Soil.
First, here is this morning's catch.

The onslaught of lemon cucumbers has begun. This evening I will put a bag of cukes at the end of the driveway with a sign encouraging passers by to take some.

When I started talking with locals about gardening, they all told me that the soil here does not favor the growth of vegetables. The soil has to be imported. An old timer whose garden I admired, advised me not to try to grow tomatoes other than Early Girl. Tomatoes around here do not grow to normal size.

In my yard there are a few inches of something like top soil, which I diligently cull and then treat with manure, compost, and an occasional bag of soil from the nursery. To set up the yurt I dug fourteen holes, each more than two feet deep and about twenty inches in diameter, and all I could set aside was about a cubic foot of "top soil."

Under this thin layer of brownish dirt there are layers of caliche.

At http://ag.arizona.edu/pubs/garden/mg/soils/caliche.html you will find a document that spells out the facts.

 Caliche is a layer of soil in which the soil particles have been cemented together by lime (calcium carbonate, CaCO₃). 

Caliche is usually found as a light-colored layer in the soil or as white or cream-colored concretions (lumps) mixed with the soil. Layers will vary in thickness from a few inches to several feet, and there may be more than one caliche layer in the soil.

What Does Caliche do to plants? 
Caliche causes three problems in the yard or garden.

1. The caliche layer can be so tight that roots cannot penetrate through it. The result is that plants have only the soil above the caliche to use as a source of nutrients and water and normal root development is restricted.
2. The same conditions that restrict root penetration also reduce water movement. Water applied to the soil cannot move through the profile if a restrictive caliche layer is present. The restricted water penetration can contribute to problems arising from inadequate root aeration and can lead to accumulations of salt in the soil surface. Both problems, lack of aeration and salt accumulation, reduce the vigor of growing plants.
3. The pH (acidity or basicity) and free calcium carbonate in a caliche soil are often high enough to cause iron to become unavailable for plants. The symptoms of iron deficiency are a yellowing of the youngest plant leaves while the veins in the leaves remain green. The resulting striping appearance is slightly different from that of nitrogen deficiency symptoms, which are a general yellowing including the veins of older leaves. Iron deficiencies are additionally aggravated by the water saturation of the soil. Check with country Extension agents for more information about how to correct iron deficiency in the yard or garden

The recommendations for flower beds is they should be 1.5' deep. A small tree needs a hole 5' deep! I'm tired just thinking about the digging I've done. Gotta go and rest.