Improve Your Garden Soil with Organic Matter

When planting trees and shrubs, should I or should I not add any organic matter into the planting hole?

In recent years, many of the problems associated with trees in urban and suburban areas have been traced to poor planting techniques. Essentially, planting trees in the old-fashioned hole in dense compacted soils like those often found around new construction sites and filling the holes with amendments such as peat moss or compost can cause the roots to grow in tangled circles in the amended area. Any roots trying to grow out, can’t, when they reach the denser original soil.

It is important to select either the right tree for the location, or if your heart is set on a particular tree, to make sure that is is planted in a suitable spot. Once you’ve found the right spot, loosen the soil by digging or tilling to a depth of at least 12 inches over an area as much as three times the diameter of the root ball. If your soil is very difficult to work with, then you could add some amendment at this time. Use it uniformly throughout the area and mix it thoroughly into the soil. There’s no need to add any fertilizer at this time.

Adding Organic Matter Improves Soil

The addition of organic material improves looseness and workability of soil. Heavy, tight clay soils benefit from the loosening effects of organic materials. And, compost also contains nutrients that help plants grow.

There are many debates as to whether you should use organic matter when planting new trees. A number of studies have shown that the more similar the soil in the planting area is to the native soil, the better the long-term health of the tree.

To decide if you need to add organic matter, consider the following factors:

1) Assess the size of the tree when mature. Contrary to popular belief, a tree’s roots are not a mirror image of its shoots. Root systems can extend three and four times the diameter of the drip line. For large specimen trees like oaks or standard fruit trees, it would be impossible to amend the entire root system of the tree. By comparison, many smaller trees and shrubs, including roses and fruit trees on dwarfing root stocks, have extremely compact root systems at maturity that are confined to a small area. With these plants, it is quite practical and affordable to amend the planting area.

2) Assess the condition of the native soil. Adding compost or organic matter to the planting area can contribute significantly to the success of the planting if the soil has poor structure, drains poorly, or is excessively sandy. Incorporate the amendment in a large area around where you’re going to plant the tree. A simple rule of thumb is, the heavier the soil, the less amendment should be used (as long as the drainage is good.) If you are lucky enough to have rich, fertile soil, then no amendments should be required.

Once the area has been loosened, dig a hole no deeper than the depth of the root ball of the tree you are going to plant. To prevent the tree from settling, do not loosen the soil beneath the root ball. However, if you want, the bottom of the planting area around the root ball could be loosened where practical. The tree can be set in the hole with just the upper surface of the root ball at the same level as the existing soil. Care should be taken when handling the root ball or it could be damaged. Roots circling the root ball should be carefully spread out, being careful not to break off any major roots. If you are planting bare root plants, gently spread the roots out in the hole. Cut off any damaged roots.

Fill in around the root ball with the soil from the hole, and gently pack it down to get out any air pockets. A good watering will help settle the soil and ensure contact between the soil and roots. Rake the soil around the area, and cover it with two to four inches of mulch. Be careful to keep the mulch away from direct contact with the trunk.