Investigating the Effect of Gypsum Powder on Chemical Constituents of Soil and Selected Crops, Adigudem, Tigray, Ethiopia

This paper investigates the effects of gypsum powder from the gypsum plant in Adigudem on chemical properties of soil as well as the yield of two major crops, wheat (Triticum aestivum) and barley (Hordeum vulgare). Three mixes of 10kg of soil with 0%, 10%, 30%, and 50% proportion of gypsum powder were used for pot experiments under glasshouse conditions at Mekelle University. One bulk soil sample was collected from a spot at 4 km from the eastern side of the plant. The chemical concentration of major elements Ca, K, Na, Mg, and Mn, and trace elements, Cd, Zn, Cu, Pb, Cr, and Fe in soil and plant parts were determined using an Atomic absorption spectrometer as well as NO3, PO4 and SO4 using UV-spectrometer. The results suggest that the gypsum powder enhances metals and anion content in soil and in crop parts compared to the control sample. The chemical constituents in soil and crop parts showed negligible variation with increasing proportions of gypsum powder. Gypsum loaded Ca, SO4, Mn, and Pb onto the soil, which exhibited higher Mg, Cu, Mo, Cd, NO3, and PO4 but the comparable concentrations of Fe, K, Zn, and Cr in decreasing order. However, a direct relationship was noted in chemical constituent loadings along the pathway: powder-soil-crop in a similar fashion in the three mixes. Factor analyses revealed that wheat parts have a higher accumulation of nutrients than the barley parts with higher content in its growth soil blends. As an extension of this research, the in-situ investigation is recommended to assess the direct impact of the gypsum powder emitted over the soil and crops.


INTRODUCTION
Sustainability of soil and crop production is a function of soil physical, chemical and biological properties. Small changes in these properties cause large impacts on crop growth and development, and thus crop yields. To maintain or improve soil properties for continuous longterm agricultural production, it is important to add inputs or soil amendments. Soil chemical characteristics are affected by soil amendments and the crop production system (Ekholm et al., 2012). Gypsum has been used as a soil amendment for a long time to provide Ca and S for plant nutrition, and this occurs any time gypsum is used as a soil amendment. Gypsum amendments significantly increased the growth and yield components of Wheat crop under soil salinity (Mamun et al., 2019). Other uses include remediating sodic soils by displacing Na with Ca; ameliorating subsoil acidity by displacing Al 3+ with Ca 2+ followed by the Al 3+ combining with © CNCS, Mekelle University 165 ISSN: 2220-184X SO4 2− from gypsum to form a less toxic entity; serving as an electrolyte source to promote rainwater infiltration and percolation and reduce soil swelling, dispersion, and crusting); and reducing water-soluble P coming off of fields (Mamedov et al., 2009;Marchesan et al., 2017).
Gypsum application reduces the levels of Al 3+ and Mg 2+ and increased soil pH, the levels of Ca 2+ , and S-SO4 2- (Leandro et al., 2014). The effect of gypsum on soil nature and Barley as well as Wheat has been studied by a number of researchers (Qadir et al., 2006;Elrashicl et al., 2010;Ekholm et al., 2012;Leandro et al., 2014;Vicensi et al., 2016;Khalil et al., 2017;Herrero et al., 2018). However; no studies were undertaken on gypsum dust particulate impacts on the growth of plants around gypsum factories in Tigray and elsewhere. This study, thus is critical to fill the information gap.

Study Area
Adigudem is located at 30km south of Mekelle city close to the Adigudem-Hiwane asphalt road in Hintalo-Wejerat Woreda (Fig 1). The Woreda lies between latitudes 12º 55'N and 13º 20'N and longitudes 39º 20'E and 39º 55'E. The elevation of the area ranges from 1400 to 2850 m a.m.s.l. The relatively flat northern area around Adigudem forms part of the Mekelle Plateau.
The Adi-Shoha Highlands to the south and west and Desa Escarpment to the east are characterized by rugged mountains and deeply incised valleys. Average annual rainfall is up to 850 mm, decreasing to 300-400 mm in the east. Average temperature in the Woreda is about 18oC. However, in the highlands, the temperature drops upto 2-5 o C during November to January (Gebrekidan and Samuel, 2011). Wind in the area is normally easterly for nine months of the year (September to May) and reverses its direction in the rainy season, June to August (Benjamin, 2005

Soil Sampling and Crop Growth Experimental Set-up
The overall steps taken in the experimental stage of the research are shown in figure 2. were planted per pot and then thinned out to 3 plants soon after emergence according to Abate et al. (2013). All the seeds were sown by embedding about 3-4 cm depth into the soil and watering was done during evening with 1-liter per pot for the first 20 days and half-liter on subsequent days for 60 days. Watering of plants were done based on optimum humidity moisture level indicators.

