No need to amend soil when planting new trees or perennials

Gardeners who follow conventional wisdom and nursery recommendations to mix organic amendments into the soil when planting new trees or perennials in their landscapes are making a mistake, according to UC Cooperative Extension environmental horticulture experts.

“This is one of the garden myths that I'm trying to dispel,” said Jim Downer, UCCE environmental horticulture advisor in Ventura County. “We recommend residents not amend the soil when they are planting based on outcomes we have observed in research.”

Downer and Ben Faber, UCCE advisor for water, soils and subtropical crops in Ventura County, summarized this and other information about the use of organic amendments in home landscapes in a six-page publication now available for free download from the UC Agriculture and Natural Resources catalog at https://anrcatalog.ucanr.edu/Details.aspx?itemNo=8711.

The publication says research has not shown that adding amendments to planting holes for perennial plants provides a significant advantage compared to using native backfill.

With perennial plants, the roots do not stay in the planting hole for long, so amendments would only be effective for a short time. The practice of amending the soil further harms the plant by creating an interface where the soil in the planting hole is different from the native soil.

“When that happens, roots and water don't move as well through the soil,” he said.

While there are some situations where amending poor soil can improve tilth, increase water-holding capacity and add nutrients, by and large most unamended soils allow for adequate plant growth of trees and perennials. Potential problems introduced by amending the soil include nitrogen immobilization, which makes the nutrient unavailable to plants; toxicity from residual chemicals, such as herbicides applied to plant material in the amendment; addition of weeds or root pathogens; damage or destruction of the soil structure; harm to the soil food web; and increased salinity.

While there are few reasons to mix organic amendments into the soil, Downer said mulching the soil surface with uncomposted organic matter is almost always beneficial.

“If your goal is to get organic matter into the soil, we recommended topping the soil with fresh, undecomposed wood chips. It will give you microbial stimulation and suppress disease. Arthropods will slowly grab pieces of the mulch and incorporate it into the soil at a gentle rate,” he said.

The publication also provides information about various common organic amendments – such as coconut fiber, coffee grounds, horse manure, peat moss and green waste compost – with details about each product's benefits and detriments.