Irrigation De-Mistified

Exclusives from Urban Ag News

By Godfrey Dol

It is said that managing irrigation is a bit like being married. No matter how hard you try, you never do it right. But it is because we try that we succeed in having a happy marriage.

Plants need water and fertilizer to grow. In the hi-tech glasshouse industry, we apply both at the same time. A fertilizer injection system provides the fertilizer, and the drip irrigation system distributes the water evenly to the plants. How hard can it be? When I was a student studying Chemistry, I was never interested in Agriculture. You plant a seed, add water, fertilizer, sunshine, and a little while later, you harvest. How hard can this be? A lot harder than I could have ever imagined.

We base irrigation strategies on the measurements from the day before. Then we look at today’s weather and try to adapt the irrigation strategy to suit. As a result, we are always one step behind. It is why it is so hard to get the irrigation right every day. 

The Tools We Need

There are some basic tools that we need to collect the right information. A scale or moisture content sensor is important. If they are not available, a manual drain station such as pictured below is a must. Even if a scale or moisture sensor is available, a manual drain station is still recommended. Use at least one slab and collect all the days drain water in a bottle or bucket. Be sure also to measure the volume irrigated per dripper. It is a good check to know if the volume of irrigation water calculated by the climate computer is the same as the measured dripper volume.

A dirty river

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Figure 1; Manual Drain Statio

The key components we want to measure are the %dry-down, the EC in the drain, and the timing of the first drain. The dry-down is the loss of moisture from maximum saturation during the day to a minimum just before the first irrigation in the morning. The dry-down tells us if we are steering the plant into a vegetative or generative direction. The EC of the drain indicates whether we give the right amount of water. The timing of the first drain tells us if we have the growing medium hydrated enough so the plant can transpire at a maximum rate.

Now that we have the right tools in place let’s look at what are the most important rules in irrigation. 

  1. A healthy, mature tomato plant uses approximately 1.7 ml/Joule/M2

Light determines about 80-90% of the water uptake. Therefore, triggering irrigation based on light sum deserves preference. The Humidity Deficit of the glasshouse air determines the remaining 10-20%of the water uptake. Many growers open the vents in the spring so that plants get used to low humidity. It is a fallacy because a plant that can handle a high light intensity does not need to take up much more water to deal with low humidity. A healthy root system is the most important.

  1. The drain percentage emerges from the desire to maintain a certain EC.

I get often asked how much drain should be achieved. It is an irrelevant question. The desired drain EC is the key parameter; the percentage drain is how the EC is controlled. If the desired drain EC is 4.5 and it reaches 5.0, the best way to lower the EC is through irrigating more. The best time to irrigate more is through the middle of the day. If the EC in the drain is too high, increase the frequency of the irrigation between 11 am and 2 pm. If the drain EC is too low, decrease the frequency of the irrigation between 11 am and 2 pm. Some growers use light intensity to decrease the Drip EC during the middle of the day. I’m not a great fan of this, as I don’t think it helps the plants transpire more easily, and it creates imbalances in the nutrient concentrations, especially when recirculating. 

The following guidelines can be used to generate a drain that maintains a steady EC, based on a drip EC of 3.0 and a drain target EC of 4.5.

              Less than 500 Joules;       0-10% drain

              500-1000 Joules;              10-20% drain

              1000-1500 Joules;            20-30% drain

              More than 1500 Joules;   30-40% drain

The important message here is that the drain percentage itself is not what should be         targeted. The EC in the drain is the best indication of whether the plant gets enough water. The values in the table above change considerably if the grower targets a higher drain EC. 

Decreasing the EC by irrigating more on a dark day is not a good strategy. On a dark day, we must reduce the amount of water significantly. The trigger for irrigation should still be light. It keeps the growing media dry, and the roots are encouraged to “find” water elsewhere.

  1. The desired dry-down determines the timing of the last irrigation.

Generally, a dry-down of 10-15% is recommended. A 10% dry-down is a vegetative action, whereas a 15% dry-down is a generative action.  If the desired dry down is 10% and the sensors show 9%, the timing of the last irrigation needs to be earlier and vice versa. I prefer to be careful with the last irrigation. If the weather becomes cloudy during the late afternoon, the desired dry-down cannot be achieved. As it is impossible to take one irrigation away, it is better to be careful, and if it the dry-down is looking like it becomes too large, night irrigations are always an option.

