Q & A October 2001
by Ed Perry, Farm Advisor - Stanislaus County

Q.We are planning to remodel the front entry to our home without causing irreparable harm to a mature shade tree. If some tree roots are cut, will it hurt the tree?

A. First let's discuss a tree's root system, then the affect of cutting roots. It is well accepted now that tree roots do not normally grow more than 3 or 4 feet deep. Most grow radially outward and mostly horizontal to the surface. Vertical growing roots near the base of the tree may grow down to 5 feet or more in sandy, well-watered soil. These roots take up water during dry periods and provide structural support, especially if the lateral roots are cut.

The large buttress roots which originate at a tree's base decrease rapidly 4 to 10 feet from the trunk to form a network of long, thin ropelike roots 8 to 12 inches below the surface. These roots branch and rebranch to form an extensive network of absorbing roots, extending outward 1 to 2 times the height of the tree. The greatest concentration of active roots is beneath the leaf canopy, also called the drip zone.

Cutting large roots near the trunk is much more injurious than cutting smaller roots beyond the area of rapid taper. Few large roots are found beyond 10 feet of the trunk. Many of the tree's water absorbing roots are under and close to the tree's base. Because of their depth and location, these roots are less likely to be accessible to cutting or damaged by construction.

Root loss due to root pruning reduces tree growth, health, aesthetics and stability. The destruction of roots just outside the drip zone on one side of a tree represents about a 15% loss. If roots are cut midway between the drip zone and trunk on one side, about 40% of the roots will be lost. Severe root loss (50% or more) usually causes immediate water stress and reduces photosynthesis (food production). Growth is reduced, dieback and even death may result.

Young, healthy, vigorous trees can survive severe root loss while large, old or declining trees may not. Recovery following the stress of severe root loss depends on rapid root replacement. Root growth requires adequate food reserves, growth stimulating hormones, water and minerals. If these are available, and there are no other restrictive influences or construction impacts, root growth and replacement will generally proceed rapidly. Low or depleted food reserves will delay root replacement. If the soil conditions have been seriously altered by landscaping or building construction, root replacement will be slowed or stopped. A delay in recovery from root loss will result in growth loss, dieback or tree death.

The worst time to cut roots is just prior to bud break in the spring because growth hormones are not present in the roots to stimulate root growth. Avoid cutting roots later in the spring when food reserves have been nearly depleted by leaf growth. Root growth proceeds most rapidly in the summer and fall when top growth has slowed, food reserves are high and growth hormones are present in the roots.

Q.How can I lower the pH of my soil?

A. Soils with a pH of over 7.0, generally referred to as alkaline soils, are common in parts of the San Joaquin Valley. Some favorite ornamental plants such as azalea, camellia, gardenia, pin oak, red oak, rhododendron and sweet gum are better adapted to acidic soils and when grown in alkaline soils are often iron or manganese deficient.

It is easier to use plant material adapted or tolerant of your soil condition than it is to change the soil. However, it is possible to lower the soil pH by the use of certain amendments. Soil sulfur is an amendment used by landscapers to acidify soils, often in existing landscapes where plants are declining because of iron or other micronutrient deficiency.

The amount of sulfur needed to lower pH is dependent upon the soil's texture and present pH. Table 1 below gives approximate amounts of soil sulfur to use under differing soil conditions.

Table 1. Approximate amounts of soil sulfur (95%) needed to increase the acidity of the 6-inch layer of a carbonate-free soil.

Change in pH	Desired Pounds of Sulfur per 1000 square feet
	SAND	LOAM	CLAY
8.5 to 6.5	46	57	69
8.0 to 6.5	28	34	46
7.5 to 6.5	12	18	23
7.0 to 6.5	2	3	7

Source: Western Fertilizer Handbook, Hort. Ed. 1990. CA Fertilizer Assoc. Interstate Publishers, Inc.

If you use sulfur, keep in mind that the purity of the material and degree of fineness are important. Sulfur is usually 95% pure. Sulfur must undergo chemical changes in the soil (oxidation) before it can be effective in lowering pH. The finer the material, the faster it will be oxidized, since finer materials have greater surface area. Finely divided sulfur particles can oxidize in one season, while coarse material may take years to oxidize.

October 11, 2001