Soil & Tillage Research, 26 ( 1993 ) 69-78 69 Elsevier Science Publishers B.V., Amsterdam

Effect of soil compaction on leaf, stem and fibrous root growth of cassava ( Manihot esculenta, Crantz )

H.O. Maduakor School of Agriculture and Agricultural Technology, Federal University o[Technology, P.M.B. 1526,

Owerri, IMO State, Nigeria

(Accepted 3 July 1992 )

ABSTRACT

Maduakor, H.O., 1993. Effect of soil compaction on leaf, stem, and fibrous root growth of c a s s a v a

(Manihot esculenta, Crantz). Soil Tillage Res., 26: 69-78.

The effect of soil compaction on the growth and proliferation of fibrous roots of c a s s a v a w a s inves- tigated in southwestern Nigeria using specially constructed boxes. Compaction increased leaf area production but decreased the efficiency of storage root production, indicating that more photosyn- thates were preferentially allocated to the top in plants growing in compacted soil. Fibrous root growth w a s also reduced by compaction especially at the early stages of growth. However storage root weight after 134 days of growth was unaffected by compaction. The need to review pre-planting ploughing for cassava is suggested.

I N T R O D U C T I O N

Cassava is the seventh largest producer of staple food in the world (USAID, 1972) and is grown in 31 African countries where consumption per capita averages over 100 kg year- ~ (Hahn, 1989 ). It also has an enormous potential as an industrial crop (Robinson and Kutianawala, 1979). There have been some advances in the understanding of the growth and physiology of the crop and the effect of environmental factors on these processes (Williams and Ghazali, 1969; Wholey and Cock, 1975; Hunt et al., 1977; Onwueme, 1978; Cassava Research, 1979; Cock et al., 1979; Lal, 1981; Connor et al., 1981; Keating et al., 1982a, b). However, there is still insufficient information on the growth, proliferation and configuration of the fibrous root systems and how they are affected by changes in soil physical and chemical properties.

The few studies in this area include that ofCampos et al. ( 1975 ) who grew

Correspondence to." H.O. Maduakor, School of Agriculture and Agricultural Technology, P.M.B. 1526. Owerri, IMO State, Nigeria.

70 H.O. MADUAKOR

sweet cassava in a well structured Brazilian Oxisol and observed that the roots penetrated to depths of 90 and 140 cm after 210 and 365 days of growth. Up to 95% of the roots were located in the top 30 cm layer. Connor et al. ( 1981 ) also reported that cassava roots penetrated beyond 2.6 cm during 7 months of growth and observed the low root density of cassava compared with other crops. However, Aresta and Fukai (1984) obtained rooting densities com- parable with other crops in their shading experiment at a relatively high lati- tude region in Australia. They also observed that maximum root depth in- creased from 60 to 80 cm, at 29 days after planting, to 100-120 cm 21 days later. By 72 days of growth the roots had surpassed 120 cm.

The effect of environmental factors on the fibrous root system of cassava has been little investigated. Aresta and Fukai (1984) reported that shading reduced both the rate of increase and the total root length of cassava. Connor et al. ( 1981 ) observed that cassava fine roots proliferated more in deep layers when water was excluded by the use of plastic mulch.

It has been shown that soil compaction limits both root and top growth of most agronomic crops, especially at the early stages of growth (Meredith and Patrick, 1961; Taylor, 1971b; Russell and Goss, 1974). Such information for tuberous crops, especially cassava, is scanty. In one such investigation, Fer- guson and Gumbs (1977) grew White Lisbon yams in soil compacted in a special box, and found that compaction either in the fine root or tuber com- partment did not affect rooting densities. Vine ( 1980 ) and Vine et al. ( 1981 ) reported the restriction of cassava root growth by high soil strength and mois- ture deficit.

It is obvious that information on the response of the fibrous root system of cassava to soil compaction in the forest zone of southern Nigeria is needed, especially as large-scale farmers in this region are increasingly employing bull- dozers to clear their land and subsequently use the tractor to prepare it for planting cassava. Such practices lead to compaction of the soil (Lal and Cum- mings, 1979).

The objective of this study, therefore, was to investigate the effects of dif- ferent levels of soil compaction on the development and proliferation of fi- brous root of cassava. Top growth, dry matter production and partitioning as well as the relationship among some growth attributes were also investigated.

MATERIALS AND METHODS

The study was conducted in specially constructed wooden boxes. Each box (46 cm X 48 cm × 90 cm) had three collapsible sides hinged to a base perfo- rated to allow easy drainage. The fourth side could be replaced by a wooden pin board of the same dimensions to facilitate root washing and sectioning.

