| | Water is the most important component in growing plants, greenhouse crops are no exception. Water is needed by plants to carry out several functions. It dissolves the necessary minerals and makes them transportable through the plant water transport system from the root zone to areas where cells are elongating like the root zone or shoot tips or expanding fruit. Water is also transported to leaves where it is lost to the surrounding air through stomata, which are small openings on the underside of the leaves. The opening and closing of these stomata depends on the water status of cells surrounding those openings.
Water transport from the root zone to the leaves is a process which requires lots of energy. This energy comes from the carbohydrate supply of the plant. Water is absorbed in the root zone in a liquid form and by the time it reaches the leaves and is lost to the surrounding air it is converted into vapor. This process is called transpiration which is a very powerful force. Water is moved inside the plants through root pressure to a small extent. The pumping action of roots is a weaker force than transpiration and it will continue up to approximately two hours after the last watering.
The growers are also familiar with the term moisture deficit, which describes the moisture status of air surrounding the leaf and moisture status inside the leaf cells. Under normal conditions the leaf cells are full of water and the air surrounding of leaf is drier thus encouraging a loss of water from leaves to the surrounding air. The higher the air relative humidity the lower the moisture deficit. Conversely the lower the air relative humidity, the higher the moisture deficit.
What determines the water quality?
The water quality is determined by the sum of cations and anions present in water and intended water usage. Cations are positively charged ions of calcium, magnesium, potassium, sodium and ammonium while anions are negatively charged ions like sulfate, chloride and nitrate. In addition to cations and anions the water quality is determined by the amount of bicarbonates and trace elements like iron, manganese, boron, copper, zinc and molybdenum. Excessive amounts of zinc may be present where water is collected from greenhouse surfaces. This is due to galvanized steel used for greenhouse gutters. Many growers monitor the amount of zinc on a regular basis and make nutrient adjustments accordingly.
A basic rule to determine water quality is to look at the Electrical Conductivity (E.C.) and SAR value. SAR stands for Sodium Absorption Ratio which is calculated from the milliequivalents of calcium, magnesium and sodium. SAR values are provided with the laboratory analysis. If the E.C. value is below 0.8 millisiemens/cm (ms) and the SAR value is less than 4, the water is considered to be good for greenhouse irrigation. E.C. values higher than 0.8 ms and SAR over 4, the water is considered to be marginal for greenhouse irrigation and its use can present problems. Special management practices are required to use such water supplies. These management practices include the design of a growing medium with good drainage, leaching with every watering and counterbalancing the sodium with calcium and magnesium.
Water with higher than 100 ppm of bicarbonates may clog the drip lines and fogging nozzles. Bicarbonates can be neutralized with different acids.
Water quantity
One of the challenges is to come up with reasonable water use figures for various crops. These figures are required in the planning stages of greenhouses. Water use is determined not only by the actual water moved through the plant but also leaching requirements to maintain a certain E.C. in the root zone, surface evaporation which may take place from exposed surfaces and growth stage of plants. Established growers have good records of water usage for the plants they are growing. We are trying to put some information together for greenhouse planning purposes.
Some useful measurements
A cubic meter is 1000 liters
One cubic foot is 28.3 liters
One imperial gallon is 4.546 liters
One imperial fluid ounce is 28.4 milliliters
M. Mirza and M. Younus, CDCN
Greenhouse Coverings - July 1999 |
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