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Soil& Tillage Research, 22 (1992) 1-11 1 Elsevier Science Publishers B.V., Amsterdam Effects of soil wetness at the time of land clearing on physical properties and crop response on an Ultisol in southern Nigeria B.S. Ghuman a,1 and R. Lal b aHTA, PMB 5320, Ibadan, Nigeria bDepartment of Agronomy, The Ohio State University, Columbus, OH 43210, USA (Accepted 8 March 1991 ) ABSTRACT Ghuman, B.S. and Lal, R., 1992. Effects of soil wetness at the time of land clearing on physical prop- erties and crop response on an Ultisoi in southern Nigeria. Soil Tillage Res., 22:1-11. Land clearing of a tropical rainforest implies cutting and felling of existing vegetation by manual or mechanized techniques, with or without windrowing, followed by burning in situ or in windrows. The objective of this study was to evaluate effects of land clearing at three levels of initial soil wetness on soil physical properties, and on growth and yield of maize and cowpea for a tropical Ultisol in south- ern Nigeria. One of three main-plot treatments measuring 6 × 40 m was cleared mechanically in Feb- ruary, April and June 1987, respectively. These months corresponded to the dry, moist and wet sea- sons, giving three different initial levels of soil wetness. Each main plot was divided into six sub-plots, 6 × 5.5 m each, with buffers in between. Maize (Zea mays) and cowpea (Vigna unguiculata) were grown in a sequence with uniform management in all sub-plots. Sub-plots were subjected to two tillage methods, i.e. tilled and no-till systems. Soil wetness at 0.05 m depth at the time of clearing was 0.11, 0.16 and 0.20 m 3 m-3 in the February, April and June clearing, respectively. In July 1987 at 0.05 m depth, soil bulk density was significantly higher in the February (by 28%), April (by 35%) and June (by 41%) clearing than that of the forested control. The increase in bulk density in the February clearing compared with the forested control was significant to 0.35 m depth. However, this difference persisted to 0.65 m depth for the April and June clearings. The steady-state infiltration rate measured soon after clearing was 62.0, 2.2 and 3.3 cm h-i in the February, April and June clearings, respec- tively, as compared with 200 cm h- l in the forested control. Tillage reduced soil bulk density in the tilled layer and increased the infiltration rate. Bulk density of tilled plots increased with time after cropping began. In contrast, however, bulk density in no-till plots decreased after cropping began. Infiltration rate increased with cropping duration in all plots irrespective of the tillage method. Maize grain yield was not affected by clearing at either level of initial soil wetness during 1987 when yields were low. In ! 988, however, grain yields of maize were lower in the February and April clearings than in the June clearings. Cowpea ( Vigna unguiculata) yield was also significantly lower in the February- cleared than in the April- or June-cleared plots. Yield observations were, however, too variable to draw valid conclusions. Furthermore, crop yields were also affected by factors other than alterations in soil properties caused by land clearing. The data suggest that dry-season clearing is better for soil physical properties than wet-season clearing. However, the soil surface must be kept covered by the felled biomass to protect soil against high temperatures and intense rains. ~Present address: 133-D, Bhai Randhir Singh Nagar, Ludhiana, Pb., India. © 1992 Elsevier Science Publishers B.V. All rights reserved 0167-1987/92/$05.00

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Page 1: Effects of soil wetness at the time of land clearing on physical properties and crop response on an Ultisol in southern Nigeria

Soil& Tillage Research, 22 (1992) 1-11 1 Elsevier Science Publishers B.V., Amsterdam

Effects of soil wetness at the time of land clearing on physical properties and crop response on an

Ultisol in southern Nigeria

B.S. G h u m a n a,1 a n d R . L a l b

aHTA, PMB 5320, Ibadan, Nigeria bDepartment of Agronomy, The Ohio State University, Columbus, OH 43210, USA

(Accepted 8 March 1991 )

ABSTRACT

Ghuman, B.S. and Lal, R., 1992. Effects of soil wetness at the time of land clearing on physical prop- erties and crop response on an Ultisoi in southern Nigeria. Soil Tillage Res., 22:1-11.

