The pH of water is measured on a scale of 0 to 14
- pH of 7.0 is neutral
- pH levels below 7.0 are acidic
- pH levels above 7.0 are basic
Each whole number difference represents a ten-fold difference in acidity. The pH of water along with alkalinity affects the solubility and availability of nutrients and other chemical characteristics of irrigation water. In general, most plants prefer slightly acidic conditions in a pH range of 5.0 to 7.0.
The pHc on a water test is an indicator of whether irrigation water is classified as a “stripper” or a “precipitator.”
- If the pHc number is greater than 8.4
- It is classified as a stripper – meaning as the water goes through the soil profile, it will dissolve the calcium and magnesium nutrients.
- The nutrients are stripped out, making them soluble.
- If the pHc number is less than 8.4
- It deposits and as the water comes through the soil, bicarbonates grab the calcium.
- As it dries up and is taken up by the plant, it becomes a solid, plugging up the soil and becomes calcium carbonate, acting as a precipitator.
Hardness is determined by the calcium and magnesium levels in your water. Since calcium and magnesium are essential plant nutrients, moderate levels of Hardness of 100 to 150 mg/L are considered ideal for plant growth. These levels of Hardness also inhibit plumbing system corrosion but are not high enough to cause serious clogging from scale formation.
Electrical Conductivity (EC or Soluble Salts)
Electrical Conductivity is a measure of electrical current carried by substances dissolved in water. Conductivity is also often referred to as “soluble salts” or “salinity”. As more salts are dissolved, water will better conduct electricity resulting in a higher conductivity reading
Sodium has many sources in water including:
- Road salt applications
- Waste waters
- Water softening wastes
- Naturally high pH waters dominated by sodium bicarbonates
High levels of Sodium can damage the soil and cause various plant growth issues. If water with excess Sodium and low calcium and magnesium is applied frequently to clay soils, the Sodium will tend to displace calcium and magnesium, resulting in the breakdown of the structure, precipitation of organic matter and reduced permeability.
Calcium concentrations in water are most often a reflection of the type of rock where the water originates.
- Groundwater and streams in limestone areas will have high Calcium
- Sandstone or sand/gravel water supplies will typically have low Calcium
Calcium below 40 mg/L typically need fertilizer applications of Calcium to prevent deficiency.
High levels of Calcium above 100 mg/L may lead to antagonism resulting in deficiency of phosphorus and or magnesium. High levels of Calcium may also lead to obstructions in irrigation equipment from scale formation (Clogged Heads).
Like calcium, Magnesium in water tends to originate from the rock and generally only causes problems when it is below 25 mg/L necessitating the addition of Magnesium though fertilizing. Magnesium can also cause scale formation.
Sodium Adsorption Ratio (SAR)
SAR is used to assess the relative concentrations of sodium, calcium, and magnesium in irrigation water and provide a useful indicator of its potential damaging effects on soil structure and permeability. Typically a SAR value below 2.0 is considered very safe for plants especially if the sodium concentration is also below 50 mg/L.
Alkalinity as CaCO3
- High Alkalinity above 150 mg/L tends to be problematic because it can lead to elevated pH of the soil which can cause various nutrient problems
- Low Alkalinity below 30 mg/L provides no buffering capacity against pH
This is especially problematic where acid fertilizers are used. Alkalinity in pond water can vary a great deal throughout the day if photosynthetic algae and plants are present.
The presence of high levels of Bicarbonates will precipitate with calcium when the soils are dry. The result is an increase of sodium relative to calcium. This will lead to the development of thin surface crusts where the sodium-dominated layer may be only 1/8’ thick, but can impede water infiltration and increases runoff.
- Bicarbonates are toxic to the roots and reduce the shoot growth of the turf.
- High Bicarbonates can also affect the effectiveness of fungicides and particularly insecticides that are sprayed. This causes the half-life of the product to often be reduced by high pH
- Bicarbonates also reduce the uptake of phosphorous and many other micronutrients that a variety of turfgrasses need.
- Bicarbonates react with calcium to form calcium carbonate. Every time a Bicarbonate attaches to calcium and magnesium, it keeps it in a carbonate form. In this carbonate form, it is hard for calcium to work into the soil profile.
High Potassium is generally not a concern for plant growth. Levels above 10 mg/L may indicate water contamination from fertilizers or other man-made sources. Water concentrations are useful simply for determining the overall fertilization requirements for plants receiving the irrigation water.
Chloride can occur in water supplies naturally or from various activities such as;
- Road de-icing
- Gas well drilling wastes, etc.
Chloride can damage plants
- Excessive foliar absorption (sprinkler systems)
- Excessive root uptake (drip irrigation)
Most plants can tolerate Chloride up to 100 mg/L although as little as 30 mg/L can be problematic in a few sensitive plants.
- Iron can be a complex water quality problem that not only affects plant growth but also can clog irrigation equipment.
- For micro-irrigation systems, Iron levels need to be below 0.3 mg/L to prevent clogging.
- Iron levels above 1.0 mg/L can cause foliar spotting in overhead irrigation systems.
- Very high Iron above 5.0 mg/L can cause severe staining and plant toxicity in sensitive species.
- Iron toxicity problems are most likely to occur where growth media is acidic (below pH 6.0).
- Induced Iron deficiency can also occur in sensitive species if pH is greater than 7.0 to 7.5.