Irrigation Water for Greenhouses
Nutrients in Irrigation Water
Irrigation water can be an excellent source of plant nutrients. The concentrations of nutrients in irrigation water vary considerably between greenhouse operations depending on the source of the water and the local geology. Irrigation water rarely contains high enough concentrations of the primary macronutrients (nitrogen, phosphorus, or potassium); however, water can contain significant concentrations of the secondary macronutrients—calcium, magnesium, and sulfur and micronutrients including iron, manganese, zinc, boron, molybdenum, chlorine, copper, and nickel.
Macronutrients
Nitrate in groundwater has the potential to provide a substantial amount of plant-available nitrogen to crops. Accounting for the nitrate in irrigation water allows for substantial decreases in fertilizer costs. To measure the concentration of the irrigation water, the grower can have the nitrate content analyzed at a commercial test lab. It is important to note that results from these tests can be expressed in units of ppm NO3¯ or ppm NO3¯–N.
Micronutrients
Waters can contain small concentrations of aluminum, boron, copper, fluoride, iron, manganese, molybdenum, and zinc. With the exception of aluminum these elements are essential to plant growth and are required in small quantities. Aluminum in irrigation water is rarely found in concentrations high enough to lead to toxicities, and should not be a major concern for most growers. Micronutrient toxicities are more probable when the pH of the substrate solution is low, rendering the micronutrients more available for plant uptake.
Chloride
Wells and municipal water sources may contain high chloride levels in association with sodium. Though it not usually listed as an essential micronutrient, chlorine (as chloride) is needed in small quantities by plants. However, in excess, greater than 2 meq/L, chloride can become a production problem.
Sodium
WHigh sodium is a concern to growers since it can contribute to salinity problems, interfere with magnesium and calcium availability in the media and cause foliar burns. Sodium and chloride problems are observed when irrigation water is run through a water softener that is using sodium chloride to “soften” or exchange the calcium and magnesium in the water. “Softened” water is generally not recommended for greenhouse or nursery irrigation .
Iron
In some areas, irrigation water containing excess iron and/or iron bacteria can cause problems such as unsightly brown stains or bluish sheen on foliage and flowers. High levels can also cause an accumulation on irrigation equipment that leads to plugged emitters (Figure 18.5).
Boron
Like sodium and chloride, boron can sometimes be found in high concentrations which are toxic to plants. High boron levels are commonly associated with alkaline soil formations in areas of low rainfall and are, therefore, frequently found in many western states.
Fluoride
ASeveral important floriculture crops including Easter lilies, freesias, spider plant (Chlorophytum), Maranta and Dracaena are known to be extremely sensitive to even low levels of fluoride in irrigation water.
Click on the following topics for more information on irrigation water for greenhouses.
