26 soil samples from five soil profiles were collected from different climatological and ecological regions in central Sudan, which included; Gedaref area, Khartoum area, Wad Madani area and Khartoum North area (recent Nile terrace). Each soil profile was studied in the field, and described following the format of the [9] FAO (2006), guidelines of soil profile description and sampled according to diagnostic or genetic horizons and classified following the American system for soil classification [10] (Soil survey staff, 2014a).

Each sample was placed in a cloth bag, labeled with; collected data, area, soil profile number, sample depth, then, subjected to physical and chemical analyses at the soil laboratories in Khartoum University. Soil pH was determined on the saturated paste using digital pH meter model (3510, Jenway) according to [11] (Blakemore et al., 1987), and the electrical conductivity of the saturation extraction was used as a measure of soil salinity according to [12] (Rhoades, 1990). The organic matter (OM) was determined used Walkley-Black method [13] (Chapman and Pratt, 1961).

Soil texture was determined using particle size analyzer model (Mastersizer 2000, Malvern) and textural classes were obtained using USDA textural Triangle according [14] (Soil Survey Staff, 2014b). Soil phosphorus was analyzed using spectrophotometer model (Lambda EZ 150, PerkinElmer, USA) according to [15] (Olsen and Sommers, 1982).

Percent calcium carbonate (%CaCO₃) was estimated by Calcimeter: 2 g of soil sample was treated by 0.1 N HCL; the volume of CO₂ from pure calcium carbonate and samples were recorded. Then, the percent calcium carbonate was calculated according to [8] (Balázs et al., 2005).

5 g of fine crushed soil sample was placed into conical flask, 10 ml of 1 N HCl and 50 ml distilled water were added, heated until boiling for 2 mins. Then, left until cool and 3 drops of phenolphthalein indicator was added and titrated against 1 N NaOH. Then, the percent calcium carbonate was calculated according to [8] (Balázs et al., 2005).

Statistical differences between samples were determined using statistical analysis according to [16] (Snedecor, 1965), using T-test with multiple samples where differences were calculated from various measurements. The means of these differences were obtained (), the deviation from each measurement was used to obtain the standard deviation (s_d). Then the T value was calculated from the equation below:

where:

T ≡ Calculated T value

≡ Means of differences.

s_d ≡ Standard deviation.

N ≡ Number of samples

The description of the study sites and selected morphological properties of representative soil profiles are presented in Table 1 and Table 2, respectively. The parent material of Profiles I and II was alluvium/colluvium and old alluvium of the Bule Nile, respectively. While the parent material of profiles III, V, and VI were alluvium. Soil texture of all profiles belong to five textural classes; loam, sandy clay loam, clay loam silty clay and clay. All profiles showed angular/sub-angular blocky structure in the surface horizon and the lower horizons were massive. The quantity of roots in the soil profiles decreased with depth and the boundary between horizons was generally diffused and smooth.

Some of the physical and chemical properties of the representative profiles were presented in Table 3. Gadarif area soil was non-saline, non-sodic and calcareous, while Soba area soil at Khartoum state was saline-sodic and calcareous. The soil of Agricultural Research Corporation Farm, Gezira State is non-saline at the depth of 0 - 84

Texture^a; C: clay; Scl: sandy clay loam; Sic: silty clay; Cl: clay loamy; L: loam. Structure^b; 1: weak; 2: moderate; 3: strong; f: fine; m: medium; c: coarse; sbk: subangular blocky; abk, angular blocky; ma: massive. Roots^c; 1: very few; 2: few; 3; moderate; 4: common; f: fine; m: medium; c: coarse. Boundary^d; a: abrupt; c: clear; d: diffuse; i: irregular; s: smooth; w: wavy.

cm, slightly saline at the bottom depth 84 - 150 cm, sodic and slightly calcareous. The Nile flood plain soil was non-saline, non-calcareous and non-sodic; it was very suitable for agriculture. The soil of Shambat area―Col- lege of Agricultural Studies Farm is non-saline at the top surface 0 - 83 cm, and slightly saline at the bottom 83 - 150 cm, sodic and slightly calcareous soil. The soil CaCO₃ content of the studied samples ranged from 3.99% to 28.76% in all studied locations.

Table 4 shows the statistical analysis between the two methods. Results revealed that there are no significant differences between the two most in all studied locations, except in samples from Gedaref location which shows significant difference and recorded as (5.598 and 5.760) respectively. Carbonates mainly forms of, Ca⁺⁺ as (CaCO₃), Mg⁺⁺ (MgCO₃) magnetite, Na⁺ (NaCO₃) Fe⁺⁺ (FeCO₃). Carbonates were deposited by sedimentation in marine or Lake Environment. Carbonates can also originate within organic materials resulting from living organisms (Products resulting from the degradation of limestone rocks through erosion), pH measurements between approximately 7.5 and 8.5 indicate the presence of carbonates. A higher pH (Towards 10) can indicate the presence of sodium carbonate [17] (Janitz, 1986).

Readings observed from titration method were higher than that of calcimeter method (Figure 1). Although, there were no significant difference between the calcimetric and titrimetric methods, except in the case of (El- daim Fruits and vegetables Farm―Gedaref area) and that probably due to addition of HCl to the soil in titration method, which lead to ferrous and aluminum oxide displacement, and later precipitation in form of hydroxides when the titration done by sodium hydroxide.

The enough consumed acid to dissolve oxides was equal to NaOH precipitating hydroxides. So the estimation of calcium carbonate by calcimeter was not affected, but some of the acid also lost by; reaction with exchangeable calcium and magnesium ions at clay surface, or reaction with other clay minerals. As the soil of Gedaref area is heavy clay, this explained the result of significant difference when the two methods were used. In general; to avoid that error, it is better to put in mind the several advantages of calcimeter method than that of titration method.

^N.S = Non − Significant at P > 0.05, ^* = Significant.

[18] Maulood et al., (2012) studied the comparison between the titrimetric and calcimetric for CaCO₃ determination in 84 different locations of soils from Iraq. They obtained no significant differences between the two methods and the two method were highly correlated (r = 0.993 to 0.998). In contrast, [19] Kassim (2013) mentioned that, the results of calcium carbonate equivalent estimated by Calcimeter were lowest as compared to acid neutralization and acetic acid methods. [4] El Mahi et al., (1987) reported that the values of carbonate equivalent estimated by acid neutralization were corrected as:

CaCO₃ equivalent = acid neutralization % CaCO₃ - 0.05 CEC.