DIGITAL MAPPING OF SOIL FERTILITY INDEX CALCULATED FOR ARABLE SOILS OF THE CIS-SALAIR DRAINED PLAIN

The article describes comparison of soil fertility indices (SFI) calculated for the ploughed layer of arable soils differing in soil organic matter (SOM) content: with high (5-8%) and medium (3-5%) SOM content. The studied soils were located on erosion-prone slope at different altitudes: soils with high SOM at 280-310 m, and soils with medium SOM at 190-280 m a.s.l. The study area was located in the interfluve area between the Irba and the Hiruzovka rivers in the Cis-Salair drained plain in the south-east of West Siberia. The soil types common there are Greyzemic Luvic Chernozems (Siltic, Aric, Pachic), Phaeozems (Siltic, Аric), Greyzemic Phaeozems Colluvic (Siltic, Taptomollic). The SFI were calculated in two steps. First, the relative indices (RI) for every agrochemical property, i.e. pHKCl, SOM, acetic acid extractable phosphorus (P) and exchangeable potassium (K), were calculated on the basis of respective minimal and optimal soil concentrations and crop requirements. Second, the SFI were calculated as sums of the relative indices divided by 4. The resultant SFI values were rated according to the scale where values below 0.4 indicate low fertility; values ranging 0.41-0.60 indicate moderate fertility, whereas values ranging 0.81-1.0 indicate high fertility. The digital maps of the RI and SFI were created using raster calculator in ENVI software, which allows performing mathematical operations with quantitative parameters of raster image pixels in GeoTIFF format. The values of relative indices in raster cells exceeding 1 (that is when factual content is higher than the optimal one) were substituted with 1 by specially developed program called ZChanger. Then the SFI map was created for erosion-risky slopes using as a basis the digital maps of the relative indices for pHKCl, SOM, P and K. Phosphorus was extracted by two different extragents, namely 0.5 М acetic acid (PAA) and 0.1 М ammonium malate (PAM). Soil fertility index was rated as medium irrespective of SOM content and soil type due to the high PAA content in soils with less SOM. It was found that when SFI were calculated using the data on easily available soil P, i.e. extracted with 0.015 М К2SO4 (PPS), then for most of the study area SFI can be rated as low, indicating the deficit of plant available phosphates in soils. High- and medium-SOM soils did not differ in рНKCL and PPS. The medium-SOM soils, such as agrochernozems, agro-grey and agro-dark-grey ones, had 1.7-2.2 lower SOM content as compared with the high-SOM soils such as agrochernozems. The reverse relation was found in labile P content: in soils with medium SOM level the PAA and PAM contents were 2 and 1.3 times higher than in high-SOM soils, respectively. In high-SOM agrochernozems PAA was lower than the optimal level (<20mg/100 g soil), while in medium-SOM soils it was higher. The PAM could be rated as medium. Exchangeable K in the arable soils was found to be unevenly distributed: in medium-SOM agrochernozems and agro-dark-grey soils exchangeable K was on average 1.2 times lower as compared with high-SOM agrochernozems, the difference with medium-SOM agro-grey soils being statistically not significant. Overall exchangeable K was lower than the optimal level (<30mg/100 g soil) Above-ground phytomass was found to be positively correlated with soil labile PPS (rs=0.41) and PAM (rs=0.33). The difference in aboveground phytomass produced by oats and peas mixture between high- and medium-SOM soils was not statistically significant.