Land clearing of a tropical rainforest implies cutting and felling of existing vegetation by manual or mechanized techniques, with or without windrowing, followed by burning in situ or in windrows. The objective of this study was to evaluate effects of land clearing at three levels of initial soil wetness on soil physical properties, and on growth and yield of maize and cowpea for a tropical Ultisol in south- ern Nigeria. One of three main-plot treatments measuring 6 × 40 m was cleared mechanically in Feb- ruary, April and June 1987, respectively. These months corresponded to the dry, moist and wet sea- sons, giving three different initial levels of soil wetness. Each main plot was divided into six sub-plots, 6 × 5.5 m each, with buffers in between. Maize (Zea mays) and cowpea (Vigna unguiculata) were grown in a sequence with uniform management in all sub-plots. Sub-plots were subjected to two tillage methods, i.e. tilled and no-till systems. Soil wetness at 0.05 m depth at the time of clearing was 0.11, 0.16 and 0.20 m 3 m-3 in the February, April and June clearing, respectively. In July 1987 at 0.05 m depth, soil bulk density was significantly higher in the February (by 28%), April (by 35%) and June (by 41%) clearing than that of the forested control. The increase in bulk density in the February clearing compared with the forested control was significant to 0.35 m depth. However, this difference persisted to 0.65 m depth for the April and June clearings. The steady-state infiltration rate measured soon after clearing was 62.0, 2.2 and 3.3 cm h - i in the February, April and June clearings, respec- tively, as compared with 200 cm h - l in the forested control. Tillage reduced soil bulk density in the tilled layer and increased the infiltration rate. Bulk density of tilled plots increased with time after cropping began. In contrast, however, bulk density in no-till plots decreased after cropping began. Infiltration rate increased with cropping duration in all plots irrespective of the tillage method. Maize grain yield was not affected by clearing at either level of initial soil wetness during 1987 when yields were low. In ! 988, however, grain yields of maize were lower in the February and April clearings than in the June clearings. Cowpea ( Vigna unguiculata) yield was also significantly lower in the February- cleared than in the April- or June-cleared plots. Yield observations were, however, too variable to draw valid conclusions. Furthermore, crop yields were also affected by factors other than alterations in soil properties caused by land clearing. The data suggest that dry-season clearing is better for soil physical properties than wet-season clearing. However, the soil surface must be kept covered by the felled biomass to protect soil against high temperatures and intense rains.

~Present address: 133-D, Bhai Randhir Singh Nagar, Ludhiana, Pb., India.

© 1992 Elsevier Science Publishers B.V. All rights reserved 0167-1987/92/$05.00

Page 2: Effects of soil wetness at the time of land clearing on physical properties and crop response on an Ultisol in southern Nigeria

2 B.s. GHUMAN AND R. LAL

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

Although it is recommended that forested land should be cleared during the dry season only (Lal et al., 1986), this recommendat ion is not always ob- served. The pressure of developing new land for establishing large plantations and the non-availability of land-clearing equipment necessitates clearing dur- ing the rainy season. The problem is equally severe in several transmigration schemes in the humid tropics (e.g. Sumatra, Brazil) as government agencies are forced to clear land all the year round to keep to a schedule of transmi- grating a pre-determined number of households every year (Scudder, 1985; Suwardjo et al., 1985 ). It has been demonstrated that in some soils, mechan- ical land clearing adversely affects soil physical properties through compac- tion by the machinery involved (Van der Weert and Lenselink, 1972; Seubert et al., 1977; Lal and Cummings, 1979; Alegre et al., 1986). The most com- monly used machinery in land clearing includes a track-type tractor with front- mounted dozer blade, shear blade, tree-pusher or tree-extractor attachments. Heavy chains to pull trees or root-rakes to extract roots are usually attached to the back of the tractor. However, the magnitude of adverse effects on soil properties in relation to soil moisture content at the t ime of clearing is not widely determined. Furthermore, Hulugalle et al. (1984) observed that ad- verse effects of mechanized land clearing can be somewhat mitigated by sow- ing the newly cleared land to a quick-growing and deep-rooted cover crop such as Mucuna utilis. Nonetheless, the degree of alleviation of compaction and the t ime needed for fallowing to restore soil structure depends on the degree of soil compaction and structural degradation caused by mechanized land clearing.

