Mulching Practices in Avocado
Orchards
M. Matava
Agronomist, Ag-Service, Vista,
California
This paper is the author's condensed
reconstruction of an address presented at the annual meeting of the
Avocado production, for the past several
decades, has relied primarily on the use of chemical fertilizers to supply the
necessary nutrients for optimum production. These man-made fertilizers rapidly
replaced animal manures and mulches as the main source of nutrients due to
their low cost and ease of application.
Although these materials adequately meet the
chemical needs of the plants, little attention was given to the biological and
physical phases of the soil. As every avocado grower knows, the feeder root
systems of avocado trees are very shallow and tend to localize in the upper
portion of the soil profile which is designated by soil scientists as the 'A'
horizon and contains the greatest amounts of soil organic matter.
Most of our
A healthy root system is evidenced by large
cream-colored feeder roots immediately below the leaf mulch layer. Moving further
into the soil profile, the soil becomes less organic, and fewer feeder roots
are found even six inches below the soil surface.
Trees not receiving adequate irrigation have
a thick layer of fallen leaves that are not readily decomposing. The trees have
leaves that are limp and tip-burned and fruit that is small and drops
prematurely. When the leaf mulch is brushed away, few feeder roots are found
beneath the surface, and those roots tend to be darkened and shriveled. No
amount of fertilizer, either chemical or organic, will improve this condition
until sufficient water is applied.
Fortunately, as an orchard matures, the
natural leaf drop eventually breaks down, supplying a constant source of
organic material. Although eventually a thick humus layer will form in a
healthy orchard, there is evidence that this can be enhanced through the use of
a variety of organic products other than traditional animal manures.
Locally [in
The Fallbrook Sanitation district mixes the
sludge with green yard waste and composts thoroughly to destroy human
pathogens. It also produces a product which uses special composting worms to break
down the material to make a fine, uniform mulch.
Spent mushroom compost has also been used
successfully. This material is a combination of straw, manure, gypsum, lime,
peat moss, and cottonseed meal. It is thoroughly mixed and broken down during
the mushroom growing process.
All these products, which contain manures,
have high levels of salts, which can cause premature leaf and fruit drop.
Application should be made during the winter months when the trees are in a
state of quiescence. This allows excess salts to be flushed from the root zone
before the trees are actively growing in the spring.
Although the initial salt levels are high,
four to six inches of water is usually sufficient to reduce the salts in the
soil extract to an acceptable level. This can be done with irrigation water if
no rain occurs. If there is any doubt that the soil has been completely
leached, it is advisable to test the soil for excess salts.
In addition to the benefits of adding
chemical nutrients, recent research has focused on the use of organic material
to help control plant pathogens, such as Phytophthora, in replant
situations. Preliminary results of research done by
John Menge, of the
Although organic materials may prove to be
beneficial to both avocado production and the environment, care should be taken
to use these products properly. No fertilizer or mulch will replace consistent
and appropriate farm management practices.