Irrigation

1. The role of water in the plant production process

One of the control parameters in plant production is the water supply by irrigation (Figure $$). The irrigation has an influence on the water content in the substrate and subsequently on the water content in the plant. As control mechanisms of action, decisions about the amount of water given per application and the frequency of the irrigation measures have to be made.

1.1. Leaf Expansion Rate and Stomatal conductance affected by water supply

In the water usage cascade of a production system from water in the substrate to leaf transpiration, the physiological important parameters water influences are leaf expansion rate (LER) and stomatal conductance (gs) (Figure $$). LER is determining plant leaf area and gs is one of the determinants for the CO2_2 influx into the leaf, both having a decisively impact on photosynthesis.

1.1.1 Leaf expansion rate

In the phase of leaf growth, we have to distinguish between cell division and cell expansion. In plant organs cell division is only active for a relatively short period, it is finalized long before the organ reaches its final size by the process of expansion. For cells to expand the pressure in the cells is important. This pressure is called turgor and depends on the water status of the plant. A low turgor caused by restricted water supply decreases the turgor and decreases cell expansion. As a result the leaf organ will be of a smaller area. An ample supply of water is a prerequisite for high plant leaf area increasing the light interception of the plant canopy.

1.1.2 Stomatal conductance

The leaf lamina of plants is equipped with stomata. These stomata function as a bridge between the plant tissue and the ambient air environment. Water is transpired via the stomata to the environment (for the function of transpiration see 1.2.2). Prerequisite for this transpiration is a sufficient water potential in the leaf lamina so that the stomata of the leaves keep open to allow the water to evaporate and leave the stomata pore as water vapor. Physically this control function is expressed as stomatal conductance or as inverse stomatal resistance. As rule of thumb around 95 % of the transpiration is realized through the stomata, around 5 % via the leaf cuticula (Taiz and Zeiger 2006). To keep up a high water potential in the leaf and consequently a high stomatal conductance again an ample water supply plays the decisive role. But why is a high stomatal conductance so important?

1.2 The role of stomatal regulation for photosynthesis

In the chapter above the role of the stomata was more focused on transpiration and energy balance. There is a third and maybe even more important function of stomata: the gas exchange.

For photosynthesis plants have to take up CO2_2 into their tissue. The CO2_2 enters the plant via the stomata. Because the opening of the stomata depends on the water situation in the plant, there is a clear link to the stomatal conductance. Besides the opening of the stomata also the concentration gradient of CO2_2 between the outside air and the air inside the stomata plays an important role. As we know the ambient CO2_2 concentration is rising due to climate change. At the moment it is ca. 0.04 % (400 vpm) (Wikipedia, 2022, Carbon dioxide in Earth's atmosphere). To increase the gradient and consequently increase the rate of CO2_2 influx, the grower can increase the ambient CO2_2 concentration in the greenhouse by adding CO2_2 via technical gas or by burning fuel using fired heaters. The desired setpoint for this increase depends on a lot of parameters, but values of 800 vpm CO2_2 are not unusual. Of course, this supply only makes sense during daytime when the photosynthetic apparatus is active. The intensity of photosynthesis controls the decline of CO2_2 in the stomata, leading to a gradient of CO2_2 between inside and outside of the leaf. In the end this concentration gradient and the stomata opening width determine the CO2_2 supply for photosynthesis.

It has to be mentioned that the CO2_2 concentration itself has an effect on the stomata opening. A decreasing CO2_2 concentration in the stomata results in an opening reaction of the stomata. Evolutionarily it makes a lot of sense to keep stomata open under lower CO2_2 concentration to enable as much photosynthesis as possible under this condition. Within limits this CO2_2 effect even counteracts the closing reaction by decreasing water potential.

1.3 Transpiration

Plants are exposed to energy incidence by light and ambient temperature. This results in increasing temperatures of the plant surface. As long as this increase has positive effects regarding the biochemical processes of the plant metabolism this is a benefit for plant growth. To avoid excessive plant surface temperature plants are able to transpire, meaning they use the physical process of evaporative cooling. The evaporation of water is energy demanding. The evaporation of 1 g of water needs the energy of 2.26 kJ (Wikipedia, Enthalpy of vaporization). This energy is taken from the surrounding air or tissue leading to a decrease of their temperature. So how does this work in plants?