Surface soil (0-15 cm ) from the Apomu series (Psammentic Ustorthent ) was collected from a plot under a secondary forest regrowth of about 15 years.

SOIL COMPACTION AND FIBROUS ROOT GROWTH OF CASSAVA 71

The soil was developed on a semi-recent slope colluvium and has a loamy sand texture and loose almost single-grained structure. The soil contains 79% sand, 11% silt, 10% clay and 2.5% organic carbon. This soil has a low water- holding capacity with 18% and 8% gravimetric moisture content at 0.1 bar suctions and 15 bar suctions, respectively (Moormann et al., 1975). Under secondary forest regrowth, the bulk density ranges from 1.2 to 1.3 g cm-3, but it increases to 1.5 or 1.6 g cm -3 within 5-6 years of cultivation.

The soil was sieved through a 4 mm mesh screen to remove stones and roots, and then was compacted at a moisture content of about 8-10%. Twenty- four boxes were divided into three groups for packing bulk densities (B.D.) of 1.4, 1.6 and 1.8 g cm -3 (on an oven dry mass basis).

The boxes were kept under natural field environments at the experimental farm of the International Institute of Tropical Agriculture (IITA) located about 30 km south of the northern limit of the lowland rainforest zone of the west African tropics. The bimodal character of rainfall distribution leads to two distinct growing seasons, with a total annual rainfall varying from 900 to 1300 mm. The first growing season is from late March to late July, ending in a short dry spell of approximately 1 month. The second, shorter season begins in late August and ends in early November.

Stem cuttings ( 15 cm long) of cassava cultivar 'TMS 30572' were planted, one per box in a slanting position. Cassava was grown under natural rainfall without supplementary irrigation. Root and shoot growth measurements were made at 48, 77, 106, 134 and 185 days after planting (DAP). At each sam- pling, shoot growth was separated into stem, leaf blades and petioles which were then dried at 60°C and weighed separately. Leaf area was determined during initial stages with a leaf area meter (Lambda Instruments Co. ). How- ever, at later stages of development, it was estimated from the regression equation:

LA=210.1 W+610.6

where LA is leaf area in cm 2, and W is dry weight of leaves in g. The roots were sampled as follows: the removable side of the box was replaced with a wooden pin board of the same dimensions. Pins (0.4 cm in diameter) were inserted through holes perforated on the board. The pins helped to keep the feeder roots in place and to mark the 10-cm depth intervals from the surface of the soil. The box was inclined at an angle with the pin board on the lower side and the soil was carefully washed off the roots. The tuberous roots were removed and sliced into small sections. They were dried to a constant weight at 60 ° C and weighed. The feeder roots were sectioned into 10 cm depth inter- vals and stored in a mixture of alcohol and water. Root length was determined by the intercept method of Newman ( 1966 ). Because of the large amount of roots, only intercepts of subsamples were counted. After the determination of root length, the roots were dried in the oven at 60 °C and weighed.

72

RESULTS

H . O . M A D U A K O R

Leaf area production

Figure 1 shows the effect of the different levels of compaction on leaf area production during the growth period. The relationship can be described by straight line models, and shows that 0.013 cm 2 of leaf area was produced in a day by plants under the lowest level of compaction (B.D. 1.4 g cm-3). This amount increased by 69% and 15.4% as the compaction level increased to B.D. 1.6 g cm -3 and 1.8 g cm -3, respectively. It also shows that leaf area production had not attained its maximum value by the end of the experimen- tal period.

Dry matter production

Boerboom ( 1978 ) described a model of dry matter production in cassava. In this model, he identified a parameter which he called the efficiency of stor- age root production (ESRP) as the slope of the straight line obtained when the storage root dry weight is plotted against the total dry weight of the plant. Figure 2 shows the plots of storage root dry weights against total dry weights and confirms the linear relationship irrespective of the level of compaction of the soil. It also shows that by compacting the soil from 1.4 g cm -3 to 1.8 g cm-3, ESRP decreased from 0.72 to 0.57, indicating that compaction results in more photosynthates remaining in the stems and leaves. The pattern of dry matter allocation to the different top components (leaves, petioles and stems ) was not significantly affected by compaction. The greatest amount went to the stem, followed by the leaves and then the petioles.

E 3.0- %

o ~ 1.0 g _1

0.~- ¢.8

7, BD=t.t, y=0.Ot3x-0.32 rz=0.93-'~ e- - -eBD= l .6 y=0.22",'-0.9/, r2=0.97, O- - -OBD =1.8 y=0.015x-0.391 rL~/~4~ ~^

! i 8"8 128 168 208

Doys o f t e r p lon t l n 9

Fig, 1. Leaf area product ion as affected by soil compact ion dur ing the season.

