Cyberconference: Water Quality for Woody Plants, Part 2 -- Mathers

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Other sessions:
Monitoring quality of irrigation water

How air, water and media work together

Managing soluble salts

Water quality for woody plants, Part 1

Subirrigation: An irreversible trend

Subirrigation and nutrition for poinsettias

Irrigation considerations for container plants

Alkalinity control for irrigation water

What You Should Know
About Water Quality
For Woody Plants: Part 2

By Hannah Mathers
Oregon State University

In certain areas of Oregon, a grower may encounter three general problems in water quality: high salt content, high pH and high alkalinity. In Part 1 we discussed high salinity and the desirable ranges of specific elements. This article will review high pH and high alkalinity.

Remember alkalinity and pH are not the same. The characteristic of water that will have a bearing on the change in medium pH is alkalinity, not pH. Alkalinity for all practical purposes, is the "buffering capacity" of the water. A buffer solution is a solution that has the capacity to resist change in pH (usually a decrease). The greater the alkalinity of water, the greater is the buffering capacity of that water and the tougher it is to acidify. Alkalinity is measured in units expressed as mg/l of calcium carbonate. As the units imply, the source of alkalinity is usually from bicarbonates (HCO₃^-) and carbonates (CO₃^-2). Both of these components will cause growing media to increase in pH if present in sufficient amounts.

How Bicarbonates Increase Soil pH
When soils dry that have been irrigated with water high in bicarbonate, calcium and magnesium combine with the bicarbonate to form calcium and magnesium carbonates. These are insoluble salts and are not readily leached from the soil. With each irrigation, more calcium and magnesium carbonates are added to the soil. As they accumulate, they ease the soil pH upward, above pH 7. High soil pH levels make zinc, iron and manganese less available to plants. Because the relatively small soil volumes involved, the pH rise is more rapid in containers and in greenhouse bench soils than in field soils.

The rate and extent of pH increase will depend upon media formulation, watering practices and fertilization practices. The most important point is "high pH does not determine the capacity of irrigation water to increase a potting medium's pH during production – it is the water's alkalinity."

Waters that contain less than 50 mg/l of bicarbonate usually do not have a serious effect on soil pH. Water containing concentrations of bicarbonate above 50 mg/ l cause pH increases in growing mediums and need to be acidified. Work closely with a consulting laboratory if acid injection is necessary.

Determining Acidification Needs
If you experience periodic iron or micronutrient deficiencies, total carbonates may be one of the causal factors. If your media pH values constantly run too high, the carbonate and bicarbonate levels in your water may be the cause of this problem. If certain crops that prefer a lower pH (see Table 2 for some crop pH preferences) turn yellow, evaluate the total carbonates in your water. See Table 1 for additional information on acid injection.

Table 1: Acid Injection into Irrigation Water
Formula: A X B X C = ounces of acid / 1000 gallons of water to adjust pH to approximately 6.4.
"A" is a factor determined by water pH.
Water pH	A	Water pH	A
6.7	0.249	7.7	0.475
6.9	0.342	7.9	0.484
7.1	0.400	8.1	0.490
7.3	0.437	8.3	0.494
7.5	0.460	8.5	0.496
"B" = the sum of bicarbonate and carbonate expressed as milliequivalents / liter (meq).
"C" = A factor determined by the type acid used.
Acid Source		C
75% Phosphoric		10.60
85% Phosphoric		8.74
93% Sulfuric		3.72
61.4% Nitric Acid		15.6
Example: Water pH = 7.5, carbonate + bicarbonate = 3.4 meq A X B X C = ounces of acid / 1000 gallons 0.460 X 3.4 X 10.6 = 16.5 ounces of 75% Phosphoric acid / 1000 gallons of water Source: Vaughan's Seed Company 1993

