Correlation analysis of texture, physico-chemical properties and nutrient availability in red cotton-growing soils of Nagarkurnool District

Introduction

Cotton (Gossypium hirsutum L.) is one of the most important commercial crops in India and plays a vital role in the agrarian economy, particularly in semi-arid regions. Soil fertility and nutrient availability are critical determinants of cotton productivity, as the crop has a long growth duration and high nutrient demand. In cotton-based production systems, imbalanced fertilization, continuous cropping and limited organic matter inputs often lead to depletion of essential nutrients and deterioration of soil health, thereby affecting yield sustainability (Fageria et al., 2011) [5] . Soil texture, physico-chemical properties, and nutrient status are closely interrelated and collectively govern nutrient availability to plants. Among textural fractions, clay plays a major role in nutrient retention due to its higher surface area and cation exchange capacity, particularly for micronutrients such as zinc (Zn), copper (Cu), iron (Fe), and manganese (Mn) (Alloway, 2008; Kumar et al., 2016) [1,10] . Conversely, sandy soils are generally poor in nutrient-holding capacity and are more prone to nutrient losses through leaching (Schapel et al., 2023) [19] . Soil reaction (pH) is another key factor influencing nutrient solubility, especially micronutrients, whose availability sharply declines under alkaline conditions due to precipitation and adsorption reactions (Gondal et al., 2021) [6] . Organic carbon (OC) serves as an important indicator of soil fertility and is strongly associated with nutrient cycling processes. Soil organic matter contributes to the supply of available nitrogen through mineralization and also influences phosphorus availability by forming organic complexes that reduce fixation (Celi et al., 2022) [3] . Electrical conductivity (EC), though primarily indicative of soluble salt content, may indirectly influence nutrient uptake by affecting soil structure and root activity under irrigated conditions (Chen et al., 2009) [4] .

Black soils and red soils are the dominant soil groups supporting cotton cultivation in southern India. Black soils are characterized by high clay content, shrink-swell properties, and relatively high nutrient reserves, but often suffer from micronutrient deficiencies due to alkaline reaction and calcium carbonate accumulation (Satyavathi and Reddy, 2004; Shukla and Behera, 2017) [17,19] . Red soils, on the other hand, are generally lighter in texture, low in organic matter, and prone to nutrient depletion, making nutrient management more challenging under rainfed conditions (Jatav et al., 2020) [9] . Correlation analysis is a useful statistical tool for understanding the interrelationships among soil properties and identifying the dominant factors controlling nutrient availability. Such analyses help in diagnosing nutrient constraints, understanding nutrient interactions, and developing site-specific soil fertility management strategies (Singh et al., 2015; Negash and Mohammed, 2014) [20,13] . Despite the importance of cotton in Telangana, systematic information on the interrelationships among texture, physico-chemical properties, and nutrient availability across different soil types, depths, and moisture regimes remains limited. Therefore, the present study was undertaken to evaluate the relationships among texture, physicochemical, and chemical properties of irrigated and rainfed cotton-growing red soils of Nagarkurnool district. Understanding these interactions will aid in identifying key soil factors governing nutrient availability and provide a scientific basis for improving nutrient management practices for sustainable cotton production in the region.

Materials and Methods

Cotton-growing soils in the study area were categorized into eight groups based on soil depth (shallow <50 cm; deep >1 m), management system (irrigated or rainfed), and sampling depth (surface or subsurface). A preliminary survey guided the random selection of 120 representative farmer fields in key cottonproducing areas of Nagarkurnool district. Fifteen fields per category were sampled at two depths, yielding 120 total soil samples (Table 1). Sampling occurred during the 2023 kharif season before cotton planting, with surface (0-15 cm) samples collected via the standard zigzag method to account for field heterogeneity. Soil samples were air-dried, lightly crushed, and sieved through a 2 mm mesh prior to analysis. Samples were stored in labeled plastic bags to avoid cross-contamination and analyzed using established protocols. Physicochemical properties included pH and EC measured in 1:2.5 soil-water suspensions with calibrated digital meters (Jackson, 1973) [8] . Organic carbon was quantified by the Walkley-Black wet digestion technique (Walkley and Black, 1934) [23] . Available N was assessed via alkaline permanganate distillation (Subbiah and Asija, 1956) [21] , P by Olsen's method (Olsen et al., 1954) [14] , and K by neutral 1 N ammonium acetate extraction. DTPA-extractable micronutrients (Fe, Cu, Zn, Mn) were determined using atomic absorption spectrophotometry (Lindsay and Norvell, 1978) [11] .