The extent of the transpiration depends, besides the plant’s internal water situation, on the following climatic conditions:

  • temperature
  • solar radiation
  • wind speed
  • vapor pressure deficit of the air (VPD)

High temperature and high solar radiation warm up the leaf lamina. Increasing leaf lamina temperature increases evaporation in the stomatal pore and consequently transpiration. The other driving force is VPD in the air boundary layer near the lamina surface. In the stomata pore the VPD is assumed to be zero (the air is saturated with water vapor, 100 % relative humidity). A higher VPD (meaning drier air) outside the leaf results in a higher transpiration rate. The transpiration itself subsequently decreases the VPD in the boundary layer of the leaf and therefore has a decreasing feedback loop on the transpiration rate. Now the role of the wind speed comes into effect: A high wind speed leads to a faster exchange of the boundary layer air volume. So, the air with the lower VPD is replaced by surrounding air which is in most cases drier. This air exchange has a positive effect on transpiration. The difference of VPD between inside and outside leaf increases faster with higher wind speed resulting in a higher transpiration rate.

Transpiration has been modeled by several authors. A comprehensive review can be found in Katsoulas and Stanghellini (2019).

1.4 Water in the substrate

As stated above plant production needs water. Plant tissue consists of dry matter produced by photosynthesis using air CO2_2 and water and by water itself. Both sum up to the fresh weight of a plant. But which water sources are used for this tissue production? Are there also losses in the production system? And taking the ecological footprint of plant production into account, how can we produce plants of the desired quality in a water efficient manner? The agricultural sector accounts for around 70 % of the global freshwater withdrawal (World Bank 2022). The availability of water in an acceptable quality for plant production for a reasonable price will decrease more and more due to higher use for industry and a higher municipal demand. Additionally, in some areas natural water resources are getting scarce due to climate change.

In greenhouse production many substrates are used. Crops can be planted directly in the natural soil of the greenhouse area or outside this soil. In the latter case inert substrates like e.g. rockwool or organic materials like e.g., peat, composts, coconut fibres, wood fibres, are used. One of the important characteristics of these materials is the water holding capacity (WHC). The WHC determines the amount of water which can be stored in the substrate and thereby determines the amount of water which can be given per dressing without inducing leaching and possible water loss. To avoid the need of intensive monitoring, reduce the number necessary irrigation actions and still assure an ample supply of water for the crop, substrates with high WHC are preferred.

Water loss in a substrate occurs, besides the uptake by the roots, by drainage and evaporation. In a greenhouse drainage happens if the substrate is supplied with a water amount which exceeds the WHC. In general, this can be avoided by a correct irrigation control. Sometimes drainage is done on purpose to leach high salt concentration from the substrate. A drainage must not be a loss of water if the irrigation system is constructed as closed system in which the drainage water is recirculated. Evaporation is a real loss and should be avoided or at least reduced by measures like covering or even wrapping the substrate using foils.

The transpiration of the plant and the evaporation of the soil sum up to the so called evapotranspiration.

As stated above, plant production needs water. Plant tissue consists of dry matter produced by photosynthesis using air CO2_2, ingested nutrients and water itself. All sum up to the fresh weight of a plant. But which water sources are used for this tissue production? Are there also losses in the production system? And taking the ecological footprint of plant production into account, how can we produce plants of the desired quality in a water efficient manner? The agricultural sector accounts for around 70 % of the global freshwater withdrawal (World Bank 2022). The availability of water in an acceptable quality for plant production for a reasonable price will decrease more and more due to higher use for industry and a higher municipal demand. Additionally, in some areas natural water resources are getting scarce due to climate change.

