Effect of Rock Phosphate Enriched Compost on Soil Micro Nutrient Status and Uptake of Finger Millet-owpea Cropping System in Sandy Loam Soils of Southern Dry Zone of Karnataka

Effect of Rock Phosphate Enriched Compost on Soil Micro Nutrient Status and Uptake of Finger Millet-owpea Cropping System in Sandy Loam Soils of Southern Dry Zone of Karnataka

Jagadeesha G. S^1* , Prakasha H. C¹ , Shivakumara M. N¹ , Govinda K¹ , Yogananda S. B²

¹Department of Soil Science and Agricultural Chemistry, UAS, GKVK, Bangalore, India

²Department of Agronomy, VC Farm, Mandya, Karnataka, India

Corresponding Author Email: jaggu.agri@gmail.com

DOI : http://dx.doi.org/10.53709/CHE.2021.v03i01.002

Abstract

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INTRODUCTION

Overuse of inorganic fertilizers without organic inputs and intensive cropping may degrade the soil quality and cause environmental pollution. Depletion of soil quality could be avoided by proper and careful soil management practices. The application of organics along with inorganic fertilizers is an important practice to improve soil quality, enhance microbial activity and nutrient recycles to produce quality crops. Organic manures act not only as a source of nutrients, but also increases microbial activity, influence structure, nutrients turnover and many other related physical, chemical and biological parameters of the soil [1].

Composting is gaining interest as suitable option for chemical fertilizers with environmental benefit, since this process eliminates or reduces toxicity of municipal solid waste (Kaushik and Garg, 2003; Araujo and Monteiro, 2005) and leads to a final product which can be used in improving and maintaining soil quality. Conventional organic fertilizers, such as urban solid waste compost (USWC) vermicompost and farmyard manure (FYM), are widely recommended for agricultural production as nutrient source and soil conditioner. Use of such compost to the crops could able to save about 50 per cent of chemical fertilizers and save millions of dollars on exchequer from importing of chemical fertilizers to the country. It can be used as an alternative to other organic matter in agriculture and as a soil conditioner. The quality of the USWC can be improved by use of beneficial microorganisms, nutrient additives and organic amendments [25].

Phosphorus (P) is one of the major essential macronutrients required for the plant growth. Globally more than 5.7 billion hectares of land contain very low available P [10]. Recently, rock phosphate (RP), which is used as a raw material for phosphatic fertilizers, is appeared to be a potential source of plant P nutrition [43]. Unfortunately, P in RP is not easily available in soils with an alkaline pH and even when conditions are optimal, plant yields are lower than those obtained with soluble phosphate [13]. Availability of P from relatively insoluble RP can be improved by integrating it with organic residues [4] and phosphate solubilizing microorganisms [6]. Many studies have shown that the enriched compost improves physical and chemical properties of soils by increasing nutrient content, organic matter, water holding capacity and cation exchange capacity. Thus contributing to improvement of crop yield and quality [20, 11]. Keeping in view, the benefits of organic manuring as well as its inherent limitations such as analysis and slow action, a study was taken up to investigate the possibility of conversion of compost (USWC, vermicompost and FYM) into phosphate enriched compost through RP and to evaluate their nutritional quality of the crop.

Finger millet-cowpea is a major cereal-pulse based cropping system followed in southern dry zone of Karnataka (Zone 6). Finger millet (Eleusine coracana (L.) Gaertn) is an important cereal that belongs to the grass family Poaceae, sub family Chloridoidae. It is estimated that finger millet accounts for some 10 per cent of the 30 million tons of millet produced globally [7]. In India, it is cultivated on 1.8 m ha with average yields of 1.3 t ha^-1. In Karnataka, finger millet is grown in an area of 0.76 m ha producing 1.32 m t with a yield of 1715 kg ha^-1 [8]. Cowpea (Vigna unguiculata (L.) Walp.) is a legume mainly grown in tropical and subtropical regions in the world for vegetable and grains and to lesser extent as a fodder crop. It also serves as cover crop and improves soil fertility by fixing atmospheric nitrogen.

