Effect of in situ incorporation of legume green manure crops on nutrient bioavailability, productivity and uptake of maize

Abstract

Maize, being a nutrient exhaustive crop and also grown as a monocrop season after season, is leading to drastic deterioration of the soil fertility status. Hence green manure legumes initially grown in situ and later incorporated before flowering stage into the soil will help to rejuvenate the soil health. An experiment was conducted at Agricultural Research Station, Vizianagaram for three consecutive years during kharif 2015, 2016 and 2017 to evaluate the effect of green manures on soil health, nutrient bioavailability and yield attributes of the succeeding maize crop. Three green manure legumes viz., sunhemp, dhaincha and green gram were grown in situ and incorporated into the soil before sowing of the maize crop in the respective treatments and different doses of fertilizer levels were applied to study the cumulative effect of green manures and inorganic fertilizers on the growth and yield attributes, economics and nutrient uptake of succeeding maize. Pooled data of all three years proved that the grain and straw yield was found to be the highest in the treatment of in situ incorporation of dhaincha with 100% RDF (65.2 q/ha and 74.0 q/ha). Moreover, the soil available nitrogen was also found highest in the same treatment (230 kg/ha), whereas the available P₂O₅ (73 kg/ha) and K₂O (318 kg/ha) was found highest in the treatment with in situ incorporation of sunhemp with 100% RDF, but these treatments are at par with in situ incorporation of dhaincha with 75% RDF. The soil available Fe was found high in the treatment with in situ incorporation of dhaincha with 100% RDF (12.98 ppm), whereas soil available Mn was found highest in the treatment with in situ incorporation of sunhemp with 100% RDF (12.43 ppm) but at par with in situ incorporation of dhaincha with 100% RDF and in situ incorporation of dhaincha with 75% RDF. The N and K uptake by maize plant was found highest in the treatment with in situ incorporation of dhaincha with 100% RDF (110.0 kg/ha and 87.5 kg/ha) whereas the uptake of P was found highest in the treatment with in situ incorporation of sunhemp with 100% RDF (39.1 kg/ha), but both the treatments are at par with each other in the uptake of N, P and K nutrients and significantly higher than sole crop of maize with 100% RDF.

Keywords:

• biomass
• green manures
• in situ
• incorporation
• nutrient uptake
• soil available nutrients

Introduction

Maize, a crop of worldwide economic importance, provides approximately 30% of the food calories to more than 4.5 billion people in 94 developing countries and considered to be the third most important food crop among the cereals in India and contribute to nearly 9% of the national food basket (Dass et al. 2012). Strong market demand and resilience of maize to abiotic and biotic stresses have increased the area and production of maize in the country over the past decade. The productivity of maize however has not increased proportionately and significant yield gaps are evident across the maize growing areas. Hence, maize yields in India need to be increased significantly to sustain this growth rate to meet India’s growing food, feed and industrial needs.

Maize being an exhaustive crop removes large amounts of plant nutrients from the soil to support high biomass production. Nutrient removal is far excess of their replenishment under intensively cropped cereal systems in India, which has led to widespread multi-nutrient deficiencies in soils, continuously increasing the response of crops to nutrient application (Majumdar, Jat, and Shah 2012). Maize crop has better yield response to chemical or inorganic fertilizers, hence heavy doses of these fertilizers are applied to maize. Though these practices help to a temporary increase in the production of crop, deterioration of natural resources (viz. soil, water and air) is also another side of such high input intensive cultivation. Over-reliance on the use of chemical fertilizers has been associated with a decline in soil physical and chemical properties and crop yield (Hepperly et al. 2009) and significant land problems, such as soil degradation due to over-exploitation of land and soil pollution caused by high application rates of fertilizers and pesticide application (Singh 2000), which generally happens in maize cultivated soils.

