phoenix soil - Knowledgebase Question

phoenix, az
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Question by stpetshaw
April 6, 2007
I am new to the southwest from the northeast. Is there any one thing you can tell me to help me understand this soil?
Thank you for your help,

Answer from NGA
April 6, 2007
Our desert soils are indeed different from the northeast, but that doesn't make them "bad," just different! Local soil has high alkalinity (8.0 to 8.5), high salts, low organic matter, and relatively low nitrogen levels. It usually has sufficient potassium and micronutrients. Native and desert-adapted plants do just fine, as they have adapted to deal with these conditions. People tend to have trouble when they try to grow plants that are native to regions where the soil is acidic, contains alot of organic matter (i.e., is dark brown/black), nutrients and moisture. These non-natives root systems haven't adapted to obtain nutrients from our salty, alkaline soil. Many of our native trees and shrubs are members of the legume family, meaning they manufacture their own nitrogen to get around the fact that there isn't much in the soil. Clever!

Desert soil and water both contain salts, which can accumulate in the root zone over time. This salt buildup forms where the water stops penetrating. Short periods of watering cause salts to build up in the top layers of soil and damage or kill your plant. Salt burn shows up as yellowing, browning along leaf edges, and leaf drop. Deep watering?or leaching?prevents this by flushing the salts past the root zone. Always water slowly, deeply and as infrequently as possible.

If you choose to grow annual flowers and vegetables, garden beds need plenty of organic matter, which must be replenished 2 times per year, for each of our growing seasons. It breaks down and disappears so rapidly! As salts accumulate in clay soils, they impede water penetration. Gypsum is a soil amendment that helps sodium be dispersed and leached beyond the root zone, and it can be helpful added to garden beds. With our high pH soils, it isn't really possible to reduce the pH significantly or permanently. The addition of organic matter reduces it temporarily on a localized basis. Soil sulfur can do the same. Sulfur also acts the way gypsum does (but uses a different set of chemical reactions), so it isn't necessary to add both. Here's a page that contains more info on preparing garden soil:

After planting, add a 1-2 inch layer of organic mulch. Mulch is great to help retain soil moisture, reduce weeds, and as it breaks down it provides nutrients to the soil. Any organic matter can be used as mulch. Try compost, bark, wood chips, straw, or pine needles. As it breaks down, dig it into your soil and add more. Well, that's a crash course on our soil. If you have other questions, please send another email to the website. Good luck with southwestern gardening!

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