Water: The Essential Element for Plant Growth
Water with the right parameters and provided in balanced amounts is basic to optimal plant growth.
Introduction
Water is a catalyst in plant functions, water is the internal transport system and it is a fundamental part of the plant itself. Water evaporation from the plant parts provides cooling.
While some species thrive with minimal quantities of water and others require more, even those that enter periods of semi-dormancy during drought demonstrate remarkable adaptability.
However, for optimal growth and high productivity in cultivated plants, a consistent and balanced water supply is crucial. Achieving this precision and timeliness in irrigation can be challenging.
The many aspects of water and its application are the focus of the following article.
Water Quality: A Most Important Aspect
Before acquiring land for cultivation, assessing both the quantity and quality of available water is paramount, as options for improving water quality are often limited. Not all water sources are suitable for plant growth and can significantly impact plant health and crop yield.
Rainwater is generally of excellent quality, but it is not always available when needed. Collecting rainwater from greenhouses or other building roofs is highly recommended. Water from rivers, groundwater, and other sources shows significant variability in its composition, necessitating laboratory analysis to determine its suitability. A high Electrical Conductivity (EC) or Total Dissolved Solids (TDS) value is generally detrimental. While an EC below 1.0 mS/cm or TDS below 500, is typically preferred, other plants can tolerate 2.0 mS/cm or even higher.
Crop sensitivity to EC values: Note that plants sensitive to higher EC values are also prone to water stress.
Laboratory reports will identify the elements contributing to a high EC, but effective remediation can be difficult. If the EC is high, more frequent irrigation and higher drainage may be necessary to prevent salt buildup in the soil. Blending lower-quality water with rainwater or other suitable sources can improve average water quality and increase the available volume. In extreme cases, reverse osmosis filtration can be employed, though this method is relatively expensive and generates significant water waste.
Water purified by reverse osmosis is depleted of minerals and substances. To make it suitable for irrigation (or drinking), specific elements must be added to provide the necessary nutrients for plant health.
The water’s acidity or alkalinity, measured by pH value, is another critical parameter. A pH range between 6.0 and 7.0 is generally optimal for most plants. A higher pH will be detrimental to the uptake of elements by the plants. If the pH exceeds 7.0, the addition of nitric, phosphoric, or sulfuric acid is recommended to neutralise carbonates in the water and consequently decrease the pH.
Water Storage
Open water storage may lead to the formation of algae and loss of water through evaporation.
Factors Influencing Plant Watering Requirements
Numerous factors determine the appropriate amount of water for plants, and this quantity is not a fixed or standardized measure. Observations of neighbouring growers cultivating the same crops often reveal successful growth under varying irrigation regimes, highlighting that consistent average soil moisture, whether slightly higher or lower, is key.
A significant portion of the water absorbed by plant roots is transpired (evaporated) through the leaves. This process facilitates water and nutrient uptake and transport within the plant, while also providing a cooling effect. The rate of transpiration is strongly influenced by climatic conditions. Increased sunlight, wind, low air humidity, and higher temperatures accelerate evaporation. Conversely, dull, cloudy weather with limited air circulation and high humidity reduces it. Irrigation schedules must be adjusted based on these dynamic conditions. On sunny, windy days, water application may need to be increased by as much as 30% compared to overcast days. Furthermore, the size and overall biomass of the plants are primary determinants of their water consumption.
Soil characteristics also play a crucial role in water supply due to variations in drainage rates and water-holding capacities. Sandy soils allow water to drain rapidly and have a low water-holding capacity, leading to quicker drying. Clay-rich soils, on the other hand, retain water effectively, drain more slowly, and thus dry out much less rapidly than sandy soils.
Soils with good water-holding capacity provide more stable moisture availability, allowing for longer intervals between watering. In soils with low water retention, more frequent applications of smaller water volumes are necessary to ensure consistent moisture levels.
Determining When and How Much to Water
The maximum amount of water that should be applied is up to field capacity. That is the point at which the soil is saturated and excess water begins to drain out. The opposite extreme is the wilting point, where the plant can no longer extract sufficient water from the soil.
An indication that field capacity has been reached is when the soil is visibly wet at a depth of approximately 30 cm or when water drains from the bottom of containers. Wilting, characterised by drooping tips or leaves, signifies that the wilting point has been approached or surpassed, and watering should have occurred well before this stage. The appropriate amount of water to apply is that which penetrates to a depth of 25-30 cm in the soil, or, for potted plants, until a small amount of drainage is observed.
