Planting in Native Soil May Be Best Bet : Conservation: Some experts feel that organic amendments may be harmful when they lower the soil’s water-retention capabilities.
Planning a water-wise garden, may begin with how you prepare the soil.
There is a growing body of evidence that adding organic amendments to the soil may not contribute anything and may, indeed, be a bad idea--at least in some cases.
Said a leading proponent of xeriscape--Randall Ismay of Landscape and Water Management Consultants in Los Angeles and Laguna Niguel: “We recommend planting in 100% native soil.”
Caltrans is trying this idea on some sites, planting in what they call an “unprepared hole.”
According to the latest thinking, not only do organic amendments not help, they may hurt, because they diminish the soil’s ability to store water--by lowering its water-retention qualities.
Even in poor or rocky soils, proponents of “no amendments” prefer picking plants suited to the soil and the site. “Change what you plant, not the soil,” they say.
The organic amendments we are talking about are things like processed wood shavings or sawdust. They are not fertilizers, but are designed to make a soil more manageable by physically separating the soil particles.
Research carried out in the Midwest suggests that planting in unamended soil also makes for healthier plants that are less likely to topple years later--plants that grow stronger, longer.
In many soils, amending the soil that goes back in the planting hole produces an unacceptable interface between native soil and amended soil. This interface turns back roots, so they tend to circle and not move out into the surrounding soil.
If the surrounding soil is a heavy clay or other relatively impermeable soil, the planting hole filled with amended soil might also become a “bathtub,” as one soil scientist calls it. Water will collect in the planting hole, killing the plant by suffocation.
This new thinking does not mean that there is no use for organic amendments, but Ismay recommends that they be used as a mulch, “on top of the soil, not in it.”
He suggests that they work better on the surface, where they can break down into the form most useful to plants and then slowly work their way down into the soil. A 3- to 4-inch layer is usually recommended.
“No organic amendments” applies mostly to good, valley-bottom soils--sandy loams and clay soils, even clay soils that “you could throw on a potter’s wheel and fire,” Ismay said.
Clay soils are composed of very tiny particles that pack together. Water penetrates this mass slowly, but exits it just as slowly. It may take hours to get a clay soil wet, but it takes weeks for it to dry out.
Getting clay wet is the trick, and amending a soil helps water soak in, but it also exits quickly. The secret here is to use drip irrigation or any of the other “low-flow” devices that put water on slowly so it can soak in.
Clay is also naturally stubborn, unyielding and intractable, if a soil can be said to be so. This means that in flower beds and vegetable gardens, where one is constantly working in and with the soil, amendments are the heroes, making a clay soil manageable.
“No amendments” applies mostly to trees and shrubs--permanent plants with far-reaching roots. Some also think it wise for drought-resistant ground covers, and even lawns, but not flowers or vegetables.
Mike Henry, turf and landscape adviser with the UC Cooperative Extension in Orange County, said “delicate plants such as vegetables and bedding plants that don’t have a tough, vigorous root system can’t penetrate a heavy soil. This is one case where organic amendments can help.”
“And in very rocky soils or very sandy soils, there may be times when organic amendments are called for because the soils can be very porous.”
But even in these cases the amendments are used not to hold onto water so much as they are used to hold onto nutrients--fertilizers you add to the soil, which otherwise might wash right on through.
“You can’t make a general statement and cover all the bases,” Henry said, “but in most instances, on level land that has not been bulldozed out of the side of a hill, it is not necessary to add organic amendments, and may indeed be a bad idea.”
George W. Schmitz, soil scientist and former chairman of that department at Cal Poly Pomona, and now a consultant with Donald Eberhard & Associates of Fullerton, consulting agronomists, said in hillside soils we are “obligated to amend because these are not soil.”
Cut from the sides of hills, or brought in as fill from some other construction sites, he likes to call these “formations” because they have little or no qualification as soil, but may be solid rock, loose rock or very sticky clay subsoils. And they all need amending, he said.
Schmitz recommends getting a soil test if you are not sure what kind of soil you have. “All sorts of interesting things might turn up,” he says, such as very poor fertility, unusual minerals or high salt concentrations.
“A soil test can explain all sorts of problems in gardens,” he said.
Kellogg Supply Co. has soil text kits available at nurseries--the Soil Test Box costs $29.95, or $32.95 for the version that also tests for salts in the soil.
You fill the box with soil and they have a soil lab run the tests. You get back figures and recommendations on what to do. Or call the County Cooperative Extension. If you are pretty sure you have an ordinary clay soil, even a heavy clay, or a nice loam, here are the new recommendations for plantings trees and shrubs:
--Dig a shallow hole, only as deep as the nursery container or root ball, but make it three to four times as wide.
Break up and pulverize the soil as you dig it out. No clods allowed. Set the plant in the hole and fill the hole with the pulverized native soil, tamping it down as you go.
It helps if you thoroughly water the ground three to four days ahead of time. This makes digging a whole lot easier, and the damp clods will shatter when hit with the back of the shovel.
--Use the extra soil to build a watering basin around the hole and put a 2- to 3-inch mulch of organic amendment inside the basin, keeping it away from the very base of the plant.
Water by flooding the basin or with drip emitters installed in each basin. The basin is necessary in clay soils to hold the water long enough for it to soak in. Sprinklers will not work on plantings that have not been amended because they apply the water too quickly and it cannot soak in.
Lydia Deets, an assistant transportation engineer with Caltrans, said this last trick--the watering basin and mulch--is a “winning combination.” Very simple, it has nonetheless cut the mortality rate of new plantings considerably.
For ground covers (even lawns, say some), one would simply rototill the soil to loosen it so roots and water can penetrate easily while the grass or other plants becomes established. For flower gardens and vegetable patches, look on the back of the packages of soil amendments at nurseries to see how much to add to the soil.
Usually, you can add up to 50% by volume, though a 4-inch layer tilled into the top 12 inches of soil usually does the trick.
One final note: Planting with no amendments does make for slower growth initially, and one must be careful not to overwater, but after a few years, plants in native soil will become larger and stronger than those in amended soil.
Robert Kourick, distributor of Water Cycle, the device to recycle “gray water” mentioned in the first of the water saving series Aug. 19, may be contacted at P.O. Box 1841, Santa Rosa, Calif. 95402.
