Among all the factors that influence the correct management and development of a crop, and more especially in the case of crops in substrates, water management (fertigation) is the most important.
If we want to know what irrigation needs our plants have, we must know what the water losses that are going to occur will be and what variables influence them:
WATER EXPENDITURE = WATER ABSORBED BY THE PLANT + EVAPORATED WATER + DRAINAGE WATER
- Water absorbed by the plant. It is the sum of the water breathed by the plant and the water of cellular constitution. The first is basically determined by solar radiation and the difference in vapor pressure (DPV) between the surface of the leaf and the atmosphere that is in contact with it; it reaches values of 95%. The second, although it may seem otherwise, is only between 3 and 4% of the total water absorbed.
- Evaporated water. In our case and being plants confined in pots or plastic bags, this component does not suppose significant losses.
- Drainage water. The amount of extra water that we must add to our plants will depend on the quality of our waters and the electrical conductivity that results from adding our fertilizer to the water. It will depend on the tolerance of our plant to salinity, the rate of transpiration, the phenological state (more drainage in young seedling) in which our crop is located and the quality of the irrigation water.
ELECTRICAL CONDUCTIVITY (dS/m)
- Supply the right amounts of readily available water.
- Supply the necessary amounts, and in a correct balance, of each of the necessary nutrients (primary macronutrients, secondary macronutrients and micronutrients).
- Maintain an adequate level of oxygen that allows adequate breathing of the root.
- Maintain an adequate temperature in the root medium.
- Homogenize the four previous points throughout the volume of our pot or culture bag.
From now on every time we refer to the contribution of water we will mention the term fertirrigar, since in a crop in substrate each time we provide water we will provide the necessary nutrients in the appropriate amount and proportion.
Parameters to take into account when fertirrigar.
- Electrical conductivity. As we have seen before, this term refers to the amount of salts dissolved in our fertigation solution. To know if we are watering properly we will take as a general rule that the drainage water can not leave the substrate with an increase in conductivity >1 with respect to the conductivity of the solution with which we are watering, that is, if our fertigation solution that we provide has an EC of 1.8 dS / m, drainage water cannot leave with more than 2.8 dS/m.
We can perform the control by putting under a representative pot a culture tray and collect the drainage water, measure the percentage it represents with respect to the amount that enters, and measure the EC.
If the water comes out with an increase of EC<1 with respect to the EC of the input solution, it means that we are draining excessively; if on the contrary the increase in EC is >1 it means that we are draining less amount than we should.
In summer the EC will increase (since it is directly proportional to the temperature) so we will have to drain more; in winter the EC decreases so we will have to provide more fertilizer.
- pH. We have to take into account that the pH range in which our growing substrate must be between 5.5 and 6.5 and for this our fertigation solution must be between 5.0 and 6.0. This range guarantees maximum absorption of nutrients.
Throughout the year there will be variations in the pH of our drainage as follows:
- In periods of vegetative growth the most absorbed ion will be the nitrate anion (NO3–) and as we already know, when an anion is absorbed the plant yields an OH– group with the consequent increase in pH.
- In periods of flowering and fruiting the most absorbed ions will be the cations and especially potassium (K +) with the consequent transfer to the proton medium (H +) and a decrease in pH.
Drastic increases or decreases in the pH of the drain can denote some problem with the substrate.
- Oxygenation. Fertigation is the way to provide oxygen to the roots. Keep in mind that as our substrate decomposes the particles become smaller and the porosity decreases, with the consequent decrease in the normal aeration capacity of a substrate that must be around 20-30%.
Oxygenation decreases in summer since the temperature increases the speed of diffusion of oxygen from water to the atmosphere.
- Water availability. It is not recommended that our plants absorb water at high voltages. We will define two concepts that summarize the appropriate way in which we must apply water to our pots: irrigation endowment (how much fertigation solution to apply each time we water) and frequency of irrigation (how many times to apply the irrigation endowment in a given period of time, for example, a week).
- Irrigation equipment: it is the volume of dissolution (liters or milliliters) that we will apply to our pot every time we water. It will depend on the volume of our pot, the water retention capacity of our substrate and the drainage that we want to obtain depending on the EC of our fertigation solution, which in turn is determined by the quality of our water.
- Frequency of irrigation: number of times we apply our irrigation equipment in a given period of time; in our case it can be a week.