Physicochemical Analysis
Physicochemical analysis was conducted for the soil sample (before used for crop growth and served as reference soil), water used for watering and gypsum powder obtained from the plant in geochemical laboratory at Mekelle University. Soil samples were taken from the 24-pots after 90days. Three sub-samples were prepared from each blend weighing one gram for analysis of major and trace elements/ Two samples, one gram each were prepared for each analysis to verify the precision of the Atomic Absorption Spectrometer (AAS) (Varian spectrometer AA-50B, Flame AAS, Stand-alone system) whose detection limits are given in Appendix 1. One gram sample was mixed with 20 ml aqua-regia stirring at 100 0 C for 20 minutes heated on a hot plate. Then 20 ml of 0.1N HCl was added drop by drop to the digested samples and shake for 2 minutes. The resulting solution was filtered by adding distilled water until the filtrate gets 50 ml volume before cation and major and minor elements measurements were done. For sulphate analysis, 2.5 ml of each filtrate was mixed with 2drops of reagent SO4-1 and 1 level micro-spoon of reagent SO4-2; heated up to 40 o C in water bath for 5 minutes and shacked occasionally. Then 2.5 ml of reagent SO4-3 was added into each sample with pipette and mixed thoroughly. 4 drops © CNCS, Mekelle University 168 ISSN: 2220-184X of reagent SO4-4 were added into the filtrate of each sample and heated up to 40 0 C for 7 minutes and shacken occasionally. 10 mm of each sample was filled in a cell and sulphate concentration was measured in the UV photometer. For nitrate analysis, 50 mg of amidosulfonic acid was added to each 5 ml of the filtrate and dissolve by maintaining pH within 1-3 range. Then, 1 micro-spoon of reagent NO3-1 and 5 ml of reagent NO3-2 was added and shaken vigorously for 1 minute until reagent NO3-1 is completely dissolved. 1.5 ml of the solution of each sample was taken and waited for 10 minutes reaction time before analyzing the nitrate values of the samples in the UV. For phosphate analysis, 5 ml of each original sample was mixed with 1.2 ml of reagent PO4-1. The solution of each sample was filled into 10mm cell and put into UV photometer for measurement.

Plant Tissue Chemical Analysis
Wheat and barley were treated to assess the accumulation of metals from soil under gypsum dust impact. The seeds were investigated separately to examine the storability of the toxic heavy metals in the edible and non-edible parts of the crops. The seed as well as composite of root and shoot parts were prepared from all batches. These were dried in stove at 100 0 C for 48 hours and were ground using electrical and manual grinder. To ensure homogenization and same particle size, the ground crop parts were sieved using 0.25mm net sieve. 0.5 g of each ground samples were digested with 5ml of nitric acid and 3 ml of hydrogen peroxide at 160 0 C for 1 hr. Then the digested 48 plants samples were analyzed in geochemical laboratory of Mekelle University using AAS and sulfate content was determined using UV photometer. Root and shoot fresh parts were weighed with analytical balance in Mekelle university botanical laboratory before and after drying at 80-105 0 C. Precautions were taken to avoid contaminations during the sample preparation and laboratory analysis. Variation in the measurements by the AAS is below 10%.

Data Analysis Techniques
Comparisons were made between the chemical constitutes of control (soil-only, gypsum-powderonly and water only) and blended groups.

Effect of Gypsum on Soil constituents
The result of chemical analysis for major, trace and anions constituents determined from the four levels of soil-gypsum mixes, gypsum powder and water used for watering the crops are presented in table 1 and figure 3. The major elements whose concentration generally increases from control (only soil) to the soil-gypsum mix are Mn, Na, K, Fe, Mg and Ca. All the parameters except copper do not show significant variation in the three levels of blends in the pots used for wheat growth (WS1, WS2 and WS3    The chemical characteristics of soil-gypsum mix used to grow both crops and composition of gypsum are presented in figure 4. Figure 4B shows that Barley soil blends have higher trace metals accumulation that might imply the resistance of Barley crops to uptake the metals. Similar is true for the anions, NO3, PO4 and SO4 (Fig 4C). The soil blend of both crops is found to have more Mg, Mo, Cd, NO3 and PO4 content than the gypsum powder (Kost et al., 2018). Matula and Pechová (2007) pointed out that the insignificant variation in the main cation nutrients (Na, K, Ca, Mg and Fe) can be assumed that the impact of the gypsum treatment on their uptake was mainly influenced by the establishment of equilibrium between the sorption © CNCS, Mekelle University 172 ISSN: 2220-184X complex of soils and the liquid phase of soils after a radical supply of calcium to soils through the gypsum dose. Gypsum is known to affect Mg concentrations in soil and this is reflected in the difference between control blend and the other blends in both crops due to the replacement of Mg by Ca in the soil, thus mobilizing the Mg and allowing it to move into the interstitial soil water (Kost et al., 2018).