In greenhouse production many substrates are used. Crops can be planted directly in the natural soil of the greenhouse area or outside this soil. In the latter case inert substrates like e.g. rockwool or organic materials like e.g. peat, composts, coconut fibers and wood fibers are used. One of the important characteristics of these materials is the water holding capacity (WHC, see Figure xx). The WHC determines the amount of water which can be stored in the substrate and thereby determines the amount of water which can be given per dressing without inducing leaching and possible water loss. To avoid the need of intensive monitoring, reduce the number necessary irrigation actions and still assure an ample supply of water for the crop, substrates with high WHC are preferred.

image

Figure xx: Water holding capacity by soil type. Source: New Mexico State University Climate Center (http://weather.nmsu.edu/models/irrsch/soiltype.html). Found on: https://www.specmeters.com/assets/1/7/water_holding_capacity_chart.pdf

Water loss in a substrate occurs, besides the uptake by the roots, by drainage and evaporation. In a greenhouse drainage happens if the substrate is supplied with a water amount which exceeds the WHC. In general, this can be avoided by a correct irrigation control. Sometimes drainage is done on purpose to leach high salt concentration from the substrate. A drainage must not be a loss of water if the irrigation system is constructed as a closed system in which the drainage water is collected and recirculated. Evaporation is a real loss and should be avoided or at least reduced by measures like covering or even wrapping the substrate using foils.

The transpiration of the plant and the evaporation of the soil sum up to the so-called evapotranspiration.

1.5 Water uptake by roots

The water uptake by roots follows the potential gradient between the water potential in the substrate and the water potential in the roots. Water always flows from the higher to the lower potential. Be aware that the water potentials in both elements are negative. To suck water from the substrate, the roots need a more negative water potential than the substrate. This leads to the effect that not the whole water volume of the soil can be taken up by plants. Herbaceous plants have water potentials from around -0.2 to -1 MPa, trees and bushes down to -2.5 MPa and plants in arid regions can reach -10 MPa.A substrate which is filled with water (including the pores) has a water potential of 0 MPa.

There are two important statutory thresholds of substrate water status: water holding capacity and permanent wilting point. Plants can access the amount of water between these two substrate conditions. How much water this is depends on the physical substrate properties as the distribution of substrate particle sizes and proportion and size of pores in the substrate. Looking from the side of the root also the intensity of rooting (expressed as root length density) and the root age determines the volume of water available for the plant. Concerning the root age there is the fact that older roots are more ineffective in taking up water. The highest uptake rate can be found in young growing roots and there especially in the root apex.

1.6 Irrigation & Fertigation

Plants need water to keep up their tissue turgor as well as to transport nutrients, assimilates, phytohormones and other organic or inorganic substances through their vessels. Water is also necessary for numerous chemical reactions in the plant. So, in addition to an adequate above ground environment, it is important to supply sufficient water to the substrate so that the plants don't experience any deficiency. Sounds easy, but in practice a lot of parameters have to be taken into account to decide when, how much and how to irrigate. It even becomes more difficult if by irrigation also nutrients should be applied. The latter is called fertigation (see chapter ‘Fertigation’).

1.6.1 When to irrigate?

Plant indication

It would be easy to start irrigation the moment that deficiency symptoms like beginning wilting of leaves start to be visible. But this is much too late. What could be other symptoms a plant shows on the way to drought stress? A first sign is the increase of leaf temperature due to the beginning closure of the stomata followed by lower transpiration cooling. This is difficult to measure because there are often much bigger short-term changes in leaf temperature due to varying radiation by e.g. changing clouds. Another indication is the decreasing water transport in the vessels. There are systems available to measure the ‘speed’ of the water column in the stem, but they are very sensible in the application. The transport rate also depends on other factors like changing radiation, leading to the fact that to derive existing stress is not trivial. Additionally only one or some few plants of the whole crop are measured. These facts show that the plant itself is a complicated indicator to derive daily irrigation measures.

Substrate indication

For a farmer there are two soil conditions which should be avoided. One is the so-called waterlogging. In this situation there is the danger of root damage by a too low concentration of oxygen in the root environment. Crops differ in their sensitivity against waterlogging. The opposite situation is a too low water content of the substrate so that the roots are only able to take up an suboptimal water amount from the substrate. So, the water content has to be in the range between these extremes. How can we measure and evaluate the substrate water condition? A good indicator is the percentage of the water holding capacity (WHC, %). The WHC defines how much water the substrate can hold against gravity in % of its volume. For a soil this is called field capacity (FC). Due to their different physical properties different substrates have different WHC values (s. Table $$). For production a substrate with a high WHC is positive because the irrigation frequency can be reduced. There is no fixed value for the WHC to be sufficient for unrestricted water uptake of the plant, because due to the physical properties of the substrate the water potential of the substrate by a given WHC is different (s. Table $$, to be added). As herbaceous plants have a water potential of -0.2 MPa to -1.0 MPa the water potential of the substrate should not be more negative to avoid drought stress. Remember from above the value of -1.5 MPa as the permanent wilting point (PWP).