The low cost and readily available chemical fertilizers have pushed the use of organic manures, including the use of green manures which are the traditionally important source of nutrients. However, in the phase of continuous energy crises, increasing fertilizer prices and growing concern for environmental quality, in recent years, a tremendous renewal of interest was in the old practice of green manuring. Another issue of great concern associated with this practice is the sustainability of soil productivity as it is subjected to intensive cropping systems. Numerous studies have confirmed that excessive application of chemical fertilizers is now a dominant force behind many environmental threats, such as soil degradation and freshwater eutrophication (Sutton et al. 2013). In this concern, Integrated Nutrient Management is a holistic production management system that promotes and enhances health of agro-ecosystem related to bio-diversity, nutrient bio-cycle and soil biological and microbial activities (Subba Rao et al. 2013). Green manuring is one such practice of enriching the soil by plowing under or soil incorporation of any green manure crops while they are green or soon after they start flowering and the value of green manuring lies in the fact that organic matter is incorporated into the soil. The leguminous green manure crops viz., sunhemp, dhaincha, green gram, black gram etc. are highly valued for their fast growing, producing large quantity of plant biomass as excellent cover crops with root nodules and capability to fix atmospheric nitrogen. Green manures are fertility-building crops and may be broadly defined as crops grown for the benefit of the soil. Green manure in situ incorporation can contribute to the production of more food without increasing chemical fertilizer applications (Nawaz et al. 2017; Xia et al. 2018; Yang et al. 2019), and can also alleviate the soil degradation caused by intensive and continuous conventional tillage (Lou et al. 2011). The extraordinary benefits credited to these leguminous cover crops include an increase in the organic matter content and improve the humus, organic carbon, nitrogen and soil microbial growth. Green manure being legumes can provide N to soils through biological N fixation and can increase the soil N supply to subsequent crops (Thorup-Kristensen, Magid, and Jensen 2003). However, leguminous green manure with high N concentrations and a low C/N ratio often decomposes quickly, and peak net N mineral accumulation in soil occurs at approximately 2–4 weeks after incorporation (Zhou et al. 2019). Green manuring crops also help to increase the supply of available plant nutrients and improvement in the microbiological and physical properties of the soil. These crops improve soil structure and help in reducing soil erosion as well as leaching losses. It is also reported that green manuring crops help to suppress weeds, reducing pest and disease problems, providing supplementary animal forage. Among all these the role of green manures grown in situ and incorporated into the soil is supplying plant nutrients, particularly nitrogen is most prominent.

Besides all these, it is also the fact that the optimum yield level of maize production cannot be achieved by using only organic manures because of their low nutrient content. Efficacy of organic sources to meet the nutrient requirement of crops is not as assured as mineral fertilizers, but the joint use of chemical fertilizers along with organic sources is capable of improving soil quality and higher crop productivity on long-term basis. The highest productivity of crops in a sustainable manner without deteriorating the soil and other natural resources could be achieved only by applying an appropriate combination of different organic manures and inorganic fertilizers (Chandrashekara et al. 2000). It is important to identify the best type of available organic resources which can be used as fertilizers and their best combination with the appropriate proportion of inorganic fertilizers. Keeping this point in view, the present investigation was conducted to study the cumulative effect of in situ incorporation of green manures (sunhemp, dhaincha, green gram) in combination with different doses of inorganic fertilizers for maximum production of maize with higher income level in sustainable manner by improving the soil qualities.

Material and methods

A field experiment in Randomized Block Design which consists of 10 treatments with three replications was conducted at Agricultural Research Station, Vizianagaram, for three consecutive kharif seasons during 2015, 2016 and 2017. The district has a tropical climate with mean temperatures ranging from 26° C to 38° C and an average annual rainfall of 1033 mm (Table 1). The experimental site was located at18°-‘7”-25N latitude and 83°-‘23”-56E longitude and the soils of the experimental field was sandy loam in texture, neutral pH (6.89), low in organic carbon (0.45%) and available N (193 kg/ha), high in soil available P₂O₅ (61 kg/ha) and medium in available K₂O (266 kg/ha). Maize hybrid DHM 117 was sown in the second fortnight of July on flat bed at a spacing of 60 cm × 20 cm with a seed rate of 20 kg/ha. Different treatments of the experiment were T1: Sole crop of maize with 100% RDF, T2: In situ incorporation of sunhemp with 50% RDF, T3: In situ incorporation of dhaincha with 50% RDF, T4: In situ incorporation of Green gram with 50% RDF, T5: In situ incorporation of sunhemp with 75% RDF, T6: In situ incorporation of dhaincha with 75% RDF, T7: In situ incorporation of Green gram with 75% RDF, T8: In situ incorporation of sunhemp with 100% RDF, T9: In situ incorporation of dhaincha with 100% RDF, T10: In situ incorporation of Green gram with 100% RDF. The RDF used in treatments was 200-60-50 kg/ha of N, P₂O₅ and K₂O.