As a general rule we should not let the water of our substrate be depleted excessively, especially in flowering and fruiting plants. In young seedlings we can let it run out more and thus force the root to explore a greater volume of substrate and achieve a correct rooting of our young seedlings.
When we must increase the water supply to our plants we will not do it by increasing the endowment (quantity) of irrigation because we remember that it was calculated based on the water retention capacity of our substrate (which is what it is and will not increase) and the EC that we want in our drainage; We will do it by increasing the number of weekly irrigations. Likewise, if the cold season arrives we have to reduce the water supply we will not do it by decreasing the endowment but the frequency, that is, the number of irrigations.
- Temperature. We should not apply our fertigation solution to our plants at either a very low or very high temperature. As a general rule we will water at noon in the cold season and two hours after sunrise or two hours before sunset, in the hot season.
Methods of controlling the contribution of fertigation.
At the level of intensive agriculture there are different methods of irrigation control:
- Associated with the plant; control of the diameter of the stem, control of the rhythm of transpiration.
- Associated with climate; measurement of solar radiation, DPV.
- Associated with the substrate; control of the ce and % of the drainage, weighing methods, etc.
We will apply a methodology that will take into account:
- Thecharacteristics of our container or pot (volume and material with which it is made).
- The characteristics of our substrate (fully available water ATD).
- Features of our fertigation solution (pH and CE).
I am going to show you an example of an irrigation in a blonde peat pot with a volume of 5 liters:
IRRIGATION EQUIPMENT: VOLUME OF WATER ABSORBED BY THE PLANT + DRAINAGE
Volume of our pot = 5 Liters
Blonde peat ATD = 30% = 5 x 0.3 = 1.5 liters
Required drainage volume: 20%
Irrigation equipment: 1.5/0.8 = 1.87 liters
Our irrigation endowment (amount of solution to apply each time we water) is 1.87 liters; with this endowment we must obtain in our tray a surplus of water of approximately 370 ml. How many times we must apply the irrigation equipment weekly it will be a matter of observing our crop and what we want to deplete the water reserves of our substrate. Normally a visual appreciation of the surface of our pot will give us an orientation of how many times water weekly.
We will also monitor the amount of drainage; if between irrigations less drainage comes out than desired it will mean that we are letting the water retained in the substrate be depleted a lot, so we must increase our frequency of irrigation (number of weekly irrigations). If, on the other hand, the drainage is higher than calculated, we are watering ahead of time and we must reduce our frequency of irrigation.
Always remember that to increase or decrease the contribution of irrigation to our plants we should never change the irrigation endowment but the frequency with which we apply this weekly.
If we use substrate mixtures we will calculate the ATD based on the percentage of each substrate in the mixture; for example, for a mixture of 50% blonde peat, 25% coconut fiber and 25% perlite we will calculate our ATD as follows:
ATD blonde peat: 30%
ATD coconut fiber: 27%
ATD perlite: 32%
Mixture ATD: 0.5*0.3 + 0.25*0.27 + 0.25*0.32 = 29%
In short, we can establish some basic rules to manage irrigation in our pots:
- We will calculate our irrigation endowment based on the ATD of our substrate and the % of drainage we want to obtain.
- The % drain is set based on the electrical conductivity of our fertigation solution; the more CE the greater the % drainage.
- If the amount of our drainage is less than calculated, we are letting the readily available water be depleted too much, with the consequent overexertion for our plants when it comes to absorbing water; if on the contrary it is excessive, we are watering ahead of time.
- The electrical conductivity of our drainage must not exceed in one unit the EC of the fertigation solution that we are applying, taking into account that:
- In summer the EC tends to go up and in winter it goes down.
- When our plants are in bloom or fruiting the EC will go down.
We will only have to worry about drastic ups and downs.
- The pH of our drainage should be between 5.5-7 taking into account that:
- In periods of vegetative growth the pH will rise and in periods of flowering and fruiting it will fall.
- In winter the pH will rise slightly as temperatures drop and in summer it will drop.
As with electrical conductivity, we will only have to worry about drastic ups and downs.
- If we have to raise or lower the contribution of fertigation we will not touch the calculated endowment for our pot, but we will increase the number of weekly waterings.
- If we have our pots where rainwater can fall, whenever it rains we will take into account this contribution; it is usually water without electrical conductivity.