A wide range of tillage methods are used in different regions of humid tropical Africa for cowpea (Vigna unguiculata Walps). In region of alfisols such as found in Southwest Nigeria, farmers plant cowpea and other grain crops on ridges, heaps and manually cleared soils. However reduced tillage is gaining ground especially in areas susceptible to water erosion. In order to sustain reduced tillage methods which include planting on old ridges and manual clearing.
Studies conducted in the humid tropics dealt with comparison of mechanized and herbicide-based zero tillage (Lal, 1976; Olaoye, 2002) and manual clearing (Ojeniyi and Adekayode, 1999; Ojeniyi et al., 2000). Most of the studies found that cowpea benefited from tillage (Ojeniyi, 1989, 1991) although Olaoye (2002) who worked in the derived savanna zone of Nigeria found that zero tillage soil had least bulk density and highest cowpea yield compared with ploughed soils. There is need for further investigation into effects of soil conserving reduced tillage methods such as use of old ridges. Therefore this work was conducted to the selected soil properties and cowpea yield given by different frequency of manual ridging, cleaning and zero tillage in rainforest zone of Southwest Nigeria.
MATERIALS AND METHODS
Field experiment was conducted at Akure in the rainforest zone of Southwest
Nigeria on a sandy loam Oxic Tropudalf. The tillage treatments compared were
||Zero tillage in which paraquat (1-1 dimethyl 1-4-4 bipyridium)
was sprayed at 2.5 L/ha after manual clearing and one week before planting
each of three cowpea crops (ZT).
||Manual clearing with cutlass for each of three cowpea crops (MC).
||One initial manual ridging (with hoe) for three crops (IR3C).
||Two alternate manual ridging for three cowpea crops.
||Three manual ridging for three-cowpea crops i.e., ridging for each crop
The five treatments were replicated three times in a randomized complete block
design. Cowpea seeds were planted single stand in each 16 cm2 plot
at 90x90 cm. The first, second and third cowpea crops were established in July
2000, April 2001 and July 2001, respectively.
Determination of Soil Properties
Soil bulk density and gravimetric moisture content were evaluated at 1,
5 and 10 weeks after planting. Core samples collected at 5 to 15 on depth were
used for evaluation of bulk density and moisture content after placement in
oven for 24 h at 100°C. Five core samples were collected from a plot at
each sampling. Mean values of soil parameters were calculated.
For the third crop, composite surface (0-15 cm) soil samples were collected before planting and at harvest per plot. The soil samples were air-dried and pass through 2 mm sieve. Organic matter was determined by Walkey-Black dichromate method and total N was determined using Kjeldahl approach. Available P was extracted using Bray-PI solution and determined using molybdenum blue colorimeter (Bray and Kurtz, 1945). The pH in soil-water (2:) was determined using a glass calomel electrode. The exchangeable k, Ca and Mg were extracted with ammonium acetate. k was determined on flame photometer and Ca and Mg by EDTA titration (Tel, 1984).
Ten plants per plot were used for determination of grain yield on air-dry basis.
Table 1 shows data of soil bulk density under first, second and third cowpea
crops. Ridging treatments reduced soil density compared with zero tillage and
manual clearing from the first to third crop. Manual clearing gave higher soil
bulk density than zero tillage. As from second crop, increase in frequency of
ridging reduced soil density. The overall mean densities for zero tillage, manual
clearing one, two and three ridging were: 1.42, 1.45, 1.26, 1.24 and 1.21 Mg
In Table 2, values of soil moisture content are shown.
In case of first, second and third cowpea crops, ridging significantly reduced
soil moisture content compared with manual clearing and zero tillage (p = 0.05).
As from the second crop, soil moisture content was reduced with increased frequency
||Effect of frequency of tillage on soil bulk density (Mg m-3)
Effect of frequency of tillage on preplanting soil chemical
|| Effect of frequency of tillage on grain yield of a cowpea
Hence the soil that was ridged thrice had the least moisture content. Therefore zero tillage enhanced soil moisture status while increase in frequency of ridging reduced soil moisture status. Manual clearing gave lower moisture content compared with zero tillage.
Table 3 shows pre-planting soil nutrients content for the third crop and
Table 4 contains soil nutrient contents at harvest. Compared with ridged soils,
zero tillage and manually cleared soils had higher soil nutrients especially
with regard to Ca and Mg. Hence pH values of untilled soil were slightly higher.
Total N in untilled soil was higher. Frequency of tillage (ridging) did not influence soil chemical composition significantly (p = 0.05) with regard to organic matter, pH, N, K, Ca and Mg. However, available P increased with frequency of ridging.
Compared with ridging treatments, zero tillage and manual clearing gave
lower values for cowpea grain yield. Therefore cowpea benefits from ridging.
Also increase in frequency of ridging increased grain yield. Hence yield increased
in the order manual clearing, zero tillage, one, two and three ridging. The
overall mean values of grain yield per plant were 31.8, 36.9, 46.6 and 65.3
g, respectively (Table 5). However, yield of the third crop given by three ridging
was not significantly higher than that of two ridging. Therefore, number of
ridging could be reduced to two per three crops.
Ridging tended to reduce soil moisture content compared with zero tillage. This finding could be due to increased presence of organic matter and mulch on zero tillage and manually cleared soils (Ojeniyi and Adekayode, 1999), which should have reduced temperature and conserved moisture. Ridging which reduced soil density should have increased soil macro porosity thereby reducing conduct of heat into soil during the day. Ojeniyi et al. (2002) and Agele et al. (2002) found that ridging accelerated soil drying and increased soil temperature compared with surface hoeing.
Increased porosity and oxidation of soil under ridging compared with zero tillage should have enhances loss of organic matter. Also the fact that ridging brought less fertile sub-soil to surface should have contributed to lower nutrient of ridged soil compared with unridged soil.
Untilled soil with bulk densities above 1.40 Mg m-3 had lower cowpea yield compared with ridged soil. With critical strength for cowpea. Also increased frequency ridging reduced soil density and increased cowpea yield. Therefore the soil that received the highest number of ridging had the least density and highest yield.
It is implied that cowpea was very sensitive to soil density and that high soil density adversely affected cowpea yield. For example, manually cleared soil with highest density had least cowpea yield although it had relatively high moisture content. Also the increased looseness of soil due to frequent ridging should have enhanced soil aeration, a situation which increases N fixation, presence of rhizobium and nodulation in leguminous crop (Osundina, 1998).
The present research relative effect of frequency of ridging, zero tillage and manual clearing on selected soil properties and yield of cowpea under humid tropical condition on alfisols in Southwest Nigeria. Although untilled soils had higher nutrient status and moisture content, loose soil given by ridging significantly enhanced cowpea yield. Yield of cowpea given by two ridging for three crops was not significantly different from that of three ridgings.