Table 2. Soil Reaction (pH) Preferences for Selected Plants

Common Name Botanical Name pH Preference

4 5 6 7 8

Shade and Flowering
Trees

Crabapple, showy Malus floribunda
XXXXXXX

Dogwood, flowering Cornus floribunda
XXXXXXXXXX

Holly, English Ilex aquifolium XXXXXXX

Honey locust Gleditsia triacanthos

XXXXXXXXXXXXX

Magnolia, star Magnolia stellata
XXXXX

Maidenhair Ginkgo biloba

XXXXX

Maple, sugar Acer saccharum

XXXXXXXX

Oak, red Quercus rubra XXXXXXXX

Oak, white Quercus alba
XXXXXXXX

Sweet gum Liquidambar styraciflua

XXXXX

Sycamore, American Platanus occidentalis

XXXXXXXX

Tulip tree Liriodendron tulipifera

XXXXXX

Witch hazel Hammamelis virginiana

XXXXXX

Evergreens

Arborvitae, American Thuja occidentalis

XXXXXXXXXX

Fir, Douglas Pseudotsuga menziesii

XXXXX

Fir, Fraser Abies fraseri XXXX

Pine, red Pinus resinosa XXXXXX

Pine, Scots Pinus sylvestris XXXXXXXX

Spruce, Colorado Picea pungens
XXXXXX

Spruce, white Picea glauca
XXXXX

Fruit Plants

Apple, common Malus pumila
XXXXXXX

Cherry, sweet Prunus avium

XXXXXXXX

Peach Prunus persica

XXXXXXXX

Pear, common Pyrus communis

XXXXXXXX

Source: Davidson et al., 1998

Warning
If acid injection is necessary, use proper eye and skin protection. Add acid to water never water to acid. Remember that acids also supply plant nutrients (see Table 3) and you may have to modify your fertility program. Acid injection may change the solubility of trace elements in your water. Analyze your water after acid injection and confirm that you have achieved the desired results.

Table 3: Additions of various elements to irrigation water, based on acid type

1 ounce 75% Phosphoric / 1000 gallons water delivers 2.8 ppm –P
1 ounce 85% Phosphoric / 1000 gallons water delivers 3.4 ppm –P
1 ounce 93% Sulfuric / 1000 gallons water delivers 4.2 ppm -S

Source: Vaughan's Seed Co., 1993

pH Monitoring an Important Part of IPM
Probably no single characteristic of soil is more significant to disease severity than pH (Chapman 1970). Solubility and availability of many chemicals are strongly influenced by soil pH. For example, initial effects of acidification are usually to increase solubility of calcium, magnesium, sodium, and phosphorous, as well as zinc, manganese, boron, lithium, copper, iron, nickel, and other elements. Effects of nitrate and ammonium on Fusarium spp. are apparently related to soil pH (Schuerger and Mitchell 1992); nitrate causes an elevation in soil pH, whereas ammonium causes a reduction (Woltz and Jones 1981). Plants grown in soil receiving nitrate plus lime had less disease than those receiving ammonium plus lime (Woltz and Jones 1973a). Effects of lime and nitrate in controlling Fusarium are most likely related to lowering the concentrations of other nutrients in soil solutions, mostly phosphorus, magnesium, sulfur and copper. Lacking these nutrients and iron, manganese, and zinc in adequate amounts, Fusarium spp. are less likely to establish significant inoculum levels (Woltz and Jones 1981).
High pH usually results in more severe disease caused by Phytophthora spp. (Schmitthenner and Canady 1983). Application of sulfur to reduce soil pH has been quite effective in controlling some Phytophthora diseases. For example, diseases caused by P. cinnamomi are controlled when soil pH is below 3.8 (Bingham and Zentmyer 1954). Of course, lowering pH with sulfur can only be used when acid-tolerant plants are grown.
Growers need to carefully monitor disease and make sure that fertilizer regimes and water quality are not aggravating diseases. Excessive fertilization may not be detrimental to crops, but may aggravate disease by providing nutrients for buildup of soil-borne pathogens. Therefore, careful controls of fertilization and water quality are important parts of integrated pest management in nurseries and greenhouses.
Summary
The take home message of this and the first article is have a "Water Quality Action Plan." That "Action Plan" should contain these five steps: 1) have your irrigation water laboratory tested at least once a year; 2) compare your test results with the standards stated in the May article; 3) consider whether acid treatment will improve your water quality and which acid is best for your situation; 4) make adjustments to your water and fertilizer practices based on steps one to four; and, 5) always use pH and EC meters to make well-informed decisions, don't guess.

References
Bingham, F.T. and G.A. Zentmyer. 1954. Phytopathology 44: 611-614.
Chapman, H.D. 1970. In: Ecology of Soil-borne Plant Pathogens: Prelude to Biological Control. University of California Press. Berkeley. pp. 120-139.
Davidson, H., Mecklenburg, R. and C. Peterson. 1988. Prentice Hall. Englewood Cliffs, NJ. pp. 190-194.
Farnham, D.S., Hasek, R.F. and J.L. Paul. 1985. Cooperative Extension University of California. Berkeley, CA. Leaflet 2995. pp.1-14.
Schmitthenner, A.F. and C.F. Canady 1983. In: Phytophthora: Its Biology, Taxonomy, Ecology and Pathology. American Phytopathological Society. St. Paul, MN. pp. 189-196.
Schuerger, A.C. and D.J. Mitchell. 1992. Canadian Journal of Botany 70: 1798-1808.
Woltz, S.S. and J.P. Jones. 1973. HortScience 8: 137-138.
Woltz, S.S. and J.P. Jones 1981. In: Fusarium: Diseases, Biology and Taxonomy. The Pennsylvania State University Press, University Park. pp.340-349.