African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 6, Num. 1, 1998, pp. 29-37

MAIZE YIELD REDUCTION DUE TO EROSION IN A HIGH POTENTIAL AREA OF CENTRAL KENYA HIGHLANDS

C.K.K. GACHENE, J.P. MBUVI, N.J. JARVIS^1 and H. LINNER^1

Department of Soil Science, University of Nairobi, P.O. Box 30197, Nairobi, Kenya
^1 Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O. Box 7014, S-750 07 Uppsala, Sweden

(Received 10 June, 1996; accepted 27 October, 1997)

Code Number:CS98004
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ABSTRACT

The effect of cumulative soil loss on maize (Zea mays L.) growth and yield were investigated on a humic nitisol in Kenya during the 1993 long-rains (LR) and short-rains (SR). The runoff plots had been subjected to different levels of erosion from 1991 to 1992. The maize grain and above-ground dry matter (AGDMY) yields, crop height, and leaf area index (LAI) were measured in fertilised and non-fertilised plots. Crop growth parameters were always greater in the least eroded plots. In the fertilised plots in the 1993 LR, maize grain yield in the most eroded plot was 83% less than in the least plots. On average, there was a crop height and LAI difference of about 143 cm and 3.18 between plants grown in the least and most eroded plots. In the non-fertilised crops, there was a crop height and AGDMY reduction of 52% and 90%, respectively, due to the loss of the first 2.5 cm of topsoil in the least eroded plots. No grain yields were obtained from plots where fertiliser was not applied in either season. The differences in crop growth due to erosion were larger in the non-fertilised compared to fertilised crops indicating that fertilizer application masked the effect of erosion on crop growth. During the 1993 SR, the effects of erosion on crop response were similar to 1993 LR. Maize grain and AGDMY were highly and negatively correlated with cumulative soil loss, while LAI and crop height also decreased significantly with cumulative soil loss for both fertilised and non-fertilised crops during both seasons.

Key Words: Runoff, soil erosion, yield, Zea mays

RESUME

L'effet de l'erosion des sols sur la croissance et la production du mais a ete etudie au Kenya pendant la longue et la courte saisons de pluie. Le sol d'essai etait un Nitisol Humique. Les parcelles d'erosion ont ete soumises a differents niveaux d'erosion de 1991 a 1992. L'essai comportait des parcells fumees et celles non fertilisees. Le rendement en mais grain et en biomasse aerienne, la hauteur des plants et l'index de surface foliaire ont ete mesures. Ces parametres ont ete trouves plus grands dans les parcelles moins erodees. En longue saison de pluie 1993, le rendement en mais grain des parcelles ayant recu la fumure etait de 83% inferieur a celui de la parcelle la plus erodee. Dans la parcelle la plus erodee et dans celle la moins erodee, la hauteur et l'index de surface foliaire du mais differaient en moyenne de 143cm et 3.18 respectivement. En parcelles non fumees, la hauteur de plants de mais et la production de biomasse ont ete reduites de 52% et 90% respectivement. Cette reduction etait due au decapage de 2.5 premiers cm de sol arable dans la parcelle la moins erodee. Pendant toutes les saisons, aucune production n'a ete obtenue dans les parcelles n'ayant pas recu de fumures. La reduction de croissance des plants par l'effet de l'erosion etait plus marquee en parcelles sans fumures que dans celles avec fumures. Cela indique que l'apport de fumures a masque l'effet de l'erosion sur la croissance du mais. Aussi bien en longue qu'en courte saisons de pluie 1993, l'effet de l'erosion sur le developpement du mais etait pareil. Le mais grain et la biomasse produite, de meme que la hauteur et l'index de surface foliaire du mais etaient negativement correles avec les pertes en terre cumulees tant en parcelles fumees qu'en celles sans fumures.

