September 15, 2010
By Jen Llewellyn
OMAFRA nursery crops specialist

As we passed through the latter part of August, it was difficult not to wonder where all those summer rains went? Although precipitation patterns were been hit-and-miss, several regions in south-western and south-central Ontario received an incredible amount of rain through regular intervals, since early June.  

My lawn didn’t start to go dormant until late August, and I didn’t water it at all. Along with the frequent rains, came very high relative humidity. And yes, it’s been hot. Stinking hot. The number of days that saw temperatures reach or exceed 29.5 C up until mid-August, makes my head spin: 22 days for Toronto (Pearson), 22 for Hamilton, 18 for London and 38 for Windsor

Lay off fertilizer

It’s a good idea to lay off the fertilizer applications when it’s this dry, especially in the field and landscape. Also, field-grown woody perennials shouldn’t receive nitrogen this late in the summer. The plant’s system needs to switch from active vegetative growth to preparing for leaf senescence and acclimation for winter dormancy. Another opportunity for nitrogen application is when leaf senescence begins, in early autumn.
Lower water volumes in late summer often mean high sediment, soluble salts and bicarbonates in irrigation water sources. Salt levels, including sodium, chloride and sulphates (Na, Cl, SO4), can have a negative impact on plant health if they reach certain thresholds (See table). High salts are becoming more of a problem in irrigation water sources close to roads (road salt), or areas that have been recently developed for residential or commercial purposes. Make sure you test your irrigation water for a full complement of salts. It is also paramount to test for the level of bicarbonates (HCO3).

Although pH is helpful to know, it is really the concentration of bicarbonates that gives the truest indication of the water’s alkalinity. You also need to know the level of bicarbonates if you are considering the treatment of your irrigation water with an acidifying agent. High levels of bicarbonates in irrigation water could lead to high pH levels in the water, container media and soil. And that could lead to problems.  

Since pH significantly influences the availability of nutrients, an alkaline pH (pH > 7) may provide scenarios where iron, manganese and zinc change to forms that are unavailable to the plant. We often see symptoms of iron and manganese deficiencies where soil and/or ground water pH is high.  

This is a great time of year to test irrigation water sources for quality and appropriateness for their use on ornamentals.  A water test at the driest part of the season will give you a worst-case scenario. Take a 500 ml sample near, or at the intake line and refrigerate the sample until it is received at the lab. Refer to the chart at right when trying to interpret results of your irrigation water test. Remember, the smaller the container volume, the lower the threshold for salts and bicarbonates in irrigation water.  

In container production, regular overhead irrigation has been necessary to keep crops going through the high daytime temperatures. Remember, the root zone temperature in containers is much higher than the air temperatures, especially in smaller pot sizes (one gallon and smaller). Irrigation water is cooling the plants in addition to replenishing the soil solution. Where irrigation water is high in bicarbonates, you will also see high pH values (often pH>7.5). Iron and manganese deficiency can be quite common in these scenarios and are especially prevalent in container production.  

Symptoms of iron/manganese deficiency appear as chlorotic (yellow) leaves with green veins, something we call ‘interveinal chlorosis.’ In container production, iron and managanese are most commonly the nutrients that are deficient. Quite often manganese is the nutrient that is deficient (iron is quite common in southern Ontario soils). During leaf emergence and before leaves harden off, interveinal chlorosis can be partially corrected with applications of chelated iron and manganese.  

Chelation is a form of the nutrient that is relatively resistant to high pH water and is available for plant uptake. Chelated iron and manganese can be applied as a 1) foliar in small droplet sizes in early morning or 2) root zone drench. The earlier you try to correct it, the faster you will see a response. Where you noticed problems with interveinal chlorosis and high bicarbonate irrigation water this year, consider remedial applications of chelated iron and manganese in May/June next year.      

Some fertilizers, chelated nutrients and pesticides can be inactivated by high bicarbonate water. Growers will often use acidified water to carry out nutrient and pesticide applications.  
Water analysis property *Acceptable range for most container woody crops  *Acceptable range for most herbaceous perennials/greenhouse crops
pH 5.0 - 7.0 5.0 – 7.0
EC (soluble salts) < 1.75 mmho/cm < 1.0 mmho/cm
Calcium carbonates < 150 ppm < 120 ppm
Bicarbonates < 150-200 ppm < 150-200 ppm
Sodium (Na) < 70 ppm < 60 ppm
Chloride (Cl) < 140 ppm < 100 ppm
Sulphur (S) < 30 ppm < 30 ppm
Sulphates (SO4) < 200 ppm < 200 ppm
Boron (B) < 0.8 ppm < 0.5
(*These ranges are a general guideline. Ornamentals vary greatly in sensitivity to salts and water chemical properties.)
Jennifer Llewellyn may be reached at 519-824-4120, ext. 52671, or by email