African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 8, Num. 3, 2000, pp. 233-242

African Crop Science Journal, Vol. 8. No. 3, pp. 233-242

CHANGES IN WATER INFILTRATION ALONG A CATENA PRIOR TO MECHANIsED CLEARING OPERATIONS AND AFTER TWO CROPPING SEASONS

M.M. Tenywa, J.Y.K. Zake, S. Sessanga, J.G.M. Majaliwa, J.B. Kawongolo¹ and D. Bwamiki²
Department of Soil Science, Makerere University, P.O. Box 7062, Kampala, Uganda
¹Department of Agricultural Engineering, Makerere University, P.O. Box 7062, Kampala, Uganda
²Kawanda Agricultural Research Institute (KARI), P.O. Box 7065, Kampala, Uganda

(Received 16 October, 1998; accepted 10 April, 2000)

Code NUmber: CS00025

INTRODUCTION

Ploughing or any other form of cultivation is done to loosen and break up the soil in order to increase aeration, water infiltration and to prepare a seed bed of suitable tilth for the crop to be grown (Voorhees, 1979). In Uganda, preparatory cultivation was predominantly manual using hand hoes until ox-drawn ploughs were introduced in 1909 (Hayes, 1938). The cultivation is usually shallow and limited to that essential to the preparation of a rough, more or less weed-free seedbed and does less damage to the soil (Zake, 1993). For hoe farmers and, to a great extent, for those with ox-drawn implements, it is difficult to open up land from forest or bush fallow, or to do preparatory cultivation during the dry season when many soils are very dry. Traditionally, the practice is to wait until the earliest showers and then try to complete the work within a few weeks of the end of the dry season, which means that the land is not usually prepared early enough for the best results (Zake, 1993). Preparatory cultivation carried out manually often fails to achieve timely and early planting. Yet numerous field experiments using annual test crops have demonstrated that early planting generally gives the best yields (Francis and Stern, 1987; Chemeda, 1997). Mechanised clearing has a great advantage in this respect since it provides the power needed to break up hard soils in the dry season and to hasten the process of land preparation for sowing.

A major thrust in Uganda today is finding practical ways by which great improvements in yield and profit may be realised from predo-minantly rain-fed production systems. The approach entailing using a combination of inputs (e.g. timely planting, improved varieties, credit, fertilisers, optimum plant density, integrated pest and disease management, weed control, favourable price policy) rather than the single factor approach will be used. According to Baker (1975), the success of this approach is very much dependent on early planting. Preparatory cultivation carried out manually by smallholder farmers and, to some extent, using hand hoes, ox-drawn ploughs often fails to allow timely early planting and limits the per capita hectarage prepared for sowing. Therefore, the wider use of mechanical clearing and subsequent continuous cultivation manually will be inevitable. Indeed, many farmers growing maize in some parts of Uganda such as Masindi and Kapchorwa already use tractors for land clearing. This system is in marked contrast to traditional hand and animal powered cultivation and the consequences of changes in land conversion and management are not completely understood (Russel, 1962; Pereira, 1973).

Clearing with heavy machinery may have disadvantages resulting from much disturbance of the surface soil and destruction of its structure and is often accompanied by undue compaction, especially if the tillage is done in wet weather (Webster and Wilson, 1980). Tillage effects on soil properties and processes (e.g. infiltration, runoff, soil loss) can vary considerably in both time and space (Zhu et al., 1997) and affect not only soil hydrology but also crop production (Cerda, 1997). The rate of water infiltration depends on rainfall intensity, soil profile characteristics including soil water content, particle and aggregate sizes, natural horizonation, and continuity and stability of soil pores (Lal and Van Doren, 1990). Except for soils of very stable structure, any beneficial effect of tillage on water infiltration are only transitory. In the long term, all forms of cultivation tend to aid the rainfall in slaking soil aggregates, sealing the surface, enhancing the rate of soil organic matter decomposition, exposing bare soil to external stresses and causing the elluviation of finer particles to lower horizons and increasing overland flow (Webster and Wilson, 1980).

