The effect of soil management method on some characteristics of limestone soils in the Muthanna desert

: Four areas were selected in the Al-Rehab region, the desert of Muthanna Governorate, which differs in the management style, the duration of agricultural exploitation, and the quality of irrigation water. It was taken in each Pedon region, and the morphological and chemical measurements were revealed and conducted. It was found that the thickness of the A horizon ranged between 10-15 centimeters and the thickness of the B horizon between 25-45 centimeters in all study pedons. The administration style did not affect Hue, as all horizons were within the color (10YR) and chroma, as the colors ranged from light white to very bale brown (10 YR7/4) and white (10YR8/1), the soil texture was be-tween Sandy loam, Sandy clay loam, Loamy Sand. As for the chemical properties, the percentage of lime CaCO3 ranged between (250-400) gm/km -1 and the percentage of gypsum ranged between (1.2-1.8) g. km -1 The electrical conductivity EC ranged between (3.9-18)ds.m -1 , while the organic matter was between (2.1-13.5) and the CEC values ranged between (12-18Coml.kg).


Introduction
Calcium carbonate is one of the essential carbonate salts common in the soils of most dry and semi-arid regions 1,2 . Carbonate minerals are one of the most critical components of the earth's crust, accounting for 4% of the earth's crust weight. It is either inherited from the material of rocks of origin or secondary, formed pedologically in the form of knots that the soils of dry areas and where the rainfall rate is less than 500 mm annually in which carbonates, especially carbonates, accumulate Calcium and magnesium. 3 found that the proportion of calcite in Iraqi soils constitutes 90% of Iraqi soils, while the proportion of dolomite and calcite-bearing magnesium reaches 10%. Soil is a natural resource not equivalent to other natural resources, so it occupies an important place because of its wealth. Agricultural production depends mainly on the soil, as the excellent and permanent exploitation of agricultural soils means full use of it, raising its fertility and increasing crop productivity through technical means (Al-Ani, 2006). Morphological studies are the basis on which the rest of the sciences related to soil are based, as it provides an essential and reliable database. Soils, in reality, are independent physical entities with a specific geographical distribution, and their chemical, physical and mineral properties and characteristics overlap in terms of influence. This interference is reflected in the apparent morphological characteristics of interest to soil surveyors (Al-Ani, 2008).
The soils of the Al-Samawa desert in the Al-Rehab area are fertile and productive and provide groundwater of acceptable quality for agriculture. It is considered among the limestone soils, as they suffer from many problems, such as the fixation of elements such as phosphorous and the formation of compaction layers in addition to the alkaline conditions 2 . Calcium carbonate in the Al-Rehab area between 250-400 gm.kg -1 is considered a high percentage, so it needs exceptional management to reduce the adverse effects on production. The researcher (Al-kassy) has shown that most of the carbonate minerals are distributed in the Iraqi soil in the form of separate particles within the soil content, and the other percentage of these minerals is in the form of binding materials for other soil separations. Calcium carbonate's prevalence rate varies from one soil to another. Therefore, these soils need exceptional management and different cultivation methods to be productive and can be cultivated and preserved from deterioration and desertification, especially since these lands have vast areas, amounting to 20 million dunums in the Samawah desert alone. Most of them are suitable for agriculture. Others need exceptional management to become productive, and some lands need a specific reclamation. Part of the lands of the Badia can be used for grazing, and the other part can be for recreation and wildlife while preserving, sustaining and developing the existing resources.

Materials and methods
The study area was selected within the administrative boundaries of the Muthanna Badia in the Al-Rehab area, some of which are planted for long or medium periods. The pedon is not planted with the availability of natural vegetation. All pedons are irrigated from artesian wells, and the locations of the pedons representing the selected sites were determined using a GPS device and were described fundamentally according to 4 Soil surveying and sampling began on 30-4-2021 Figure 1. Soil samples were air-dried, crushed manually and with a wooden hammer passed through a sieve with holes 2 mm in diameter and kept in plastic containers, after which the following laboratory analyses were carried out:

Physical properties
Particle size distribution: Hydrometer method according to 5 . Bulk density was estimated by the Core method for the soil models described in 6 . Chemical properties pH: The degree of soil reaction in the soil sample extract with water was estimated at 1:1, according to what was mentioned in 7 . Electrical conductivity: The electrical conductivity of a soil sample extract with water was estimated at 1:1 using the Conductivity bridge device according to what was mentioned in 7 . Calcium Carbonate minerals: They were estimated by Calcimmeter according to 8 . Gypsum: The gypsum was estimated by the method of sedimentation using an acetone solution, and then the electrical conductivity of the formed precipitate was measured, as stated in 7 . CEC was estimated by the simplified methylene blue method presented in 9 . Organic matter was determined by the wet oxidation method according to the Walkely and Black method described in 10 . Calcium and magnesium ions were estimated by 1:1 extraction by slaking method with Versenate, according to what was mentioned in 10 . Sodium: The dissolved sodium ion was estimated with 1:1 extract, and a photometer flame was used according to the method described in 7 . ESP exchanged sodium ratio: The ratio was calculated as in equation 11 . ESP= 6.28+0.64*SAR-----(1) Morphological characteristics: Morphological characteristics (color, texture, structure, and texture) were converted to quantitative according to 12