Mots Cles: Runoff, Rendement du mais, erosion du sol, productivite des terres

INTRODUCTION

Loss of topsoil due to erosion has been demonstrated to have a depressive effect on crop growth and this loss is viewed as a major constraint to food production, especially in the tropics (Yost et al., 1985; Lal, 1990). Although the effect of erosion on crop growth may be difficult to assess due to other external factors, a number of studies show that erosion reduces crop yields, and that much of this reduction is currently masked by improved technology and soil management (Young et al., 1985). Once productivity losses occur, expensive management practices such as the use of fertilizers and high levels of manure application may be necessary in order to restore yields (Aina and Egolum, 1980; Lal, 1985; Yost et al., 1985; Kilewe, 1987; Pesant and Vigneux, 1992). Studies have indicated that soil amendments may only partly restore crop yields lost due to erosion. For example, Pesant and Vigneux (1992) observed that application of inorganic fertilizers on soils with 20 cm of topsoil removed proved to be inadequate to maintain maize yield and that the use of beef manure on an annual basis was therefore necessary in order to increase yields. Yost et al. (1985) indicated that maximum maize grain yields could not be obtained where 35 cm of soil had been removed, even when optimum fertilizer was applied to the soil. In Kenya, the removal of 0, 3, 6, 9, 12 and 15 cm of topsoil of an alfisol resulted in corresponding maize grain yields of 1920, 1670, 1060, 810, 230 and 150 kg ha^-1 (Kilewe, 1987). In the same study, it was noted that after removal of 12 cm of topsoil, application of higher inorganic fertilizer and manure rates to restore soil productivity was uneconomic.

Reductions in crop yields on eroded soils have been attributed to the resulting poor physical and chemical properties (Dormaar and Lindwall, 1985; Yost et al., 1985; Belay, 1992). This results in severe reduction of plant height (Pesant and Vigneux, 1992) and decreased nutrient content in the plant (Lal, 1988). These findings indicate that soil erosion, if allowed to continue unchecked, can reduce soil fertility leading to a decline in yields.

Water erosion has been considered a major risk to agricultural sustainability in Kenya, but this has been based on qualitative observations. The objective of this study was to quantify the effects of erosion on growth and yield of maize (Zea mays L.) in a high potential area of Central Kenya and to determine the critical factors causing decline in maize growth and yield following erosion. Such information is needed to enable proper land use planning decisions.

MATERIALS AND METHODS

The study was conducted at the College of Agriculture and Veterinary Sciences, University of Nairobi. The farm is approximately 12 km north-west of Nairobi city and is at an altitude of 1940 m above sea level. The site is representative, in terms of soils and climate, of large areas of the Central Kenya highlands.

The soil is a humic nitisol (FAO, 1990), equivalent to a paleustalf in the USDA soil taxonomy system (Soil Survey Staff, 1990). The soils are underlain by Nairobi trachytes of Tertiary age and are well-drained, very deep ( > 180 cm), dark red to dark reddish brown, friable clay (C. K. K. Gachene, 1989, unpublished). The physical and chemical characteristics of the soils are given in Table 1. The climate of the study area is semi-humid (Sombroek et al., 1980) and experiences a bimodal rainfall distribution. The long-rains start in mid-March and taper off in May, while the short-rains fall between mid-October and mid-December. The mean annual rainfall is 1006 mm. Before the start of the experiment, the area was covered with shrubs (mainly Lantana camara) and grasses (Red oat grass, Themeda triandra) and had been under fallow for a period of more than fifteen years. During dry spells, cattle from the University farm were allowed to graze freely.

The effect of erosion on maize growth and yields was assessed using sixteen runoff plots which had been subjected to varying levels of erosion for four seasons (hereafter referred to as the 'erosion cycle'), i.e., during 1991 and 1992. The runoff plots, each measuring 3 m wide and 10 m long, were installed at the site in mid-December 1990. The plots were adjacent to each other on a single catenal position with an average slope of 31%. To vary the levels of erosion during these four seasons, meshes of different hole dimensions were stretched over the plots at a height of 15 cm above the soil surface. Use of mesh covers to assess the effects of erosion on soil productivity has been recommended by FAO (1985). The following types of meshes were used; M1: fine mesh with 140 holes per square cm; M^2: medium mesh with 4 holes per square cm; M3: coarse mesh with 1 hole per square cm and M4: bare soil with no mesh cover.

