Good Nutrition

What does it take to make a tree?  Part of the answer lies in the soil.  It was my job last week to guide a group preparing to become Certified Arborists into the world of tree nutrition.  We started with the Periodic Table, noting that fourteen soil-borne elements are essential and four are beneficial to the happiness of a tree.  I wanted to sing my way through the Table, a la Tom Lehrer in 1959, but I spared everyone that pleasure and just concentrated on the dry facts of plant science.

Like most other plants, a tree’s greatest nutrient need from the soil is nitrogen, which is a key component of many functions, including photosynthesis.  Nitrogen can also be a limiting factor, since it is a rather slippery character and sometimes in short supply.  Microbes and lightning can put nitrogen into the ground, but it can leach below the roots or return skyward through volatilization.  Among its many other benefits, organic matter (such as compost) is beneficial to plants since it supplies nitrogen.  Plants deficient in nitrogen will show  stunted growth, reduced root systems and yellowing older leaves, since the little nitrogen existing in the plant will move to the newest leaves and shoots in an attempt to keep them green.  

Another important element is iron.  At a young age, I remember my oldest living relatives (born in the nineteenth century!) speaking of having “tired blood,” which may have been iron deficiency.  Plants can experience this, too, especially those we call acid-lovers, which include blueberries, rhododendrons, azaleas, and mountain laurels.  When soil pH is relatively low, or acidic, iron is freely available and these plants do well.  Raise the pH, however, and the iron in the soil becomes less readily available, and the acid-lovers suffer.  The primary symptom, once again, is yellowing foliage, this time throughout the plant  and in concert with distinctly green veins.  Horticulturists call this “interveinal chlorosis,” a term I encourage you to drop at your next cocktail party in order to raise the comment, “this guy must be some sort of plant doctor.”

In the tree world, the classic example of iron deficiency is pin oak (Quercus palustris).  This normally attractive tree is sometimes planted along city streets, since it can take some of the abuses urban life hurls at green plants.  Unfortunately, many factors raise the soil pH in an urban tree pit, including the aging of sidewalks, which are made from lime-based materials.  It is therefore not uncommon to see a bright yellow pin oak in a state of full, glorious iron chlorosis. 

Fixing an iron deficiency in both people and plants is possible.  While great-grandmother took a swig of the potion bought from the traveling medicine man, gardeners can lower the soil pH with sulfur, which makes more iron available and yellow plants greener.  A faster fix is to spray the plant with chelated iron, which will re-green it almost overnight, a cool trick.  It’s no wonder plant people are the life of any party.