Soil wetness at the t ime of land clearing may alter soil structure and cause severe soil compaction. In the absence of irrigation, soil wetness depends on soil properties, land use, and the amount and frequency of rainfall. Land clearing in different seasons, therefore, may affect the soil properties differ- ently. There are few data on the effects of degree of soil wetness at the time of land clearing on soil compaction and on degradation of soil physical proper- ties. The objective of this experiment, therefore, was to evaluate the effects of soil wetness at the time of land clearing on alterations in soil physical prop- erties and on crop growth and yield. Soil physical properties evaluated were bulk density, infiltration rate and soil moisture profile.

M A T E R I A L S A N D M E T H O D S

Experimental design

The experiment was conducted in southern Nigeria, at the Okomu Oil Palm Plantation. Okomu (5 ° 12'E; 6°24 'N; 76 m above mean sea level). Mean

Page 3: Effects of soil wetness at the time of land clearing on physical properties and crop response on an Ultisol in southern Nigeria

EFFECTS OF SOIL WETNESS ON A NIGERIAN ULTISOL 3

annual rainfall of the area is 2.0 m with a range of 1.5 to 2.4 m. The rainfall distribution is monomodal , with a rainy season from March to October fol- lowed by a 4-month dry season.

A 0.5-ha field was selected within the high rainforest reserve in the Bendel State of Nigeria. This land had not been disturbed for about 100 years. The soil, classified as Ultisol (Lal, 1987), contained 80-82% sand, 3-4% silt and 14-17% clay (Table 1 ). Soil pH (1:1 soil water) was 4.9_+0.4 and 4.7_+0.4 in the 0-0.05 and 0.05-0.10 m layers, respectively. The forest vegetation of the study site consisted of several tree species, including Afzelia, Antiaris, En- antia and Strombosia. Mean tree density ( trees> 0.15 m girth at 1 m height) was 1285 trees ha-~ with mean basal area of trees between 386 and 462 m 2 h a - ). An area 6 m wide and 40 m long was cleared in February, a second in April and a third in June 1987 by using the shear blade with a stinger (Roman KG) front-mounted on a D-8 Caterpillar. Trees were cut off at ground level and removed from the plots. Times of land clearing were selected in relation to onset of the rains, thereby attaining contrasting levels of soil water content, i.e. dry (February), moist (April) and wet (June) soil water regimes. The soil moisture profile at different times of land clearing is shown in Fig. 1. The data in Fig. l show that soil moisture content was lowest in February and ranged from 10% by volume in the 0-0.10 m layer to about 0.18% in the 0.60- 0.70 m layer. In April and June, soil moisture contents were about similar from 0.20 to 0.70 m depth. However, soil moisture content in the surface 0- 0.10 m layer was lower in April than in June - 16 vs. 20%.

At each t ime of clearing, bulk density was measured on six 100 cm 3 cores taken from 0 to 0.70 m depth at 0 . l -m intervals (Blake and Hartge, 1986). Cores were taken at random, three each from tilled and untilled sub-plots totalling six over the main plot and three from forested control. The steady- state infiltration rate was measured at six locations per clearing with a double- ring infiltrometer (Bouwer, 1986). The inner ring was 0.20 m in diameter

TABLE1

Soil properties at two depths of the experimental site

Soil properties 0-0.05 m 0.05-0.10 m

pH ( 1 : 1 in water) 4.9 4.7 Organic carbon (%) 2.7 1.1 Total nitrogen (%) 0.35 0.11 Available phosphorus (ppm) 6.5 4.5 CEC (c mol kg- t ) 6.1 2.6 Sand (%) 82 g0 Silt (%) 4 3 Clay (%) 14 17

Page 4: Effects of soil wetness at the time of land clearing on physical properties and crop response on an Ultisol in southern Nigeria

WATER CONTENT (m3/m 3)

0.10 0.14 0.18 0.22 0.26

ing

O.

E

"1" O. Q. W 0

,-I

4 B.S. GHUMAN AND R. LAL

O.

'ing

O.

O . ¸

Fig. 1. Initial soil water distribution with depth in the profile at three times of land clearing.

and the outer ring 0.28 m in diameter. The above properties were measured within l week of each clearing.

Effect on crops

Each main plot was sub-divided into 6 sub-plots measuring 6 × 5.5 m each. Three plots were randomly chosen from each main plot and tilled (T) man- ually with a shovel to a depth o f0 .15 -0 .20 m. The remaining plots were kept untilled (NT) . Treatments were laid out as per the randomized split plot de- sign. Tillage treatments were applied for each crop in each year.

