Fish Are Friends, Not Food...

May-June 2009

YOU KNOW HOW LINES from favourite movies pop into your head sometimes? Well I have a 5 year old, so I watch a lot of animated films. My favorite is Finding Nemo and we watched it again last week. It’s amazing how many things remind me of this movie.

Recently, I unknowingly conducted an experiment on water deprivation of some violas – they were laying in the container in protest. I soaked them for 30 minutes and amazingly they sprung to life the next morning (again!). I performed a high pitched sing-songy “Just keep growing, just keep growing, grow-ing, grow-ing.”
I have such a mind-block when it comes to indoor plants. I can maintain anything outside but put it on my window sill and you may as well give it its last rights. There, I’ve admitted it, my dirty little secret. “Hello, my name is Jen. It’s been three weeks since I killed my last houseplant. I swear on my honour as a horticulturalist or you can use me as an organic fertilizer.” See what I mean?

Fertilizing trees in the landscape can be a fantastic opportunity to help our landscape plants overcome deficiencies and support growth. Frequently, arborists will carry out a soil test for the macronutrients N-P-K (nitrogen, phosphorus and potassium) and some micronutrients like iron, manganese and zinc (Fe, Mn, Zn). Many landscape companies already have a database of soil samples from several neighbourhoods in the areas they serve.

Typically, nitrogen is the nutrient that plants use the most of and it should be the main focus of any fertilizer. Nitrogen is not usually reported on a soil test because soil nitrate nitrogen can be quite variable and is not reflected accurately in soil tests. Recommendations for nitrogen usually run up to 1.5-2.0 actual nitrogen per 100 m2. Woody and herbaceous perennial landscape plants do not need large amounts of nitrogen because it can stimulate excessive vegetative growth leaving plants weak and susceptible to pests, winter injury and breakage.

Although phosphorus is a macronu-trient, about 95% of the soil tests that I’ve seen indicate that there is already an adequate amount of phosphorus in the soil. The days of 10-52-10 and other high phosphorus fertilizer formulations for landscape soils should be over now that we know better. We also know that high levels of phosphorus can actually inhibit successful inoculations of mycorrhizal fungi. Plants only require small amounts of phosphorus at a time and if we add more than that, they can’t take it up. (It is important to note that phosphorus is still an important component for soilless container mixes.) Typically soil tests that are found to be <15 ppm for phosphorus are considered to be low.

Potassium is an important macronutri-ent and many soils can benefit plant growth through the addition of low to moderate amounts of this nutrient, especially at the end of the season. Soil tests will help determine amounts to be added. Typically soil tests that are found to be <100 ppm for potassium are considered to be low.

What about other macronutrients? In most cases, a typical southern Ontario soil test usually reports adequate levels of calcium and magnesium. Calcium levels are usually higher than magnesium. Typically, these two nutrients may be low in coarse sandy soils that have an acidic pH, and are low in organic matter, or in prepared soil mixes. Where a deficiency in either calcium or magnesium has been identified, it is important to provide the deficient nutrient. Despite moderate to high levels of calcium in our alkaline soils, quite often we’ll find that fertilizers containing some calcium (and potassium) may impart some added disease resistance to our ornamental plants. This effect will be more pronounced on soils that hover around pH 7 and below.

What about micronutrients? Many soils in southern Ontario are considered alkaline having a high pH (greater than pH 7) because of the underlying parent material which is composed of calcium and magnesium. Incidentally, this is also what makes our ground water so “hard” or alkaline. Sometimes we find that manganese (Mn) and iron (Fe) may not be available to plants because they are in an unavailable form in our alkaline soils. Iron and manganese deficiency are fairly common in our high pH soils and symptoms of their deficiency include interveinal chlorosis (see photo). Red oak, pin oak, paper birch and rhododendron are common examples of plants showing interveinal chlorosis on our alkaline soils.

How will you know whether the deficient nutrient is iron or manganese? You will need to do a foliar nutrient test. By adding additional iron and manganese to the soil, you would think the problem would be solved right? But no! The high soil pH will cause these nutrients to become unavailable before the roots have a chance to absorb them. The solution is to use chelated forms of iron and manganese. Chelated iron and manganese remain available to plants at high soil pHs. In some cases, using a chelated product containing both iron and manganese will help keep landscape plants green and healthy looking. You can add diluted chelated products to the soil directly or as multiple foliar sprays.

Applying Fertilizer
The best way to add fertilizer is in small amounts, splitting up applications between two or three times per season can help maximize root uptake of nutrients and minimize loss. We used to think that applying fertilizer as soon as the snow was gone was the best way to ensure nutrients were available for spring growth. Now we know that the nitrogen is actually lost to the air in those cold, wet soils. Or even worse, the soluble nitrogen runs off to other areas.

Many perennial plants (including trees) rely on the nutrients they absorbed the previous growing season to support the spring flush of growth. This makes fall fertilizing a practical option for maximizing nutrient uptake. So by splitting annual fertilizer applications between May, June and October, we can really help plants get what they need and reduce the amount of waste. If that seems too labour intensive, another option is the use of controlled release fertilizers. These products are coated to ensure a more gradual release of nutrients over the growing season.

A current list of OMAFRA accredited soil and tissue testing laboratories can be found at:

Contact Information

This column is written by Jen Llewellyn, Ontario Ministry of Agriculture and Food and Rural Affairs (OMAFRA) Nursery Crops Specialist

Email any questions you have directly to Jen and we'll publish her response.

P: 519-824-4120 ext. 52671 • F: 519-767-0755

OMAF website:

Nursery Landscape Agriphone: 1-888-290-4441

Our mission is to enhance and promote the care and benefit of trees for present and future generations in Ontario through education, research and awareness.