Soil Amendments and Backfilling Fruit Trees

Q. I'm planting new fruit trees this year. I've noticed there seems to be a consensus that backfilling should be done with only native soil, without any amendments. However, is there an exception in Las Vegas where the soil is exceptionally poor?

Planting fruit trees in Las Vegas directly in amended desert soil.

A. Yes there is, and you are correct. Our desert soils normally need amending at planting time but they are full of nutrients if they are not poisonous to plants. If you watch online videos or get information from outside sources, they may tell you it’s a waste of money to improve the soil at the time of planting. They may be correct in other locations but under our desert conditions it usually isn’t true.

Planting 3000 fruit trees directly in amended desert soil at an orchard in Las Vegas, Nevada.

In cases cited by outside sources, the soil already had enough organics in it that it made no difference; 2% organic content or higher. The research at Oklahoma State University during the 1970s clearly showed that no amendment to the soil was needed and mixing organics such as compost into the soil at the time of planting was a “waste of money”.

This original research was repeated at Arizona State University where the researchers used an agricultural soil with a similar organic content; 2% or more. The researchers came to the same conclusion; the addition of organic matter (compost) was a waste of money. Ipso facto, no soil amendments are needed!

Very practical original research often proves what we already know

It is common knowledge that if the soil has an organic content more than 2% that no organic addition is needed and it’s a waste of money to mix in additional compost when planting trees and shrubs. What about soils with organic content much lower than 2%. Is organic matter still needed? That research is never been done. Many of our Mojave desert soils have an organic content much much lower than 1%.

How do you know if your soil has an organic content above 2%? 

Send it to a soil testing lab (pay $70 and wait two weeks) where they can accurately measure the organic content of your soil and tell you its percentage. Or you can look at its color and approximately judge for yourself. The color of a soil darkens as the organic content increases. If your soil has a light tan color, the color of “creamy coffee” then it has a very low organic content; probably less than 2%. You can also do a "jar test" of your soil. The organics of a layer (if they are large enough) will float to the surface of the water.

Soil jar test