Going with the Flow

January 17, 2008
Written by DR. MOHYUDDIN MIRZA
30The importance of proper management of water in greenhouses is well understood by growers. However, I find it so interesting that in many greenhouses, watering is handled by the person who is paid the least. I think the highest paid person should be the person who “carries the hose.”

 Water management in greenhouses has many components from quality to quantities and methods of application. A basic understanding of water and its role should help to make better decisions.

Water is essential for plant growth. It influences plant growth in four major ways:

1.    Water is the major constituent of a plant, comprising 80 to 90 per cent of the fresh weight.

2.    Water is the “solvent” providing nutrient transport within the plant.

3.    Water is a biochemical reactant in many plant processes, the most important being photosynthesis and respiration.

4. Water is essential for maintaining turgidity in plant cells, promoting cell elongation and plant growth.

Water is used as a coolant by the plant through the transpiration processes. 


PLANT WATER CONTENT
Water content is what is present inside the plant at a given time. Basically, plant water content will be determined by how much has been absorbed through roots, how much is being lost through transpiration, and how much is being stored by the plant itself.

Plant water content is in a constant change during the day, when transpiration losses through leaves usually exceeds the rate of water absorption through the roots. This lag between water uptake and water loss creates a condition of internal water stress within the plant. This stress is normal during daylight hours within normal limits. If the stress is allowed to reach extreme levels for extended periods, the plant growth rate declines and eventually the plant dies.

Good growers understand this water stress concept and manage plants accordingly. The use of environmental control computers has helped growers understand the moisture deficit relationship to plant growth. Moisture deficit is a calculation, based on temperature and air relative humidity, that gives a numerical value that is related to the amount of water loss from a crop.

Too high or too low a level of deficit can affect the growth of the plant. 

The moisture deficit is measured in many units but the most commonly used is grams/m3 of air. Under high humidity conditions, the moisture deficit is low and there is no need by the plant to produce more roots. Consequently there is less root development.

Under high deficit situations, the transpiration rate is high, and if roots cannot meet the demand for water, then stomata start closing, which slows down the photosynthesis. It is suggested to use a deficit range of between 3 and 7 grams/m3.

UNDERSTANDING WATER QUALITY
Up till recently a chemical analysis was considered to determine water quality for irrigation. As the demand for food safety increases, there is an increased emphasis for testing microbial loads, e.g., bacteria and fungi especially coliform bacteria. Many growers have included oxygen determinations as part of water quality.

A greenhouse water analysis includes the following: pH and Electrical Conductivity, total soluble salts, nitrate, nitrite and ammonium nitrogen, phosphorus, potassium, calcium, magnesium, sodium, sulfur, iron, manganese, copper, zinc, boron, molybdenum, chloride, carbonates, bicarbonates, fluoride and silicon. Many growers will also ask for a scan of commonly used herbicides at least once a year. Sodium Absorption Ratio (SAR) is also reported. The SAR is a calculated value that describes the relationship between sodium, calcium and magnesium. The analysis will also include information on ion balance of the water.

MANAGING HARD WATER
Hard water is suitable for most greenhouse crop irrigation, but some treatment is required to solubilize bicarbonates to free up calcium and magnesium. Our experience is that all of that calcium and magnesium is not available for plant use. For calculation, use 30 per cent as available for plant growth. Water is treated with acid to neutralize the bicarbonates and in this way pH is brought down to a desirable level.

The quantity of acid needed to neutralize bicarbonates depends on their amount present in the water supply. Phosphoric, nitric, sulphuric and citric acid can be used for this purpose, although commercially the first three acids are more commonly used. The table at left describes the amount of acid needed to neutralize 60 ppm of bicarbonates. When calculating the amount of acid make sure that about 50 ppm are left in the water. For example if you have a total of 400 ppm of bicarbonate in water, then work on neutralizing 350 ppm.

Note that pH is logarithmic, so it can drop very rapidly after a certain point, so don’t depend on the calculation alone. Check the pH after the desired amount of acid has been added and then make corrections accordingly. We have seen cases where the pH dropped to a level of 3 and the grower did not realize it until the damage was done.

Automatic acid dosing systems are reliable, but calibration of electrodes is necessary on a regular basis. All growers should prepare a pH curve to educate their new employees and to have a handy reference guide. A pH curve can be prepared by taking one litre of water, adding a known volume of acid, and measuring the pH drop after each addition. Plot the numbers on graph paper or plug the information into your computer and you have a pH curve. A curve will occur when there is a sudden drop after a certain amount of acid has been added. This is due to the fact that pH is measured on a logarithmic scale.

Growers need to know that point where the pH drop is sudden.

Acids are corrosive, so proper care should be taken in handling them. When diluting the acid, add acid into the water not water into the acid. Wear proper clothing, gloves and safety glasses. Calibrate your pH meter frequently and obtain new buffers every year.

METHODS OF APPLICATION
Water is applied to plants in greenhouses by a variety of methods, ranging from handwatering to highly regulated drip irrigation and based on evapo-transpiration and controlled by computers. An understanding of growing media properties is essential to make watering decisions. In the vegetable sector, growing media such as coir have become more common. They have high water-holding capacity and high porosity.

In spite of the fact that emitter design has changed so that when the irrigation cycle is stopped there are no water droplets at the tip, we see plugged emitters due to deposit of bicarbonates and calcium. One of the successful approaches is to use pH 4.0 water during the last watering on a daily or weekly basis.

UNUSABLE WATER?
The water may not be usable due to several reasons. Most common reasons and treatment strategies are given below:
High sodium: Sodium is not required for plant growth and can destroy drainage of growing media and create toxicity to plants. Here is a recent example where sodium caused serious crop damage in tomatoes (top left photo).

The Reverse Osmosis (RO) unit membrane needed replacement and the grower did not do it. Serious economic loss occurred. Sodium can be removed by reverse osmosis or distillation. RO is a practical and economical option. There are many types of water treatment systems in the market place that do not remove sodium, but only some growers have installed them. If you are not sure, ask questions and seek more information on such systems before investing large sums.

Herbicide contamination. This has serious implications especially when you are using dugouts and collect water from a large run-off area. The recommended approach is to install charcoal filtration system with adequate back washing and set up plant bioassays. Chemical lab analysis many a times cannot detect these herbicides. Plant bioassays mean growing sensitive plants like lentils with a suitable control.

 The picture on page 40 shows what a bioassay looks like. Lentils were grown in new soil using contaminated pond water and the new growth was abnormal. In this case, it was a Tordon-based herbicide.

Organic contamination: If water shows contamination from fecal mate-rial due to run-off from animal waste sources, then first evaluate the problem. Environmental Farm Plans are good resource material to study and plan to avoid contamination sources. Many growers successfully use hydrogen peroxide, ozone, heat and UV sterilization systems. Installation of these systems will also help with recycling issues.

Fine clay suspended in water: Many growers using dugout water in rural areas can face this problem. Chemical quality may be good but suspended clay particles may render it unusable. Different physical filtration systems should be considered.

SUMMARY
Water is the single most important factor that will determine the success or failure of a greenhouse business. Water quality and quantity must be understood and applied in relation to sunlight, growth stage, type of growing medium and other uses within the greenhouse.


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