Table 1. Total rainfall (mm) and maximum temperatures (0 C) from kharif 2015–2017.

	2015		2016		2017
	Total RF	Max Temp	Total RF	Max Temp	Total RF	Max Temp
January	6.0	27.1	0.0	28.1	0.0	26.9
February	0.0	30.1	0.0	30.8	0.0	29.8
March	31.0	32.5	0.0	33.6	87.6	32.1
April	34.6	34.1	0.0	37.9	1.0	35.2
May	23.4	36.8	183.6	35.2	48.0	35.7
June	391.6	30.8	215.6	31.2	58.6	31.7
July	109.9	33.0	150.0	29.8	72.0	31.2
August	268.1	30.9	169.4	30.5	291.0	29.9
September	138.1	30.7	358.2	28.8	142.0	30.2
October	43.0	31.6	43.6	30.1	143.6	29.7
November	80.0	28.8	2.0	29.0	6.0	28.3
December	0.0	28.5	0.0	27.9	0.0	26.5
Avg/Total	1125.7	31.2	1122.4	31.1	849.8	30.6

In situ incorporation of green manures was done with leguminous green manure crops such as sunhemp, dhaincha and green gram in their respective treatment plots. These leguminous green manure crops were sown 60 days before the sowing of maize crop, grown in situ and then incorporated in the soil at the time of flowering. Prior to incorporation, the biomass of each green manure crop and the nutrients supplied were quantified. Plant protection measures and irrigations whenever required were provided in the same manner for all the treatments. Doses of NPK nutrients were supplied to each plot respectively as per the treatments. Nitrogen was applied in the form of urea in 3 equal split doses at the time of sowing, knee high stage and at the time of tasseling (i.e., at 60 DAS). Phosphorus was applied as Single Super Phosphate as basal dose and potassium in the form of Murate of Potash in 2 split doses, at the time of sowing and tasseling. Regular biometric observations were recorded at specific time intervals by selecting randomly five plants in each treatment. Crop was harvested at 115 days after maturity of the cobs and growth parameters viz., plant height, number of grains/cob, cob length and yield attributes were recorded from net plots. The initial soil samples were collected before sowing of the crop at 0–15 cm depth and the final soil samples were collected after harvest of the crop during all three years. The soil samples were analyzed for pH, EC, Organic Carbon (Walkley and Black 1934), available Nitrogen (Subbiah and Asija 1956), available Phosphorus (Olsen et al. 1954) and available Potassium (Merwin and Peech 1951). The DTPA – extractable Zn, Cu, Mn and Fe in soil were estimated using Atomic Absorbtion Spectrophotometer as per the procedure of Lindsay and Norvell (1978). Plant samples were digested in diacid HNO₃: HCLO₄ (9:4) mixture (Jackson 1973) for estimation of P, K and micronutrient contents, whereas as the plant N content was analyzed by digestion and distillation using Kelplus N auto analyzer. The trend of observations was similar in all three years, hence data was subjected to pooled analysis for interpreting the results.

Results and discussion

The experiment was conducted basically to study the cumulative effect of in situ incorporation of legume green manure crops (sunhemp@50kg/ha/dhaincha@30kg/ha/green gram @20kg/ha) integrated with different doses of inorganic fertilizers (50%, 75% and 100% RDF) on growth, productivity, uptake of maize and changes in the physicochemical properties, organic carbon build-up and available macro and micronutrients of the soil.

Green manure biomass incorporation

Before incorporation, the biomass of the green manure crop incorporated was quantified and the total nutrients supplied by each legume crop was also analyzed through standard procedures (Table 2). Moreover, a comparison between the three legume green manure crops was also studied based on the biomass produced and incorporated. It was observed that the fresh biomass incorporated (Table 4) and the quantity of nutrients added to the soil after incorporation was significantly higher in dhaincha crop (T3, T6 and T9) in all three years followed by sunhemp and then green gram. Similar results were also reported by Lokesh Dubey, Dubey, and Jain (2015) stating that dhaincha and sunhemp have a higher rate of biomass production and both can produce dry matter to the extent of 16 to 19 t/ha within a short period of 45–60 days.