The cumulative soil loss for the 16 runoff plots after the erosion cycle is shown in Table 2. Soil loss from M1, M2, M3 and M4 were significantly different (P = 0.05) (Table 3.). This variation in soil loss had to be achieved before the plots were planted with maize in the 1993 long-rains. During the 1993 long and short-rains, the meshes were removed and the plots were planted with maize. Eight runoff plots, i.e., plots 1 to 8 (Table 2), were cropped to fertilised (DAP, grade 18:46:0 at the rate of 200 kg ha^-1) maize while the other 8 plots, i.e., plots 9 to 16 (Table 2) were cropped to non-fertilised maize during 1993 long and short-rains.

Agronomic aspects e.g. time of planting, plant population, and weeding were carried out according to the prevailing local practices and conditions. The recommended maize varieties, namely, H512 and H511 were planted during the long and short-rains, respectively. The seeds were planted at a spacing of 25 cm within rows and 75 cm between rows. Maize stover at the rate of 1.0 t ha^-1 was applied as surface mulch in plots 1 to 8 at the beginning of each growing season. This was applied as two trash lines across the plots at a spacing of 2 m from the top and bottom of the plot boarders. All the plots were kept clean-weeded by hand tools.

The following crop parameters were measured: crop height, leaf area index (LAI), above-ground dry matter yields (AGDMY) and grain yields. Crop height and LAI measurements were made on a weekly basis. Crop height was measured as the average of four plants per plot selected at random from the middle rows. The LAI of 4 plants per plot was calculated following procedures described by Mckee (1964), Francis et al. (1969) and Daughtry and Hollinger (1984). In addition, regular field observations of the growth of the maize were made, noting recognisable development stages such as emergence, tasselling, flowering of the tassel and full-ripeness or hard-corn. The crop was considered to have reached a particular phenological stage when 75% of the plants in the sampling area (of 1.5 m wide and 6.0 m long in the middle of the plot) possessed the features of that phenological stage.

The maize crop was harvested for grain and above-ground dry matter yields. The maize grains were air dried and weighed after attaining 13% moisture content. The maize residue was weighed in the field, chopped, subsampled and put in the oven for moisture determination and later expressed on a dry-matter basis. The above-ground dry matter and grain yields were used as indicators of crop response to cumulative soil loss. Grain yield, AGDMY, crop height and LAI were thus regressed against cumulative soil loss. During the 1993 long rains, no runoff and soil loss was generated. Hence, the subsequent crop parameters for the 1993 short-rain crop were regressed against cumulative soil loss recorded at the end of the erosion cycle. Although there was some runoff and soil loss during the 1993 short rains, the effect of this on crop response during the same season was not considered.

RESULTS AND DISCUSSION

General observations on crop growth after the erosion cycle. The maximum crop height, LAI, above-ground dry matter and grain yield for the different plots after the erosion cycle are shown in Table 3, Table 4, Table 5 and Table 6. These crop parameters varied between different levels of erosion and also from season to season. During the 1993 long-rains, grain yield, AGDMY and crop height from least eroded plots (M1) were significantly different from the most eroded plots (M2) (Table 3). During the 1993 short-rains, the effects of erosion on crop response were similar to 1993 long-rains. The 1993 long-rains started late (in the 2nd week of April) (Fig.1) and consequently the maize was planted in the first week of May on all plots instead of the usual month of March, which was dry. Gapping and replanting had to be done for some of the plots. In June 1993, visual observations of the crop indicated serious water stress, especially in the non-fertilised crops. As a result, the crop hardly reached tasselling stage and no grains were harvested in the non-fertilised plots (Table 5), while the fertilised plots gave very poor yields, ranging from 147.2 to 854.3 kg ha^-1. During the 1993 long-rains, plant height ranged from 114.3 to 257.5 cm and from 59 to 121.8 cm for fertilised and non-fertilised crop, respectively. The LAI ranged from 1.86 to 5.04 for the fertilised crop but was less than 2.0 for non-fertilised crop (Table 5). The above-ground dry matter yields were always higher in the fertilised compared to non-fertilised plots (Table 5.)