Results and Discussion

Correlation analysis in red soils Irrigated deep red soils (sub-surface)

The correlation analysis among texture, physico-chemical, and chemical properties (Table 2) showed several significant relationships. Sand was negatively correlated with clay (r = -0.683**), while soil pH showed a significant negative correlation with clay (r = -0.537*). Electrical conductivity was strongly and positively correlated with silt (r = 0.889**). Organic carbon exhibited a highly significant positive correlation with available nitrogen (r = 0.710**), but showed no significant association with texture or pH. Available phosphorus was positively correlated with sand (r = 1.000**) and negatively correlated with clay (r = -0.683**), whereas available potassium showed no significant relationships. Among micronutrients, Zn, Mn, Fe, and Cu were strongly and positively correlated with clay (r = 0.786**, 0.730**, 0.865**, and 0.929**, respectively) and showed highly significant negative correlations with soil pH. Strong positive inter-correlations were observed among micronutrients, particularly between Fe and Cu (r = 0.957**) and Zn with Mn, Fe, and Cu, indicating close associations in their availability.

Irrigated deep red soils (sub-surface)

The correlation analysis of irrigated deep red sub-surface cottongrowing soils of Nagarkurnool district revealed several significant relationships among texture, physico-chemical, and chemical properties (Table 3). Clay content was positively and significantly correlated with micronutrients, particularly Zn (r = 0.783), Fe (r = 0.698), and Cu (r = 0.877**). Soil pH showed a significant negative correlation with clay (r = -0.515) and exhibited strong negative correlations with Zn (r = -0.799), Fe (r = -0.789), and Cu (r = -0.741). Electrical conductivity (EC) was positively correlated with silt (r = 0.516) and pH (r = 0.526), but negatively correlated with clay (r = -0.570), indicating an inverse relationship between salinity and finer soil fractions.

Rainfed deep red soils (surface)

In rainfed deep red surface soils, (table 4) clay content exhibited positive correlations with Zn (r = 0.649**), Mn (r = 0.479), Fe (r = 0.068) and Cu (r = 0.807**). Soil pH showed a significant positive correlation with available P₂O₅ (r = 0.567*), but was negatively correlated with Zn (r = -0.722**), Mn (r = -0.732**), Fe (r = -0.556*) and Cu (r = -0.586*). Organic carbon (OC) exhibited a very strong positive correlation with available nitrogen (r = 0.941**) and a moderate positive correlation with P₂O₅ (r = 0.534*). Available nitrogen was also moderately correlated with P₂O₅ (r = 0.632*).

Rainfed deep red soils (sub-surface)

In rainfed deep red sub-surface soils (Table 5), clay showed positive but non-significant associations with most nutrients and micronutrients. Zinc and Cu exhibited moderate positive correlations with clay (r = 0.363 and 0.362, respectively), indicating reduced influence of texture with depth. Soil pH displayed consistent and significant negative correlations with Zn (r = -0.693**), Mn (r = -0.599*), Fe (r = -0.557*) and Cu (r = -0.550*). Organic carbon was significantly correlated with available N (r = 0.549*) and P₂O₅ (r = 0.584*). Among micronutrients, strong positive interrelationships were observed, particularly between Zn and Cu (r = 0.897**), and moderate correlations of Zn with Mn (r = 0.673**) and Fe (r = 0.633*).

Irrigated shallow red soils (surface)

The correlation analysis of irrigated shallow red surface soils (Table 6) revealed a strong negative correlation between sand and silt (r = -0.882), indicating contrasting distribution of textural fractions. Clay showed a significant positive correlation with OC (r = 0.741), while OC was strongly correlated with available N (r = 0.745). Zinc exhibited a positive correlation with clay (r = 0.599) and a strong negative correlation with pH (r = -0.664). Manganese showed a similar negative relationship with pH (r = -0.657) and a strong positive association with Zn (r = 0.820). Copper showed the strongest positive correlation with clay (r = 0.872).