Be aware of the fact that the WHC of a substrate can change during production due to compaction and by root growth. The substrate gets more dense, so the WHC decreases (Fig. xx, to be added).

As instruments for measuring the water situation in the substrate mainly the following tools are available: tensiometer (measuring the matric potential in Pa), TDR or FDR sensors (time domain reflectometry/frequency domain reflectometry, measuring the volumetric water content of a soil in Vol. %). For the latter data about the relation between the volumetric water content and the matric potential of the specific substrate is necessary.

To be added: photos of tensiometers and TDR/FDR sensors

1.6.2 How much to irrigate?

If no uncontrolled irrigation is possible (e.g. rain in open field production) it might be assumed that it is optimal to irrigate to full WHC. But to avoid a too low oxygen concentration, the substrate is normally only filled up to 90-95 % of the WHC. Otherwise there would also be the danger of producing drainage causing water and nutrient losses. In an open field there should always be a buffer for a possible rainfall event.

If the production system is constructed as a closed system (means that excessive water is sampled and reused), then it is possible to give more than 100% of the WHC. One advantage is that if some single drippers release less than expected, every plant gets sufficient water. The second advantage is that high salt concentrations in the slabs by fertigation can be flushed out (see also chapter ‘fertigation’).

1.6.3 How to irrigate?

To supply the plants with the irrigation water various methods are possible. If there is a planting done directly in the soil overhead sprinklers can be used. This leads to a more or less all-over wetting of the soil and is only used for crops like e.g. lamb's lettuce or red radish. These crops are sown out directly to the soil and have such a high plant density that a single plant irrigation is not feasible. As a consequence, there is a loss of water by evaporation and an increase in air humidity.

For row crops like e.g. tomato, cucumber or sweet pepper, the irrigation water is distributed via water hoses to drippers directly to the plant pot. This is much more water-saving.  It is possible to add fertilizer to the irrigation water. In this case we speak about ‘fertigation’ (see also chapter ‘fertigation’).

In these water hoses there are water outlets with integrted valves which open at a certain pressure (e.g. 0.7 bar) and distribute the water through thin pipes (so called ‘spaghetti’) directly to the single plant pots. It is very important to control the functionality of the drippers so that all plants get the water and nutrients needed.

Figure $$: Overhead sprinkler irrigation (https://www.taiwantrade.com)
Figure $$: Pressure compensated valve mounted to a drip line with 4 spaghettis (https://www.farmtek.com/farm/supplies/prod1;;pg110412_110409.html)

Figure $$: Rockwool pot on a rockwool slab with two drippers (‘spaghetti’) for two tomato plants (Fricke)
Figure $$: Rockwool pot on a rockwool slab with two drippers (‘spaghetti’) for two tomato plants (Fricke)

Figure $$: Greenhouse after crop removal. Soil covered by mulch foil, slabs, main pipes, and drippers still inside (Fricke)
Figure $$: Greenhouse after crop removal. Soil covered by mulch foil, slabs, main pipes, and drippers still inside (Fricke)

