South African Avocado Growers’ Association Yearbook 1987. 10:1519.
Proceedings of the First World
Avocado Congress
Root distribution of
avocado trees in different soil types
BJ
DURAND and NJF CLAASSENS
Citrus and
Subtropical Fruit Research Institute, Private Bag X11208, Nelspruit, RSA
SYNOPSIS
A healthy avocado
tree in a homogenous soil with a relatively low bulk density has roots that are
fairly uniformly distributed, vertically and horizontally. Where the roots were
affected by Phytophthora root rot, the distribution became discontinuous
horizontally and with depth. In a different soil, avocado roots did not
effectively penetrate from one soil texture and structure into another and root
mass was very low in a soil with a bulk density above 1,7 g/cm^{3}.
INTRODUCTION
Avocados may be
grown in a wide range of soils with varying rates of success. Nel (1983)
describes an ideal avocado soil as having a red colour, a clay content of
between 20 and 40 per cent, poor structure, no compacted, clayey or patchy layers,
a depth of at least 2 m and a pH (water) of 6,0 to 6,5. A soil generally
considered to be suitable for avocado growing is the Hutton soil form with an
ortic A horizon and a red apedal B horizon.
Management
need not be so intensive where avocados are planted in an ideal soil, provided
the climatic conditions are suitable and rainfall is evenly distributed.
However, for every factor that is suboptimal, the shortcoming must be
compensated for by improved management. A table was drawn up delimiting minimum
depths above certain types of restrictrive layers for different soils (Nel,
1983). The minimum depth for a soil with a highly weathered (soft) granite C
horizon is 0,7 m. Another soil characteristic stressed by Nel (1983) is that
there should be no clear transition between horizons or soil layers in terms of
texture, structure, density and colour within the avocado root depth of 1 m.
Avila et al (1984) found that avocado roots did not penetrate sand lenses in
alluvial soils, which supports the recommendation of Nel (1983) that there
should not be textural changes in an avocado soil Uniform root distributions
were found to depths of 1,2 m and 2,1 m for different cultivars in deep uniform
soils, but more than 80 per cent of the roots were found in the top 1,5 m
(Correa et al, 1984). Rowell (1979) found avocado roots growing to a depth of 3,3 m but
the main concentration occurred in the top 1,5 m which concurs with the
findings of Correa et al (1984). It has also been reported (Howell, 1979) that very few roots were
found in a compacted zone in the soil profile. Therefore, avocado roots clearly
have the ability to penetrate
deeply in a uniform soil, but are limited by impervious layers and will not
easily grow from one soil texture and structure into another.
The purpose of this study was to evaluate a
technique for studying root distribution and to determine the effective root
depth for avocado trees at Burgershall. The study at Nelspruit was added to
evaluate root distribution in a different kind of soil and to compare it with
the results at Burgershall in an attempt to define soil factors that may
control root distribution. The horizontal and vertical root distribution of the
roots were investigated in this study.
The soils at Burgershall are Hutton form soils of the Doveton series
with an ortic A horizon and a red apedal B horizon, on top of partially
weathered bedrock at a depth of 1 m. Clay content throughout the horizons was
48,5 per cent. At Nelspruit another Hutton form soil, Msinga series, was
present with an ortic A horizon (9 percent clay content) and a red apedal B
horizon (20 per cent clay) of unknown depth. There was a noticeable change in
terms of texture, structure, density and colour between the A and B horizons at
a depth of 0,5 m in the Nelspruit soil.
In order to compare the root distribution of
different avocado trees, a technique described by Huguet (1973) and later by
Moutounet et al (1977) was used and
modified. The modification was that roots were excavated, sieved from the soil
and weighed, rather than counted and measured. A trench was dug in a
logarithmic spiral shape to cover as much of the root system as possible,
without much damage to the tree. Roots were extracted from every 0,5 m x 0,5 m
x 0,1 m deep block of soil in the trench to a maximum depth of 1,1 m along the
length of the different trenches. The lengths of trenches depended on tree
size, as the formula for calculating trench dimensions adjusts the starting and
end points according to the diameter of the stem and the radius of the canopy.