Maize (Zea mays), cv. TZSR-W, was planted on 16 July 1987 at 0.30 × 0.75 m spacings. The first dose of N P K fertilizer mixture ( 15 : 15 : 15 ) was broad- cast at the rate of 60 kg h a - 1 at planting. The second application of 60 kg N ha-1 in the form of calcium a m m o n i u m nitrate was added a month later. Lime was not applied because maize was sown as a first crop after clearing a high rainforest about 100 years old. Leaf area index at different growth stages was measured by the length × breadth method and by using an appropriate shape factor. Maize was harvested for grain yield on 23 October. A second crop of maize was planted on 29 April 1988 and harvested on 16 August. Cowpea ( Vigna unguiculata), cv. IT 84S-2246-4 , was planted on 23 August 1988. The spacing used was 0.15 × 0.50 m. A low application of N at the rate

Page 5: Effects of soil wetness at the time of land clearing on physical properties and crop response on an Ultisol in southern Nigeria

EFFECTS OF SOIL WETNESS ON A NIGERIAN ULTISOL 5

of 30 kg ha- 1 was broadcast in the form of NPK ( 15 : 15: 15 ) mixture. Infes- tations by insects were controlled by applying two sprays of Sherpa-plus at the rate of 4 cm 3 per litre of water; the first spray in September and the second in October. The crop was harvested for grain yield in November, 1988.

Following the cowpea harvest in 1988, bulk density and infiltration rate were measured once again in all plots, using the procedures already described.

RESULTS AND DISCUSSION

Effect o f soil wetness at clearing on soil physical properties

In comparison to forest, soil bulk density was significantly affected by clearing at three levels of soil moisture (Fig. 2). In the dry soil profile (Feb- ruary clearing), bulk density of the cleared area was significantly greater than that for the forest to 0.35 m depth. Soil bulk densities for the moister soil profiles (April and June clearings) were higher than that for the forest treat- ment to a depth of 0.65 m. In other words, there was a greater increase in the bulk density in the moister soil profiles (April and June clearing) than in the dry profile (February clearing). For example, the increase in bulk density for 0-0.10 m depth was 0.28 Mg m -3 for February clearing, 0.36 Mg m -3 for April clearing and 0.41 Mg m-3 for June clearing. Increase in bulk density for

BULK DENSITY ( M g / m 3 )

I.O I. I 1.2 1.3 1.4 1.5 1.6 I I I I I I 1

LSD (.05) ,

0., ~

I- 0 . 3

Q

..J ~ 04 -0- Forest

0 . 5 ~-(I- February Clearing -0-- April Clearing ! L ~ -b- June Clearing

O.E

0.7_

Fig. 2. Effect o f initial soil wetness at clearing on soil bulk density.

Page 6: Effects of soil wetness at the time of land clearing on physical properties and crop response on an Ultisol in southern Nigeria

6 B.S. GHUMAN AND R. LAL

tl.I

Z o

t~

,'7 Z

3 0 0 - o

270 -~

240 r-°'o,O,o. Forest 200.0

2,o- o'o'o-o--o--o--o--o- o--o- o_o_o_ o

~T 1 5 0 - "~I 12C

90

3 0 xQL//April Clearing =~Q ..~fJune Cleoring

o - ~ E ) ~ -~-- '~ 2 =_ -.~" -~= ~, _,~.3 I I I I I .j2

0 I0 20 30 40 50 60 ELAPSED TIME (rnnts)

Fig. 3. Effec t o f in i t ia l soil w e t n e s s at c l ea r ing on in f i l t r a t ion rate.

0.10-0.20 m depth was 0.34 Mg m -3 for February clearing, 0.48 Mg m -3 for April clearing and 0.47 Mg m - 3 for June clearing. Increase in soil bulk density by deforestation was measured up to 0.70 m depth (Fig. 2). The data show that forest land at Okomu should be cleared towards the end of the dry season to restrict damage to soil bulk density by machinery.

