SUMMARYIron toxicity is a nutrient disorder associated with high concentrations of iron in soil solutions. Deficiencies of other nutrients, such as P, K, Ca, Mg and Zn, have been implicated in its occurrence in rice plants. Field experiments were carried out in 1992 and 1993 in Ivory Coast to evaluate the iron toxicity tolerance of promising rice cultivars available in West Africa, and to provide additional information for selecting breeding materials. Two sites, differing in their potential to cause iron toxicity, were used. Glasshouse and field studies were also conducted to test the role of other nutrients in the occurrence of iron toxicity. The results showed that genetic tolerance to iron toxicity can significantly improve rice production in iron-toxic soils, with some cultivars producing yields in excess of 5 t/ha. The application of N, P, K and Zn in the field decreased the uptake of iron in rice tops, and this can be a significant factor in the iron-toxicity tolerance of the cultivars.

Rice A gronomy J our n al • Volume 10 0 , I s sue 3 • 2 0 0 8 735 ABSTRACT New Rice for Africa (NERICA) cultivars are widely adopted interspecifi c crosses between Oryza sativa and O. glaberrima but their responses to fertilizer was not known. In 2004 and 2005, four released NERICAs (NERICA 1, 2, 3, 6) under two levels of phosphorus (0 and 2.6 g m -2 ) and four of nitrogen (0, 3, 6, and 12 g m -2 ) were evaluated on an acid Typic Haplustult at Ikenne (6°54´ N, 3°42´ E) in the Nigerian forest agroecosystem. In 2004, averaged over N levels, NERICA 3 attained physiological maturity most quickly (92 days aft er seeding [DAS]) at 2.6 g P m -2 . But under drought stress (2005), although NERICA 3 was the fi rst to initiate panicles (49 DAS) and to reach midfl owering (73 DAS), maturity was delayed until 100 to 101 DAS with or without P. Also, in 2005, NERICA 6 took a much longer time to mature ( 110-111 DAS) than the other cultivars. Th erefore, NERICAs 3 and 6 were the least suitable cultivars for low-input, drought-prone environments. At low N of 3 g m -2 and zero P, paddy yield was highest in NERICA 1. With application of 12 g N m -2 , NERICA 1 also yielded 20 to 41% more than the others, implying that it had a potential for tolerance to low N and was the most N-responsive among the cultivars evaluated under limited P. Moderate levels of N (6 g m -2 ) and P (2.6 g m -2 ) are recommended for NERICAs in smallholder low-input production systems.

Phosphorus (P) deficiency is one of the major limiting factor for crop production in highly weathered soils in the humid tropics. Field experiments were conducted for two years (1992 and 1993) to determine P response and efficiencies of upland rice cultivars in an Ultisol, low in available P, in the forest zone of Cote d'Ivoire. The rice cultivars tested were selected from a large number of entries tested earlier for acidity tolerance.Grain yields of the cultivars were significantly increased by P application. There was little further response in grain yield at higher rates than 60 kg P ha -1. The rice cultivars differed in agronomic and physiological P efficiencies and the efficiencies were higher at lower rates of R The rooting depths of the cultivars were increased by application of P at the lowest application rate (30 kg P ha-1).The results suggest that P fertilization of soil acidity-tolerant upland rice cultivars can significantly improve the productivity of the Ultisols.

Abstract:Inland valleys with wet lowlands are an important water source for farming communities in the sub-humid zone of West Africa. An inland valley and surrounding contributing watershed area located in the sub-humid zone near M'bé in central Côte d'Ivoire was instrumented to study surface runoff and base flow mechanisms. Four flumes at different distances down the main stream and more than 100 piezometers were installed. Measurements were taken during two rainfall seasons in 1998 and 1999. Under initial wet conditions, a typical single-peak hydrograph was observed. Under low antecedent moisture conditions, however, runoff was characterized by a double-peaked hydrograph. The first peak, which occurred during the storm, was caused by rain falling on the saturated valley bottom. The second peak was delayed by minutes to hours from the first peak and consisted of rain flowing via the subsurface of the hydromorphic zone that surrounds the valley bottom. The duration of the delay was a function of the water table depth in the hydromorphic zone before the storm. The volume of the second peak constituted the largest portion of the stream flow.

Little attention has been devoted to calibrating soil tests for phosphorus (P) in the field for upland rice in West Africa. The information is needed to improve fertilizer P recommendations. A field experiment was conducted in the rainy season of 1994 to establish the relationship between extractable P by Bray 1 and grain yield of four upland rice cultivars grown on an Ultisol in the forest zone of Ivory Coast. The critical limit of Bray 1 P in the soil at 90% relative rice grain yield varied from 12.5 to 15.0 mg P/kg of soil for the four cultivars tested. The P critical limit in the soil tended to be lower for the cultivars of rice earlier found to be efficient at converting applied P into grain yield.

An effective and economically efficient phosphorus (P) management strategy requires accounting for the residual value of P fertilizer. Despite a great deal of research on soil P much remains unknown about the residual value of fertilizer P to upland rice in the humid tropics of Africa. A field experiment was conducted for six years (1993 -1998) to determine the response of four promising upland rice cultivars to fertilizer P applied at 0, 45, 90, 135, and 180 kg P ha 21 as triple superphosphate (TSP) only once in 1993, and its residual value in 1994, 1995, 1996, and 1998. The experimental site was located on an Ultisol, low in available P, in the humid forest zone of Cote d'Ivoire. Grain yields of the rice cultivars were significantly increased by fertilizer P in 1993, and by the fertilizer P residues in the subsequent years although the magnitude of response decreased rapidly with time since the fertilizer was applied. The levels of Bray 1 extractable P in the soil fell below the critical value in the 1994 season in all treatments except those which received 135 and 180 kg P ha 21 in 1993, and fell below the critical value in all P treatments in the 1995 season. 2457 ORDER REPRINTSA decrease in available P coincided with decreased residual value of P fertilizer and the soil P test (Bray 1 P) appeared to predict well the crop response. Our results show that there was a rapid loss of available P in the Ultisol and it is suggested that annual application of light rates of soluble P is a better option than heavy application at one time.

A farmer-friendly method of determining the most suitable cultivation soils would help in transferring new integrated soil management technologies. The potential for using soil color (Munsell data) was tested by physico-chemical analysis of 1028 ferrallitic soil samples from 289 profiles unequally allocated above 7 deg N in Côte d’Ivoire. Soil hue variations in depth and along the toposequence revealed the existence of vertical and lateral gradients of soil hue. The relative contribution of the different descriptors (clay, sand, carbon, total nitrogen, total phosphorus, potassium, magnesium and calcium) to the three functions extracted using a discriminant analysis to differentiate the four groups of soils with different hues was evaluated as well as the analysis of variance to determine the possible groups number for each one of the descriptors. Differences between physico-chemical components of red (2.5YR and 5YR) and yellow (7.5YR and 10YR) soils were determined, especially for P, Mg and K in extension. A decreasing gradient of inherent soil fertility indicators with an increasing yellowness in soil hue was revealed using multiple regression models. The soils 2.5YR and 5YR were therefore deemed more appropriate for stable and sustainable agriculture.Key words: Hue, ferralsols, fertility, soil use

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