Farmers in southern dry zone of Karnataka especially Cauvery command area practice finger millet-cowpea, which is a major cropping sequence in a year. Some parts of this area having high P build up and farmers are applying only inorganic fertilizers to their field. Excessive use of chemical fertilizers has created concerns due to energy crises, stagnant yields and soil (physical, chemical and biological properties) health. Integrated nutrient management using enriched compost along with mineral fertilizers could sustain both agriculture and environment thus maintaining the long term ecological balance of soil ecosystem. In recent years, land application of enriched compost has emerged as an attractive and cost effective alternate strategy for waste management around the world. However, judicious use of enriched compost along with inorganic fertilizers not only improves soil physical, chemical and biological properties but also minimizes environmental risks as well. In view of this, the present study was initiated with the objects of effect of varied levels of RP enriched USWC, vermicompost and FYM with N and K fertilizers on soil micronutrient status and crop uptake after harvest of finger millet-cowpea cropping system in sandy loam soils of southern dry zone of Karnataka.

MATERIAL AND METHODS

The site and experimental details                                                                                   

           A field experiment was carried out to assess the effect of rock phosphate enriched compost on soil nutrient status after harvest of finger millet-cowpea cropping system in high phosphorus build up soil at ZARS, Mandya in Southern Dry Zone of Karnataka state lying between 12° 34′ 03″ North (latitude) and 76° 49′ 08″ East (longitude) with an altitude of 697 m above mean sea level. Finger millet (variety KMR 204) was taken as a main crop (kharif) and cowpea (variety C 152) was taken up as residual crop (summer) with a spacing of 30 x 10 cm.

The soil was sandy loam in texture with 87.42, 1.62, and 9.87 per cent sand, silt and clay, respectively and bulk density of 1.38 Mg m^-3. The soil was neutral in reaction (pH 7.12) and low in soluble salts (0.21 dS m^-1). The soil was low in organic carbon (0.48 %) content, low in available N (210.80 kg ha^-1), K₂O (130.20 kg ha^-1) and high in available P₂O₅ (133.58 kg ha^-1). The exchangeable Ca and Mg content of soil was 2.57 and 1.08 C mol (P⁺) kg^-1, respectively and available S was 8.85 mg kg^-1. The DTPA extractable micronutrient content viz., Cu, Zn, Fe and Mn were 0.83, 1.21, 1.94 and 14.04 mg kg^-1, respectively.

The experiment was laid out in a randomized complete block design (RCBD) with eleven treatments and replicated thrice. The treatment combination include, T₁: Absolute control, T₂: Package of practice (100 % NPK + FYM @ 10 t ha^-1), T₃: Recommended N and K + 25 % P supplied through enriched USWC, T₄: Recommended N and K + 50 % P supplied through enriched USWC, T₅: Recommended N and K + 75 % P supplied through enriched USWC, T₆: Recommended N and K + 25 % P supplied through enriched vermicompost, T₇: Recommended N and K + 50 % P supplied through enriched vermicompost, T₈: Recommended N and K + 75 % P supplied through enriched vermicompost, T₉: Recommended N and K + 25 % P supplied through enriched FYM,  T₁₀: Recommended N and K + 50 % P supplied through enriched FYM and T_11: Recommended N and K + 75 % P supplied through enriched FYM. FYM @ 10 t ha^-1 was common for all the treatments except Absolute control (T₁). Recommended dose of fertilizer was 100:50:50 kg of N, P₂O₅ and K₂O per ha and net plot size was 12 m².

Soil micronutrient analysis

            Before transplanting and at harvest of both the crops, soil sampling at a depth of 0-15 cm were collected using a soil auger. The soil samples collected were air dried and passed through a 2 mm sieve. The processed soil samples were analyzed for micronutrient cations (Cu, Zn, Fe and Mn). These cations were extracted with DTPA extractant (0.005 M Diethylene Triamine Penta Acetic acid + 0.01 M CaCl₂ + 0.1 M Triethanolamine buffered to pH 7.3) at 1:2 soil to extractant ratio as described by [17]. The concentration of these cations was determined by AAS under suitable measuring conditions [23].