Infiltration rate, like soil bulk density, was significantly affected by land clearing at different initial soil wetnesses (Fig. 3 ). For the dry-season clearing (February), the steady-state infiltration rate was about three times less than that in the forest (200 cm h - ~ ). In comparison with the forested control, the infiltration rate decreased by 61 and 91 times for the April and June clearing, respectively. Among different clearings, infiltration rate for the April and June clearing decreased by 19 and 28 times, respectively, than that of February clearing. This difference may have been caused partly by increased soil bulk density resulting from clearing at higher moisture contents. Formation of sur- face seal may be another factor responsible for low infiltration. Just as it in- creases soil bulk density, land clearing in the wet season is also deleterious to water infiltration into the soil. In other words, wet-season clearing of forest land may induce water runoff and soil erosion on sloping lands in the Okomu area if rainfall intensity exceeds 20-30 m m h - i. This threshold value of rain- fall intensity is based on the equilibrium infiltration rate for mechanized clearing (Fig. 3), which is as low as 20-30 m m h -1.

Effects of tillage and cropping on soil physical properties

Effects of two tillage systems and two years of cropping on soil physical properties are depicted in Table 2. Before planting in 1987, bulk density of

Page 7: Effects of soil wetness at the time of land clearing on physical properties and crop response on an Ultisol in southern Nigeria

EFFECTS OF SOIL WETNESS ON A NIGERIAN ULTISOL 7

TABLE 2

Effect of initial soil wetness at clearing and subsequent tillage on bulk density (BD) at two depths and steady-state infiltration rate (SIR) of field soil

Clearing Tillage 19872 month treat-

ment ~ BD (Mg m -3)

19883

SIR BD (Mg m -3) SIR (cmh - l ) ( c m h -~)

0 - 0 .10m 0.10-0 .20m 0 -0 .10m 0.10-0.20m

February T 1.28 1.39 117.7 1.31 1.34 139.0 NT 1.36 1.43 62.0 1.34 1.36 89.3

April T 1.29 1.47 12.4 1.32 1.36 45.8 NT 1.44 1.57 2.2 1.32 1.52 21.3

June T 1.24 1.53 5.0 1.29 1.38 53.4 NT 1.49 1.60 3.3 1.35 1.49 36.6

LSD (0.05) Clearing month NS 0.08 43.6 NS 0.05 29.9 Tillage 0.06 0.06 NS NS 0.04 24.4 CM × T NS NS NS NS 0.07 NS

~T, tilled; NT, no-till. 2Before planting maize in July 1987. 3After harvesting cowpea in November 1988.

the 0-0.10 m layer was significantly affected by tillage. However, bulk density of the 0.10-0.20 m layer was significantly reduced by both time of clearing (i.e. initial soil wetness) and tillage method. Similar observations were made on bulk density after the harvest of cowpea in 1988. The interaction of clear- ing month and tillage ( C M x T ) was significant only in 1988 for the 0.10- 0.20 m soil layer. However, soil bulk density of the 0-0.10 m layer in the tilled plots increased, irrespective of the clearing month, with time ( 1987 vs. 1988 ). The reverse was true for the no-till plots, probably due to the loosening effect of soil fauna, especially that of earthworms. In the tilled plots, loosened soil progressively settled, resulting in an increase in soil bulk density with time. In contrast, bulk density of the 0.10-0.20 m soil layer decreased for both til- lage methods. Harvesting of maize may have also influenced bulk density of surface layers.

The steady-state infiltration rate (SIR) was significantly greater in the dry- season cleared (February) than the rainy-season cleared plots in 1987 as well as in 1988 (Table 2 ). This may be due to the lower bulk density and drier soil profile in 1987 than 1988. In 1988, the SIR was also affected significantly by tillage; the tilled plots had higher SIR than the no-till plots. Though bulk den- sity of the 0-0.10 m layer in the tilled plots increased with time, the SIR also increased. High SIR may result from the formation of root channels, espe-

Page 8: Effects of soil wetness at the time of land clearing on physical properties and crop response on an Ultisol in southern Nigeria

8 B.S. GHUMAN AND R. LAL

cially owing to cropping with deep-rooted woody plants (Hulugalle and Lal, 1986).