The number of trees available for this study were limited and therefore, the
sizes and condition of the trees, as well as the management practices under
which they were grown, differed.
Excavated roots were weighed fresh, ovendried
and then separated into feeding roots (of less than 1 mm in diameter) and
larger roots, which were then weighed separately. Bulk density determinations
were made for every sample depth in the soil profile. Three trees of the Fuerte
cultivar were investigated at the Burgershall Experimental Station and one tree
(Sharpless cultivar) at the CSFRI in Nelspruit. All the trees were
approximately 18 years old and in production. One of the Fuerte trees was
severely affected by Phytophthora root rot. Both the healthier trees at
Burgershall had skirts lying on the ground, while the sick tree had a very
sparse canopy. The Sharpless tree was a relatively healthy tree in a cultivar
collection orchard that had declined severely, mainly due to Phytophthora root rot.
This tree was skirted to about a metre above the ground. All the trees
investigated were originally established on Zutano seedling rootstocks.
The results for the three Fuerte trees at Burgershall are discussed
together. Differences in root distributions will be highlighted and
explanations offered, where possible. Individual trees are identified by
referring to their health status, namely healthy, apparently healthy and sick.
The difference between the healthy and the apparently healthy tree was that the
latter tree had a smaller proportion of feeder roots compared to the healthy tree
(Table 1).
The distribution of avocado roots for the
'healthy' tree in a soil type suitable for avocados, is illustrated in Figure
1. Throughout the profile, root distribution is fairly uniform, with root mass
tending to increase gradually further away from the stem. The second Fuerte
tree is apparently healthy and no external difference can be detected between
this tree and the 'healthy' tree. A trend of decreasing root mass towards the
periphery of the tree, with a noticeable increase at the edge of the dripline,
is illustrated in Figure 2. The root distribution of a Fuerte avocado tree
affected by Phytophthora cinnamomi Rands,
is shown in Figure 3. The root distribution of the 'sick' tree is much less
continuous than that of the 'healthy' and the 'apparently healthy' trees.
Fig 1. Distribution of total dry root mass along the profile of a logarithmic spiral trench for a healthy avocado tree.
Fig 2. Distribution of total dry root mass along the profile of a logarithmic spiral trench for an apparently healthy avocado tree.
Fig 3. Distribution of total dry root mass along the profile of a logarithmic spiral trench for a root rotaffected avocado tree.
A decreasing root mass away from the trunk
is expected in all trees. The unusual trend in root distribution of the
'healthy' tree is ascribed to the fact that no large roots were encountered
near the trunk of the tree. Large roots near the trunk account for the
declining root mass distributions away from the trunk, as observed for the
other two trees.
Due to differences in the depths and lengths
of the trenches, all mass data were calculated as dry mass per cubic metre of
soil for comparing the root systems of the trees (Table 1).
The 50,9 per cent feeder roots of the
healthy tree is probably unrealistic, because of the absence of large roots
near the trunk, as already mentioned. There is, however, a trend for the
proportion of large roots to increase relatively, and feeder roots to
decrease, as root rotinfection intensifies. Total root mass tends to decline.
The percentage dry matter for avocado roots was lower in root rotaffected
trees.
Regression analysis suggests that the feeder
root mass increases from the stem to the periphery of the trees, while large
root mass declines. The correlations are poor and nonsignificant, due to
variance of the data. As the feeder root distribution was the primary concern,
feeder root mass was the dependent variable, large root mass and distance from
the stem the independent variables. Only the results for the 'apparently
healthy' tree is presented, as the 'sick' tree exhibited no correlation and the
data for the 'healthy' tree was considered nontypical. The equation is
y=96,24+ 0,1585X+0,5668Z, where X equals large root mass and Z equals distance
from the stem. R2 = 0,398, Rsquared adjusted for degrees of freedom = 0,312.
