High quality irrigation water is becoming more and more difficult to find in many areas of our country. Growers planning to start or expand a nurseries should examine available sources of water before beginning construction. This examination should include quality of water, quantity of water, and reliability of the source and should be backed up with lab analysis.
In many areas, surface waters are satisfactory for irrigation, however, streams and rivers are susceptible to pollution that makes them unusable. Low country areas tend to experience the most serious water quality problems. In some cases, these waters may have high salt or high pH issues that make them unsuitable for use in production. Nurseries in areas where no viable alternative is available must resort to moving their nurseries or growing salt tolerant plants.
The electrical conductivity (EC) is a measure of total dissolved salts (ions) in irrigation water. The use of irrigation waters with high EC values (greater than 1.5 mmhos/cm) can result in salt toxicity. High salt concentrations reduce plant growth by literally sucking the water out of plants, causing dehydration and leaf burn. Water pH should also be analyzed because over time it will affect the soil or substrate pH, micronutrient availability, and efficacy of applied chemicals. Water pH should be within 5.5-7.0 for optimum production.
Irrigation water analysis allows growers to be aware of ions that may be present in high levels. The irrigation water ions of greatest concern to growers of ornamental plants are:
Sodium ions in high concentrations can impair the physical condition of field soils. Sodium ions in soil become attached to (adsorbed to) individual clay particles, causing loss of soil structure and poor soil permeability. Large amounts of calcium and magnesium can help soil structure resist the effects of sodium. The relationship between calcium and magnesium and sodium is expressed as the sodium absorption ratio (SAR). A SAR value of 18 or more indicates poor quality while less than 10 is acceptable. The effect of sodium ions on physical structure of soil-less medium is not a problem unless the SAR exceeds 35.
The effect of individual constituent ions in irrigation water on plant growth varies widely:
In container production, salinity problems can often be solved by significant leaching events. Adjustments to cultural practices can minimize salt accumulation.
To minimize salt accumulation, consider:
When leaching, the equivalent of two pot container capacities should pass through the pot. Leaching with more than two container-capacities wastes water. In addition, leaching is most efficient when the water is distributed evenly over the soil or medium surface.