Maize growth and development was poor, probably owing to low pH and/ or aluminium toxicity. There was no significant effect of initial soil wetness at clearing (i.e. clearing month) on the leaf area index of maize in 1987 or 1988 (Table 3 ). However, tillage effect on the leaf area index was significant at 64 days after planting (DAP) in 1987 and at 45 and 60 DAP in 1988, though the effect of tillage differed between years. At full canopy cover, 64- 75 DAP, the leaf area index was greater in NT than in T plots in 1987, but the reverse was the case in 1988. As expected, the grain yield of maize was low. The low yield of maize is attributed to several factors, including some other than the time of land clearing. Time of sowing maize is an important factor; the optimum time of sowing is late March to mid April. Maize yield declines progressively with late sowing (Lal, 1973 ), probably owing to high incidence of pests, low levels of radiation, and high humidity at the time of harvest. Because of low yields due to other yield-restrictive factors, maize grain yield was affected neither by initial soil wetness at clearing nor by subsequent til- lage treatment (Table 4). Mean maize grain yield was 0.28 Mg ha- 1 in 1987. Such a low yield does not clearly demonstrate the effects of time of land clear-

TABLE 3

Effect of initial soil wetness at clearing and subsequent tillage on leaf area index of maize at different days after planting (DAP)

Clearing month Tillage 1987 1988 treatment I

DAP DAP

22 36 50 64 27 45 60 75

February T 0.23 1.04 1.62 1.32 0.68 2.92 3.35 2.56 NT 0.29 1.03 2.01 1.46 0.30 1.73 2.35 1.80

April T 0.27 0.84 1.38 1.09 0.53 2.39 2.78 2.59 NT 0.29 1.17 1.87 1.14 0.52 2.41 2.69 2.24

June T 0.33 0.95 1.35 0.94 0.47 2.31 2.92 2.45 NT 0.26 1.15 1.87 1.78 0.34 1.87 2.49 2.27

LSD (0.05) Clearing month NS NS NS NS NS NS NS NS Tillage NS NS NS 0.26 NS 0.39 0.35 NS CM × T NS NS NS 0.45 NS NS NS NS

tT, tilled; NT, no-till.

Page 9: Effects of soil wetness at the time of land clearing on physical properties and crop response on an Ultisol in southern Nigeria

EFFECTS OF SOIL WETNESS ON A NIGERIAN ULTISOL 9

TABLE 4

Effect of time of land clearing and tillage methods on grain yields ( Mg ha - l ) of maize and cowpea

Year Tillage Clearing month treatment

February April June Mean

Maize 2 1987

1988

Cowpea 2 1988

T 0.23 0.29 0.37 0.30 NT 0.24 0.27 0.28 0.27 Mean 0.24 0.28 0.33 0.283 T 2.33 1.82 2.76 2.31 NT 1.58 1.87 2.10 1.85 Mean 1.96 1.85 2.43 2.083

T 0.95 1.48 1.29 1.24 NT 1.17 1.38 1.34 1.30 Mean 1.06 1.43 1.32 1.27

'T, tilled; NT, no-till. 2Insignificant treatment effect except of clearing month (i.e. initial soil wetness ) on cowpea yield, for which LSD (0.05) =0.17. 3LSD (0.05) for 1987 vs. 1988 maize yield=0.41.

ing on crop growth. Grain yields of maize were favourable during 1988. Al- though there were no consistent trends, yields were generally higher for June clearing than February clearing.

Contrary to what one would expect, the cowpea seed yield was lower for the dry-season than for the rainy-season clearing (Table 4). Since the February- cleared plots remained exposed to the sun and rain for a longer period before planting than the April- and June-cleared plots, organic matter may have be- come oxidized and nutrients leached or washed out of the root zone by 3 months of rain falling on an exposed soil surface. Data presented elsewhere (Ghuman and Lal, 1989, 1991 ) indicated a rapid decline in soil organic mat- ter and total nitrogen in mechanically cleared plots. There was also a rapid decrease in available P, pH and exchangeable cation. Alterations in soil prop- erties resulting from exposure perhaps affected the performance of crops in different plots. After the maize harvest in 1987, maize stalk was left in situ" this added organic matter to the soil and encouraged soil faunal activity, re- sulting in improved soil structure. This probably explains the significantly better performance of the maize crop in 1988 than in 1987, with grain yields of 2.08 and 0.28 Mg ha- 1, respectively.

