عنوان مقاله [English]
Objective and background: Plants such as rice need to provide their nutrient elements by using fertilizers for much more production in surface unit. For this purpose, it is essential to recognize macro-elements amount in soils and prepare their ideal maps. Soil CEC is a vital indicator of soil fertility quality and pollutant sequestration capacity as well as characteristics of N, P, K as macro-elements. This research was conducted with the aim of estimating and mapping the desired properties in order to obtain the results and maps that could be used in optimum management of fertilizer use and control of groundwater contaminants.
Materials and methods: The study area with an area of about 40,000 hectares is one of the central areas of Guilan province. 247 soil samples were collected from depth 0-30cm. The values of CEC, total nitrogen, phosphorus and potassium in soil samples and their descriptive statistics were determined. The normal distribution of data was analyzed using Kolmogrov-Smirnov test. Data that did not have normal distribution was converted to normal with appropriate transformations. Before the use of interpolation method, trend and anisotropy analysis were performed. Semi-variograms were calculated using ordinary Kriging and maps were plotted.
Results and discussion: The amount of K and P varied from 78 to 269.5 mgkg-1 and from 2.3 to 56 mgkg-1, respectively. The average contents of K and P were 192.03 and 16.51 mgkg-1, respectively. The amount of total N changed from 0.02% to 0.8%, which its average was 0.26%. Also, the content of CEC varied from 10.6 to 47.1 cmolckg-1 and its average was 26.72 cmolckg-1. The fitted model was based on semi-variograms of total nitrogen was exponential and those of phosphorus, potassium, and CEC were spherical. Determination coefficient (R2) of models had high value and the nugget effect/threshold is less than 25%. These characteristics showed that semi-variograms of properties had strong spatial structure. After specifying the semi-variograms, a map of their values was prepared using ordinary Kriging. Evaluation criteria values R2, RMSE and MAE derived for K 0.79, 27.84 and 0.106, P 0.73, 8.17 and 4.63, total nitrogen 0.72, 0.059 and 0.025 and CEC 0.76, 4.06 and 3.09. Criteria values R2, RMSE and MAE showed that accuracy of prepared maps was ideal. With attention to interpolation maps, spatial distribution of K was good in western, north-western, and central area of studied region. K deficiency was concentrated in southern and north-eastern areas. The amounts of P and total N were suitable in central and northern areas which their deficiencies were observed in southern area. With regard to soil nitrogen and P maps, usage of more than optimum limit of nitrate and phosphorus fertilizers causes ground waters pollution. Potash fertilizers application in land with high CEC results its fixation, too. Precise attention to CEC map and on-time fertilizer application can solve this problem. Therefore, accurate notice to different amounts of these parameters in maps, critical and optimum limits can well manage fertilizers application, prevents additional costs to farmer and pollution of ground water resources.
Conclusion: Since the investigation of N, P, K, and CEC is important for determination of soil fertility quality, so, the maps of spatial distribution of mentioned parameters were prepared via determination of experimental semi-variogram with strong spatial structure using kriging. The criteria of R2, RMSE and MAE showed that maps accuracy was acceptable. Spatial distribution of K was good in western, north-western, and central area of studied region. K deficiency was almost concentrated in southern and north-eastern areas. The contents of P and total N were suitable in central and northern areas which their deficiencies were observed in southern area.