SOIL COMPACTION AND FIBROUS ROOT GROWTH OF CASSAVA 73

1400-

1200-

1000-

c~ v

800

-o

o 600

g o

z.oo-

200-

.~. ~ BD:I.~ y:0.72x-45.09 r2=0.99

0-----0 BD=I.6 y= 0.57~c- 36.5/, r2=0-99

c>-.-.o BD=I.5 Y = 0.57•- 36.53 r 2 = 0.99

~o 4~o 6b0 80'0 1o~ 12~o i~oo To to t D r y w t ( 9 )

Fig. 2. Relationship between the total and storage root dry matter as affected by soil compaction.

Fibrous root growth

Variation with time Under the lowest level of compaction 1.4 g cm-3, cassava produced 1640

m of fibrous roots weighing 16.4 g dry weight during 185 days of growth. Compacting the soil to 1.6 g c m - 3 reduced the mean seasonal root production by 14% (550.0 m) . On further compaction to 1.8 g c m -3, it was reduced by 26% to 474.24 m. In the case of the root dry weight, the reductions were 7.4% and 24.8% for soil densities of 1.6 g cm -3 and 1.8 g cm -3, respectively.

The variation of root length density (RLD) (cm root cm-2 of ground sur- face) with time can be described by linear equations at the three levels of compaction; a two stage linear relationship, however, obtains at the lowest level (1.4 g cm -3) (Fig. 3). During the first stage which occurs at the early part of the season (0-77 DAP), the rate of fibrous root production (0.59 RLD day- i ) is 30 times higher than that during the second stage (0.02 RLD day -1 ) which occurs at the later part of the season (77-185 DAP). Com- pacting the soil to 1.6 and 1.8 g cm -3 decreased the first stage but increased the second stage rate to 0.24 RLD day- l and 0.16 RLD day- 1, respectively. Compacting the soil, therefore, during the early part of cassava growth de-

74 H.O. MADUAKOR

50

?oE40 .

• G 3o-

ac ~2o-

~1o-

7, 7, BD= 1.4: o: y =0.59~c-12.2:r2=1.00 b: y = 0. 02~c * 3|,5:r2--O.9{

o-.-.-o BD= 1.6:y=0.243c*0.66 = 0.98 + / * / r2

o..----o BD=I.8: y = 0 . 1 6 X 5 .70 f " / r 2 : 0 ' 8 9 b " " /

f * . / . / ° / / io/

o/ le /"

, v !

o 40 a0 12o :~o 2~o Deys e f { e r p l e n t l n g .

Fig. 3. Variation of root length density with time as affected by compaction during the season.

3o , • /

25- , ~ /I

~o× 15- / ' / / / / 7~ X BD: 1.4 y=O.28x-3458 r2=0.17 z o /,

E . ~ o..---4 BD=l.6 y=0.38x-9844 r2=0.96 ~ ' 0 - . / / ~ - - - O B D = t ~ y : 0 . 4 1 ~ - 7 9 5 7 r2=0.9a

o / / ~ / / x x

S ~ 5 - •

0 / z / , 0 2b 40 6'0 8b a 1~0 12'0

ROOl leng|h ¢m x 10

Fig. 4. Relat ionship between root length and leaf area as affected by soil compaction.

creases the rate of fibrous root production while compacting it at the later stage increases it.

Root length rather than root dry weight is correlated with water and nu- trient uptake which in turn contribute to the production and expansion of the leaves in which photosynthesis occurs. Therefore, the relationship between leaf area and root length is important. Figure 4 shows that it is non-linear at

SOIL COMPACTION AND FIBROUS ROOT GROWTH OF CASSAVA 7 5

the lowest level of compaction but becomes significantly linear as the com- paction level increased. The leaf areas produced per cm of root length were 0.38 cm 2 and 0.41 cm 2 for B.D. 1.6 g cm -3 and 1.8 g cm -3, respectively. The difference was not significant, indicating that these levels of compaction had a similar effect on the relationship between leaf area and root length.

V a r i a t i o n w i t h d e p t h

More roots were produced in the upper part of the soil compared with the lower part (Fig. 5). At 134 DAP, 69% and 81% of all roots produced were in the upper 0-60 cm depth in soils compacted to 1.4 g cm -3 and 1.8 g cm -3, respectively. Also the total amount of roots produced per cm 2 of surface was higher in B.D. 1.4 (27.9) than 1.8 g cm -3 ( 18.5 ). However, the lateral distri- bution of the roots did not differ appreciably indicating that compaction had little effect on it (Fig. 5 ) probably as a result of the confinement of roots in the boxes.