In days where the light sum is only a couple of hundred Joules, there is no need to achieve drain. If we consider a day of 300 Joules, then the amount of water the plant uses is 300 Joules x 1.7ml/Joule/M2= 510 ml/M2. Most standard slabs contain 10-15 liters of water, so there is no danger of the plant running out of water. If we irrigate twice at 0.25 liters/M2, the slab is saturated. It means that we have driven most of the oxygen out of the slab, which creates an ideal environment for soil diseases. One or no irrigation is a better option. It also means that during dark days, the saturation point on the moisture content meter is never achieved. As a result, the difference between the maximum and minimum moisture content is less. On these days, the dry down should be measured as the difference between the minimum moisture content of that day and the maximum saturation point of previous days. 

Roots are like muscles; if you don’t use them, you lose them. Three days of less than 500 Joules per day results in dying off of the root system. It is important to keep the above calculation in mind. Saturating the growing medium on these dark days results in Pythium and root dieback. In the graph below, you can see the irrigation strategy of a sunny day followed by a cloudy day. In this case, a scale was used. The maximum weight was 36.6 Kg (dark blue line). The minimum weight the next morning was 33.0 kg. The dry-down was (36.6-33.0)/36.6 = 9.8%. The next days’ light sum was 432 Joules. The maximum weight increased to 35.4 Kg, so the saturation point was not achieved. No drain was achieved, which allowed for oxygen to remain in the root zone. The last irrigation was at 3 pm, and the weight decreased to 33.14 Kg by the next morning. It means that the timing of the last irrigation was perfect. It is a good way to manage the irrigation on a cloudy day. Also, note that the EC (light blue line) is rising. It means that during the next sunny day, the irrigation frequency has to be increased during the middle of the day to decrease the EC.

  1. The first drain should be achieved at 500 watts or 1.2-1.8 ltr/M2

During periods of high transpiration, we want to make it easy for the plant to take up water. The growing media should be saturated, and the EC becomes lower as the drain increases. Therefore, it is important to have the first drain at 500 watts. This rule of thumb can be applied regardless of where in the world you are. The second rule that can be applied is that the first drain should be achieved at 1.2-1.8 ltr/M2. This value depends on the desired dry down. If a dry down of 10% is required, the drain should come at 1.25 liters per square meter. At a dry down of 15%, the first drain should come at 1.8 liters per square meter. If the drain starts before that, the frequency in the morning is too fast.

During low or no radiation, we want to make it more difficult for the plant to take up water, so the roots are encouraged to spread through the growing media, looking for moisture. It is why we want to achieve a minimum dry down. In general, the plant sends assimilates to the warmest part of the plant. During the late afternoon, the growing media often is the warmest part. Having enough air in the growing medium at a time when the assimilates are directed to the roots results in optimum root growth.

The above strategies help implement the four key parameters for irrigation; a start time, drain time, the total amount of irrigation water, and stop time. By following these rules of thumb, the grower creates an environment for the roots where they are kept healthy and aerated so that they can perform at maximum transpiration when required.

Figure 2 shows a typical rootzone temperature, moisture content, and EC graph. In the morning, the moisture content is quickly brought up to the saturation level, and the drain starts at the 8th irrigation (cycle size 0.2 ltr/m2). The EC decreases quickly during the day and starts rising again after the last irrigation. At the same time, the moisture level decreases, meaning air is entering the root zone. Moisture measurement within the slab has greatly enhanced the understanding of the irrigation requirements of plants. It is important to remember that the measurement needs to be representative of the whole irrigation zone. One measurement per hectare doesn’t seem enough to represent one hectare. However, using one measurement for multiple irrigation zones in the same glasshouse for the same variety gives a better statistical average. It is especially true if the electronic data is backed up by a manual drain station. It is recommended to perform manual EC measurements of the surrounding growing media to verify that the single points of measurement are valid representations of the irrigation zone. It is also important to make sure that the slabs of growing media contain a representative number of plants. When the planting density is increased, it can happen that the measured slab does not have the right amount of plants. Equally important is that any equipment that is used is maintained to a proper standard. It includes the EC and pH meter.

