to let the irrigation run on a light setting until 250-200 W/m2 outside light remains to keep the EC stable as illustrated from practice in Figure 2.0. In this example, taken from a commercial greenhouse, the stop time can be seen to vary by +1.5 hrs on a bright versus darker day, yet EC and the decrease in WC overnight remain stable despite the widely fluctuating light levels.
There should be no reason to lower the dripping EC too far, i.e., for cluster tomatoes less than 3.0mS with sufficient irrigation in line with crop activity towards the end of the day. With a targeted stop time on a bright day, EC can remain 3.0-3.5mS to help the balance in the crop.
Stopping too early on bright days will result in an uncontrolled increase in the substrate EC. Lowering the drip EC too far during the warmer months can have a negative impact on plant development, making it harder to achieve the correct balance in the crop for maximum summer production.
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