Tilage and cropping systems effects on soil moisture, nitrogen fixation and yield of cowpea intercropped with maize in Northern Ghana

Results from research work on tillage systems conducted mostly in the rainforest and coastal Savanna zones under monocultural crop production systems have been conflicting. Against this background, experiments were conducted at Nyankpala within the northern savanna ecology to assess the influence of different tillage systems on soil properties, water storage, nitrogen fixation and crop yield. The experiments were laid in split-plot design with four replications. Tillage systems which consisted of Conventional (CON), Bullock Plough (BP), Hand hoe (RH) and Zero-tillage (ZT) were the main treatments. The sub-treatments were sole maize, sole cowpea, maize/cowpea inter-row cropping system and bare fallow in 2000 which was replaced in 2001 with maize/cowpea intra-row cropping system. The results showed that tillage loosened the soil and thus reduced soil bulk density, increased porosity and enhanced surface roughness. Bulk density and surface roughness were in decreasing order of ZT>HH>BP>CON and CON>BP>HH>ZT respectively. The ZT, which had a sparse vegetative cover due to bush tires in the preceding dry season, recorded the lowest surface roughness among the tillage treatments. The reduction in soil bulk density and the increase in surface roughness due to tillage reduced runoff and more water were stored in the deeper tilled than the shallow and the non-tilled treatments. The average reduction in estimated runoff over the two years experimentation was 54% in cowpea and 43% in maize under CON. More than 50% of the water in the 105 cm deep profile was stored at the 0-30 cm depth in all the treatments. Among the tillage treatments, CON and BP with greater working depths of not less than 12 cm stored more water than HH and ZT treatments. There was also a mean reduction of 50% by CON and BP in weed infestation compared to RH and ZT treatments. The ZT treatment significantly (P<0.05) reduced plant height and LAI of crops, which in turn reduced the amount of light intercepted, and the subsequent low dry matter yield produced by the crops on it. Calculated water use (ET) was higher for maize than cowpea. Water use efficiency (WUE) of maize for dry matter production at 8 WAP was significantly (P<0.05) higher under CON and BP than under HH and ZT treatments. A similar trend was observed for cowpea in 2000 but in 2001 ZT significantly (P<0.05) recorded a greater reduction of 42.8% in WUE. Grain yields of maize and cowpea did not differ between CON and BP but were significantly higher (P<0.05) than the grain yields of the RH and ZT treatments. The differences in land equivalent ratios (LER) among tillage treatments were not significant but each was more than 1 (>1) implying a substantial agronomic advantage over sole cropping system. Generally, there were no consistent increases of both nodule weight and nodule number per cowpea plant due to tillage at the 4 and 6 WAP but each of them declined at 8 WAP. Nodule weight was not affected by tillage but nodule number per plant was significantly lower (P<0.05) on ZT than CON, BP and HH treatments at 8 WAP. The percent nitrogen fixed by cowpea under each of the tillage treatments was higher than 50% but the differences among the treatments were not significant. Cropping systems had no significant influence on soil bulk density and water storage but bare fallow in 2000 significantly reduced (P<O.05) total water stored by about 27 and 6% compared to sole maize and cowpea at 4 WAP. Similarly, bare fallow increased runoff by 40% and 15% as compared to sole cowpea and maize respectively. Sole cowpea and maize/cowpea mixture significantly reduced (P<0.05) runoff better than sole maize. Cowpea nodule number per plant and nodule weight were each significantly higher in the sole than the intercrops but percent nitrogen fixed did not differ significantly between the sole and the intercrop. The differences in these variables between the inter- and the intra-row cropped cowpea were not significant. The economics of production in terms of benefit cost ratio (BCR) among the tillage treatments was in decreasing order of BP>CON>HH>ZT while in terms of cost it was ZT>CON>BP>HH. Among the cropping systems, the BCRs in sole crops were higher than their intercrops but the BCRs of inter-and intra-row cropped maize and cowpea were similar. Based on the results, the recommended tillage practice was BP. which is comparable to CON in soil water conservation and crop yields. Maize intercropped with cowpea was better than the sole cropping system since LERs were more than 1 (>1) but either inter- or intra-row cropped maize with cowpea would give similar results.

Source: KNUST