Plant tissue analysis and nutrient uptake

            The above-ground samples (grain, straw and haulm) collected at physiological maturity for grain, straw and haulm yield determinations were also grounded in a Willey mill with stainless steel blades and subjected for nutrient analysis. The micronutrient concentrations in the digested plant sample were determined after making proper dilution by AAS. The nutrient uptake by finger millet and cowpea at harvest was worked out using the following equation.

Nutrient content (%) × Dry matter production (kg ha^-1)

Nutrient uptake (kg ha^-1) = ———————————————————————

       100

Statistical analysis

            The experimental results were tabulated, analyzed and the data interpretation was done by Randomized Completely Block Design (RCBD) of analysis of variance (ANOVA) for further comparison between the treatments [33].

RESULTS AND DISCUSSION

Soil micronutrient status of finger millet

DTPA extractable copper (Cu)

Among the treatments, significantly higher DTPA extractable Cu was recorded in treatment T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) (1.59 and 1.62 mg kg^-1) in first season (2017) and pooled data, respectively and was on par with T₄ (recommended N and K + 50 per cent P supplied through enriched USWC) (1.50 and 1.53 mg kg^-1) and T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (1.55 and 1.59 mg kg^-1) in first season and pooled data, respectively (Table 1). In second season (2018), again T₅ registered significantly higher DTPA extractable Cu (1.65 mg kg^-1) compared to other treatments but at par with T₈ (1.63mg kg^-1). It may be due to the application of USWC which increased the microbial activity in the soil, consequently release of chelating agents that form stable complex with organic substances thus reducing the fixation and favoring the release of micronutrients. [42] and [44] also reported an increase in total and extractable soil Cu concentrations when soil was amended with USWC compost. A significant increase in total soil Cu concentration was also detected in the acidic sandy soil when amended with municipal solid waste compost [29].

DTPA extractable zinc (Zn)

            At harvest of finger millet, significant increase of DTPA extractable Zn was noticed in treatment T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) (1.83, 1.87 and 1.85 mg kg^-1) and was on par with T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (1.77, 1.85 and 1.81 mg kg^-1) in first season, second season and pooled data, respectively (Table 1). Soil organic matter exerts a significant and direct impact on the availability of Zn (Zhang et al., 2001). Zinc ions are bound to organic material of the soil firmly and the variable zinc amount in the soil increases in parallel to the organic matter content. For these reasons, the amount of DTPA Zn increases on the topsoil where organic material is abundant [22]. [9] and [44] also reported an increase in total soil Zn concentrations when compared to unamended controls with the application of municipal solid waste compost.

DTPA extractable iron (Fe)                           

            Similar to Zn, significantly higher DTPA extractable Fe was recorded in treatment T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) (5.30, 5.39 and 5.35 mg kg^-1) compared to rest of the treatments (Table 1). The range of Fe content was 1.87 (T₁: absolute control) to 5.35 mg kg^-1 (T₅) pooled data. It might be due to enhance the microbial activity in soil upon addition of composts and consequent release of complex organic substances which form ligands with Fe and would have prevented it from precipitation, fixation and oxidation. The results are in consonance with findings of [32].

DTPA extractable manganese (Mn)

           Treatment T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) had significantly higher (17.67 mg kg^-1) DTPA extractable Mn in pooled analysis and was on par with T₂ (POP: 100 % NPK + FYM @ 10 t ha^-1), T₃ (recommended N and K + 25 per cent P supplied through enriched USWC), T₄ (recommended N and K + 50 per cent P supplied through enriched USWC), T₇ (recommended N and K + 50 per cent P supplied through enriched vermicompost) and T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (16.93, 16.81, 17.41, 17.22 and 17.63 mg kg^-1, respectively) (Table 1). In 2017, DTPA extractable Mn was significantly higher in T₅ (17.53 mg kg^-1) and was on par with T₂, T₃, T₄, T₆ (recommended N and K + 25 per cent P supplied through enriched vermicompost), T₇, T₈ and T₁₁ (recommended N and K + 75 per cent P supplied through enriched FYM) (17.05, 16.67, 17.29, 16.17, 17.39, 17.42 and 16.33 mg kg^-1, respectively) but in 2018, T₈ recorded significantly higher (17.83mg kg^-1) DTPA extractable Mn compared to other treatments but at par with T₄ (17.54 mg kg^-1) and T₅ (17.82 mg kg^-1). It could be attributed to higher Mn content in the initial soil and application of enriched composts upon mineralization might have released Mn. [41] also reported an increase in total soil Mn concentrations with addition of municipal solid waste compost.

