GET CULTURED: Fertilizer Management When using Secondary Water Sources

by Donald J. Merhaut

Historically, fertilizer management has been based on clean water sources which contributed minimal amounts of the essential plant nutrients. In addition, these clean water sources usually had insignificant concentrations of impurities such as sodium, and other nonessential elements. Now that high quality water sources are limited, less pure secondary water sources such as reclaimed water, recycled water or well water containing high concentrations of salt may need to be utilized in plant production. If the quality of these waters cannot be improved, then there are some plant nutrition concerns that will need to be monitored and/or corrected so that plants receive proper nutrition. While there are many pretreatment options of improving the quality of secondary water sources, which we have discussed in past articles, this portion of the newsletter will focus only on the fertilizer options that may help alleviate some problems associated with poor water quality.

Secondary water sources may compromise nutrient availability in 4 primary ways:

1)    Water pH – If pH is too high or low, it may impact the stability and thus the solubility of compounds, especially chelates.

2)    Electrical conductivity (EC). High salt concentration (>1 mhos/cm or dS/m EC) or a Total Dissolved Solid (TDS) level > 640 ppm, will become a problem, especially if fertilizer is supplied through the irrigation supply.

3)    High sodium (Na⁺) and chloride (Cl^-) – High concentrations of Na⁺ and Cl^- will accumulate in oldest leaves and cause tissue necrosis. Also, Na⁺ is a positively-charged element, and therefore may limit the uptake of positively-charged essential nutrients such as potassium (K⁺), and ammonium-nitrogen (NH₄⁺-N).

4)    Boron (B) – Some groundwater supplies are high (0.5 ppm) in boron and can cause plant toxicity. 

The symptoms and solutions

Water pH –

Problem symptoms. If the water pH gets too high (>7.0) or too low (<4.5), micronutrients may precipitate out. Low pH scenarios may occur when water is acidified before passing it through heat exchangers, a heating treatment done to sanitize water sources. In either case, if precipitation is a problem, it will usually be evident in the plants as chlorosis of the new growth, primarily because of iron deficiency. Evidence of micronutrient-chelate precipitation may be found in water storage tanks, especially in hydroponic systems – there will be a rusty- orange film developing on the bottom of the tanks. This orange slime is the denatured chelates and the micronutrients. When a chelate is ‘denatured’, its’ molecular structure is irreversibly changed by heating, which inactivates its’ ability to bind micronutrients and causes the chelate and micronutrient to form a solid. The analogy of this would be similar to egg whites, which, when raw, are a clear white liquid; however, when egg whites are cooked, they become a solid white ‘omelet’!

Treatment – If water pH is high, and acidification is not an option, consider the selection of chelates that are stable at a higher pH. (See Table 1). As seen in Table 1, chelates such as CDTA, DTPA and EDDHA will be stable at pH 7.0 or higher. Another option would be to choose acid forming fertilizers. These usually contain sulfates and ammonium-nitrogen, which will acidify the media.

Table 1. Chelates, chemical formula, molecular weight (M.W.), formation constants, and the pH range at which the chelate usually forms a stable complex with iron (Fe). The formation constant indicates the ability of the chelate to bind each of the nutrients: the higher the formation constant, the higher the ability of the chelate to or ‘hold onto’ the micronutrient and keep that micronutrient in solution, rather than the micronutrient binding to another compound and precipitating out of solution. (Bachman G.R. and W.B. Miller. 1995. Iron chelate inducible iron/manganese toxicity on zonal geranium. Journal of Plant Nutrition 18(9):1917-1929; Norvell, W.J. 1971. Equilibria of metal chelates in soil solutions. pp. 115-138. In: J.J. Mortvedt, P.M. Giordano, W.L. Lindsay (eds.) Micronutrients in Agriculture. Soil Science Society of America. Madison, WI.).

High Electrical Conductivity (EC).

Problem symptoms. The concentration of salts in irrigation water can cause several problems. An ideal EC is 0.2-0.5, especially for seedlings and other young plants and plug production. This lower EC range also allows most operations to safely incorporate fertilizer into the irrigation water without major elevations in EC. Most crops can tolerate >1.0 mS/cm. Usually crops in the families: Ericaceae, Theaceae and Proteaceae have close mycorrhiza associations and therefore have a lower EC tolerance ~1.5 mS/cm because of apparent issues with mycorrhiza and soluble salts. Many other more common crops, especially woody perennials, will tolerate a water EC ~2.0 to 3.0. Symptoms of high salts include root dieback, necrosis of oldest leaves (where salts will accumulate), and plant wilting.

1. From a fertilizer perspective, keeping plants well fertilized so that growth continues will sometimes appear to ‘mask’ the symptoms of the necrosis of the oldest foliage. Some growers will do this on faster-growing crops. Fertilization through Controlled Release Fertilizers (CRF) rather than injecting fertilizer into irrigation water will minimize EC elevations. Another recommendation is using media which is very well drained, such as substrates containing cedar shavings, and resorting to more frequent irrigation episodes. This allows the grower to supply water needs of the crop, while simultaneously leaching the salt-laden irrigation water from the media.

High Sodium (Na⁺) and Chloride (Cl^-) –

Problem symptoms. Excess salts, including Na and Cl, will accumulate in the oldest leaves causing necrosis, leaf curling, marginal leaf scorch and leaf drop. In severe cases, plants may also wilt.

Treatment. The treatment options are similar to those recommended for water with high ECWhere possible, select fertilizers that have no Na- or Cl-containing compounds.

High Boron –

Problem symptoms. Boron (B) toxicity will be expressed in plants with symptoms usually on older leaves: brown blotches on leaf tips, necrosis, cupping and curling of leaves, or marginal chlorosis in more severe cases. Occasionally, even new growth may exhibit B toxicity symptoms. Boron toxicity in plants can occur at concentrations in water as low as 0.5 ppm. Knowing the B sensitivity of the crop is important, as some plant species are much more tolerant of higher B concentrations. Usually recommendations are to conduct a tissue analysis; however, , even within a species, boron levels can be quite variable and may not provide any helpful information.

Treatment: Boron is taken up into plants as electro-neutral boric acid (H₃BO₃). There are no known adjustments in fertilizer programs to mitigate boron toxicity. The only method to remove B from the water supply is to increase water pH above 9.2. At this high pH, the majority of the boric acid is converted to borate (H₂BO₃^-), a negatively charged compound which can then be removed through anion exchange systems. A more feasible solution, , where B toxicity occurred with zinc (Zn) deficiency in the field, foliar and soil applications of Zn did alleviate boron toxicity symptoms (Nable, R.O., Bañuelos, G.S. and J.GH. Paull. 1997. Boron toxicity. Plant and Soil 193:181-198.

In conclusion, there are management options for the fertilizer programs to optimize plant growth that will help alleviate issues associated with poor water quality (improper pH and high salts, boron, sodium and chloride). Depending on the water quality concern, these options include:1) selecting fertilizers with lower levels of nonessential salts (Na, Cl). 2) choosing chelates with suitable pH tolerances, 3) utilizing controlled release fertilizers rather than fertigation.

Caution!! In all cases, close monitoring and proper record keeping of plant growth and water quality are important. It is always recommended to conduct a small trial to evaluate the performance of new fertilizer, media, and irrigation methods on one’s cropping system before investing on changing these horticultural programs on the entire operation.

Don Merhaut is a UC Cooperative Extension Specialist for Nursery and Floriculture Crops, Department of Botany and Plant Sciences, UC Riverside.