Morphological characterization of the study pedons
The results of the morphological description showed relative differences due to the environmental conditions and the material of origin, As well as the difference in management systems, the quality of crops and the length of cultivation. There was a difference in the natural plant, where Schanginia arbuscular and Tamraxix Aphylla appeared in the uncultivated soils. What are the long-cultivated soils? Artemisia and Malva appeared. The depths of horizons varied in the study pedons. They also varied, as the depth of the horizon A ranged from 10-15 cm in all the pedons, and the surface horizon was 15 cm in the uncultivated soils. As for the rest of the pedons, it was 10 cm. As for the other horizons, the thickest was B in all the pedons due to the accumulation of Calcium carbonate transferred with irrigation water. Regarding soil color, all study areas were within Hue of 10YR only. Chrom values and chromium values differed, as the color ranged between Very pale brown (10 YR7/4), Light Yellowish (10YR6/4 white (10YR8/1), where the colors were light between light white and yellowish, and this reflects the desert conditions and the lack of organic matter as a result of the hot climate and the oxidation of organic matter, as well as the accumulation of calcium carbonate and salts, especially in uncultivated soils where limestone soils are a result of the original material and irrigation water. Soil texture is an essential morphological characteristic that directly affects soil management and other characteristics. The textures ranged between Loam, Silt loam, Silty clay loam, and sandy loam. These tissues are within the coarse and medium textures as a result of sedimentation conditions and the coarse-grained limestone parent material, as well as the materials transported by torrents and rain, where pedon was distinguished Cultivated for a long time with a relatively soft tissue where the horizon was A for all without studying Silty loam. The composition is generally characterized as blunt-angled or devoid of structure due to the accumulation of salts and calcareous materials. The roots of the plants appeared on the cultivated horizons. They were generally medium and soft due to cultivation with fibrous-rooted field crops and some annual natural plants. Some horizons showed a granular structure with no sub-angular blocky due to long cultivation and the availability of percentages of organic matter and plant roots.    Table 3. Morphological description of the studied pedons 1-4.

Chemical characteristics of the study area
Electrical conductivity EC Table 4 shows that the electrical conductivity values of the study peduncles ranged from 3.8-18 ds.m _1 , and these values varied according to the length of the cultivation period; it was 18 in the uncultivated soils down to 3.8 in the soils that were cultivated for 30 years, and it was clear the effect of cultivation and irrigation on Reducing the degree of soil salinity, where the pedon No. 4 was distinguished by a somewhat low electrical conductivity, ranging between 4-3.8, according to the horizons of the pedon. The degree of reaction pH The degree of reaction in all study pedons and horizons was almost neutral to slightly basal, where the values ranged between 8.7-8 Table 4, which is consistent with drought conditions, high temperatures and conditions of the limestone material. Calcium sulfate Gypsum (gypsum) The values were generally low and indicate that the soil is not gypsum, and the results in Table 4 ranged between 1.2-1.8 gm.kg-1. It was the lowest value in the uncultivated soil bed and increased significantly in cultivated donuts. The increase was proportional to the length of the period. Agriculture This refers to the accumulation of percentages of gypsum from the irrigation water containing this substance. In general, the percentage of gypsum is few and harmless. Organic matter OM

72-100 Ck1
It was observed that the organic matter decreased in general and in all the study pedons, and this reflects the conditions of the dry region and the high temperatures where the organic matter was oxidized (Konen et al. 2003, Al-Azzawi 2017, and Jaib 2019). The values of the organic matter ranged between 0.2-50 and 13. gm.kg -1 Table 4, the lowest in the horizon was Pz of the uncultivated pedon and the highest value on the horizon AP in the cultivated pedon for 30 years. The length of the cultivation period and soil management were clearly on the increase. The accumulation of organic matter and the percentage of organic matter decreased with increasing depth, and this is due to the depth reached by the roots of the cultivated plants.

CEC Cations Exchange Capacity
The study soils were characterized by a lack of exchange capacity, and this characteristic increases the proportion of clay, type of clay and organic matter, and the ratio ranged between 12-18 Cmol. Kg -1 Table 4. This percentage is considered low, and this is due to the coarse sandy soils and the low percentage of clay and organic matter, as well as the increase in the percentage of Coca 3, which encapsulates the soil particles and reduces the surfaces subject to interaction. Calcium Carbonate (Lime) Coca3 The study showed a high percentage of calcium carbonate in all the pedons and horizons of the study and that the soil is clear limestone, which is attributed to the limestone mother and the existing lime deposits, as well as the irrigation water and torrents carrying these compounds. The proportions in Table 4 ranged between 250-400 gm.kg -1 The percentage of carbonates increased in The middle horizons of the study soils, which refers to the pedogenic movement of lime minerals and the formation of the calcic and procyclic horizons. This is within horizon B; despite the increase in the percentage of lime, its effect on production is limited due to the lack of lime melting and its limited osmotic effect.

Discussion
The degree of reaction in all study pedons and horizons was almost neutral to slightly basal, which works on the accumulation of lime 2,13 . This characteristic is essential, as it shows the ability of the soil to retain nutrients, prepare it for plants continuously, and retain nutrients and preserve them from washing and loss 14 .
However, there was an increase in CEC with the increase in the number of years of cultivation, and this is due to Increased accumulation of organic matter 15 .

Conclusion
The results showed apparent differences in the organic substance, cation exchange capacity, carbonate distribution, and increased gypsum accumulation due to soil management.