CASE STUDY: IRRIGATION MANAGEMENT OF TOMATO CULTIVATION IN SOIL

by András Ujszászi – CTO

Gremon Systems has set the goal, based on its extensive market experience and the feedback of professionals working in cultivation, to develop along with its existing products a solution, specifically for soil growers, which helps them a lot in their daily work.   

Over the past decades, with the breaking away of the hydroculture cultivation there had been an unworthily few number of researches and developments in this segment.          
However, there have been generational changes in the lives of growers working in this sector, young professionals started to feel and perceive that the experience based know-how inherited from previous generations is professionally insufficient and their need for their own professional path is important to them.               
Changes in weather conditions also make this necessary, that in addition to experiential knowledge we make daily decisions based on as many sensory measurements as possible.             
A further challenge for growing in soil contrary to inactive substrate is that the soil cannot be changed from year to year. Professionals have to pay extra attention to preserve the soil composition and soil life, since restoring these is not an easy task. This sector plays an extremely important role in supplying mankind and these growing areas must be maintained for future generations. These professionals need to find the right sustainable balance in respect of yield, soil life and soil composition. It is in our common interest to grow it sustainably with the least possible use of water and fertilizer.

In the following, we will examine examples of irrigation management of tomato cultivation in soil.

Generally speaking, in soil cultivation without sensors, growers are over-irrigating. They always strive for the less bad, since the signs of over-irrigation are less spectacular than those of under-irrigation. With the introduction of the Trutina Soil system, according to our gained experience, even 30% of water saving can be reached with smart irrigation.

 Let’s have a look at how this can be done.

When should we start irrigating:

According to our measurements, the water uptake dynamics of plants is very similar to that of soil-free cultivation. Here, too can be said that it is worth waiting for the plants to start their activity with the first irrigation after sunrise. This is well measurable with the soil moisture sensors. This time, under general circumstances, is approximately when 200 Joule/cm2 radiation summary is reached. This can even mean 2.5-3 hours after sunrise. If irrigating earlier the plants are not active enough for the water uptake. There are several reasons for this: low plant temperature, low soil temperature, high relative humidity of the environment, low irradiation. Irrigating too early increases the root pressure gratuitously, this initiates passive transpiration in the plant, which may increase the chance of botrytis or even cause edematic symptoms.            
In addition, the irrigated water largely leaves the root zone vertically before the plants could obtain it, this is typical for less hard soils.

Irrigation frequency:

The soil moisture sensors of the Trutina system should be positioned so that one of the sensors is at a depth in the middle of the root zone, the second sensor should be located just below the root zone, to detect the degree of over-irrigation. From the point of view of the water uptake of the plants, only the increased water content in the immediate vicinity of the roots means that the water uptake is easier because it causes an increase in root pressure here. So, we also have to choose the water dose per irrigation accordingly.  
After starting the irrigation let’s have a look at the rate at which the sensor located in the centre of the root zone measures the increasing water content while also observing the sensor placed deeper. When the deeper sensor shows an increase in water content it means that the water layer in the soil layer above it has reached its saturation level, so it cannot be further moistened.
At this point, the irrigation must be suspended, because this irrigation water is already dispensed unnecessarily, and the fertilizer ingredients contained therein will be inaccessible to the plant as they are washed away and leak into deeper layers causing harmful accumulation. With this method additional water doses can be set and the tendency of water content in the immediate vicinity of the roots can be followed.
During the daytime active period it is worth irrigating as much which maintains easy access to water in the upper layer for the plants. It should be mentioned here that in most cases we also use acids to adjust and reduce the pH value in soil cultivation. This is usually necessary to provide the appropriate pH value for the uptake of the nutrients in the root zone. Since altering the pH value of the soil persistently is very difficult, therefore, reducing the pH of irrigation water is effective when it occurs in the immediate vicinity of the roots and lasts during the active period of the day, that is from the first irrigation until the last. This usually involves applying several smaller doses of water, which is not common in the practices of soil growers. Then we can be sure that the plants with the correct pH will be able to absorb the applied nutrients completely.

Table: shows the effect of soil pH on the uptake of individual elements in soil and in soil-free cultivation

The following figure shows the correct water dosage settings on the Trutina Soil software graph:

In the first two days the grower irrigated one large dosage of water each day. It is noticeable that according to the data of the soil moisture sensor 15 cm deep in the root zone the moisture quickly reaches full saturation and the water also appears deeper, 20 cm deep, to the extent that it saturates there as well and continues to seep into the deeper layers. It is clear that this water and the nutrients will no longer be available to the plants.          
The next day the grower irrigated the one large dose of water in two halves. It can be seen that the sensor placed 20 cm deep in the root zone reaches the saturation level relatively quickly, but the sensor 25 cm deep only shows a significant increase in water content during the second irrigation. This increase in water content in the deeper layer is still a waste and is undesirable.
On the fourth day the grower has irrigated three times, using smaller amounts of water per irrigation. The sensor placed at a depth of 15 cm shows similar saturation as in the previous days, but this saturation lasts longer and the sensor at 25 cm depth shows only a slight change in humidity, which was the goal. As a result, there is no waste of water and nutrients, the roots can take up much more water, and the pH value can be kept in the optimum range steadily during the day.

In my opinion the number of irrigations, if our equipment is suitable, can be safely increased further of course, by proportionally reducing the rations of water if the composition of our soil allows it, we can irrigate small dosages every time we reach 200 Joules /cm2 of radiation sums, this will greatly help our plants with the transpiration during the active period.
It is important to do this only with active plants and stop irrigating in time in the afternoon to leave time for the soil moisture to decrease to the slow late afternoon water uptake. Late afternoon irrigation is extremely harmful to plants because the excessively maintained root pressure causes cell wall damage and increases vegetative elongation at the end of the day resulting in soft, light green plants for the night. These plants due to the greater turgor pressure will stay moist at the wounds caused by the daily cultivation jobs, they have no chance to dry out and will be exposed to fungal diseases.

Irrigation should be finished so that between the last irrigation and the sunset, there should be approximately 250 Joules/cm2 of cumulated radiation. This leaves the possibility of a proportional reduction in the water content in the afternoon.

The following graph shows a well-chosen irrigation technique where the blue line shows the sensor data at a depth of 15cm.

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