1.6.4 Fertigation and fertilization

In greenhouse production water is given mostly together with nutrients as a ‘fertigation’. The nutrients from commercial complex fertilzers or from selected different chemical substances are solved in water and this nutrient solution is stored in water tanks. Usually three tanks are necessary. One tank contains an acid (e.g. HNO3_3) to control the pH of the solution. For the stock solution of the nutrients, which is highly concentrated (e.g. 100 fold), calcium and phosphate have to be separated in two different tanks. Otherwise calcium phosphate (Ca3_3(PO4_4)2_2) would occur as precipitate. The stock solution is diluted to the desired concentration by a fertilizer mixer. Depending on crop and developmental stage of the crop the combination of the single elements should be adapted (see Fig. $$). This can be done by changing the withdrawal ratio from the single tanks, or by directly changing the composition of the chemicals used for the stock solutions. The overall concentration of the nutrient solution leaving the fertilizer mixer is controlled by an EC meter. Concentrations of around 1.5-2.0 dS m^-1^1 are used. If the weather is sunny the concentration is reduced, because the plants need relatively more water in this situation than nutrients. Scheduling of dressings is done by radiation sum. If the sum reaches a given setpoint the valves open for a given duration. Especially during the first growing period this setpoint has to be adjusted frequently. The second control option is the duration of a single dressing. A little surplus is necessary to have a flush out effect of the older solution out of the substrate slabs. The aim is to have only a low surplus of the solution. The surplus solution is collected and redirected to a storage tank. This tank could be just a normal tank or it is equipped with a heating to warm up the solution. It can also work as a disinfection unit (e.g. sand filter). But there are also other disinfection methods on the market. An often used one is a UV-filter. After disinfection the solution is pumped to the fertilizer mixer for adding nutrients and the circle starts again. In such closed recirculating systems it is very important to control the composition of the single nutrients. Plants change their uptake according to their developmental stage. Analyses of the solution are necessary to adapt it accordingly.

Table $$: Final delivered nutrient solution concentration (ppm) and EC recommendations for tomatoes grown in Florida in rockwool, perlite or nutrient film technique (Hochmuth and Hochmuth, 1995). Numbers in bold denote changes from the previous stage. (Heuvelink, 2005)
Table $$: Final delivered nutrient solution concentration (ppm) and EC recommendations for tomatoes grown in Florida in rockwool, perlite or nutrient film technique (Hochmuth and Hochmuth, 1995). Numbers in bold denote changes from the previous stage. (Heuvelink, 2005)
Figure $$: Fertigation mixing room (Fricke)
Figure $$: Fertigation mixing room (Fricke)
Figure $$: UV-desinfection (Fricke)
Figure $$: UV-desinfection (Fricke)

Organic farmers are not allowed to work with mineral fertilizers in the fertigation solution. So they work with a combination of base fertilization direct on the soil by organic fertilizers as e.g. horn shavings, horn meal, pig bristle pellets, supplemented by top dressings especially for N via drip fertigation using e.g. sugar beet vinasse. The latter contains N, P, K, Ca, Mg and some micronutrients. Vinasse is a viscous material which is diluted to a certain extent in order not to clog the drippers. A pure supply with vinasse for crops like e.g. cucumber and tomato is not possible. The base fertilization using solid fertilizers onto the soil leads to the need of an additional sprinkle irrigation to wet the whole soil surface with the scattered fertilizer. The drip fertigation moistens only a small area around the dripper. Using only drip lines here would result in undissolved solid fertilizer on the soil surface. The sprinkler irrigation can be mounted on soil level (see Fig. $$) or over the top of the canopy (see Fig. $$).

Fig, $$: Sprinkler Irrigation on soil level (IGPS)
Fig, $$: Sprinkler Irrigation on soil level (IGPS)

Fertigation (1)

Fertigation is a composition of the two words ‘fertilization’ and ‘irrigation’. In greenhouse production water is given mostly together with nutrients as a fertigation.

Air boundary layer

The air boundary layer of a leaf is the layer of unstirred air around the leaf surface. The extent of this layer depends on wind speed (thinner layer under higher wind speed) and the leaf size (Taiz and Zeiger 2006).

Evaporation

The transition of liquid water (here water in the soil) to water vapor. Evaporation occurs on the soil surface, especially if the surface is wet. A dry soil surface acts as an isolation barrier against evaporation.

Evapotranspiration

The evapotranspiration is the sum of the evaporation of the substrate and the transpiration by the plant.

Fertigation

Fertigation is a composition of the two words ‘fertilization’ and ‘irrigation’. In greenhouse production water is given mostly together with nutrients as a fertigation.