While manufacturers of moisture meters claim that their meters are compensated for temperature, the reality is often different. In the graph above, it appears that the moisture content is increasing. That is not true. The increase in water content is caused by an increase in temperature and EC over the three days of measurement. It complicates the interpretation of the data. However, the most important information from the moisture content meter is the difference between minimum and maximum. This difference is less prone to fluctuations. 

If no moisture content measurement is available, we can still get the dry-down information from manual drain stations. By physically checking the manual drain stations for the first drain every day, we can backward calculate the dry-down. For instance, if we know at which irrigation cycle the first drain arrived, we know the volume of water added to the slab at that time. If we know the saturation weight of the slab, we can calculate the dry down that was achieved. For instance, if the drain arrived at the 5th cycle and we give 100 ml per cycle, there are four drippers per slab, and the slab saturation volume is 15 Liters, we can calculate that it took 5 x 4 x 100 = 2,000 milliliters to re-saturate the growing medium. We should allow for the water uptake for the plant during that time as well. If the first drain came after 200 Joules, then we can calculate the water uptake from the plant as 200 x 1.7 ml/Joule/M2 = 340 ml/M2. If the dripper density is 2,5 plants/M2, the four plants on one slab have taken up 340 x 4/2.5 = 544 ml per slab. The real dry down is (2,000-544)/15000 = 9.7%. It seems like a lengthy calculation, but the only variables are the number of irrigations before the first drain, and how many joules have passed at the first drain. If the grower in the above example wants to maintain a 10% dry-down, he only has to make sure that the first drain comes at the 5th irrigation at 200 joules light sum. If he wants to increase dry down to 15%, the first drain must arrive at the 7th irrigation at 200 joules.

Different Strategies for Different Stages of the Crop

A tomato crop has 5 distinct periods that require a different approach to irrigation. I discuss the best strategies for each period, for a crop of tomatoes grown in a high light climate.


In high light climates, young plants tend to grow vegetative. The block can be dried down to 40-50% of the saturation weight when the roots emerge from the bottom. Usually, a 10 x 10 x 7.5 cm block weighs about 500 grams when it is saturated. It means that the block can be dried down to 250-300 grams. The irrigation must be given in the morning so that the blocks are not too wet at night. When the plants suck water out of the blocks, air replaces it, providing the roots with necessary oxygen. The EC in the block can run up to 12 applying this practice, which makes the plant more generative and encourages the roots to fill the block. It also prevents the long, stringy roots that form when too much water is applied, and puddles form on the surface.

Planting to Flower

Most growers in warm climates do not have access to nurseries that can deliver a plant at flowering stage. When the plants arrive from the propagator and the plants are still small, the grower needs to complete the propagation cycle. When there are conditions of high light and low humidity, it causes the plant to make large leaves to cool itself. There are no fruits on the plant that function as a sink for assimilates. If allowed to grow without intervention, this plant grows very vegetative. The grower must give generative impulses that force the plant to flower and fruit. One well-known aspect of fruiting plant species is that threatening environmental conditions causes the plant to act reproductively. Most growers make use of this fact by not allowing the roots of the plant to grow into the growing media. An example of this is shown in Figure 8.10 and 8.11. The green sheet prevents the roots from going from the block into the slab with more volume.

Fig 3; Controlling the Plant

By doing this, the volume of the growing media is restricted. Even a little plant can suck the moisture out of the block, which gives the grower control over moisture and salt content of the root zone. Drying down the weight of the block to 50% of its saturation weight is a generative impulse through which the plant starts producing hormones that steer it towards reproduction. 

The irrigation must be carefully controlled. Weighing the blocks multiple times per day allows the grower to dry the blocks down to 50% of their saturation weight. The plants should not go into the night with a wet growing medium. If the plants need irrigation, irrigate in the morning when the temperature is still cold. Check those areas of the glasshouse that are warmer or receive more light (wall or gable rows) are not drying out (and wilting) sooner. It is preferred to have the irrigation hoses under the gutter so that the sun does not have a chance to heat the irrigation water.

The EC in the drip should be 4.0. When the blocks are irrigated, the water system can be stopped when the first drain is visible. If it is evident that there is too much variance in EC between blocks, more drain needs to be realized. The EC in the block can rise to EC = 10-16. It creates a generative impulse.

