1 - Raised bed gardens may be useful while developing soils, but may require special effort in some situations.

Yard and Garden March 29, 2014

Q.

Last year I tried to start a garden on our property for the first time. I hauled some mulch from the landfill and tilled that into the soil. Our garden did not do well. This year I hauled in some horse manure.

It took me quite a few years when I lived in Pecos to get the soil nutrient rich enough for a garden and I expect that it will take a few years in this soil.

A friend warned me that horse manure can cause some problems. She recommended that I get a soil test and told me that raised beds are a good way to garden in my area.

Do you have any recommendations? The soil on our property is caliche and has gypsum. It is similar to what all properties have in that area. Would adding sand from the mountain side to the soil help?

A.

Your plan to try to build your soil is good if there is enough soil above the caliche, and you are probably correct that it will be a multi-year process. The horse manure usually does not have as much salt as cattle manure, but does add additional mineral salts to an already salty soil. Because horses are not ruminants, there will be weed seeds that make it through their systems and into the manure. This will create weeding challenges, but the weeds can be used to build the organic matter level of their soil by tilling them into the soil or composting them. Just do not let the weeds form seeds or develop strong perennial roots. These weeds and legume "green manure" crops can help with the development of his garden soil by adding organic matter the soil. Undigested straw in the horse manure will decompose in the soil and add additional organic matter. If you can get sheep manure, it is also useful. Your friend’s recommendation to get a soil test is a great recommendation. The soil test results will provide valuable information in guiding your soil building program.

However, while you build the soil for your garden spot, it will be good to build the raised beds that were recommended. This will increase the probability of immediate gardening success. A secondary benefit is that the soil under the raised beds will begin improving with water and acids from the organic matter leaching through the beds into the soil underneath. So, in time, if the caliche is not too severe, those sites may become good garden sites even without raised beds.

If the caliche is right on the surface, and there is little soil, then raised beds may be the best short- and long-term solution. Raised beds provide numerous of advantages for gardening in New Mexico.

If the soils are extremely salty, there is a possibility that salts may migrate upward by capillary action into raised beds. As water evaporates from the surface of the beds water will be drawn upward from soil below the raised beds. The soil test will tell you if this is a potential problem. If there is a problem you can create a barrier to upward movement of water and minerals from the salty soil below. This is especially important in salty river valley soils where salts often accumulate. The barrier consists of a bottom layer of large rocks, a layer of porous weed barrier (filter fabric), then a layer of smaller rocks (pea gravel), a layer of filter fabric, and finally the engineered "good" soil for the raised bed garden. Air space between the large rocks prevents upward capillary movement of water into the bed from below. Since the capillary water would be carrying salts, this barrier protects the bed from rapid salt accumulation. The soil depth in the bed must be appropriate for the plants being grown, and there may be a "perched" water table that forms at the bottom of this soil, so a little extra depth of soil in the raised bed would be good. There will be drainage, but that perched water table will still form where finer textured soils with small pore spaces exist just above the rock layers with larger pore spaces. Water moves less readily from small pore spaces into larger pore spaces, so some water will accumulate in the zone just above the change in soil texture.

Over time, salts from the irrigation water and fertilizer will accumulate in the raised beds and the soil will need replacing, but the barrier at the bottom will delay the accumulation of salt from capillary water if there is a salt problem in the underlying soil.

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.

Links:

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.

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