Q. I’m planting
new fruit trees and landscape trees this year. I noticed there is a
consensus out there that fruit trees, trees and shrubs should be backfilled
with native soil without using any amendments. However, is there an exception
in Las Vegas with the soil is exceptionally poor?
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| Typical “native” or “natural” desert soil in Las Vegas. Would you plant directly in this? I hope not. |
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| Here is the SAME soil (25 feet away) ten years later after wood chips were spread on the soil surface and water was applied for fruit trees. Now which soil would plants be “happier” growing in? |
A. You are right and my advice is bucking conventional advice from most places. For instance, using native soil for backfill in North Carolina will be fine in most cases. But it also depends on what is planted. Many urban landscapes have “fill” that was brought in by the developer or general contractor. Most fill…No, I take that back…ALL fill… used in urban desert settings is junk. Using soil amendments depends on
the soil and also the plants. Let me explain why.
the soil and also the plants. Let me explain why.
Desert Soils Vary in Organic Content
Some desert soils are okay to plant
into directly and you will have few problems. Others are not. Much of it
depends on the organic matter content of the soil and the type of plants. If your soil contains at least 5% organic matter at the time of planting, the addition of organics to the soil as a soil amendment probably won’t do much. If the plants going into the soil prefer growing in highly organic soils, you are probably going to see a problem if the organic content is only 5%.For instance, a soil with low organic content but suitable for lawn grasses or some trees and shrubs will not be suitable for annual flowers or vegetables.
into directly and you will have few problems. Others are not. Much of it
depends on the organic matter content of the soil and the type of plants. If your soil contains at least 5% organic matter at the time of planting, the addition of organics to the soil as a soil amendment probably won’t do much. If the plants going into the soil prefer growing in highly organic soils, you are probably going to see a problem if the organic content is only 5%.For instance, a soil with low organic content but suitable for lawn grasses or some trees and shrubs will not be suitable for annual flowers or vegetables.
Soils are a Mixture of Sand, Silt, Clay AND Organics
Soils
are a mixture of minerals and organic content that results from dead plants and
animals that decompose into the soil. Desert soils with very low rainfall like
ours.
are a mixture of minerals and organic content that results from dead plants and
animals that decompose into the soil. Desert soils with very low rainfall like
ours.
 |
| This is a jar test. I have students do this to their soils at home. First of all, the soil is darker in color which means it has organics in it unless it is a soil like a Latersol in the tropics. This is NOT a desert soil. I can tell from the color. The organic component has mixed in with the soil particles and colored it darker but the larger stuff either floats on the surface or is the very top layer on top of the clay layer. |
When
planting without soil amendments such as compost, soil organic matter content
should be at least 5% if you don’t want the plants to be “unhealthy”. If it is lower than this, add organic content to the soil
such as a good quality compost. Mix it with the soil taken from the planting
hole.
planting without soil amendments such as compost, soil organic matter content
should be at least 5% if you don’t want the plants to be “unhealthy”. If it is lower than this, add organic content to the soil
such as a good quality compost. Mix it with the soil taken from the planting
hole.
Another
option is to use a soil mix for filling the planting hole around the tree roots
or container roots. Soil mixes are like Hamburger Helper; they contain organics and it is convenient and
easier to use than mixing the soil yourself.
option is to use a soil mix for filling the planting hole around the tree roots
or container roots. Soil mixes are like Hamburger Helper; they contain organics and it is convenient and
easier to use than mixing the soil yourself.
 |
| Soil color can tell you alot about a soil. This cark soil color tells me there is a good amount of organics in it and will not need to be amended for lawns and some trees and shrubs. |
Be
careful of adding too much organic content to the soil. This can work against
the establishment of the plant in the surrounding soil. 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 already high enough in organic matter to make little difference after platning.
careful of adding too much organic content to the soil. This can work against
the establishment of the plant in the surrounding soil. 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 already high enough in organic matter to make little difference after platning.
 |
| This is the same desert soil you saw above. Organics are added to this desert soil AND the soil is covered in wood chips to add organics to the soil over time as long as there is rain or irrigation. |
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 usually
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.
are extremely low in organics. Soils in the desert that are relatively high in
rainfall or were previously farm land (under irrigation). These are usually
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.
 |
| Map of the US showing organic content of soils and how it varies with rainfall. Desert soils are always lower in soil organics than soils in wetter climates unless they are amended. |
How Do You Know the Organic Content of a Soil?
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, then it may be fairly accurate. But, garbage in, garbage
out. If the sample is NOT a good representation of the soil that interests us
then it is garbage.
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, then it may be fairly accurate. But, garbage in, garbage
out. If the sample is NOT a good representation of the soil that interests us
then it is garbage.
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.
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.
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.
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.
(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.
Las Vegas soils, most of them, are extremely low in
organics. ADD compost to these soils at the time of planting. You have one time
to do it and after that it is very hard to do if not done at planting.
organics. ADD compost to these soils at the time of planting. You have one time
to do it and after that it is very hard to do if not done at planting.
Would You Like to Learn More?
Sign up for my half day class on the Basics of Desert Soils and Growing Horticultural Crops offered in early March 2018. Read all you want in books and the internet but this class focuses on growing plants in desert soils of the Mojave. Class size is limited. Enroll now using the link below.