It’s important to note that wilting can also be a symptom of root disease. In such cases, the roots are unable to function properly, and adding more water can exacerbate the problem.
Another phenomenon to be aware of is temporary wilting following a sudden shift from days of cloudy weather to sunny conditions. During prolonged periods of low light and high humidity, plants may reduce root growth and activity. When, in the following days, bright sunlight suddenly appears, the inactive root system may not be able to meet the increased transpiration demands, leading to visible wilting. A light overhead watering can often alleviate this stress without further inhibiting root function through excessive moisture.
While the topsoil may appear dry due to surface evaporation, the moisture content deeper down is typically more stable. Therefore, a dry topsoil layer should not be the sole indicator for irrigation. Assess soil moisture at least 2-5 cm (1-2 inches) below the surface. For deeper assessment (12-25 cm or 5-10 inches or more), a simple soil probe can be made using a 45 cm (1.5 feet) long PVC pipe with a diameter of 4-5 cm (1.5-2 inches). Cut out a lengthwise section of the lower portion of the pipe. By twisting the pipe into the soil and extracting it, the soil moisture can be observed at different depths within the exposed section of the pipe.
It is crucial to remember that soil moisture can vary across a single cultivation area. Therefore, moisture checks should be conducted at multiple locations to obtain a representative understanding of the overall soil moisture.
Beyond Hydration: Other Functions of Water in Plant Growth
Water plays several vital roles beyond simply providing moisture to plants.
Soil contains air within its pores and pockets, which is essential for root respiration. Irrigation temporarily fills these spaces with water, displacing the air. As the water drains, fresh air replenishes the pore spaces. Adequate watering is therefore necessary to facilitate this gas exchange. Maintaining excessively wet soil conditions leads to a lack of oxygen, causing plants to exhibit signs of chlorosis (yellowing of leaves due to impaired nutrient uptake) and eventual suffocation.
Within the soil, a dynamic exchange of nutrients and elements occurs between soil particles and water. Irrigation dissolves these elements, making them accessible for uptake by plant roots. Drainage also prevents the accumulation of salts and other elements in the soil. Especially in the topsoil, water evaporation leaves dissolved elements behind, leading to increased EC and salinity, which is detrimental to plant health.
For commercial hydroponic or substrate cultivation, drainage is a particularly critical aspect of nutrient management.
Water Distribution Methods
Water should be distributed as evenly as possible across the cultivation area, while acknowledging that certain areas may dry out more quickly than others. Examples include the edges of raised beds, especially those exposed to direct sunlight, and the ends of drip lines where water pressure may be lower. In such cases, drip lines can be slightly repositioned towards the drier areas, or additional external drippers can be installed to provide supplemental water. On uneven terrain, drippers at the lowest points may continue to release residual water from the laterals even if they are pressure-compensating. Pressure-compensating drippers contain a small internal membrane that regulates water flow, ensuring consistent output within a specific pressure range.
Irrigation Devices and Automation
Consistent watering is essential to maintain optimal soil moisture levels, sometimes requiring daily applications or intervals of no more than two days. Intensive cropping systems may necessitate watering twice daily, and substrate cultivation often demands small, frequent applications (e.g., every hour). These schedules are often impractical to maintain manually.
Various automated irrigation systems are available to address this need. The simplest setup involves connecting a lateral tube (typically 12 or 16mm in diameter) to a water valve. Emitters or drippers are inserted into the lateral at intervals corresponding to the plant spacing. Opening the valve delivers a predetermined amount of water to each emitter based on the duration the valve remains open. Drippers with different flow rates and spacing options are available. With proper installation and maintenance, these drip lines are a reliable solution for irrigation.
For lawns or crops like carrots and spinach, overhead sprinklers are often used instead of drippers, providing fine overhead irrigation that covers a wider area.
A small pump can be installed to ensure sufficient water pressure for the simultaneous operation of multiple drippers or sprinklers. Automation can be further enhanced by using a time clock and solenoid valves (electrically operated valves) to control irrigation schedules. However, these timings and intervals need to be adjusted based on prevailing weather conditions. For more sophisticated automation, soil moisture and solar radiation sensors can be integrated with a software program to create a fully automated irrigation system that responds dynamically to environmental conditions.
All these automated systems offer significant water savings compared to manual watering methods.
For container gardening on balconies and terraces, lateral tubes can be concealed amongst the pots, with smaller, less visible tubes connecting to individual containers for a more aesthetically pleasing arrangement.