Proceedings
of The World Avocado Congress III, 1995 255 - 258
SOIL
MANAGEMENT OF AVOCADOS. EFFECTS ON GROWTH AND CROPPING
J.N. Hermoso, J.T. Soria and J.N. Farré
Estación
Experimental La Mayora
Algarrobo
Costa. Malaga 29750
Spain
A long-term experiment on
avocado soil management was established with recently planted Hass/Topa-Topa
trees at 8 x 8 m. The treatments were clean soil with shallow cultivation
(cultivation), clean soil with herbicides CbLerbicide), regularly sowed natural
weed cover (sward) and sugar cane bagasse (mulch). Mulching was discontinued in
1983 allowing sward to grow.
A shorter term experiment,
lasting four seasons, included mulched or unmulched soil, with or without
preemergence herbicides from planting. Also transparent polyethylene and
expanded polystyrene board.
Mulched trees grew
significantly better than unmulched trees, in both experiments, for the first
four growing seasons. Synthetic mulches performed as well or better than
bagasse. Preemergence herbicides did not affect growth.
In the long term experiment,
mean yield far the first 8 years (6 crops) was in the order Mulch >
Cultivation > Herbicide = Sward. The order for tree efficiency (yield per
unit trunk sectional area) was Cultivation = Sward = Mulch > Herbicide. At
the adult stage, two trees out of sixteen under sward were killed by Rosellinia
necatrix.
From 11 to 18 years of age (8
crops) yield was in order Herbicide > Cultivation > Sward. There was no
difference in yield between trees with sward from planting and trees,
previously under mulch, with sward from 8 years of age. The order for tree
efficiency was Cultivation = Herbicide = Sward ) Sward (following Mulch).
Fruit size was similar far
all treatments.
1. Introduction
In the early 70's clean soil with frequent shallow cultivation was the most common soil management method for fruit trees in the Mediterranean coast of Spain. In contrast, under similar climatic conditions, in California and Israel herbicides were widely used. Before the start of the experiments several preemergence herbicides had been screened by means of field and pat studies for low tree toxicity and good weed control under dry winter conditions (Farré y Hermoso, 1984).
Maintenance of the natural wood cover as a out sward
is a common way of soil management for deciduous fruit tress along a wide range
of climatic conditions (Farré, 1979). It does not involve the use of
potentially polluting chemicals and its energy requirements are slightly lower than
cultivation. Additionally it may increase soil organic matter and improve soil
surface physical conditions.
Mulches usually improve soil
physical conditions without the competition for nutrients and water of the
sward.
2. Materials and Methods
2.1. Treatments
The long term experiment on
avocado soil management was established in December 1974 with recently planted
Hass/Topa-Topa trees at 8 x 8 m. The treatments were clean soil with shallow
cultivation (cultivation), clean soil with herbicides (herbicide), regularly
mowed natural wood cover (sward) and sugar cane bagasse (mulch). For the first
two seasons the tram under sward were kept weed free in a 1 m radius with
herbicides. Bagasse, with approximately 50 % dry matter, was applied at the
rate of 10 kg m-2 over the whole surface and reapplied as needed to maintain
good soil cover. Mulching was discontinued in 1983 allowing sward to grow.
A short term experiment was
started in May 1979 with recently planted trees at 7 x 4 m. It included bagasse
mulch, over 2 a wide strip along the row and unmulched soil, both with or
without preemergence herbicide from planting. Also transparent polyethylene and
expanded polystyrene board (Porexpan).
2.2. Experimental design
Treatments were applied an a randomized
complete block design of 4 tree plots, with 9 replicates in the short term
experiment and 4 repli- cates in the long term experiment. From year 7, two trees under sward showed
reduced growth. A new plot of four trees from a guard row had their 5 cm
topsoil changed for virgin soil. From year 12 the records of the plot with
missing trees was calculated adding the new four trees to the two old trees
still alive. Trunk cross sectional area war. measured at 25 cm above ground in
the short and 20cm in the long term experiment.
2.3. Herbicides
Caragard (CIBA-GEIGY), a
mixture of the triazines terbumetone and terbutilazine, was used bemuse of its
good performance in preliminary experiments in the short term experiment and
all but the last two seasons of the long term experiment.
Roundup (glyphosate) was used
as postemergence herbicide. Paraquat was occasionally used in the short term
experiment.
2.4. Irrigation,-soil and
fertilization
All trees were originally drip irrigated. In the long
term experiment they were changed to microsprinklers at 12 years of age.
Except for the first 2 years of the long term experiment soil matric potential measured with tensioneters was maintained above -15 kPa in summer and -30 kPa in winter.
The soils have been described
by Farré (1979). N, P, K and Zn were applied according to leaf content,
uniformly to all treatments.
3. Results
3.1. Growth- of-young
trees
In both experiments (table 1)
young mulched trees grew consistently better than unmulched trees.
Synthetic mulches were as good or better than sugar
cane bagasse.
Under clear polyethylene
trees grew slightly but not significantly better than under expanded
polystyrene.
Under the experimental
conditions, the preemergence herbicide did not reduce growth of very young trees.
3.2. Cropping
Cropping in the long term
experiment is summarized in table 2. For the first six crops mean yield and
tree size were largest under mulch. Tree efficiency war. similar under all
treatments except clean roil with herbicide that had slightly lower values.
For the last eight crops here reported, tree efficiency was similar for all treatments except for the sward trees previously under mulch that had the lowest values. Mean tree size and yield were highest under the clean soil with herbicide. Mean fruit size was within the range 200 - 210 g. similar for all treatments.
4. Discussion
The improved growth of young
trees with mulches recorded here has been frequently reported (Farré, 1979). The
present results show that some pre- emergence herbicides can be safely used
from planting on light roils in low rainfall areas. This is probably safer
nowadays with the availability of low solubility herbicides.
The trees under sward had,
some years, reduced leaf nitrogen levels that may have reduced their growth.
Summer soil matric potentials were similar for all treatments. Under drip
irrigation the wetted area is less than 1.5 % of the surface soil. Cut sward
transpiration from this reduced area is therefore negligible. Adult avocado
trees completely shaded the ground which was further covered with a thick layer
of leaves. This reduced the density and growth of the sward even when microsprinklers were used.
The root system of the avocado under the conditions
of the experiments is extremely shallow. Munoz (1988) showed that 49 per cant
of the avocado root length under drip and 59 per cent under microsprinklers
were in the top 15 cm of soil. The improved exploration of this fertile and
well aerated top soil by the mulched and, to a lesser extent, the clean with
herbicide trees may have been the main cause for their improved growth and
cropping. The increased susceptibility of the avocado root system to Rosellinia
necatrix under sward is a further drawback of this method.
Faré, J.X., 1979. Water use
and productivity of fruit trees: Effects of soil management and irrigation. Ph.
D. Thesis. Univ. of London.
Faré, J.X. y Hermoso, J.X.,
1984. Herbicide y mantenimiento del suelo en la fruticultura subtropical
de la Costa del Sol. I Congreso S.E.C.H.:239-245.
Munoz, A.J., 1988.
Comparación del sistema radicular del aguacate bajo dos sistemas de riego:
goteo y microaspersi6n. Trabajo fin de carrera E.U.I.T.A. Sevilla.