Amendments to the Soil Constitution

Southwest Yard & Garden by Dr. Marisa Thompson

Bosque soil at Los Lunas Riverside Park, December 21, 2017. Huge cottonwoods nearby, but not shown (photo credit M. Thompson).

Short video of huge tree at that same Riverside Park in Los Lunas in January 2018.

Question: Should I amend my soil when planting a tree?

-        Common Question from Tree Lovers Across the State

Answer: The short answer is no. The less-short answer is still no, assuming you’re planting a tree that’s recommended for your area. The recommended tree species are the ones with roots that are well adapted to our native soils, so they’re more likely to live long, sustainable lives.

Before I get into the weeds on why soil amendments, including fertilizer, are not recommended when planting trees, let me first explain that this week I’m talking about landscape perennial trees, shrubs, flowers, grasses, cacti, succulents, and vines. I’m not addressing the needs for annual flowers, garden vegetables, or commercial fruit trees. Another important distinction is that non-native, non-adapted trees may benefit from modifying the planting site, but even then only in the short term.

Why not amend the soil around your tree? There are a bunch of reasons depending on the problem—or perceived problem. The number one issue we see around the state with dying trees is just plain water—where and how you apply it. Another common reason trees don’t make it is that they’re planted too deep, so amending the soil wouldn’t help with that issue. For more on how to plant a happy tree and figuring out the whole watering conundrum, visit these previous columns:

• Irrigating in Winter: Same Depth, Less Frequent Irrigation
• Knowing How Much to Water PART I #itscomplicated
• Knowing How Much to Water PART II #itsSTILLcomplicated 
• Planting Trees: the HOLE Story

We want secondary (smaller, finer) roots to grow out laterally 2–4 times the height of the mature tree. They’re the ones doing all the water and nutrient absorption work. Amending the soil in the planting hole restricts the outward growth of roots beyond the edges of the hole and encourages circling roots to form. Research on trees and soil amendments has shown that by adding organic matter to your soil backfill, the initial results are good. But that interface between the amended soil and the native soil is bad news. Water doesn’t move well across the interface, and neither do roots. With time, the roots just stay in the original hole area and spiral in there, so you end up with a potted tree with reduced growth rate, constant water and nutrient stress (because those lateral roots never fully developed), and heightened vulnerability to pests and diseases.

My favorite answer to most horticulture questions is asking more questions. What are you trying to achieve by amending your soil? What type of soil exists in the tree’s native habitat? What kind of soil do you have? What is the soil history in the planting spot? How will you be watering the tree? What’s wrong with your soil, again?

Let’s take an example of a Texas red oak tree (Quercus buckleyi) in a 15-gallon container that was donated by the Los Lunas El Jardin Encanto Garden Club and planted this week here at the NMSU Agricultural Science Center at Los Lunas. This tree is native to sandy soil areas of West Texas and the Pecos River Valley. The USDA Web Soil Survey of the planting site shows that the soil at our planting site is a Glendale clay loam (click HERE for a little video I made on how to use the Web Soil Survey tool to know your soil better). Sure enough, on NMSU Extension Horticulture Agent Jeff Anderson’s suggestion, I filled the hole with water a few days before planting to get an idea of the water-holding capacity, and it was still mucky at the bottom today. This will become a problem if the tree is overwatered, and trying to increase organic matter by amending the soil might make it worse. A major reason for adding organic matter when planting is that it increases the water-holding capacity of soil. That’s why it is recommended for veggies and annual flower plantings in sandy soils where the water drains too quickly.

Slow and steady wins the tree race. Trees that grow too fast are more likely to develop structural problems and have other issues. Fertilizers are not recommended at the time of planting perennial plants because we want the plant’s energy to go into root establishment at a healthy rate to support the aboveground growth. Hold off on fertilizing for at least a few years while your tree is setting down roots. Even then the best practice is “Test, don’t guess.” Having your soil tested can help you decide which nutrients are deficient, if any. Furthermore, a nutrient deficiency isn’t necessarily remedied by adding more. For example, calcium in tomatoes can be deficient in the fruit, causing blossom end rot, but calcium levels in our soils are usually adequate to extreme, so adding calcium won’t help—water is the key in that case. NMSU Extension Guide A-114, “Test Your Garden Soil” (http://aces.nmsu.edu/pubs/_a/A114/welcome.html), has more info on soil testing.

Nutrient toxicity is a potential issue to consider too. Soil amendments are not regulated, so fertilizers can be extremely high in salts. According to a helpful Colorado State University Extension publication on soil amendments for vegetable gardening (http://extension.colostate.edu/docs/pubs/garden/07235.pdf), “Plant-based composts are low in salt. These may be applied at higher application rates, more effectively improving the soil. Plant-based composts are typically higher in price.” By “plant-based composts,” they’re talking about compost made from kitchen scraps. Contact your local city government or county Extension office (http://aces.nmsu.edu/county/) to find out about compost options for your community. Also, ask about composting workshops. The Bernalillo County Extension Master Composters volunteer program is another helpful resource and fun group to work with. Or check out NMSU Extension Guide H-110, “Backyard Composting” (http://aces.nmsu.edu/pubs/_h/H110/welcome.html), to learn how to make your own compost.

As Community Forester for the City of Las Cruces Jimmy Zabriskie said when I asked if he ever recommends using soil amendments when planting a tree, “Save the money for mulch.”

Stay tuned for more on the goodness of mulch in future columns.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the NM Desert Blooms Facebook page (@NMDesertBlooms)

Please copy your County Extension Agent (http://aces.nmsu.edu/county/) and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms (http://desertblooms.nmsu.edu/) and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505- 865-7340, ext. 113.