TABLE 1 A comparison of dry root mass and
percentage feeder roots for three Fuerte avocado trees with different health
statuses. Results for a Sharpless tree at Nelspruit in a different soil is
included. 


% Dry matter 
Large roots g/m3 
Feeder roots g/m3 
Total root mass (g/m3) 
% Feeder roots 






Healthy 
48,7 
732,7 
759,5 
1492,2 
50,9 
Healthy 
48,2 
978,8 
510,3 
1489,1 
34,3 
Sick 
44,6 
1038,2 
124,3 
1162,5 
10,7 
Nelspt 
53,0 
1591,5 
220,6 
1812,1 
12,2 
Fig 4. Plot of dry feeder root mass over dry, large root mass and sample
distance from the stem for the healthy avocado tree.
Fig 5. Plot of total dry root mass for every depth sampled over bulk density for every sample depth and the depth of every layer.
Fig 6. Distribution of total dry root mass along the profile of a
logarithmic spiral trench for the avocado tree at Nelspruit.
Fig 7. Plot of dry feeder root mass over dry, large root mass and sample
distance from the stem of the tree.
Fig 8. Plot of total dry root mass for every layer over bulk density and
the depth of every layer.
The abovementioned raw data are plotted in
Figure 4. A multiple linear regression was performed on rootmass, as the
dependent variable, bulk density and the depth of every sample layer as
independent variables, for the 'healthy' tree. The correlation was very poor
and nonsignificant and the raw data are plotted in Figure 5. Root distribution
is fairly uniform in depth, but declines rapidly at the soil/rock interface, as
can be expected. No root preferance for a range of bulk densities could be
established. Bulk density decreased from 1,27 g/cm^{3} at the surface
to 1,14 g/cm^{3} at 600 mm, and 1,04 g/cm^{3} at 900 mm.
Root distribution of a relatively healthy
Sharpless tree under unsuitable soil conditions at Nelspruit, is illustrated in
Figure 6. Most of the roots occurred in the top 200 mm depth of the soil
profile, decreasing sharply with depth. Root mass also decreased toward the
periphery of the tree. The root mass found in the coarse gravelly clay layer
was very low in comparison with the rest of the profile, and in general the
roots had a stunted appearance. A summary of the root mass distribution and
percentage feeder roots is included in Table 1.
Regression analysis indicated that feeder
root mass and the mass of larger roots decreased with increasing distance from
the stem. The correlation was nonsignificant. The relationship between the dry
mass of feeder roots, larger roots and sample position in the trench, is
depicted in Figure 7.
A multiple linear regression was performed
on root mass as the dependent variable, while bulk density and soil depth were
the independent variables. The equation is y=2,659E+ 19639,69X+104297Z, where
X is equal to large root mass and Z equals distance from the stem. R^{2}
= 0,710 and is significant at the five per cent level of confidence. Rsquared
adjusted for degrees of freedom = 0,628 and is not significant at the five per
cent level. The correlation is not dependable because the Rsquare value is
significant at the five per cent level of confidence, but the adjusted Rsquare
is not.
Bulk density increased from 1,62 g/ cm^{3}
at the surface, to 1,66 g/cm^{3} at 600 mm and 1,84 g/cm^{3} at
900 mm, Avocado roots occurred mainly in soil with a bulk density less than 1,7
g/cm^{3}. The increase in bulk density affected root growth, but a
change in soil texture and structure had an even greater effect, as illustrated
in Figure 8. Total root mass per cubic metre of soil was higher in the
Nelspruit soil than at Burgershall. The percentage dry matter for the roots was
also higher. No explanation can be offered for these results.
A comparison of the 'healthy' and the
apparently healthy' trees in relatively deep soils at Burgershall and the tree
in shallow soil at Nelspruit (Figure 9), indicated that two depths in the
profile at Burgershall were more favourable for the development of avocado
roots than other depths. These depths vary from tree to tree according to the
exposure of the soil to sunlight, wind, desiccation and factors unknown at
present. In the shallow soil at Nelspruit, the only region where significant
amounts of roots developed at all, was in the upper 200 mm, where the bulk
density was relatively low.