Irrigated shallow red soils (sub-surface)

In irrigated shallow red sub-surface soils (Table 7), sand was strongly and negatively correlated with silt (r = -0.781**). Clay exhibited strong positive correlations with Zn (r = 0.856**), Mn (r = 0.664**) and Cu (r = 0.948**). Soil pH did not show significant relationships with texture but was negatively correlated with Mn (r = -0.707**) and Fe (r = -0.603*). Organic carbon showed weak and non-significant correlations with available N and P₂O₅. Available P₂O₅ exhibited negative but non-significant associations with Zn (r = -0.507) and Cu (r = -0.416). Copper showed the strongest association with clay among all micronutrients.

Rainfed shallow red soils (surface)

In rainfed shallow red surface soils (Table 8), sand showed strong negative correlations with silt (r = -0.794**) and clay (r = -0.541*). Clay was significantly and positively correlated with Zn (r = 0.732**), Mn (r = 0.704**), Fe (r = 0.580*) and Cu (r = 0.862**). Soil pH exhibited strong negative correlations with Zn (r = -0.556*), Mn (r = -0.672**), Fe (r = -0.753**) and Cu (r = -0.541*). Organic carbon was strongly correlated with available N (r = 0.767**).

Rainfed shallow red soils (sub-surface)

The correlation analysis of rainfed shallow red sub-surface soils (Table 9) revealed highly significant negative correlations of sand with silt (r = -0.922**) and clay (r = -0.678**). Soil pH showed strong negative correlations with Zn (r = -0.714**), Mn (r = -0.611*), Fe (r = -0.703**) and Cu (r = -0.698**). Electrical conductivity did not exhibit significant correlations with nutrients or organic carbon. Organic carbon was strongly and positively correlated with available N (r = 0.871**), while its association with P₂O₅ was moderate and non-significant. The correlation analysis across irrigated and rainfed black soils of Nagarkurnool district clearly demonstrates the dominant influence of soil texture, particularly clay content, soil reaction, and organic carbon on nutrient and micronutrient availability. The consistently strong positive correlations between clay and micronutrients such as Zn, Cu, and Fe across surface and subsurface layers indicate that finer-textured black soils enhance micronutrient retention through greater specific surface area, higher cation exchange capacity, and stronger adsorption sites. These findings are in agreement with earlier reports that emphasize the role of clay minerals in regulating micronutrient dynamics in Vertisols and associated black soils (Alloway, 2008; Satyavathi and Reddy, 2004; Meena et al., 2006) [1,17,12] . Soil pH emerged as a key controlling factor for micronutrient availability in black soils. The negative correlations observed between pH and Zn, Mn, and Cu in both irrigated and rainfed conditions suggest reduced solubility of these micronutrients under alkaline soil reaction. Such relationships are well documented in black soils, where increasing pH promotes precipitation and adsorption of micronutrients as hydroxides or carbonates, thereby limiting their availability to crops. The significant negative association of Zn with pH in particular indicates a strong risk of pH-induced Zn deficiency in these soils, as also reported (Thakur et al., 2011, Jatav et al., 2020, and Shukla and Behera, 2017 [22,9,19] . Organic carbon showed a consistently strong positive relationship with available nitrogen across all black soil groups, highlighting the central role of soil organic matter in nitrogen mineralization and nutrient cycling. The positive correlations between OC and available P₂O₅ in several soil conditions further suggest that organic matter may contribute to phosphorus availability by reducing fixation and enhancing P solubilization through organic ligand formation (Negash and Mohammed, 2014, Singh et al., 2015, and Braos et al., 2023) [12,20,2] . However, the generally weak relationships between OC and micronutrients indicate that in black soils, clay content and soil pH exert a stronger control over micronutrient

Acknowledgement

AcknowledgementsThe authors gratefully acknowledge the Soil Health Unit, Agricultural Research Institute, PJTAU, Hyderabad, for providing research facilities and encouragement.

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