            Organic residues are good reservoir of trace elements. Organic acid and polysaccharides formed during organic manure decomposition forms soluble micronutrient chelates and helps in dissolution of soil minerals containing micronutrients. Organic manure is rich source of micronutrients and they provide convenient physical, chemical and biological environment in soil for better nutrient availability. All these factors attributed to better availability of nutrients [28]. [14] who reported higher available micronutrient status (Zn, Fe, Mn and Cu) in post-harvest submerged soils of paddy were under 50 per cent RDF + 2.5 t ha^-1 paddy straw compost and 75 per cent RDF + 2.5 t ha^-1 paddy straw compost compared to chemical fertilizers alone.

Soil micronutrient status of residual cowpea

DTPA extractable copper (Cu)

In 2018 season, after harvest of cowpea, significantly higher DTPA extractable Cu was noticed in treatment T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) (1.51 mg kg^-1) and was on par with T₄ (recommended N and K + 50 per cent P supplied through enriched USWC) (1.42 mg kg^-1) and T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (1.47 mg kg^-1) (Table 2). In 2019 season, T₈ (1.59 mg kg^-1) registered significantly higher DTPA extractable Cu but at par with T₅ (1.57 mg kg^-1). In pooled analysis, T₅ recorded significantly higher DTPA extractable Cu (1.54 mg kg^-1) compared to other treatments except T₄ (1.45 mg kg^-1) and T₈ (1.53 mg kg^-1). It could be attributed to dissolution of native Cu present in soil and due to release of Cu as a result of mineralization of enriched USWC and vermicompost. The results are in accordance with [29] and [44] showed significant increase in total soil Cu concentrations when soil is amended with municipal solid waste compost.

DTPA extractable zinc (Zn)

          Significantly higher DTPA extractable Zn was recorded in treatment T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) (1.75, 1.81 and 1.78 mg kg^-1 in first, second season and pooled means, respectively) (Table 2). This treatment was on par with T₄ (recommended N and K + 50 per cent P supplied through enriched USWC), T₇ (recommended N and K + 50 per cent P supplied through enriched vermicompost) and T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (1.60, 1.55 and 1.69 mg kg^-1, respectively) in first season and at par with T₈ (1.79 mg kg^-1) in second season. In pooled data, T₅ was on par with T₈ (1.74 mg kg^-1). It could be attributed to the enrichment of USWC, vermicompost and FYM that might have supplied additional Zn to the soil pool. [38] reported that an increase in total soil Zn concentrations when amended with municipal solid waste compost. The positive influence of Zn enriched compost on DTPA-Zn was also been reported by [16]. The marginal decrease in available Zn status in cowpea plots may be due to uptake of Zn by the crop. Similar results were reported by [35].

DTPA extractable iron (Fe)

          Among the treatments, significantly higher DTPA extractable Fe was noticed in treatment T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) (5.22, 5.31 and 5.27 mg kg^-1, in first, second season and pooled analysis, respectively) (Table 2). And this T₅ treatment was on par with T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (5.12 mg kg^-1) in first season. Higher Fe content in soil might be due to release of Fe from the native soil by the action of chelating agents which form ligands with higher oxides of Fe and convert it to soluble forms. The results are in accordance with the findings of [30] and [42].

