Magazine - (Part.3) Managing Turfgrass Using Poor Quality Water

By Jerry Spencer in Consultancy on 8th Dec 2007 7:00

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Part 2 Click Here


PART 3
Interaction of salinity and SAR. Salts and sodium do not act independently in the plant environment. The effects of sodium on soil particle dispersion (and therefore permeability) are counteracted by a high electrolyte (soluble salts) concentration; therefore, one cannot assess a water’s sodium hazard independent of its salinity. The combined effect of water ECw and SAR on soil permeability is given in Table 3. Note that the table provides general guidelines only. Soil properties, irrigation management, climate, a given plant’s salt tolerance, and cultural practices all interact significantly with water quality in the actual behavior of soils and plant growth.

Bicarbonate and carbonate. The bicarbonate content and, to a lesser degree, the carbonate content of irrigation water also deserve careful evaluation. Recycled waters and well waters are especially likely to contain excessive bicarbonate levels. Substantial bicarbonate levels in irrigation water can increase soil pH, and in combination with carbonate they may affect soil permeability. In addition, bicarbonate content may make itself obvious during hot, dry periods, when evaporation may cause white lime (CaCO3) deposits to appear on leaves of plants irrigated by overhead sprinklers.

Although high levels of bicarbonate in water can raise soil pH to undesirable levels, bicarbonate’s negative impact on soil permeability is often of greater concern. As mentioned above, the bicarbonate ion may combine with calcium or magnesium and precipitate as calcium carbonate or magnesium carbonate. This precipitation increases the SAR in the soil solution because it lowers the dissolved calcium concentration. Table 3 indicates tolerable levels of bicarbonate in irrigation waters. The bicarbonate hazard of water may be express as Residual Sodium Carbonate (RSC), calculated thus

(6) RSC = (HCO3 + CO3) – (Ca + Mg)

 

where concentrations of ions are expressed in meq/L (see Equation 2 and Table 1 for conversions). Generally, water with an RSC value of 1.25 meq/L or lower is safe for irrigation, water with an RSC between 1.25 and 2.5 meq/L is marginal, and water with an RSC of 2.5 meq/L or more is probably not suitable for irrigation.

pH (hydrogen ion activity). Water pH is easy to determine and provides useful information about the water’s chemical properties. Although seldom a problem in itself, a very high or very low water pH can be a warning that you need to evaluate the water for other constituents. The desirable soil pH for most turfgrasses ranges from 5.5 to 7.0; most irrigation waters’ pH values, however, range from 6.5 to 8.4. Depending on the properties of the soil where the grass is grown, an irrigation water pH range of 6.5 to 7 would be most desirable. Water with a pH outside the desirable range must be carefully evaluated for other chemical constituents. Irrigation water with a high pH can also directly impact on the efficacy of some pesticides which suffer from alkaline hydrolysis and break down in the tank.

Chloride. Besides contributing to the total soluble salt concentration of irrigation water, chloride (Cl) may be directly toxic to plants grown on a golf course, park, or other landscape site. Turfgrasses tolerate all but
extremely high levels of chloride as long as they are regularly mowed.
Chloride salts are quite soluble, so they can be leached from well-drained soils with good subsurface drainage. As indicated in Table 3, irrigation water with a chloride content greater than 355 mg/L is toxic when absorbed by roots, while a chloride content greater than 100 mg/L can damage sensitive ornamental plants if applied to foliage.

Boron. Boron (B) is an essential micronutrient for plant growth, though it is required in very small amounts. Turfgrasses generally tolerate boron better than any other plants grown in a landscape, but they are more sensitive to boron toxicity than to either sodium or chloride. Most turfgrasses will grow in soils with boron levels as high as 10 ppm.

OTHER PARAMET ERS
Nutrients. With the exception of municipal recycled water, the nutrient value of most irrigation waters is negligible. Recycled waters, however, always contain a range of micro (trace) elements sufficient to satisfy the needs of most turfgrasses.

MANAGEMENT
NUTRITION
Fertilisers.
The choice of fertiliser can have considerable impact on turf and this is exaggerated when salt index is considered under the conditions of high salt. The basic management rule if faced with the issues discussed above is to stop contributing to the problem! The easiest means of achieving this is to take care when fertilising to use fertilisers possessing a low salt index as considerable variation can exist as for example in the case of potassium salts.

Slow and controlled release fertilisers can reduce issues associated with ‘up front’ fertilisers in relation to salt index and because of their high analysis considerably reduced quantities have to be used in comparison to say organic fertilisers to apply the same amount of nutrient.

As the table below shows all commonly used slow release fertilisers possess a low salt index at least comparable and in many cases considerably less than organic fertilisers. See Table 4.


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Read more articles in Consultancy, by Jerry Spencer or from December 2007.