Effect of Gypsum Dust on Nutrient Intake of Crops
The mean concentration of the elements in the crop body and seed are presented in figure 4.
Among the major (Na, K, Ca, Mg, and Fe) and trace elements (Cr, Cu, Pb, Zn, Mo, Cd), the major elements show higher concentration in both the crops as well as soil and dust samples than the trace elements. Calcium remains highest in the crops and it is the second abundant next to magnesium in soil/powder samples. Calcium has the highest concentration of 4561ppm (45.91%) in soil samples and also shows maximum in barley among the crops (Kost et al., 2018). The major metals in barley seed and body (shoot and root) samples showed even distribution over the soil blends.
Potassium, sodium and nitrogen did show preferential increment with slight variation in calcium. Unlike wheat, Mg, Fe and Mn in barley showed slight decrement in crops growth with soil from crop growth-1 (Cbs1) up to crop growth-3 (Cbs3) (Fig 4D). The macronutrients: Mg and Ca in barley crop parts show clear increment in seeds harvested in crop growth-1 (Cbs1) up to crop growth-2 (Cbs2) relative to control (Cbc) (Fig 4D).
The major elements (major nutrients): Na, K, Ca and Mg are highly mobile, bioavailable and easily transferable from soil to plant systems (Rodriguez and Rubio, 2006). This natural phenomenon was evident in this study that the accumulation of nutrients in root and shoot of wheat is related with the increasing concentration of Na in soil. The highest and lowest concentrations of Na in root and shoot were found in control (78 mg kg-l) and crop growth (145 mg kg-l).  The relative bar sizes help to understand the transfer of nutrients from shoot/root to seeds.
Wheat has higher storability of all nutrients than Barley. Wheat seed stores more major nutrients in addition to some trace metals (Mn, Cu, Pb, and Zn) than its shoot/root. Leandro et al. (2014) had indicated that the nutritional status of barely crops was not affected by gypsum application.
The factor analysis conducted summarizes the interrelationships in the nutrient distribution among the crop's parts, soil, gypsum powder and water. The factor analysis produced four factors with total variance of 93.3%. The first factor accounting for 53.8% of the total variance contains Mn, Cu, Zn, Mo, Cd, Cd, NO3 and PO4. One-way ANOVA suggested that these Parameters may be associated with their accumulation within the Wheat seed, shoot and root. It can be noted that barley parts showed lesser nutrients accumulation as implied from the observation that its blend soil contain higher values of these nutrients. As one of its undesirable effects, gypsum hinders the uptake of zinc, magnesium, iron, copper, and phosphorus in plants, The third factor comprising Ca, Mn and SO4 can be directly linked with the gypsum powder chemistry. The last factor with Cr and Pb might be attributed to loadings from water used for watering the crops and was fetched from groundwater tap.

CONCLUSIONS
Accumulation of trace metals in the experimental pots is generally higher than the reference control. Barley was noted to be more resistant to nutrient intake, especially heavy metals, than the wheat crops. Generally, the present study suggested that there is a significance effect on nutrient intake of the crops was observed due to gypsum powder. Gypsum powder mix increased the major and trace metals as well as anion content in the soil and consequently in crop parts as compared with the Control. Though weak variation with increasing proportions of gypsum powder mix is noted, evidently direct loadings from the gypsum powder on soil and crop parts was observed in similar fashion in the three mixes. Such constituents as Ca, SO4, Mn and Pb were added on to the soil from gypsum powder. The soil chemical analysis reveals higher Mg, Cu, Mo, Cd, N and PO4 content making the geogenic nature of soil to be responsible rather than the powder for such constituents' intakes by the crops. Both the soil and gypsum powder have equivalent Fe, K, Zn and Cr concentration in decreasing order implying that the original soil is also a contributor.

RECOMMENDATIONS
On spot investigation is recommended to assess the direct impact of the gypsum powder emitted over the soil and crops. The regional government especially the region's land use and environmental protection agency should follow the progress through continuous assessment and enforce the factory to reduce the Dust emitted to the environment. The factory should take the initiative to assess the impact or to co-operate similar investigations and should modify the old plant to minimize the dust emitted to the surrounding environment. Comprehensive and in-situ impact assessment is recommended for detail understanding of the positive as well as negative effects of the dust gypsum on soil and plants morphology and yield.