DTPA extractable manganese (Mn)

          Treatment T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (16.95 mg kg^-1) registered significantly higher DTPA extractable Mn in first season and was on par with T₄ (recommended N and K + 50 per cent P supplied through enriched USWC), T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) and T₇ (recommended N and K + 50 per cent P supplied through enriched vermicompost) (16.15, 16.39 and 16.25 mg kg^-1, respectively) (Table 2). In second season, T₅ registered significantly higher DTPA extractable Mn (16.78 mg kg^-1) compared to others except T₄ and T₈ (16.44 and 16.73    mg kg^-1, respectively). And in pooled analysis, T₈ (16.84 mg kg^-1) showed significantly higher DTPA extractable Mn but at par with T₄ and T₅ (16.30 and 16.58 mg kg^-1, respectively). Application of enriched USWC and vermicompost upon mineralization might have released Mn from organic amendments. [31] reported that increase in Mn content might be due to release of Mn from organic amendments during mineralization process or due to dissolution of native Mn from soil. [9] recorded an increase in total soil Mn concentration with addition of municipal solid waste compost. The decreasing trend in Mn content at harvest of cowpea may be ascribed to more uptake of Mn by the cowpea since it was a residual crop [24].

Micronutrient uptake by finger millet

          Significant influence on uptake of micronutrients by finger millet was observed due to different P enriched composts and results are furnished in Tables 3 and 4.

            Irrespective of the treatments, significantly higher uptake of Cu in grain was noticed in T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) (137.77 and 140.66   g ha^-1 in first and second season, respectively) and was on par with T₂ (POP: 100 % NPK + FYM @ 10 t ha^-1) (123.21 and 124.70 g ha^-1), T₄ (recommended N and K + 50 per cent P supplied through enriched USWC) (130.15 and 136.13 g ha^-1), T₇ (recommended N and K + 50 per cent P supplied through enriched vermicompost) (123.88 and 122.64 g ha^-1) and T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (134.74 and 132.16 g ha^-1) in first and second season, respectively (Table 3). In pooled analysis, T₅ (139.22 g ha^-1) recorded significantly higher uptake of Cu in grain compared to other treatments except T₂, T₄ and T₈ (123.95, 133.14 and 133.45 g ha^-1, respectively). Treatment T₅ registered significantly higher uptake of Cu in straw (192.08, 195.99 and 194.03 g ha^-1 in first, second and pooled data, respectively) (Table 4). This treatment was on par with T₄ (190.33, 189.09 and 189.71        g ha^-1) and T₈ (188.38, 188.14 and 188.26 g ha^-1) in first, second and pooled data, respectively. In first season, T₅ was on par with T₂ (175.05 g ha^-1) also.

            Significantly higher uptake of Zn in grain was recorded in T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) (164.02, 167.59 and 165.80 g ha^-1 in first, second season and pooled data, respectively) and was on par with T₂ (POP: 100 % NPK + FYM @ 10 t ha^-1) (136.88, 139.82 and 138.35 g ha^-1), T₄ (recommended N and K + 50 per cent P supplied through enriched USWC) (152.02, 159.41 and 155.72 g ha^-1), T₇ (recommended N and K + 50 per cent P supplied through enriched vermicompost) (149.32, 149.38 and 149.35 g ha^-1) and T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (153.83, 161.05 and 157.44 g ha^-1) in first, second season and pooled data, respectively (Table 3). Significantly higher uptake of Zn in straw was noticed in T₈ (250.00, 282.74 and 266.37 g ha^-1 in first, second season and pooled data, respectively) (Table 4). This treatment was on par with T₂ (POP: 100 % NPK + FYM @ 10    t ha^-1), T₄, T₅ and T₇ (recommended N and K + 50 per cent P supplied through enriched vermicompost) (227.11, 242.66, 246.10 and 231.01 g ha^-1, respectively) in first season and at par with T₅ (276.64 g ha^-1) in second season. In pooled data, T₈ was on par with T₄ (256.98 g ha^-1) and T₅ (261.37 g ha^-1).