Although the precipitation in the 1993 short-rains was higher than during the 1993 long-rains, it was poorly distributed (Figure 1) and this affected crop performance. Maximum height, and LAI attained was 274.5 and 5.88, respectively, for the fertilised plots, while the corresponding values were 134.8 and 2.64 for the non-fertilised plots (Table 6). The fertilised crops reached tasselling stage while tasselling was less than 50% in the non-fertilised crops. As in the previous season, no grains were harvested in the 1993 short rains in non-fertilised plots. Above-ground dry matter yields ranged from 3946.7 to 8027.8 kg ha^-1 for the fertilised and from 1111.1 to 3515.6 t ha^-1 for the non-fertilised crops (Table 6). Although the LAI, crop height and above-ground dry matter yields in the fertilised crops were as large as in most of the other growing seasons, there was a very poor harvest in terms of grain yields (Table 6). This was probably a result of the uneven distribution of rainfall during the season, with low amounts received at the critical tasselling stage.

Effects of erosion on maize growth and yields. In the plots where fertilizer had been applied, maize grain yield in the most eroded plot (plot 8, Table 5) was 83% less than in the least eroded (plot 1) during 1993 long-rains. There was a crop height and LAI difference of about 143.2 cm and 3.18, respectively, between plants grown in plots 1 and 8 (Table 5). In the non-fertilised plots, in the 1993 long-rains, there was a crop height and above-ground dry matter reduction of 52% and 90%, respectively, due to the loss of the first 2.5 cm of topsoil in the most eroded plot (plot 16, Table 5). The corresponding values for the fertilised plots during the same season were 56% and 56% due to loss of the first 2.7 cm of topsoil in the most eroded plot (plot 8, Table 5). This observation indicates that fertilizer application masked the effect of erosion on crop growth and yield. During the 1993 short-rains, the effects of erosion on crop response were similar to 1993 long-rains.

Table 7 and Table 8 show regression equations relating crop growth parameters to cumulative soil loss under fertilised and non-fertilised conditions during the 1993 long and short-rains. Maize grain and above ground dry matter yields were highly and negatively correlated with cumulative soil losses, while LAI and crop height also declined significantly with cumulative soil loss. This can be attributed to the loss of the topsoil which is most favourable for the crop growth. Reduction in plant height due to erosion was also reported by Pesant and Vigneux (1992), Andraski and Lowery (1989), Lal (1985), Belay (1992) while Mbagwu et al. (1984) also found that increased erosion resulted in reduced LAI.

For the fertilised plots, many of the relationships have low correlation coefficients (r) when compared to the values obtained from plots where no fertiliser was applied. This demonstrated that fertilisation can mask or compensate for the effects of erosion on crop growth. It has always been suspected (FURP, 1987) that nitisols in Kenya have been losing significant amounts of plant nutrients through soil erosion and the effects of this on crop growth have tended to remain masked by fertilizer application. Young (1985) indicated that much of the reduction in productivity due to erosion is masked by improved technology and soil management.

CONCLUSIONS

This study shows that erosion can lead to a decline in crop yield due to deterioration in soil properties. Although erosion had no negative effects on seed emergence, crop height, LAI and above-ground dry matter and maize grain yields varied between different levels of erosion. These crop parameters were greater from least eroded plots when compared to most eroded plots. Crop growth parameters declined significantly with cumulative soil loss for both fertilised and non-fertilised crops. However, there was a strong relationship between crop parameters and cumulative soil loss for the non-fertilised crops indicating that fertilizer application tended to compensate for the effects of soil erosion on crop growth.

ACKNOWLEDGEMENTS

We thank Martha Kimani and Ferdinard Anyika for help with field work. Financial support was provided by the Swedish Agency for Research Cooperation (SAREC).

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