The trend is to use less nitrogen in the fertilizer formula at this time to stop encouraging the plant to become vegetative. It seems low nitrogen also helps in increasing Brix of the fruit.

Flower to Harvest

Once the flowers on the first truss are open, the plastic sheet is removed, allowing the roots to grow into the larger volume of root space. The longer this action can be postponed, the better it is for the generativity of the plant. However, not allowing the plants to root into the slab makes them unstable, and they might fall over, even when tied up to the string. By the time the roots are allowed into the slab, the EC in the block is 10-16. The slab must be filled with EC=4 irrigation water. This difference in EC forces the roots to go into the slab very quickly. A nice truss such as shown in Figure 8.9 is a sign that the plants are generative. Notice the extreme curl in this cherry truss. If the irrigation is given too late in the afternoon, or the growing media is kept too wet, the trusses become more elongated and stick up, as shown in figure 4.

Figure 4; Nice curl in the truss because of generative steering with irrigation

Now the plants have access to the almost unlimited water supply in the bag. It is a dangerous situation as the availability of so much water creates a vegetative impulse. To reduce this effect, apply the following rule of thumb; restrict the irrigation after planting and reduce the moisture content in the slab by 1-2% per day. It means that after 10 days, the water uptake of the plants creates a dry down of 15%. The decrease in water content must be realized with zero drain. After two to three weeks, the drain starts, and the EC in the drain from the slab is at 6-10. From this time on, reduce the drain EC by 0.5-1.0 EC unit per week. The EC should be at EC=4.5 by the time harvesting commences. Bring the drip EC down gradually to EC = 3.0.

Three weeks after allowing the roots into the slab, the dry down is 20%, and it should remain above 20% until one week before harvesting or earlier if the plants show a more generative appearance. Achieve this by stopping early with the irrigation and giving more water during the day between start and stop times to control the EC. Late irrigations have an extraordinarily strong vegetative impulse, causing trusses to stick up straight (see figure 5) instead of the nice curl shown in figure 4. If it seems the dry-down is too much overnight, night irrigations are a better choice. But in most cases, it is better to stop early. You can always give extra irrigations, but you cannot take one away after its given. The grower has an essential input in irrigation management. The weather is different every day, and the stop time of the irrigation varies as a result.

Figure 5; Truss sticking up due to late irrigation.

Harvest and Beyond

If everything went right, we should have a well-balanced plant, loaded with fruit, ready for harvest. The highest fruit load on the plant takes place approximately one month after harvesting begins. At maximum load, the plants required nurturing. The grower must make it as easy as possible for the plant to grow. The majority of the assimilates that the plant makes are used for fruit production. Nurturing the plants helps stimulate vegetative growth and guarantee a future harvest. Now is the time to switch from generative to vegetative actions. The focus switches from the plant making fruits, to making leaves.


The dry-down of the slab must quickly decrease to 10% or less. It means irrigating later into the day. By now, the EC in the drain should be 4.5, while the drip should be at EC = 3.0. With less light, the plants require less irrigation. The following rule of thumb can be applied;

The last irrigation should come at 200 joules before sunset. If the dry down is too high, night irrigation can be given between 9 and 11 pm. It is important to remember that the guidelines above emerge from a desire to maintain the correct EC. In other words, keeping the right EC is the prime directive. The drain percentage is a means to achieve this. If the EC is too high, Some companies put a high priority on Brix and believe that they can achieve a higher Brix by maintaining a higher EC. 


About 6 weeks before the end of the crop, the growing point is removed. The remaining seven to eight trusses ripen one by one over the next 6 weeks until the plant is empty. Removing the growing point results in the plant needing less assimilates for growth, making leaves and roots. The majority of assimilates that the plant creates are used for fruit growth. Due to the decreasing fruit load, the plant has a smaller buffer to direct water to the fruits when root pressure is high. Therefore root pressure must be reduced. We do this by increasing the EC and dry-down. The dry down can be increased slowly from 10% at topping to 25% at last harvest. The EC can be increased to 8. Due to the reduced fruit load, there are more assimilates available for the last couple of trusses. Even with a high EC, the fruit size up properly. 

Irrigation is a very important tool in the arsenal of a grower. 

If you like to be copied in on future articles or would like to know more and have questions, follow me on LinkedIn, Godfried Dol, or email or go to my website; You can also download previous posts from this website.

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