            Among the treatments, Treatment T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) (177.07 g ha^-1) had significantly higher uptake of Fe in grain compared to other treatments except T₄ (recommended N and K + 50 per cent P supplied through enriched USWC) (162.21 g ha^-1) and T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (175.30 g ha^-1) in first season (Table 3). In second season, significantly higher uptake of Fe in grain was recorded in T₈ (194.45 g ha^-1) but at par with T₅ (193.38 g ha^-1) only. With respect to pooled means, T₅ (185.22 g ha^-1) registered significantly higher uptake of Fe in grain and was on par with T₈ (184.88 g ha^-1). Significantly higher uptake of Fe in straw was noticed in T₈ (250.00, 282.74 and 266.37 g ha^-1 in first, second season and pooled data, respectively) (Table 4). This treatment was on par with T₂ (POP: 100 % NPK + FYM @ 10 t ha^-1), T₄, T₅ and T₇ (recommended N and K + 50 per cent P supplied through enriched vermicompost) (227.11, 242.66, 246.10 and 231.01 g ha^-1, respectively) in first season and at par with T₅ (276.64 g ha^-1) in second season. In pooled data, T₈ was on par with T₄ (256.98 g ha^-1) and T₅ (261.37     g ha^-1). Treatment T₄ registered significantly higher uptake of Fe in straw (445.71, 453.67 and 449.69 g ha^-1 in first, second season and pooled data, respectively) (Table 4). This treatment was on par with T₈ (436.81 g ha^-1) in 2017 season and at par with T₅ (451.76 and 443.64 g ha^-1) and T₈ (447.06 and 441.94 g ha^-1) in 2018 season and pooled means, respectively.

            Similar to Cu, Zn and Fe, Manganese uptake also differed significantly among different treatments. In 2017 season, T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (178.32 g ha^-1) registered significantly higher uptake of Mn in grain compared to others except T₂ (POP: 100 % NPK + FYM @ 10 t ha^-1), T₃ (recommended N and K + 25 per cent P supplied through enriched USWC), T₄ (recommended N and K + 50 per cent P supplied through enriched USWC), T₅ (recommended N and K + 75 per cent P supplied through enriched USWC), T₆ (recommended N and K + 25 per cent P supplied through enriched vermicompost), T₇ (recommended N and K + 50 per cent P supplied through enriched vermicompost) and T₁₁ (recommended N and K + 75 per cent P supplied through enriched FYM) (165.70, 145.20, 170.16, 167.91, 142.81, 171.44 and 147.71 g ha^-1, respectively) (Table 3). But, in 2018 season, T₅ (199.58 g ha^-1) had significantly higher uptake of Mn in grain compared to others except T₂, T₃, T₄, T₆, T₇, T₈ and T₁₁ (176.05, 163.30, 189.97, 160.35, 181.51, 196.55 and 170.02 g ha^-1, respectively). In pooled data, significantly higher uptake of Mn in grain was noticed in T₈ (187.44 g ha^-1) and was on par with T₂, T₄, T₅, T₇ and T₁₁ (170.88, 180.06, 183.74, 176.48 and 158.87 g ha^-1, respectively). Treatment T₅ recorded significantly higher uptake of Mn in straw (312.97, 351.12 and 332.04 g ha^-1in first, second season and pooled data, respectively) (Table 4). In first season, this treatment was on par with T₂, T₄, T₇, T₈ and T₁₁ (284.43, 302.95, 295.06, 303.44 and 267.54 g ha^-1, respectively). And this T₅ treatment was on par with T₄ (338.60 and 320.78 g ha^-1) and T₈ (343.85 and 323.65 g ha^-1) in second season and pooled means, respectively.

            Uptake of micronutrient is associated with the metabolic activities of plants and depends on the concentration of nutrients ions and yield. Higher Cu uptake due to high Cu in USWC and vermicompost which increased available Cu content in the soil due to mineralization. [5] reported higher Cu uptake in wheat with application of sewage sludge and compost, due to increased availability of Cu by the composts through mineralization with the successive plant growth stages and the roots ability to retain Cu under conditions of both Cu deficiency and excess. Significant increase in Zn uptake in finger millet might be due to residual effect of urban compost, which upon mineralization released nutrients in the later stages of crop growth thereby resulting increased supply and uptake.

            Higher uptake of Fe by finger millet grain and straw was due to chelating effects of organic compounds produced during mineralization of composts and formation of soluble chelates from the compost that helped in mobilizing the nutrients and increase their availability [2]. [27] reported higher Fe accumulation in residual soybean. Higher Mn uptake in finger millet grain and straw was recorded due to residual effect of P enriched composts which increased availability of nutrients in soil due to mineralization resulting in higher uptake of Mn [2]. [37] reported that the soils with five years’ application of enriched USWC provided more nutrients and microelements to the plant as compared to the control and less than five years’ application of fertilizer. [36] also reported the higher micronutrient uptake by rice using Zn enriched compost.