Although information on tillage effects on infiltration is presently available, very little is known about the change in hydrological properties of the soil along a slope cleared by heavy machinery. The question then is whether the generic effects of tillage on soil and hydrologic properties are also nested along the slope, lending them a possibility of prediction. Since it is well known that the catena model is useful in coping with the wide biophysical variability in the great Lakes region of East Africa, it is desirable to understand the effects of mechanised clearing on soil hydrological properties. These are difficult, labourious and costly to measure yet they are indispensable in quantifying the degradative processes (e.g. run-off, soil erosion). The objective of this study was to characterise soil hydrological properties on a slope that was subjected to mechanical tillage and subsequently cropped for two seasons.

MATERIALS AND METHODS

The study was conducted at Makerere University Agricultural Research Institute, Kabanyolo (MUARIK). The institute is located at 0⁰ 28’N and 32⁰ 37’E at an elevation of approximately 1200 m above sea level and lies about 20 km north of Kampala within the Lake Victoria Basin.

The mean annual rainfall of 1160 mm is distributed bimodally (March to June and September to November) and the mean monthly temperature is 24.5 °C (Yost and Eswaran, 1990). The surrounding topography is undulating with flat-topped laterite caps on the hills. The soils are dark reddish brown, sandy to sandy clay loams overlying clayey subsurface horizons derived from pre-cambrian schists and quartz. They were classified within the USDA system as Typic Kandiudults (Yost and Eswaran, 1990) equivalent in the FAO systems to Rhodic Nitisols (M. Isabirye, pers. comm.). Soils were characterised prior to clearing and after each of the two maize cropping seasons by measurement of pH, soil organic carbon, N, K, Na, Ca, Mg and texture following established guidelines (Okalebo et al., 1993).

Prior to conversion to agriculture, the vegetation was a 10 year-old secondary forest with scattered patches of napier grass (Pennnisetum atropurpureum). The land was bulldozed to fell trees and bushes, followed by disc ploughing and harrowing. In order to establish the changes in water infiltration along the landscape, three study plots of 9 by 40 m each were established at the lower backslope of a west-facing ridge with an average slope of 21%. The plots were subdivided into upper, middle and lower slope segments.

Zea mays variety "Population 29" from the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria was grown for two seasons. Planting was done by hand. Infiltration measurements were made using double ring infiltrometers (Bouwer, 1986) prior to opening of land (April, 1991) and following the first (January, 1992) and second (September, 1992) maize crops. One infiltration test lasting one hour was made per segment in each of three replicated slope segments. The infiltration data were evaluated for fitness according to the Green-Ampt, Kostiakov, Philip models (Table 1). These models vary considerably in their predictive capacity of the soil-water infiltration characteristics, depending largely on the assumptions made in their derivation (Majaliwa and Tenywa, 1998).

The temporal variation of infiltration rate, ic subsequent to the introduction of the new land use on the ridge was described using the Meek et al. (1990) and Kay et al. (1994) equation:

i_c (t)= i_c (t=0) + Di_cmax [1- e^-Kt]

where: i_c (t)= value of infiltration rate at time t (year), Di_cmax = [i_c(T)- i_c(2T)]/(1-exp(-kT)] is the maximum projected change in the steady state infiltration rate caused by the land use practice. T = sampling time interval (in this case 3/4 years), and K = -1/T{[i_c(T)- i_c(2T)]/[i_c(t=o)-i_c(T)]} is a constant.

Data were subjected to ANOVA and multiple regression analysis using Systat, a statistical package (Wilkensen, 1990).

RESULTS AND DISCUSSIONS

Site characterisation. Soil characterisation data are presented in Table 2. Generally, values of soil properties prior to site clearing were above the threshold values suggested for Uganda soils (Tenywa, 1998). After two continuous maize cropping seasons, these soil properties decreased significantly (P<0.05), including clay (Table 2).

Cumulative infiltration. Mechanised clearing followed by disc harrowing and manual cultivation significantly reduced cumulative water infiltration. Point cumulative infiltration after 1 hour varied widely between 47 and 150 cm depending on the time of sampling and location along the slope (Fig. 1). The mean cumulative infiltration for upper and middle segments prior to land clearing was higher than the second post-clearing value (P<0.05). Many researchers have also observed that land clearing and subsequent cropping can cause dramatic deterioration in the soil properties particularly in the edaphologically inferior slope pedons (Webster and Wilson, 1980). The major processes implicated in this degradation include rapid reduction in soil organic matter (SOM), accelerated soil erosion, surface sealing and/or crusting, and compaction (Lal et al., 1975).

