1 - Unproductive field soil can be amended with compost and used in raised bed gardens.

Yard and Garden April 22, 2017

Q.

I am thinking of switching to raided garden beds this year. Do you have any recommendations on how to improve my soil? It is mainly field dirt and I noticed nothing really grew well last year. I plan on making the beds 4 x 10 and about 12 inches high. Any advice would be greatly appreciated.

- Darryl C.

Isleta

A.

Raised beds offer a lot of advantages, one advantage is that you will "engineer" the soil in a small area tailored to the specific needs of the plants you are growing. A disadvantage is that (especially if you irrigate with well water) you will need to replace the soil every few years as minerals from the ground water accumulate in the garden bed. The best way to improve the soil is to add compost to your field soil. Manure may help, but if it is too fresh the salts in the manure will damage the plant roots and the plants themselves. Old manure aged in the open (not in piles under a roof) is safer than fresh manure. Even better, you can make compost with the manure, kitchen wastes, landscape wastes / weeds and even old alfalfa. Old spoiled alfalfa (from bottom bale that got wet and rotted) can be good for adding to the soil in the raised bed also - just shred it well before mixing it in the soil.

Take the compost, old manure, or shredded alfalfa and mix it with your field soil. You will want the ratio to be 20 to 50 percent compost mixed with the field soil. If you have had a lab test to determine the nutrient needs of the soil, you can mix in fertilizer based on the laboratory recommendations as you prepare the soil. Mix the components as well as possible then fill in the raised beds. Water well and, if it settles, add some more mixed soil.

Do you have gophers in the area? If so, hardware cloth (1 / 2 inch mesh) under the bottom of the raised bed and coming up the outside a little ways will help keep the gophers out. I wrote about gophers in the January 14, 2017 Yard and Garden column (http://aces.nmsu.edu/ces/yard/archives/011417.html) and much more information can be found in NMSU Extension publication Guide L-109: Controlling Pocket Gophers in New Mexico.

The depth of the raised bed depends on the plants you will grow. A depth of 2 to 3 feet is good. If possible loosen the soil under the raised bed to give more root depth, especially with shorter raised beds that you mentioned.

If you have salty soil (white crust on the surface) you may need to create a salt barrier at the base of the bed. To do this put large cobbles (3 inch and larger) at the very bottom, then fill with smaller rocks above that. The cobble layer should not be dug down into the soil. Then put some landscape fabric (woven plastic fine mesh) on top of that. Then fill the soil in above that. This will prevent upward movement of water with salt into the raised bed. Irrigation water will add some salts, but occasionally watering enough to leach surplus salts through the cobbles will help reduce salt build up. The soil level above the rocks should be 2 to 3 feet. There will be a "perched water table" that forms above the rocks in the soil as water will drain poorly from fine textured soil into the rocks and cobbles, so you need enough soil above for the roots of the plants to avoid being waterlogged. Watering less will not help avoid this. The rock layer should be a pretty good gopher barrier so you will probably not need to use the hardware cloth.

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, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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