Micronutrient uptake by residual cowpea

          A significant influence on uptake of micronutrients by residual cowpea was observed due to different P enriched composts and the data is furnished in Tables 5 and 6.

            Significantly higher uptake of Cu in grain was recorded in treatment T₄ (recommended N and K + 50 per cent P supplied through enriched USWC) (47.34, 49.72 and 48.53 g ha^-1) and was on par with T₂ (POP: 100 % NPK + FYM @ 10 t ha^-1) (45.85, 47.21 and 46.53 g ha^-1), T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) (46.61, 48.92 and 47.77 g ha^-1), T₇ (recommended N and K + 50 per cent P supplied through enriched vermicompost) (47.14, 49.03 and 48.08 g ha^-1) and T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (46.99, 49.33 and 48.16 g ha^-1) in first, second season and in pooled data, respectively (Table 5). In second season, T₄ was on par with T₁₁ (recommended N and K + 75 per cent P supplied through enriched FYM) (47.45g ha^-1) also. Among the treatments, T₈ (89.17     g ha^-1) registered significantly higher uptake of Cu in haulm compared to other treatments except T₄ (83.94 g ha^-1) and T₅ (88.71 g ha^-1) in 2018 season (Table 6). Treatment T₅ (88.55 and 88.63 g ha^-1) noticed significantly higher uptake of Cu in haulm compared to other treatments but at par with T₄ (81.68 and 82.81g ha^-1) and T₈ (87.84 and 88.50 g ha^-1) in 2019 season and in pooled data, respectively.

            The increase in uptake of Cu by cowpea grain and haulm may be due to the higher availability of Cu resulting from increased mineralization from the applied enriched USWC, vermicompost and FYM. An increase in uptake of Cu by corn plants was noticed with the application of MSW compost as reported by [26]. Increased plant uptake of Cu was also observed in corn, potato, squash, clover, basil, and swiss chard where plants were grown in soils amended with MSW compost [19, 39, 46]. The results are in consonance with the findings of [40] who reported an increased Cu uptake with the application of urban compost.

            Uptake of Zn by residual cowpea grain was significantly higher in treatment T₇ (recommended N and K + 50 per cent P supplied through enriched vermicompost) in first season (62.73 g ha^-1) and in pooled data (65.09 g ha^-1) (Table 5). This treatment was on par with T₂ (POP: 100 % NPK + FYM @ 10 t ha^-1) (61.17 and 63.19 g ha^-1), T₄ (recommended N and K + 50 per cent P supplied through enriched USWC) (62.66 and 65.07 g ha^-1), T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) (61.66 and 64.28 g ha^-1), T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (61.01 and 63.18 g ha^-1) and T₁₁ (recommended N and K + 75 per cent P supplied through enriched FYM) (59.89 and 61.54 g ha^-1) in first season and in pooled data, respectively. In second season, T₄ (67.49 g ha^-1) had significantly higher uptake of Zn in grain compared to other treatments except T₂, T₅, T₇, T₈ and T₁₁ (65.21, 66.89, 67.45, 65.34 and 63.20 g ha^-1, respectively). Treatment T₈ (74.40 and 77.63     g ha^-1) showed significantly higher uptake of Zn in haulm compared to other treatments except T₄ (69.93 and 75.04 g ha^-1), T₅ (72.35 and 75.46 g ha^-1) and T₇ (66.31 and 70.08         g ha^-1) in 2018 and 2019 season, respectively (Table 6). In pooled analysis, T₈ (76.02          g ha^-1) registered significantly higher uptake of Zn in haulm which was on par with T₄ (72.48 g ha^-1) and T₅ (73.90 g ha^-1).

            Significant increase in Zn uptake in cowpea might be due to residual effect of enriched composts which upon mineralization released nutrients in the later stages of crop growth thereby resulting increased supply and uptake. Increased uptake of nutrients which lead to synergistic effect of Zn concentration and uptake. Similar observations are revealed by [34] who reported higher Zn uptake in corn. Higher Zn uptake by potatoes grown in soil treated with MSW compost has been reported by [29]. Increased Zn uptake in different species with the application of town refuse compost was noticed by [21].

