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The Garden Plot for November 02, 2007
3:55 PM - 4:00 PM
[Program Website]
Today's Highlight: “To Amend or to Not Amend - I”
To amend or not to amend, that is the question. When planting a tree or shrub, we always want to add something to help it along. In fact, when I was a kid, horticulturists recommended adding peat moss or pine bark to the planting hole when transplanting a tree. But there has been an about-face on the amendment issue. In 1979, researchers from Oklahoma presented experiments that busted apart the old rule of thumb, “Put a $5 tree in a $20 hole”. In fact, these researchers suggested that soil amendments added at planting might even result in worse tree growth. They compared treatments with added pine bark, peat moss, sand, or vermiculite to an untreated control with no soil amendments. After a full growing season, they found that the trees with no amendments had more roots than any of the other treatments. The Oklahoma researchers discovered that soil amendments helped water and roots penetrate easier in the area where the amendment had been added. The problem was that when tree roots hit the unamended area, they stopped growing. The result was a “potted plant” effect, with a dense mass of tree roots stuck in the amendment space, rather than growing out into the unamended area.
Researchers from Cornell tested peat moss against native soil and came to the same conclusion. But in the Cornell study, the unamended soil treatment also included a major modification of the planting hole. They dug trenches out from the planting hole on all sides of the tree in a big star shape. Not surprisingly, tree roots were happy to grow into the trenches filled with dug up and replaced native soil. The Cornell researchers found that soil type was a factor and, just as important, that different tree species responded differently to soil amendments. So the plot thickens. It is not as simply black and white as ‘amend-or-don’t-amend’ in all climates, soils, and with all tree species.
I wondered: why had no one tested compost? In 1995 researchers from Washington state did just that. Balled-and-burlapped red maple were planted in holes backfilled with (1) native soil (2) 50% aged pine bark (3) 50% compost and (4) 100% manure compost. The Washington researchers found that the 50% compost treatment increased root growth within planting holes after 5 months, but did not increase shoot growth. The treatment with 50% aged pine bark induced nitrogen deficiency symptoms and reduced shoot growth the first year. This is not too surprising because pine bark is very low in nitrogen. So, as the low nitrogen pine bark is broken down by soil microbes, the nitrogen-starved soil microbes use what nitrogen is available in the soil, essentially stealing it from the newly transplanted tree’s roots until the pine bark is completely decomposed. If the soil is low in nitrogen, yellow, nitrogen deficient leaves are often a result. In the Washington study, shoot growth of all treatments was the same after 2 years. Thus the researchers concluded that compost amendment did not help.
I disagreed. In our dry climate and short Montana growing season, I would be quite pleased with increased root growth within the first 5 months of planting, even if shoot growth was unaffected. So, we decided to test this more-complex-than-we-thought amendment issue in a Montana study. Next week we will continue to tweak the question: to amend or not to amend and present our data from 2007.
Helen Atthowe's new short program of gardening tips
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