No abstract

Studies were conducted at the Agricultural Research Centre, Kpong, of the University of Ghana, to find solution to the problem of low rice yields on the Vertisols of the Accra Plains. Rice yields from continuously cropped fields have been observed to decline with time, even with the application of recommended levels of inorganic fertilizers. The decline in yield has been attributed to low inherent soil fertility, which is partly the result of low levels of soil organic matter (OM). As part of the study, cow dung (CD) and poultry manure (PM) were separately applied to the soil at 20 t ha -1 solely and also 5, 10 and 15 t ha , on the Vertisols of the Accra Plains. Studies were also conducted on the redox potential of CD, PM and rice straw (RS). The application of 10 t ha -1 CD and urea fertilizer (at 45 kg N ha -1 ) and 10 t ha -1 PM and urea (at 60 kg N ha -1 ) both gave paddy yields of 4.7 t ha -1 , which did not differ significantly from the yield of 5.3 t ha -1 , obtained under the recommended inorganic nitrogen fertilizer application of 90 kg N ha -1 . This indicates a synergistic effect of OM and urea on soil fertility. The redox potential studies showed that RS had greater propensity to bring about reduced soil condition in paddy fields than CD and PM, while PM brought about greater reduction than CD.

No abstract

No abstract

The productivity of the Vertisols of the Accra plains is low, in spite of its high potential fertility. This could be attributed to physical constraints and low organic matter content. A study was conducted at the Agricultural Research Centre, Kpong, to assess the suitability of Mucuna pruriens, Mimosa invisa and Stylosanthes guianensis, in short fallows, for improving the fertility, physical properties, moisture conservation and weed control in the case of these soils. Mucuna had the highest biomass of 5.3tha 21 , as opposed to 4.1tha 21 under natural fallow. The mucuna and stylosanthes biomass had comparatively higher total N values (240.75 and 203.51kgha 21 , respectively) than mimosa and natural fallow (152.46 and 162.74kgha 21 , respectively). Soil organic carbon, one year after the incorporation of cover crops, was found to be relatively higher under the cover crops (mucuna, stylosanthes and mimosa) than it was under natural fallow. Bulk densities were thus lower under the cover crops than they were under natural fallow. Also, soils under the cover crops had higher total porosity, water infiltration rates and moisture storage than they did under natural fallow. Weed cover, 115 days after planting maize, was significantly lower in plots previously under Mucuna pruriens than it was under natural fallow, Stylosanthes guianensis and Mimosa invisa. There was a negative linear correlation between cover crop biomass and weed density (R 2 ¼ 0.593-0.805). Grain and stover yields of maize were higher under Mucuna pruriens than they were under natural fallow (138 and 70 per cent higher, respectively). Burning of the natural fallow resulted in greater weed growth than when the material was worked into the soil (41 and 8 per cent, respectively). Grain yield was significantly greater under mucuna than it was under natural fallow (3.1 and 1.3tha 21 , respectively).

Sesbania sesban L. (Merr) and Mimosa invisa L. (Fabaceae) were grown in rotation with lowland rice (var. KRC Baika). The experiments were conducted on three farmers' fields at the Kpong Irrigation Project (KIP) sites at Akuse (lat 6° 06' N, long 0° 07' E) and Asutsuare (lat 6° 04' N, long 0° 12' E). In Farm 1, S. sesban and M. invisa were incorporated three times, in rotation with rice, over a period of 3 years, and in Farm 2, the green manures were incorporated once and rice planted within the year. The treatments also included a control (no fertilization), recommended inorganic nitrogen (N) that is, 90 kg N ha-1 , S. sesban + 50% N and S. sesban + 75% N. Total dry matter and grain (paddy) yield of rice were higher in Farm 1 than Farm 2. Nitrogen contents of the S. sesban and M. invisa plants were 2.5 and 2.4% respectively. Total N of S. sesban ranged from 239 to 259 kg ha-1 and that of M. invisa ranged from 152 to 186 kg ha-1. The single incorporation of the green manures in Farm 2 did not have a significant impact on rice growth and grain yield, while in Farm 1, S. sesban and M. invisa gave comparable yields with those under the recommended inorganic N. Grain yields under S. sesban + 50% N (T2) and mimosa (T4) were comparable to that under the recommended inorganic fertilizer (T5) and both were significantly higher than the control (T6); grain yields were 5.8, 5.8, 6.5 and 3.9 t ha-1 , in T2, T4, T5 and T6, respectively. In spite of the good performance of M. invisa it required upland conditions to grow and establish. The application of 50% N requirement, in addition to S. sesban incorporation, enhanced rice growth and yield, and resulted in 20.7% savings on nitrogen fertilizer use.

The Vertisols of the Accra Plains of Ghana are water logged after significant rainfall due to the low-lying topography (0.1-1 %). Camber bed (Cb) drainage landforms have been developed at the Agricultural Research Centre, Kpong, for draining off excess water. Field experiments were conducted to verify if maize growth and yield gradient from the trough to the crest were the result of nutrient gradient or some other factors. Four 5 m and two 10 m Cbs and a 20-m flatland were prepared in a split-split plot design, with landform as main plot, nutrient levels as sub plot and crop row as sub-sub plot. A pot experiment was also carried out for detailed studies. Soil movement brought about a nutrient gradient from the trough of the camber bed to the crest, and also made the soil profile homogeneous. The trough was low in nutrients, compact and prone to water logging, but nutrient levels increased through the middle slope to the crest. The flatland did not have a nutrient gradient but was prone to water logging due to its low-lying nature. Total dry matter (TDM) of maize and grain yield similarly increased from the trough to the crest. Grain yield of maize on the flatland ranged from 2.5-2.6 t ha -1 , while yields on the 5-m Cb were 3.6, 4.2 and 4.8 t ha -1 on the trough, middle slope and crest, respectively. Excess application of 15-15-15 NPK and sulphate of ammonia fertilizers (150% of recommended levels) did not appreciably increase biomass and grain yield in the troughs. However, the maize crop in pots, with soil from the trough, responded positively to fertilizer application, thus confirming that low yield in the trough was the result of both low nutrient availability and the compact subsoil.

No abstract