Storage roots

At 134 days, the dry weights of storage roots (SR) were 279 g, 277 g and 330 g for soil densities 1.4 g cm -3, 1.6 g cm -3 and 1.8 g cm -3, respectively, showing that compaction had little effect on storage root production.

BD=I.4 Root length density cm crn -2

0 2.0 4.0 6.0 810 I0.0 , i i

o-2o ~////)//////////a 20 -4 0 r/I//////////////I////////J

= ~ o - 6 o l l / / / / / / / / / / / / / / / / A 60 - 8 O l / I I / I I I I / / / / / / I I I / / / / A 8o - l oor / / / / /~

I

BD= 1.8 Root length densi ty cm ¢m -2

0 2;0 at0 6;0 810 10.0 0-2.8'

E 2 0 - 4 0 ~ ~' 4 0 - 6 0 ~

30cm 20cm lOom ~ < 10era 2 0 c m 30¢m 30¢m 20crn 10cm " ~ 10cm 2 0 c m 30cm

i i' /UI':II / ?LII i ii I i..ii::"-:-.i-~. :";..'-'.:.~.::~ "-.-::--:i .~ i . ~2ocm

8o " ii..: ."i ; i : . ' - - - - - : / " " "

,o T i,ool 1,oo o

Fig. 5. Fibrous root distribution as affected by compaction at 134 days after planting.

76 H.O. MADUAKOR

DISCUSSION

Though compaction reduced the total amount of roots produced, it did not affect the ability of the fibrous roots so produced to penetrate the compacted soil and absorb water and nutrients. A similar observation was reported by Lal in 1983. A possible explanation is that compaction of a sandy soil such as that used in this study increases the moisture content especially at high suc- tion (Warkentin, 1971 ). There is therefore increased water and nutrient up- take from the compacted soil by the roots leading to increased leaf area pro- duction and expansion (Fig. l ).

Because compaction increased soil strength (Taylor, 1971 a), the bulking cassava SR will have to exert more pressure on the soil to enlarge. Under this condition, the top becomes a stronger sink than the SR for photosynthates as shown by the decrease in ESRP (from 0.72 to 0.57) with increased level of compaction (Fig. 2). The values of ESRP obtained in this study agree with those calculated for different cultivars of cassava by Boerboom, 1978.

The total length of fibrous roots produced by cassava in the least-com- pacted soil after 185 days of growth is low compared with other crops as was also observed by Conner et al. ( 1981 ). Despite this, however, cassava was able to absorb enough water and nutrients for normal growth under the pre- vailing environmental conditions. The generally observed ability of cassava to do well under harsh environmental conditions can therefore be attributed to the activity of its fibrous roots system (FRS). In this study, the configura- tion of FRS was not significantly affected by compaction (Fig. 5 ) and there- fore penetrated and proliferated because of its ability to utilise the increased moisture stored in the soil as a result of the reduction in the pore sizes of the sandy soil when compacted (Meredith and Patrick, 1961; Giles, 1983 ). This reduction should normally reduce the rate of root penetration and prolifera- tion especially that of the main axis roots (Russell and Goss, 1974). There- fore the non-significant effect of compaction on the root configuration espe- cially at the later part of the season may be attributed to the fact that the pores, though reduced in size, were still large enough for the cassava fibrous roots to penetrate. Usually, roots grow in pores formed by soil aggregation and as long as the diameter of the root is less than that of the pore, the root will penetrate the pore (Taylor, 1971 b; Cannel, 1977 ).

The greater confinement of the roots in the upper part of the profile (Fig. 5 ) has also been reported by others (Dos Campos et al., 1975; Connor, 1980; Connor et al., 1981; Aresta and Fukai, 1984), and obtains as long as moisture is adequate and evenly distributed in the soil profile. If there is moisture shortage, however, the roots tend to proliferate in areas where there is mois- ture to compensate for reduced growth in the dry parts.

Cassava SR in compacted soil can still enlarge to their maximum potential as evidenced by the non-significant effect of compaction on their final weights.

SOIL COMPACTION AND FIBROUS ROOT GROWTH OF CASSAVA 77

It follows then that compaction reduced SR enlargement in the early part of the season when the top was a stronger sink for photosynthates than the SR. Later in the season however, more phytosynthates were translocated to the SR from the now increased leaf surface causing them to enlarge and exert pressure on the soil, thus counteracting the effect of compaction. This pres- sure results in heaving up and cracking of the soil surface. These features, which are commonly observed in mature cassava fields especially in clayey soils during dry periods, may increase infiltration rate and soil moisture stor- age and reduce run-off and soil loss.

If the levels of compaction in this study do not affect SR which are the main components of cassava required for consumption and industrial use, then the necessity of cultivation of similar soils before cassava planting needs to be reviewed.

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78 H.O. MADUAKOR

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