upon Tyne NEI 7RU, U K S U M M A R YPure calcite (AR grade CaCO,) was treated with ferrous perchlorate solution to give a surface coating of iron (Fe) oxide. Maximum sorption (x,) of phosphate (P) by the calcite increased from 18.2 to 160 mg P kg-' as the amount of coating increased from 0.00 to 16.0 g Fe,O, kg-' CaCO,. Evidence for Fe oxide coatings on carbonate minerals in two Sudanese soils was obtained by optical microscopy and electron-probe microanalysis. The relative contributions of carbonate and Fe oxide minerals, and Fe oxide coatings to P sorption in these soils were calculated, based on an assumed model of oxide distribution. Separate-phase Fe oxide was the major contributor (3&40%0) to P sorption in the soils; the Fe oxide coatings on carbonate minerals were only minor contributors (< 6%), and the contribution of uncoated carbonate minerals was found to be negligible (< 1 %). These results suggest a very minor role for carbonate minerals, even when coated with Fe oxide, in the sorption of P by these calcareous, Sudanese soils.

Abstract:One calcareous and one sodic soil were treated with solutions of either sodium carbonate (Na,CO,) or sodium perchlorate (NaCIO,) to increase their sodium (Na) saturation before extracting with sodium bicarbonate (NaHCO,) to measure the recovery of native plus added phosphate (P). The Na,CO, additions also increased pH, and by comparing with the NaCIO, treatments, it was possible to separate the effects of added Na and pH on extractable P. For the calcareous soil both added Na and increased pH led to increases in the amount of NaHC0,-extractable P. For the sodic soil the amount of extractable P was mainly affected by pH, with little, if any, effect of the added Na. The suitability of the Olsen test for soil P for Na-affected soils is discussed.Key words: soil testing, phosphate, calcareous soil, sodic soil, pH, sodium bicarbonate extraction.Previous studies of the effect of sodium (Na) saturation on phosphate (P) release (Smillie et a1 1987; Sharpley et a1 1988) have shown that replacing indigenous exchangeable cations with Na leads to a substantial increase in water-extractable P. Sharpley et a1 (1988) found that soils treated to make them Na-saturated released about 2.5 times more P, on average, than untreated ones. They concluded that this effect would have important implications for P availability in Naaffected soils.The Olsen soil testing procedure, which is widely used to assess P availability to plants (Olsen et a1 1954), involves extraction with 0.5 M sodium bicarbonate (NaHCO,) at pH 8.5. The effect of increasing soil pH on NaHC0,-extractable P was studied by Sorn-srivichai et a1 (1984), who found that increasing pH by lime addition led to decreases in extractable P. This was not reflected in either the yield or the plant uptake of P in parallel greenhouse experiments. Increasing soil pH should increase P availability to plants due to a reduction in P sorption, increased desorption of P by hydroxyl ions, and an increase in P mineralised from organic matter. Sorn-srivichai et a1 (1984) concluded that the observed decline in NaHC0,-P was due to an artefact in the Olsen extraction, caused by the increased calcium concentrations in the limed soils.Calcareous (pH 7G8.5) and sodic (pH 8.5-100) soils of arid regions are naturally alkaline. The effects of soil pH and Na ions on NaHC0,-P could be quite marked in these soils, bearing in mind the findings discussed above. They do not appear to have previously been studied.The soils used in the present study were from the Sudan, from two of the sites used by Adam et a1 (1983) in their work on the mineralogy of the major Gezira soils. One was the Suleimi series, a Vertisol with calcium carbonate content of 51 g kg-' and pH in water (1 : 2.5) of 8.4; the other was the Laota series, an Aridisol with calcium carbonate content of 22 g kg-' and pH in water of 9.3. The mineralogy of the soils is similar, with the clay fractions dominated by smectite (Adam el a1 1983). Clay contents were 537 and 399 g kg-' for Suleimi and Laota soils, respectively. The exchangeabl...

Laboratory incubation studies were carried out on four soils from the Gezira region of the Sudan to measure the effect of wetting and drying cycles, similar to those experienced by irrigated field soils, on P availability, as measured by the sodium bicarbonate (NaHCO 3 ) and resin extraction methods. These soils, which are of low P status, were used both with and without amendments of P. In general, for unamended soils, wetting and drying increased NaHCO 3 -extractable P compared to that from soils maintained dry and, for prolonged wetting and drying treatments, increased NaHCO 3 -extractable P compared to soils maintained for the same period at a constant water content. In contrast, resin extraction of unamended soils showed that the wetting and drying treatments decreased the extractability of P compared to soils maintained dry and there was no difference when wetting and drying was compared to constant moisture. For soils with P added, wetting and drying increased NaHCO 3 -extractable P compared to soils maintained moist; a similar effect was seen with resin P extraction, except that the differences were generally not significant. Thus, wetting and drying does increase P extractability in these arid region soils and NaHCO 3 extraction was more sensitive than resin extraction in measuring the effect.

Field experiments were conducted for two rainfed seasons(2006 and 2007) and one irrigated season (2008) at the Agricultural Farmof the Upper Nile University in Malakal (Sudan). The objective of theexperiments was to study the effect of each of farmyard manure, waterhyacinth compost, Rhizobium seed inoculation and in dual combinationson groundnut symbiotic performance and growth and yield under bothrainfed and irrigation systems. The organic fertilizers were applied at therate of 15 t ha-1 each. The results showed that each organic manure alonesignificantly increased shoot dry weight, number of nodules plant -1 andpod and hay yields than the control in both rainfed and irrigation systems.Rhizobium seed inoculation with either manure significantly increasedplant shoot and plant root dry weights, plant nodulation, early podformation, absorption of plant N and plant P compared to the control.Moreover, these treatments significantly increased groundnut pod andhay yields over that of the control.

The objective was to investigate the effect of treatments interactions of application of irrigation canal sediments, humate fertilizer and irrigation interval on wheat (Triticum aestivum L.) growth and yield attributes in high terrace soils. Treatments were arranged in a split-split plot design with four replicates. Application of irrigation canal sediments constituted three rates (0, 30 and 60 ton/ha), three rates of humate fertilizer were used (0, 2 and 4 kg/fed) and two levels of irrigation interval were imposed (10 and 15 days). The soil of the experimental site is characterized by high amount of sand and low chemical soil fertility and the climatic zone of the area is described as desert. The results showed significant differences (P ≤ 0.05) due to the interactions effect of these three factors on number of plants/m 2 , plant height, number of spikes/m 2 , number of seeds/spike, 1000-seeds weight, grain yield and straw yield for both seasons. Also, treatments interactions had highly significant effects (P ≤ 0.01) on number of tillers/m 2 and harvest index, and very highly significant differences (P ≤ 0.001) on biological yield of wheat in both seasons. The best combination of irrigation canal sediments, humate fertilizer and irrigation interval which gave the highest values of growth, yield and yield components of wheat for both seasons; except for number of plants/m 2 in the second season and straw yield in both seasons, was the interaction effect of application of 60 ton/ha irrigation canal sediments, 4 kg/fed humate fertilizer and 10 days irrigation interval.