Leave the leaves!

By Andrew Land

Having just held our annual Fruit Tree Giveaway, and been asked by several new fruit tree owners whether they should be concerned about heavy metals in their soil, it seems like a good time to put down a few words about trees and how they relate to soil health and fertility.

Tree roots intake water containing water-soluble mineral nutrients from deep in the soil, which are then pulled up through the tree into leaves via xylem. Moreover, as leaves age and approach the end of their lives (a process called “senescence”), they re-absorb some of their constituent nutrients from the larger plant. That mineral nutrition is showered down onto the soil surface when trees go dormant, making it available as fuel for the the soil food web as part of the process of nutrient cycling.

The Soil Food Web (courtesy USDA Natural Resources Conservation Service)
The Soil Food Web (courtesy USDA Natural Resources Conservation Service)

Those leaves also have a profound effect on the quality of soil folks are using to grow veggies in their urban home gardens. Heavy metals such as lead are among the average urban veggie gardeners’ greatest concerns. However, according to Ruby Blume of the Institute of Urban Homesteading in Oakland, CA, “[u]ptake of lead by plants…is negligible and regular amending with compost will completely neutralize the lead over time.” As we strive for balance and resilience in our garden ecosystems, fostering beneficial fungi and bacteria through regular applications of composted organic matter is one of our most powerful weapons. Not only does it offer a wide spectrum of nutrients, but compost also binds to toxic lead, thereby removing it from our list of concerns.

Kansas University’s (KU) Department of Agronomy released a compelling article on lead in soils in an e-update just this past month. In the article, they described how lead is most often held in roots and can only make its way to shoots or fruits very gradually, if ever. For those plants that produce a fruit (as opposed to an edible root or leaf), the likelihood that lead will make its way into the edible portion of the plant is even less likely. KU agronomy students ran tests to determine whether amending soils with a ratio of 1:3 leaf compost:soil affected lead uptake by the plants. Not only did the plants produce larger vegetables, but the leaf compost was shown to dilute the overall lead concentration in the soil by approximately 30- 50%.

By keeping in mind that the amazingly rich, healthy soil found on forest floors is directly related to the near constant shower of organic matter from above, we’re reminded that tree leaves play a crucial role in keeping our garden soils healthy. Now that studies are showing that composted trees leaves also mitigate lead in urban soils, the message has become clear:  leave the leaves!

– Andrew is a Neighborhood Trees Specialist with Friends of Trees