Sixty five surface soil samples were collected from the fields of Eastern Farm of Horticultural College and Research Institute, Periyakulam, Tamil Nadu. The geo-coordinates were recorded for each sample using GPS (Fig. 1).

Collected soil samples were air-dried, sieved through a 2 mm sieve, labeled, and stored for analysis. Soil samples were sieved with a 0.5 mm sieve for the estimation of organic carbon. The soil samples were analyzed with standard procedures. Soil pH and EC were estimated as per Jackson, (1973). Organic carbon was estimated as per the procedure outlined by Walkley and Black (1934). Soil available nitrogen was estimated as per Subbiah and Asija (1956) and available phosphorus as per Olsen et al. (1954). Available potassium and sulphur were determined as per Stanford and English (1949) and Williams and Steinbergs (1959) respectively. Available Zn, Fe, Cu, and Mn were analyzed with respect to Lindsay and Norvell (1978) and available Boron as outlined by Berger and Truog (1944).

The analytical results of each soil sample was categorized into low, medium and high based on the critical limits as followed in Tamil Nadu (Table 1).

Using the number of samples in each category, the per cent sample category and Nutrient Index Values (NIV) were calculated using the formulae furnished below.

Percent sample category =  No. of samples in Low or Medium or High category  Total number of samples ×100

Nutrient index value was calculated from the proportion of soils under low, medium and high available nutrient categories, as represented by, NIV=[(PH×3)+(PM×2)+(PL×1)]100

Where,

NIV = Nutrient Index Value

P_L, P_M and P_H are the percentage of soil samples falling in the category of low, medium and high nutrient status and given weightage of one, two and three respectively (Ramamoorthy and Bajaj, 1969).

The nutrient index values were rated into various categories viz., low (<1.67), medium (1.67-2.33), and high (>2.33) for OC and available N,P, and K. For available S and micronutrients, the ratings are very low (<1.33), low (1.33-1.66), marginal (1.67-2.00) adequate (2.01-2.33), high (2.34-2.66) and very high (> 2.66).

Database on soil analytical parameters was generated in Microsoft Excel and the thematic maps were created at the Department of Remote sensing and GIS, TNAU, Coimbatore by using Arc-GIS software. The thematic maps on physicochemical properties and available nutrient status were generated by categorizing the fertility status such as ‘Low’, ‘Medium’ and ‘High’ by showing appropriate legend for soil fertility parameters by krigging.

The overall data (65 Nos) indicated that pH of the soil varied from 5.09 to 8.33 with a mean value of 6.82. The electrical conductivity of the soil samples ranged from 0.01 to 0.21 dSm^-1 with a mean value of 0.06 dSm^-1 and soils were non saline. The organic carbon content of the soil samples ranged from 2.40 to 9.84 g kg^-1 with a mean of 6.56 g kg^-1. The free CaCO₃ content in the soil samples ranged from non-calcareous to slightly calcareous in nature (3.95 to 5.65 %) with a mean value of 4.85% (Table 2).

The available N, P and K status of the soils varied from 165 to 322; 11.4 to 33.0 and 220 to 620 kg ha^-1 with a mean of 291, 26.0 and 405 kg ha^-1 respectively. The available sulphur content varied from 4.57 to 24.22 mg kg^-1 with a mean of 7.26 mg kg^-1 (Table 2).

The DTPA extractable Fe, Zn, Mn, and Cu content ranged from 0.69 to 16.65; 0.77 to 3.59; 2.01 to 11.96; 0.84 to 2.54 mg kg^-1 with a mean of 6.53, 1.29, 6.85 and 1.48 mg kg^-1 respectively. The hot water soluble boron content of the soils ranged from 2.54 to 7.34 mg kg^-1 with a mean of 4.91 mg kg^-1.

Number of samples in each fertility category were worked out and the results are furnished in Table 3. Among the 65 soil samples, 17 are acidic, 32 are neutral and 16 are alkaline in reaction. Soil EC values of all the samples found to be in non-saline category. Organic carbon content in soil samples revealed that, 13 samples under low, 30 under medium and 22 were high category. With respect to available nitrogen, 46 samples fell under the low and 19 under medium status. In the case of available phosphorus, 20 and 45 samples were coming under the category of medium and high status, respectively. Among the soil samples, 11 and 54 samples were belonging to the category of medium and high available potassium status. Available sulphur found to record 60 samples under low, 4 samples under medium and 1 sample under high category. With respect to DTPA micronutrients, 29, 15, 21; 28, 36, 1; 0, 19, 46; 9, 47, 9 nos of samples are grouped under low, medium and high category for DTPA Fe, Zn, Mn and Cu respectively. In the case of HWS-B, all the 65 samples fell under the category of high status.

Percent category samples for soil physic-chemical properties and available nutrients was worked out and the results are given in Table 4.

Soil pH values indicated that out of total samples, 26.2 per cent were acidic, 49.2 per cent were neutral and 24.6 per cent were alkaline. All the soil samples were categorized under non saline. Among the 65 soil samples, 20.0, 70.8 and 92.3 per cent of samples are grouped under the low status for O.C, available N and S respectively. In the case of micronutrients, 44.6, 43.1 and 13.8 per cent of samples were grouped under low status for available Fe, Zn and Cu respectively.

Among the 65 soil samples, 46.2, 29.2, 30.8, 16.9 and 6.2 per cent of samples are with medium status for O.C, available N, P, K, and S respectively. Among the micronutrients, 23.1, 55.4, 29.2, and 72.3 cents of samples were classified under medium status for available Fe, Zn, Mn, and Cu respectively.

High category found to be with 33.8, 69.2, 83.1 and 1.5 per cent of samples in O.C, available P, K and S respectively. Micronutrients viz., Fe, Zn, Mn, Cu and B recorded 32.3, 1.5, 70.8, 13.8, and 100 percent of samples under high category.

Nutrient index values were worked out and the values are furnished in Table 5.

Among the major nutrients, nitrogen registered the lowest nutrient index value of 0.84 followed by phosphorus (1.75) and potassium (1.84). Among the micronutrients, the order of nutrient index values was B>Mn>Cu>Fe>Zn. Nutrient index values for organic carbon and available N were low while for available P and K, the nutrient index values were medium. Nutrient index values for available sulphur found to be very low. With respect to micronutrients, DTPA-Fe, Zn and Cu were classified under very low while DTPA-Mn and HWS-B were marginal. Similar results were reported by Sellamuthu et al. (2015), Theresa et al. (2019) and Muthumanickam (2020) for assessing the nutrient index values in Tiruchirapalli District, rice ecosystem of Anaimalai Block and vegetable grown soils of Horticultural College and Research Institute, Periyakulam respectively.

Thematic maps were generated for soil pH, EC, organic carbon and available nutrient status by showing appropriate legend for the respective parameters (Fig. 2). Thematic maps generated clearly depicted the spatial variability of soil fertility parameters. Similar thematic maps were created in Veeranam Command Area, Tamil Nadu by Arunkumar and Paramasivan, (2015); mapping Soil Fertility and its Spatial Variability in Tiruchirapalli District, Tamil Nadu Using GIS by Sellamuthu et al. (2015) and for vegetable grown soils of Horticultural College and Research Institute, Periyakulam by Muthumanickam (2020).