            Irrespective of treatments, significantly higher uptake of Fe in grain was recorded in T₄ (recommended N and K + 50 per cent P supplied through enriched USWC) (92.88, 104.98 and 98.93 g ha^-1) and was on par with T₅ (recommended N and K + 75 per cent P supplied through enriched USWC) (92.16, 104.69 and 98.43 g ha^-1) and T₇ (recommended N and K + 50 per cent P supplied through enriched vermicompost) (90.72, 103.13 and 96.93 g ha^-1) in first, second season and in pooled means, respectively (Table 5). And this T₄ treatment was on par with T₂ (POP: 100 % NPK + FYM @ 10 t ha^-1) (88.29 g ha^-1) and T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (91.62 g ha^-1) in first season and at par with T₈ (95.81 g ha^-1) in pooled means also. Treatment T₈ (509.85, 565.86 and 537.86 g ha^-1) registered significantly higher uptake of Fe in cowpea haulm compared to other treatments but at par with T₅ (506.02, 550.43 and 528.22 g ha^-1) in first, second season and in pooled means, respectively (Table 6). In 2018 season, T₄ was on par with T₄ (475.54 g ha^-1) and T₇ (467.93 g ha^-1) also.

            Higher uptake of Fe by cowpea grain and haulm was due to chelating effects of organic compounds produced during mineralization of composts and formation of soluble chelates from the compost that helped in mobilizing these nutrients and increase their availability. [27] reported higher Fe accumulation in residual soybean. [15] documented increase in Fe uptake by lettuce and barley with the increased application of urban waste compost.

            Similar to Cu, Zn and Fe, Manganese uptake also differed significantly among different treatments. In first season, T₈ (recommended N and K + 75 per cent P supplied through enriched vermicompost) (72.76 g ha^-1) had significantly higher uptake of Mn in grain compared to rest of the treatments except T₂ (POP: 100 % NPK + FYM @ 10 t ha^-1), T₃(recommended N and K + 25 per cent P supplied through enriched USWC), T₄ (recommended N and K + 50 per cent P supplied through enriched USWC), T₅ (recommended N and K + 75 per cent P supplied through enriched USWC), T₇ (recommended N and K + 50 per cent P supplied through enriched vermicompost) and T₁₁ (recommended N and K + 75 per cent P supplied through enriched FYM) (68.83, 68.42, 71.87, 72.29, 71.45 and 68.14 g ha^-1, respectively) (Table 5). Treatment T₄ (78.59 and 75.23 g ha^-1) noticed significantly higher uptake of Mn in grain and was on par with T₅ (78.00 and 75.15 g ha^-1), T₇ (77.67 and 74.56 g ha^-1) and T₈ (77.34 and 75.05 g ha^-1) in second season and in pooled data, respectively. In 2019 season, T₄ was on par with T₁₁ (73.74 g ha^-1) also. Among the treatments, significantly higher uptake of Mn in cowpea haulm was recorded in T₅ (341.48, 335.75 and 338.61 g ha^-1) and was on par with T₄ (320.65, 319.63 and 320.14 g ha^-1) and T₈ (324.33, 321.62 and 322.97 g ha^-1) in first, second season and in pooled means, respectively (Table 6). In second season, T₅ was on par with T₈ (321.62 g ha^-1) also.

            Manganese uptake by cowpea grain and haulm varied significantly with the addition of enriched USWC, vermicompost and FYM which attributed to mineralization of Mn from the organics. These findings fall in line with findings of [42] and [18] who reported that application of MSW compost increased Mn uptake in blueberry leaves and spinach. [37] reported that the soils with five years’ application of enriched composts provided more nutrients and microelements to the plant as compared to the control and less than five years’ application of fertilizer.

Conclusion

            Under phosphorus rich sandy loam condition, application of recommended N and K along with 50-75 per cent P through rock phosphate enriched USWC and vermicompost had beneficial effect on enhancing soil micronutrient status and uptake after harvest of finger millet-cowpea cropping system as compared to control and package of practices.

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