Fig 9. Comparison of the vertical root distributions for two relatively healthy avocado trees (BHI and BH2), with that of the Nelspruit tree (NELS) growing in a soil with distinct soil horizons.
Avocado roots are fairly uniformly distributed, vertically and horizontally,
in a homogenous Huttontype soil. The restrictive bed rock at 1 m affected the
root mass near that depth, probably due to occasional overwet conditions,
although there were no signs of waterlogging at any stage. Bulk density had no
measurable effect on root distribution in the Burgershall soil, which implies
that other factors, such as soil temperature, may contribute more to
determining root distribution. Horizontal feeder root distributions tended to
increase toward the periphery of the trees at Bt rgershall, according to the
regression analyses. This is attributed to the canopy skirts touching the
ground and keeping the soil cool and probably to the microjet irrigation that
only wets the soil under the canopy of the tree. The loam soil retains sufficient
moisture to keep roots alive in protracted drought periods and excessive root
dieback should not occur frequently, Due to Phytophthora infection, this
general trend is obscured in infected trees.
The soil at Nelspruit exhibited a sharp
transition from sandy loam to coarse gravelly clay at a depth of 0,5 m, and
very little root mass was found beyond the transition zone. Vertical root
distribution was restricted to the upper soil layers and horizontally, root
mass decreased away from the tree trunk. Three factors may account for this
root distribution, apart from the obvious restrictive coarse gravelly clay
layer:
1 Irrigation is by dragline sprinkler system which wets the total orchard floor in every cycle. Due to the shallowness of the upper soil, roots are confined between the surface and deeper layers that may be overwet after rain and even after irrigation.
2 The sandyloam soil will dry out faster than
the Burgershall soil, because it is more exposed and its soil moisture holding
capacity is less. Roots, especially feeder roots, will dieback more
frequently, due to temporary drought.
3 Phytophthora root rot is probably partly
responsible for the decreasing root mass toward the periphery of the tree, as
the relation of feeder root mass to total root mass is similar to that of the
diseased tree at Burgershall.
A correlation can be demonstrated between
root mass, bulk density and depth in the Nelspruit soil, but it is noticeable
that a sudden change in soil texture seems to have a greater effect on' root
penetration. Bulk density in the order of 1,7 g/cm3 and higher, contributes to
restricting root penetration. This concurs with the findings of Rowell (1979)
that few, if any, roots occur in a compacted soil layer.
Where no physical restrictions or overwet
conditions occur, other factors such as soil temperature, fertility and
available oxygen probably contribute more to determine avocado root
distribution.
The method used in the study, yielded data
that made it possible to study avocado root distribution in the soil without
damaging the trees unduly. Effective root depth of the trees was determined to
be about 100200 mm above a restrictive layer or abrupt change in soil texture
and structure.
1 Avila N
Rovira, L, Chirinos, AV & Figueroa, M, 1986. Quantification of some minerals extracted from the soil by an avocado
crop. Proc of the Trop Reg, Am See Hort Sci, 23,108113.
2 Correa, L De S, Moreira, CS, &
Montenegro, HWS, 1984. Distribution of the root system of avocado (Persea spp)
cultivars in a redyellow podzolic soil. Anais do VII Congresso Brazileiro de Fruticultura.
I, 5363.
3 Huguet, JG, 1973. Novelle methode de' etude de I'enraciniment des vegetaux perennes A partir d'une tranchee spirale. Ann Agron, 26(6), 707731.
4 Moutounet, B, Aubert, B, Gousseland, J &
TiawChan, P in collaboration with Payet, 0 & Joson, J, 1977. Etude de
I'enraciniment de quelques arbes fruitiers sur set ferrallitique brun pretend. Fruits, 32(5), 321.
5 Nel, DJ, 1983. Soil requirements for Avocado
Production. Farming in South Africa, Avocados B2/1983.
6 Rowell, AWG, 1979. Avocado soil moisture
studies. S Afr Avocado Growers' Assoc Research Report 3, 3537.