Soil Prep for New Tree

(Question)

We had a pine tree die last year. I would like to plant a native tree in its place. Perhaps an Allegheny Serviceberry. It will be in full sun. Should I be concerned with the acidity of the soil? Almost all the pine needles dropped from the tree before we took it down. The soil seems sandy on top and clay about 2-3 inched down. We live in Kingston Ontario. Thank you

 

(Answer)

Thank you for contacting the Toronto Master Gardeners with your inquiry.

Allegheny serviceberry ( Amelanchier laevis) is an excellent choice if you are looking for a small native understory tree with four-season interest. The early white spring flowers, outstanding orange-red fall color, and striking gray bark makes it a lovely specimen for any landscape. As an added bonus the edible purplish-black fruit in late summer is attractive to many birds.

There have been many articles written suggesting excess soil acidity results from conifer needles, but there is little evidence to support this. An excellent article titled Garden Myth: Pine Needles Acidify the Soil illustrates how the soil pH is not much influenced by the presence of pine needles. In fact, the article goes on to state that the reason plants do not grow well under pine tress is due to the fact that conifers produce very dense, very superficial roots reaching out in all directions. These roots quickly absorb any rainwater that falls and any minerals available in the soil. The soil under a pine is therefore in a permanent state of drought and mineral deficiency.

You may wish to refer to Conifer Needles Good for the Soil  written by Judith Adams in which she states “I’ve been gardening here for 27 years, and in that time have never swept up any of the needles for disposal. Most remain where they fall on soil or lawn, making a soft mulch. I also collect fallen needles and use them as an organic amendment to improve the texture and drainage in my clay loam soil.

 The foliage that drops from conifers in autumn is my most valued material to help increase drainage and bring more oxygen into garden soil. Using the needles as a soil amendment in planting holes prevents soil from compacting around root balls. Needles incorporated into soil increase the movement of water and gases, and contribute to a biologically active soil environment that favours root growth. Conifer needles are slow to decompose underground, and will continue aerating soil for several seasons before completely degrading

Before beginning to plant it is important to understand that soil is not a sum total of it parts but is rather an ecosystem. It takes many years to create a healthy soil.

When planting your new shrub, especially if it is in a pot, it should be thoroughly soaked with water to loosen the roots and get them ready for their new environment. The addition of transplant fertilizer is designed to encourage the feeder root growth, and comes in a ratio of 5-15-5. The middle number is phosphorus, and is what is needed to encourage the roots to grow.

The size of the hole should be at least twice as wide as the current container – wider is even better. The purpose is to loosen as much surrounding soil as possible so that the roots can easily grow while absorbing plenty of water, nutrients and air.

The hole should be as deep as the container. You can measure the depth by placing the container into your hole. Don’t dig much deeper than the container. Anytime the soil is loosened, it eventually resettles. If you have dug a hole that is too deep, the tree will sink when the ground resettles. This can lead to disease because the trunk, unlike the roots, is not meant to be buried in soil.

Mulching to a depth of 10 cm around the tree, without the mulch actually touching the bark (prevents moisture build up and the possibility of fungal growth on the bark), will keep moisture in the soil.

You may also be interested in our  Gardening Guide: Planting a Tree 

Hope you have many years of enjoyment of this beautiful tree.