Desert Soils Need Organics Added!

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?

A. This does not apply to desert soils with extremely low organic matter content. Some desert soils are okay to plant in directly with few problems. Others are not. Your soil organic matter content should be about 5%. If it is lower than this, add organic content to the soil such as a good quality compost. 

This is the kind of desert soil we typically see in the Las Vegas Valley. This soil is very light brown and cannot be done dry with a shovel. You must use a pick. This soil has nearly no organics in it. To plant in the soil without adding organics is sheer disaster.Some people call this mistakenly "caliche'. It is not. It is a sandy loam with zero organics but digs like caliche if not amended.

Mix it with the soil taken from the planting hole OR bring in a soil mix and use that to fill the planting hole around the tree roots or container “ball” of roots. Be careful of adding too much organic content to the soil. This can work against the establishment of roots in the surrounding soil. 

This soil is in a valley in northern Arizona. It is also light brown and extremely hard to dig. But the soil is very productive once it has been reclaimed with organics and water. It is also a sandy loam with virtually no rocks, a perfect soil for crops once it is been reclaimed.

This is the situation with research done in Oklahoma, Arizona and other states. These practices of “not adding organic matter” to the soil at planting is from their research with soils significantly high in organic matter.

This is the orchard in North Las Vegas covered with wood chip mulch 4 to 6 inches deep. The wood chips come from trees that would normally be taken to the landfill. If these trees are chipped and applied to the surface of the soil where there is water, the soil becomes dark brown in less than one growing season. Remarkably, we even find worms.

Many soils of the Mojave Desert with very low rainfall are extremely low in organics. Soils in the desert that are relatively high in rainfall or were previously farm land (under irrigation). These are frequently already high enough in organics and adding more does little, if any, good. Using the deserts of the Southwest as an example (Sonoran, Mojave, Chihuhuan, Great Basin) they range in historical rainfall from 4 inches to over 10 inches of rainfall each year. This is a 250% difference depending on locale!!! Of course we will see different types of plants and a difference in plant density and canopy size when we compare desert environments with a difference in rainfall of 250%!!! This is reflected in soil differences there as well. We see differences in organic content, salts, pH, etc.

The Sonoran Desert is different from the Mojave Desert primarily because it receives 250% more rainfall. This rainfall leads to a higher density of plants, plants which become larger and are different from the plants in the Mojave Desert. The presence of plant life and rain lead to higher organics in the soil which also contributes to better plant growth. The Mojave is really a desert!

How do we know what the organic content of a soil is?

We can send it to a soil testing laboratory and spend maybe $75 to $100 and wait for three weeks for a reply or use our noggin and get a rough approximation. The soil testing lab will give you a precise amount in the sample sent to them. If the sample sent to them is representative of the soil that interests us, the it may be fairly accurate. But, garbage in, garbage out. If the sample is NOT a good representative of the soil that interests us then it is garbage.

Rich, productive soils are dark brown in color, not light tan. The presence of organics in soils turn light tan soils into dark brown soils. Adding 50% organics such as compost to a desert soil does not result in 50% organic matter content. It will probably be somewhere around 5 to 10% when it's all done.

Look at the soil.

Soil color is a pretty good indicator of soil organic content. Rich soils, full of organics are brown to black. The lighter the color, the less organics in it. If the soil is moist and dark brown, you probably don’t have to add anything. If it is light tan or very light colored, even when moist, it probably needs organics added despite the recommendations from Oklahoma or Arizona.

This is a true desert soil, so low in organics that its color is almost white. What to do? Add organics, in this case in the form of compost or manure. Mix it all together and water it. To get a really productive soil you will continue to do this for the next 2 to 3 years. The soil will settle down and start giving you big benefits.

Dig in the soil.

If you need a pick to dig or a shovel barely scrapes the surface, AND it is light colored….ADD ORGANICS!!!! Add organics in a ratio of about 1:1 by volume or container. Add a five gallon bucket of compost to this cement-like soil. Adding organics/compost in a 1:1 ratio (v/v) will NOT result in 50% organic matter content but probably in about 3 to 5% content after watering, settling, and growing for one season. Next year add 25% by volume (v/v) if it is a garden soil or apply about one inch of compost to the soil surface around a plant and lightly scratch it into the soil surface, and water it in. Keep compost at least (approximately) 6 to 12 inches from the “trunk” or stem of the plant. Beginning the third year, add compost around the tree/plant at the beginning of its growth cycle primarily for nutrients and improved biological activity.

Add organics to the soil when planting anything in our desert. Smaller amounts of organics are needed for lawns, a moderate amount is needed for trees and shrubs but a high amount is needed for vegetable and herb production.

Las Vegas soils, most of them, are extremely low in organics. ADD compost to these soils at the time of planting fruit trees, ornamental rees and shrubs. You have one time to do it and after that it is very hard to do if not done at planting time.