I consider those ever-popular metal tree root deep-feeders as virtually useless and mostly harmful.
Spring has been warmer than past years up here “on the mountain”. Friends are thinking about watering. I hope they think carefully. (My plants and trees receive no summer irrigation. But that’s another blog altogether.)
The roots of many plants, it turns out, don't really get most of their water and nutrients from the deep regions of the soil. Rather, most plants, tree and shrubs gather all the moisture and nutrients they require from the top 12- to 36-inches, depending on the plant. The first foot is the most critical. Here is where the soils are most aerobic and this facilitates the exchange of nutrients from the minerals into soluble form that plants can absorb. Feeding root hairs actually grow up toward those first moisture-laden nutrients. (The lettuces pictured here can send roots as deep as four feet--if you don't have gophers! This is why boxes with wire bottoms should be 24-inches high instead of 12 inches. To give the roots "room to move". Yet the lettuces still get much of their "food" from the top 12 inches. So careful irrigation is a must.)
The roots of many plants, it turns out, don't really get most of their water and nutrients from the deep regions of the soil. Rather, most tree and shrubs gather all the moisture and nutrients they require from the top 12- to 36-inches, depending on the plant. The first foot is the most critical. Here is where the soils are most aerobic and this facilitates the exchange of nutrients from the minerals into soluble form that plants can absorb. Feeding root hairs actually grow up toward those first moisture-laden nutrients. (I describe this in more detail in my newest book - Roots Demystified, Change Your Gardening Habits to Help Roots Thrive.)
An old adage advises gardeners to water trees deeply; this often means infrequent, but lengthy irrigation. Increasingly, drip irrigation has been used as a tool for deep, occasional irrigation. Infrequent, deep irrigation tend to produce two points in the total irrigation cycle where the soil life and root hairs are damaged enough to reduce plant growth—during both the drying stage and when the soil is too wet. Dry soil kills off the tender root hairs as they can’t survive too much air. When the ground is flooded, root hairs die and noxious gases can’t escape nor pure air enter—or as pure as it may or may not be.
The metal probe of a typical deep root feeder is 18- to 24-inches long. This means that the application of the water begins below almost the entire zone of maximum water and nutrient uptake.
Good pore space makes for healthier plants. An ideal mineral/humus/pore structure balance results in a crumbly soil that allows water to percolate down, harmful gases to vent out, and refreshing air to permeate the soil. Soil breathes 24/7 at a lumbering, beneficial rate we cannot see. A soil with a healthy structure allows for easy and deep root growth and will produce the best-looking lawn, garden and tree growth.
Root feeders flood the pore space of the deeper soils, which quickly becomes flooded and anaerobic. Sometimes the deep soil saturates and the irrigation water backs up toward the surface so that more shallow soils actually get waterlogged. It's a bit backwards to send the water through a 24-inch deep probe just to apply the water 6- to 12-inches deep!
I recommend you just toss your root feeder—actually, recycle the scrap metal parts. (China could use them.) A good sprinkler can do a better job. And a drip irrigation system will promote the best growth possible—better than any other method of watering.
One garden’s “deep” soil is another garden’s “shallow” soil. Another hidden assumption about tree roots concerns the depth of your garden's soil. There are places where glacially-deposited topsoil extends for dozens of feet, but these are more the exception than the rule. If you have such a deep, loamy soil, then rejoice, but remember that the majority of moisture and nutrient absorption by trees still happens in the top two feet of the soil. Typically, most suburban yards have a very shallow layer of topsoil, if there's any left at all after construction. If, for example, there is a continuous layer of rich-orange clay some 12 inches under the ground, then the 12-inch layer of topsoil is, for all practical purposes, the only place your plant’s roots will be feeding.
Most heavy clays, whatever the color—and dark blue or pale white-gray are the worst—are relatively worthless to feeding tree roots. While clayey soil has plenty of nutrients, their availability is locked up in its tight, anaerobic structure and strong chemical bonds. Tree roots can, over many years and decades, grow somewhat into clay soils, but the number and extent of roots in the looser topsoil in far greater and more important to tree nutrition.
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