A great value

Many growers believe if a water source is good enough for them to drink from, it is good enough for their plants, too. However, the needs of plants and people are not the same. In some situations, potable drinking water can kill or weaken healthy plants.

Therefore, an irrigation water analysis should be a standard practice for every grower. It should include all their various water sources, and ideally will be performed before the season starts and prior to encountering any problems. In this way, the analysis will serve as the baseline for corrective action and for the monitoring of future activities.

Limestone in the growing media
An irrigation water analysis can help determine how much limestone will need to be incorporated into the growing media, whether home-made or a custom mix purchased from a reliable supplier. The rate of incorporation will factor in the crop being grown, the length of crop time in the mix, and the amount of alkalinity (bicarbonates) in the water.

Alkalinity can be reduced by using acid injection in the irrigation water (based on the lab results) or by reducing the amount of limestone in the mix. In some cases, the alkalinity is so low that it is beneficial to increase the quantity of lime as well as the coarseness of the lime being incorporated.

The analysis can further help determine the need for acid injection as well as the best rate of injection for the crops being grown. If there are two water sources — for example, one with 240 ppm bicarbonates and another with 100 ppm bicarbonates — this information could help identify which water source to use for propagation. It would also be useful information for determining the levels of a specific grade of acid to utilize for each water source based upon crop requirements.

A follow-up analysis can help determine if the correct amount of acid is being injected into the irrigation lines. For instance, if a grower is using sulfuric acid for neutralizing the bicarbonates, it should be possible to track a corresponding increase in sulfur levels, with a decrease in bicarbonates, as the acid rate is increased. One milli-equivalent of sulfur (16 ppm S) will neutralize one milli-equivalent of bicarbonates (61 ppm HCO3), regardless of whether a grower is using 35 percent sulfuric (battery acid) or 93 percent sulfuric acid.

Soluble salts
The analysis can also help determine the concentration of soluble salts in the water source as well as the make-up or distribution of the salts. The growth of plant roots can be adversely affected by a high concentration of total salts, which includes the salts in the water and the fertilizer solution.

Gypsum (calcium sulfate) and limestone (calcium and magnesium bicarbonates) are often found in water sources. These salts are included in the electrical conductivity, or total salts, and are generally not harmful to plant roots. However, sodium and chloride can be harmful to young plant tissues.

If the water source has high EC values due to these salts, it will be necessary to choose a fertilizer source with a low salt index and a fertility rate lower than that usually recommended on a constant basis. This is very important for young plants and crops like New Guinea Impatiens that are sensitive to high salt levels in the growing media and root zone.
 

Porosity
The irrigation water analysis can also help in making decisions about how porous the mix should be. High concentrations of sodium chloride, sodium bicarbonate, or boron salts may require an “open” mix that permits frequent leaching of accumulated salts, especially near the succulent bases of young plants. Salt crystals can precipitate on the soil surface or on tender stems and cause plant damage.

Beneficial nutrients
It is important to determine the level of beneficial nutrients in the water supply. Calcium, magnesium, and sulfur are common elements in many water sources and are required for growing good, healthy plants. These elements are not, however, always found in the more commonly used fertilizers. Growers have two choices when their water sources do not have an adequate supply of beneficial nutrients. They can incorporate them into the soil mix at the beginning of the cycle or incorporate them into the routine fertility program during production.

A good irrigation water analysis can determine the relative ratios of the elements in the water source. Calcium, magnesium and sodium are positively charged ions (cations) found in most water supplies. Most fertilizers contain ammonium and potassium cations, which are required in significant quantities to produce strong, healthy plants.

All of these cations will compete for absorption sites on plant roots. Generally, the ammonium and potassium cations are found in greater concentrations in the fertilizer solution than those in the water source. However, if there is an imbalance, it may be necessary on certain crops to use fertilizers with more urea-nitrogen or higher potassium levels. For instance, a 20-10-20 fertilizer may be the better choice for delivering adequate potassium (at 200 ppm N) than a 21-7-7 fertilizer due to the cations in the water source.
 

Toxicity
The water analysis may reveal elements in the water source that are toxic to one or more of the crops being grown. Plants require boron at a very low concentration. Since the concentration in most water supplies is minimal, it is a common practice to include a small amount of boron in the formulation of a complete fertilizer.

However, in some production areas the water source supplies adequate or excessive boron. High levels of boron can stunt roots and plant growth, cause marginal necrosis, and/or kill plants. Knowing the boron concentration can help determine which fertilizer to purchase (with or without boron) as well as which water source to use for specific crops.

Other elements of concern in the water supply include chlorides and fluorides, especially when crop sensitivities play a key role in the level of success.

pH values
The soil pH is important when determining the availability of certain elements in the soil solution for plant growth. However, it is the dissolved limestone in the water source and the resulting alkalinity (bicarbonates) that influences the soil pH, and not the pH of the water source. The pH value of a water supply provides no useful information for growers to use in making plant production decisions.
 

A substantial return on investment
Many growers have a pH/EC meter and can test for pH and soluble salts, and others use alkalinity test kits that can be helpful in determining the need for acid injection. However, these tests alone cannot address the total scope of issues a laboratory-performed analysis can provide. A professional irrigation water analysis can help growers analyze potential problems with their soil, fertilizer, or crops before a problem occurs. This small investment in time and effort is well worth the return of a healthy and marketable crop.