C O N C L U S I O N S

The data presented support the conclusion that soil wetness at the time of clearing has a significant effect on clearing-induced alterations in soil prop-

Page 10: Effects of soil wetness at the time of land clearing on physical properties and crop response on an Ultisol in southern Nigeria

10 B.S. GHUMAN AND R. LAL

erties. The higher the wetness, the more deleterious is the effect on soil phys- ical properties. The results support the conclusion that land clearing should be carried out in the dry season to minimize soil damage. However, the felled vegetation should be kept on the soil surface to minimize exposure both to insolation (to guard against high soil temperatures) and to raindrop impact (to protect from accelerated erosion). I f feasible, land should be sown im- mediately to a cover crop (Hulugalle et al., 1984). Increased soil bulk density associated with land clearing could also be lessened by tilling. In no-till plots, however, it may take several seasons after clearing to alleviate the compaction problem and attain improved soil physical conditions.

Soil wetness at the time of land clearing had no significant effect on maize growth or yields. Furthermore, maize grain yield was poor in the first crop- ping year following clearing. However, grain yield increased in the second year. Low grain yield of maize was at tr ibuted to delayed planting and high incidence of pests and pathogens. In contrast to maize, grain yield of cowpea was significantly lower in the February-cleared than in the April- or June- cleared treatments. Growth and yield of cowpea in this acid soil was better than that of maize.

ACKNOWLEDGEMENT

This report is based on the I I T A - U N U - T h e Ohio State University Project.

REFERENCES

Alegre, J.C., Cassel, D.K. and Bandy, D.E., 1986. Effects of land clearing and subsequent man- agement on soil physical properties. Soil Sci. Soc. Am. J., 50:1379-1384.

Blake, G.R. and Hartge, K.H., 1986. Bulk density. In: A. Klute (Editor), Methods of Soil Anal- ysis, American Society of Agronomy, 2nd edn., Agron. no. 9 (Part 1 ), pp. 363-375.

Bouwer, H., 1986. Intake rate: cylinder infiltrometer. In: A. Klute (Editor), Methods of Soil Analysis, American Society of Agronomy, 2nd edn., Agron. no. 9 (Part 1 ), pp. 825-844.

Ghuman, B.S. and Lal, R., 1989. Soil temperature effects of biomass burning in windrows after clearing a tropical rainforest. Field Crops Res., 22: 1-10.

Ghuman, B.S. and Lal, R., 1991. Land clearing and use in the humid Nigerian tropics. II. soil chemical properties. Soil Sci. Soc. Am. J., 55:154-188.

Hulugaile, N.R. and Lal, R., 1986. Root growth of maize in compacted gravelly tropical Alfisol as affected by rotation with a woody perennial. Field Crops Res., 13: 33-44.

Hulugalle, N.R., Lal, R. and ter Kuile, C.H.H, 1984. Soil physical changes and crop root growth following different methods of land clearing in western Nigeria. Soil Sci., 138:172-179.

Lal, R., 1973. Effects of seedbed preparation and time of planting of maize in Western Nigeria. Exp. Agric., 9:304-313.

Lal, R., 1987. Tropical Ecology and Physical Edaphology. John Wiley, Chichester, UK, pp. 90, 658-660.

Lal, R. and Cummings, D.J., 1979. Clearing a tropical forest. I. effects on soil and micro-cli- mate. Field Crops Res., 2: 91-107.

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EFFECTS OF SOIL WETNESS ON A NIGERIAN ULTISOL 11

Lal, R., Sanchez, P.A. and Cummings, Jr,. R.W. (Editors), 1986. Land Clearing and Develop- ment in the Tropics. A.A. Balkema, Rotterdam, Netherlands, pp. 425-426.

Scudder, T., 1985. Strategies for socio-economic aspects of land clearing and development. In: Tropical Land Clearing for Sustainable Agriculture. International Board for Soil Research and Management Proc. Ser. no. 3, Bangkok, Thailand, pp. 45-58.

Seubert, C.E., Sanchez, P.A. and Valverde, C., 1977. Effects of land clearing methods on soil conditions and resulting effects on root growth. Trop. Agric. (Trinidad), 51:325-33 I.

Suwardjo, H., Sudjadi, M. and Ross, M.S., 1985. Potential of, constraints to, and development strategies for agricultural land development in Indonesia. In" Tropical Land Clearing for Sustainable Agriculture. International Board for Soil Research and Management Proc. Ser. no. 3, Bangkok, Thailand, pp. 113-140.

Van der Weert, R. and Lenselink, K.J., 1972. The influence of mechanical clearing of forest on some physical and chemical soil properties. Surinaamse Landbouw, 20" 2-14.