In terms of location along the slope, higher cumulative infiltration was observed at the upper-slope segments and there was a relatively decreasing trend down the hillslope prior to clearing and after one maize cropping seasons (Fig. 1). After the second cropping period no difference was detected between the segment cumulative infiltration. The lower cumulative infiltration observed downslope might be a result of residual effects of an inherently clayey material deposited after two maize cropping seasons. This is postulated to be a result of accelerated and selective detachment, transportation, deposition and accumulation of soil sediments over a more permeable clay-impoverished horizon.

Model validation. All of the models overestimated the infiltration rates. The Green-Ampt’s ic was significantly (P<0.05) higher than the Philips’s A value (Table 3). Amongst the three models tested, the Kostiakov model was the most accurate, precise and unbiased in predicting the infiltration process on the ridge (Fig. 2). In 4 % and 8% cases the Green-Ampt and Philips models did predicted negative value for i_c and A, respectively.

Temporal variation of infiltration rate, I_c. Mean infiltration rates along the slope prior to clearing and after cropping to maize are presented in Figure 3. The infiltration rate showed considerable variation especially after clearing the land with drastic alterations in the upper and middle slope segments. The average infiltration rate was very rapid according to the BAI classification system (1979) cited by Landon (1991) and ranged between 18 and 60.6 cm h^-1 but without a predictable trend down the slope. According to Kay et al. 1994, steady state infiltration rate on the different segments changed over time as follows:

T_1/2 being the half-life of the steady state infiltration rate. It represents the time it takes for the steady state infiltration rate of a soil under given conditions to fall by 50% as a result of soil structure degradation.

The temporal change in the steady state infiltration rate (I_c) was dependent on slope segment with a tendency for I_c to decrease rapidly on the upper and middle slope segments in comparison to the lower slope segment. The infiltration rate after the second cropping season was significantly (P<0.05) lower than the value of the first cropping season on both the upper and middle segments. According to the model of Kay et al. (1994), the time (half life) in years required for the steady state infiltration rate to decrease to half the initial value for the upper, middle, and lower slope-segments were 7/6, 17/12 and infinity, respectively. The 50 % decrement of the steady state infiltration rate on the upper and middle segments were reached in a period less than two years, while in the lower slope-segment it was not attained in the study period.

Alterations in infiltration rate as a result of continuous cultivation are largely expected to affect the two segments: upper and middle. Going by the trends in the model of Kay et al. (1994) three seasons (9/4 years) are sufficient to induce drastic deterioration in soil structure of the two segments. However, changes in the steady state infiltration rate along the slope before and after the two cropping seasons were not significant (P<0.05). Perhaps, few seasons were considered. Significant loss of clay particles and soil organic matter along the slope-segments support the postulation that a decrease in macro-pores and soil structure deterioration occurred (Lal, 1976; Stocking and Peake, 1986).

Many studies on the relationships between soil-water parameters and other soil properties have stressed the importance of clay content, in influencing the capacity and availability of moisture in soils (Abrol et al., 1968; Lal, 1978; Coote and Ramsay, 1983; Mochoge and Mwonga, 1988). In addition, cumulative and steady state infiltration rate changes were severe, when contrasted with Moura and Boula (1972) report of a decrease in infiltration rates of a Brazilian Eutrustox from 82 to 12 cm hr^-1 after 15 years of intensive annual cropping. This can be attributed to the fact that the Brazilian Eutrustox were younger with a comparatively more stable structure compared to the highly weathered and vulnerable study soils.

CONCLUSIONS

Overall, water infiltration was higher at the upper slope and decreased in lower slope positions with more pronounced negative impacts of continuous cultivation on water infiltration of the middle and upper segments along the slope. The Kostiakov model gave comparatively better predictions of infiltration rates for the study soils. Based on the infiltration rate decay trends, it was deduced that the rate of soil degradation following initial land opening may be extremely high and devastating particularly on the upper and middle slope positions. It is recommended that further studies be done to establish the long-term effects of different forms of tillage on the soil properties and processes in general to guide decisions for sustainable productivity management.

ACKNOWLEDGEMENTS

The authors thank Rockefeller Foundation and International Board of Soil Research and Management (IBSRAM) for funding the research. M. Ajonye, R. Muziira, C. Najjuma, A. Ndaba, P. Katabazi and C. Oryem assisted with the field activities.

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