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Myths About Amending Desert Soils: Benefits of
Compost
Andrew Backhaus, Ph.D
Artistic Arborist, Inc, 4519 N. 7th Ave, Phoenix, AZ 85013
602-263-8889 (office); rabackhaus@yahoo.com
Guides to Amending Desert
Soils - The DON'Ts....
Soil amendments are often added in an
attempt to improve native soils. Unfortunately, many of these suggestions
actually make the situation worse. Here are some examples of what NOT to do.
1. DO NOT Add sand to improve the
porosity of a clay soil. Mixing sand with clay makes ADOBE! Soil compaction
actually gets worse rather than better if you mix sand with native soil. In
order for sand to have a benefit you need to add it in a ratio of 80-90% sand to
20-10% soil. This is a huge quantity of sand! This is how golf greens are made.
However, golf greens are made to grow grass (not trees), withstand thousands of
foot traffic and drain well. They also have to be watered 1-2 times per day!
This is hardly what you want to grow trees in our desert. Avoid adding sand to
your gardening needs.
2. DO
NOT Add gypsum to your planting hole. Gypsum is a slowly soluble salt of
calcium sulfate. It is used in certain agricultural situations when the
irrigation water on a farm has a high level of sodium chloride. However, the
amount of sodium in our residential water systems is rarely ever high enough to
justify using gypsum. Most of the salts in our water is made up of calcium and
magnesium with and usually have very little sodium. Thus when you add gypsum
your actually making the problem worse by adding more, unneeded calcium.
3. DO NOT use ammonium sulfate (21-0-0) or
N-Furic acid (33-0-0) as your sole nitrogen source. Yes, it is cheap.
Yes, it makes your plants green up quickly. Yes, it lowers soil pH after
repeated use. However, we have found a number of situations where exclusive and
continued use of these two fertilizers leads to serious nutrient deficiency
problems. The most surprising is the disappearance of calcium from the soil. As
strange as it sounds, we have encountered several situations in old, established
residential desert landscapes where people's trees were actually suffering from
calcium deficiency! We were baffled by our lab soil tests that that showed
inadequate calcium levels. Further study revealed that these properties had
years of repeated applications with acid-forming fertilizers. Over time,
resulted in the leaching of virtually all of the calcium from the soil that
cause numerous problems in the trees. To replace the lost calcium we had to
apply calcium nitrate at low rates continuously over a long period to gradually
increase soil calcium levels. We also had to use special foliar applications
contain calcium to correct some problems.
Guides to Amending Desert
Soils - The DO's
Native desert soils are noteworthy for
having extremely low organic matter content and newly planted trees and shrubs
can have difficulty adapting to the low organic matter. One of the best things
you can do to amend you soil at planting time is to add compost to your
backfill. You don't need much and a little goes a long way.
Compost benefits desert soils in many
ways
· It improves soil structure, porosity, and
bulk density - making a better plant root environment
· It improves moisture infiltration and
permeability of clay soils - increasing drainage and reducing erosion
· It increases the moisture-holding capacity of
sandy soils - reducing water and nutrient loss
· It improves and stabilizes soil pH - so that
chlorosis is less common
· It improves the cation exchange capacity
(CEC) - the ability of soils to retain nutrients
· It improves soil structure
· It supplies numerous micro and macro
nutrients
· It supplies vital beneficial soil
microorganisms - these improve nutrient and suppress diseases
· It binds and degrades pollutants
· A good source of recycled, local compost is
the best for our environment
Not All Composts Are Equal
In Arizona good compost is hard to find. We have
analyzed a large number of local compost manufacturers and have found a huge
variation in quality. Here are the criteria that should always be evaluated when
choosing compost...
· Are they high in
soluble salts? - many local composts tend to be high and even toxic.
Insist on low salt levels
· Do they contain sewage
sludge? - if so they may contain hazardous human pathogens (E. coli,
Staph) and sewage sludge is also high in toxic heavy metals and salts
· Have they been treated
with sulfuric acid? - sulfuric acid is used to artificially hasten
decomposition and darken the compost so it looks "rich". However, this harsh
treatment alters the balance of beneficial microbes.
· How long has the
compost been aged? - the longer the better. Compost should be "cooked" a
minimum of 4 months. Raw, insufficiently cooked compost can cause problems to
plant roots. Look for a low C:N ratio.
· The temperature
during the cook should be high enough to kill weed seeds and pathogens. If the
temperature is too high it will reduce the microbe balance.
· Compost should not be too
dry or hydrophobic -
hydrophobic compost repels water and they will float away with watering. Good
compost should be slightly damp and have a high moisture-holding capacity
· Size matters -
finely ground compost is good for top dressing on turf. Slightly coarser
composts are better for mixing in back fill planting or in vegetable garden
beds. Coarse composts are good for nursery tree box planters. The coarsest
types, such as tree bark is good for mulch. Mulch, unlike compost, is more of a
decorative soil cover. It is applied to soil surfaces to reduce evaporation and
prevent erosion, but it is not generally mixed into a soil as compost is.
Compost overs and bark are often used as a replacement for gravel.
· Cation Exchange
Capacity (CEC's) - this value indicates how
well the nutrient holding capacity is. The higher, the better. Remember prairie
sod? It has some of the highest CEC's known. Desert soils tend to have CEC's
near zero, as does sand. That's one of the key reasons why compost rather than
sand is recommended for amending soil.
· Bulk density -
the weight of the compost is important for shipping costs and is related
to water content.
· What is the chemical composition of
the compost? - look for adequate macro- and micronutrient composition
· What is the microbial
population? One of the greatest advantages of compost is its ability to
inoculate the soil with beneficial microorganisms. Compost is a living thing.
The more diverse microbe population, the better. There are labs now that will
quantify the living components in compost. There are roughly 6-10 categories
that are examined. It has been shown that high populations in all of these
categories results in excellent crop productivity. An imbalance of one category
can give detrimental growth.
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