Characterization and classification of the major pigeon pea growing soils in the study area
Among the 18 soil series identified in the mandal, five series were the major pigeon pea growing soils. These soils include the series Gajulapet (GA), Manikonda1 (MA1), Manikonda2 (MA2) Potanpalle2 (PO2) and Kodur series (KO).
Morphological characteristics
The depth varied from shallow to very deep among the major soils. Gajulapet soils were shallow (25-50 cm), Manikonda1 soils were moderately shallow (50-75 cm),Manikonda2 and Potanpalle2 soils were deep (100-150 cm) and Kodur soils were very deep (>150 cm). The dominant soil colour was having a hue of 10 YR in all the soils series with its value ranging from 3 to 5 and chroma varying from 1 to 4 except Gajulapet. Gajulapet soils were dark brown (7.5 YR4/3 M), in the surface and yellowish red (5 YR4/6 M) in the subsurface. Soil colour depends on the mineral composition and changes in soil coating (
Schaetzl and Anderson, 2005). Dominant soil texture was sandy clay loam, which resulted due to the process of soil formation and the dominant structure was moderate subangular blocky which was well developed due to high clay content
(Verma et al., 2012). Soil consistency varied from firm, friable, slightly sticky to very sticky, non plastic to very plastic. With increase in clay content, soil consistency also increased as evident from Table 1 and 2. Soil consistency is influenced by the clay content of the soil
(Sarkar et al., 2001), organic matter and soil texture
(Wani et al., 2016).
Physico-chemical characteristics
Clay increase was evident in all the soils indicating the process of clay illuviation
(Sarkar et al., 2002). In Manikonda2 and Potanpalle2 soils, there was a clay increase towards the depth which later decreased at the deeper layers due to the influence of parent material and also the less active pedogenic processes. Surface pH varied from slightly acid (6.19) in Manikonda1 soils to moderately alkaline (8.06) in Manikonda2 soils. Surface CEC was low (9.77 cmol (+) kg
-1) in MA1 to 18.76 cmol (+) kg
-1 in MA2 soils. These low CEC values indicate the dominance of low activity clays. In general, CEC followed a similar trend of clay content in soils. Base saturation was very high (100%) in the MA2 and PO2 series, which could be due to higher Ca
+ occupying exchange sites on the colloidal complex (
Sireesha and Naidu, 2013). Exchangeable sodium percentage (ESP) was recorded in all the soils, however it was <10%, posing no hindrance to growing pigeon pea. Calcium carbonate equivalent was not recorded in GA and MA1 soils.
Soil classification
The soils were classified based on soil taxonomy (USDA). The temperature and moisture regimes are isohyperthermic and ustic respectively in the study area. Gajulapet and Manikonda1 series were classified as
Alfisols, with argillic sub-surface horizon and hence keyed out to
Haplustalfs at great group level and to
Typic Haplustalfs at sub-group level. Manikonda2 and Potanpalle2 series were classified as
Inceptisols order and as
Haplustepts at great group level. Manikonda2 series was keyed out as fine-loamy, smectitic, superactive,
Typic Haplustepts and Potanpalle2 series was a member of fine-loamy, smectitic, superactive,
Aquic Haplustepts, as redox depletions with chroma of 2 or less and also aquic conditions for some time in normal years (or artificial drainage) were observed in one or more horizons within 75 cm of the mineral soil surface. The particle size class was fine-loamy as the particles in the fine-earth fraction,18 to less than 35 percent (by weight) of clay and mineralogy class was smectitic due to the dominant smectite minerals by weight, than any other single kind of clay mineral. The CEC class was superactive due to a higher CEC: clay ratio of 0.60 or more. Kodur series was classified as a member of fine-loamy, smectitic, superactive, isohyperthermic family of
Vertic Haplustepts. Due to the presence of pressure faces in a layer 15 cm or more thick that has its upper boundary within 125 cm of the mineral soil surface these soils were classified as
Vertic Haplustepts.
Land suitability evaluation
The soils of the study area were classified to 18 soil series and 66 mapping units of phases of soil series. Of these, the major pigeon pea growing soils were mapped to 19 units of only five soil series which were characterized and discussed in this study. The GA and KO soils have 5 and MA1, MA2 and PO2 have 3 phases each. The details of these phases are given in Table 3 and the land suitability for pigeon pea (Fig 1) and management interventions needed in Mahabubnagar Rural mandal is given in Table 4.
The study area has a mean temperature of 27
oC and rainfall of 530.8 mm during the growing season and LGP of 147 days. These confirm to the suitability of climate in the region towards pigeon pea, though the average annual rainfall was 613.3 mm, which was only moderately suitable. When the major pigeon pea growing soils were evaluated for their suitability to grow the crop, it was found that the dominant class was marginally suitable with limitations of rooting conditions, followed by texture and nutrient availability due to higher soil pH. Effective soil depth of less than 40 cm is considered as not suitable and 40-75 cm is marginally suitable class
(Naidu et al., 2006). Gajulapet soils were shallow (25-50 cm) and Manikonda1 soils were moderately shallow (50-75 cm), where depth was the major rooting limitation. In these soils, it is better to choose shallow rooting varieties or adopt growing alternative crops. Manikonda2 soils were found to be marginally suitable with limitations of nutrient conditions especially because of the soil pH greater than 8.0 and were designated to S3n. A soil pH between 7.9 and 9.0 is marginally suitable class
(Naidu et al., 2006). Potanpalle2 soils had two phases which were moderately suitable with limitations of erosion and nutrient availability (S2en) and the phase PO2iB2 was evaluated to marginally suitable class with limitations of nutrient availability (S3n). To manage alkalinity and thereby to improve nutrient availability, application of gypsum may be carried out. The very deep soils of Kodur had five phases of which KOhB1, KObB2, KOiB1 were found to be highly suitable towards growing pigeon pea.
Only a very small area of 5.55 per cent was highly suitable for the crop in the entire study area. These soils were deep to very deep, with sandy clay loam or sandy loam horizons and well drained providing better infiltration, ensuring better availability of water and nutrients to crop. It was found that 16.24 and 32.79 per cent of total geographical area was moderately and marginally suitable respectively with limitations of erosion hazards, gravelliness, nutrient availability, rooting conditions and texture and 5.70 per cent of total area was found currently not suitable. Productivity of the crop is affected by soil slope, gravelliness and effective rooting depth (
Shivaramu, 2012). The limitations of rooting conditions mainly are due to shallow soils, which could be managed either by choosing shallow rooted dwarf pigeon pea varieties with better anchorage and more feeder roots, or by choosing alternative crops such as minor millets or crops such as green gram
(Ramamurthy et al., 2019). Limitations due to slope and erosion can be managed by adopting proper soil and water conservation measures in situ. This could include opening up dead furrows perpendicular to the slope or contour trench cum bunding. By adopting proper management strategies productivity of pigeon pea could be improved for sustained use.