Field capacity

Field capacity (in substrates called ‘water holding capacity’) is the amount of water per volume (L m-³) or per g of soil (g g-1) which a soil can hold against gravity. It can also be expressed as suction (kPa) at this water content.

Permanent wilting point

The permanent wilting point (PWP) is defined as the minimum water volume in a substrate that the plant needs not to wilt. By convention the PWP is defined at −1.5 MPa of suction pressure. (Wikipedia, 2022, Permanent wilting point)

Leaf expansion rate (LER)

The LER is defined as increase of leaf area per e.g. unit of time (cm² d^-1^1) or temperature sum (cm² °Cd^-1^1)

Root length density

The root length density (m m-3) describes the intensity of rooting in the substrate volume.

A stoma (Wikipedia; Stoma) is a cell structure in leaves of plants which forms a pore. Water evaporates from the leaf mesophyll cells into the air filled pocket of the stoma inside the leaf. From there the vapor is transported to the air outside the leaf. The aperture of the stoma is controlled by guard cells.

Stomatal conductance (mmol m^-1^1 s^-1^1) expresses the net molar flux of CO2_2 entering or water vapor exiting the stomata of a leaf. Its inverse is called stomatal resistance (s m^-1^1) (Wikipedia, 2022, Stomatal conductance)

Pressure of the cell sap on the cell wall. If the osmotic potential inside the cell is more negative than that of the apoplast, the cell takes up water. The resulting pressure tensions the cell wall. This pressure is intercepted by the elastic tissue pressure.

Vapor-pressure deficit (VPD)

The VPD describes the difference (deficit) between the amount of water vapor in the air under saturated condition and the actual amount of water vapor (Wikipedia; vapor-pressure deficit). The unit is Pascal (Pa). It has to be taken into account that air of higher temperature is able to include a higher amount of water vapor. So the VPD depends on ambient temperature. Water vapor is transported in the air from a lower VPD to a higher VPD.

Water holding capacity

Water holding capacity (in natural soil ‘Field capacity’) is the amount of water per volume (L m-³) or per g of substrate (g g-1) which a substrate can hold against gravity. It can also be expressed as suction (kPa) at this water content.

Water potential

Water transport in plants is described by the water potential concept.

The water potential Ψ (Psi) is defined as the potential energy of water per unit volume relative to pure water in reference conditions. It quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, and matrix effects such as capillary action (caused by surface tension). (Wikipedia; Water potential)

Water always flows from a higher to a lower potential. Unit is Pascal (Pa).

References

Hochmuth G, Hochmuth B (1995).  Challenges for growing tomatoes in warm climates. Greenhouse Tomato Seminar. Montreal, Quebec, Canada. ASHS Press, Alexandria, Virginia pp. 34-36. In: Heuvelink, E. (Ed.) 2005: Tomatoes. CAB International. Wallingford, UK. p 279.

Idso S B, Jackson R D, Pinter Jr. P J, Reginato R J, Hatfield J L (1981). Normalizing the stress-degree-day parameter for environmental variability. Agricultural Meteorology, 24, 45-55.

Katsoulas N, Stanghellini, C (2019). Review: Modelling crop transpiration in greenhouses: different models for different applications. Agronomy 9(7), 392. doi:10.3390/agronomy9070392

Nederhoff E M (1994). Effects of C0%-2% concentration on photosynthesis, transpiration and production of greenhouse fruit vegetable crops. Dissertation. Agricultural University, Wageningen, The Netherlands. p 2.

Taiz L, Zeiger E (2006). Plant Physiology, Fourth Edition. Sinauer Associates, Inc., Sunderland, MA, USA. P. 65. ISBN 0-87893-856-7.

Wikipedia (2022). Different search terms as indicated in the text. https://en.wikipedia.org.

World Bank (2022). Water in Agriculture. https://www.worldbank.org/en/topic/water-in-agriculture#1. Last retrieve 23.02.2022.

Yan H, Huang S, Zhang C, Coenders Gerrits M, Wang G, Zhang J, Zhao B, Acquah S, Wu H, Fu H (2020). Parameterization and application of Stanghellini model for estimating greenhouse cucumber transpiration. Water, 12, 517. doi:10.3390/w12020517.