March 19, 2016

1 - Although gardeners often see recommendations on the internet to add wood ash to their garden, this is bad advice for most New Mexico gardeners.

Yard and Garden March 19, 2016

Q.

On the internet I saw a gardening tip that said I would be surprised with the results if I put wood ashes into my garden. Is it OK to use ashes from juniper and piƱon pine in our gardens? I have cleaned my fireplace and I have a lot of ashes that I could use.

A.

If you put wood ashes in your garden in New Mexico, and much of the rest of the arid Southwest, you will indeed be surprised, but not pleasantly surprised. In our arid soils wood ashes add mineral salts to an already salty soil. This results in salt burn symptoms in the plants and often in the death of garden plants.

The garden tip you referred to is a good tip for gardens in locations that receive a lot of precipitation on a regular basis. Soils in these locations have had many of their minerals leached from the soil (dissolved and washed away). However, in arid lands the original minerals remain in soil formed from the weathering (decomposition) of the rock parent material because there is so little precipitation here. The good thing about this is that the minerals are still in the soil, but the bad part of that, for us, is that there are some minerals in excess that cause problems. These excess minerals cause the salt burn and can also interfere with the absorption of other necessary minerals by the plants. One such problem is the excess of calcium salts. These are minerals containing calcium which reacts with some other minerals in the soil causing them to become insoluble and unavailable to the plants, or to raise the pH of the soil (creating alkaline soil) which reduces the solubility of some plant nutrients such as phosphorus. Other excess minerals also raise the pH of the soil and create problems.

The pH of a soil is the measure of the acidity of the soil. A pH of 7.0 is considered neutral. A pH below 7.0 is considered acidic and a pH above 7.0 is considered alkaline. Many of our vegetables prefer a soil pH of about 6.8, while many of the New Mexico soils have a pH of 7.5 or greater.

The solution for arid land gardeners (and good advice for gardeners everywhere) is to have the soil tested. Your local NMSU County Cooperative Extension Service office can provide you with guidance regarding collecting and submitting soil samples. The Extension Service agent can also help you interpret the results of the soil sample and guide you in providing necessary nutrients or soil amendments.

You may find that the addition of organic matter will also be helpful for your garden. Well decomposed compost and well composted manure will add organic matter to your garden soil to slowly release nutrients to your plants. The organic matter will also help lower the pH of your garden soil to release some nutrients present in the soil but not normally available to plants. In addition to this, the organic matter will help hold moisture and any applied nutrients so keep them more readily available to your garden plants. Never the less, the soil test should come first to guide you even in the use of organic matter in your garden.

Only in extremely rare cases will the addition of ashes benefit New Mexico garden soils. However, be careful how you dispose of ashes. Some municipalities, and other garbage collection agencies, prohibit putting wood ashes in the garbage for fear that a live ember could cause a fire in the vehicle transporting the garbage.

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.

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

Please copy your County Extension Agent and indicate your county of residence when you submit your question!