Table 2. Nutrient composition of legume green manure crops incorporated.

Organic source	Fresh biomass accumulation (q ha^-1)	Dry matter ( q ha^−1)	Nutrient concentration (%)			Nutrient accumulation (kg ha^−1)
			N	P	K	N	P2O5	K2O
Sunhemp	155	47	2.30	0.50	1.80	108	24	85
Dhaincha	175	52	2.80	0.52	1.18	146	27	61
Greengram	143	42	2.21	0.26	1.26	93	11	53

Table 3. Effect of green manure incorporation on growth of maize.

Treatments	Plant height (cm)				No. of grains/cob				Cob length (cm)
	2015–16	2016–17	2017–18	Mean	2015–16	2016–17	2017–18	Mean	2015–16	2016–17	2017–18	Mean
T1	231.4	176.0	172.0	193.2	410	323	289	341	16.0	15.0	15.0	15.3
T2	209.9	190.0	181.0	193.6	311	296	270	292	13.4	14.6	14.2	14.1
T3	212.8	195.0	186.0	198.0	324	308	276	303	13.9	15.1	14.6	14.5
T4	201.0	185.0	182.0	189.2	308	277	267	284	12.6	14.5	14.1	13.7
T5	226.3	207.7	192.0	208.6	362	329	315	335	15.1	17.4	15.6	16.0
T6	228.6	214.3	202.0	214.9	389	334	328	350	15.6	17.7	15.8	16.4
T7	224.7	212.3	193.0	210.2	341	314	293	316	14.6	16.5	15.2	15.4
T8	231.7	218.0	200.0	216.4	416	351	323	363	16.8	17.5	16.0	16.8
T9	236.4	220.0	210.0	222.2	439	380	337	385	18.0	18.1	16.4	17.5
T10	230.8	209.0	198.0	212.5	402	345	319	356	16.2	16.6	15.9	16.2
Mean	223.37	202.1	191.0	205.8	370.23	326	301	332.6	15.2	16.3	15.3	15.60
SEm	7.47	7.26	5.58	3.88	16.46	18.3	10.89	8.82	1.045	0.87	0.48	0.47
CD (0.05)	22.2(ns)	21.57	16.6	11.2	48.90	54.37	32.3	25.4	3.11	2.61	NS	1.37
CV %	5.80	6.20	5.05	5.74	7.70	9.72	6.25	8.09	11.90	9.33	5.41	9.27

Table 4. Biomass incorporated and effect of green manure incorporation on grain and straw yield of maize.

Treatments	Total Fresh biomass incorporated (q/ha)				Straw yield (q/ha)				Grain yield (q/ha)
	2015–16	2016–17	2017–18	Mean	2015–16	2016–17	2017–18	Mean	2015–16	2016–17	2017–18	Mean
T1	0.0	0.00	0.0	0.0	66.7	58.8	58.6	61.3	59.0	53.4	54.9	55.8
T2	157.5	143.8	167.8	156.3	55.3	51.4	52.9	53.2	51.7	46.1	47.9	48.5
T3	181.4	152.5	186.4	177.8	54.2	53.8	54.3	54.1	53.7	48.4	51.1	51.1
T4	152.4	130.5	137.5	142.8	52.8	47.2	51.2	50.4	49.8	43.1	45.9	46.3
T5	151.2	140.2	162.2	150.2	66.3	65.3	63.4	65.0	60.7	56.5	60.7	59.3
T6	168.7	164.5	185.3	172.8	70.4	66.4	64.7	67.2	64.3	58.7	61.9	61.6
T7	148.7	127.1	144.2	142.7	62.4	60.4	60.3	61.1	56.7	52.5	59.3	56.1
T8	158.9	144.7	172.8	158.8	73.9	70.8	69.4	71.4	67.7	60.6	61.2	63.2
T9	171.1	155.8	193.1	173.4	75.6	74.4	72.1	74.0	70.1	62.9	62.8	65.2
T10	151.6	129.9	136.7	142.2	70.7	69.4	67.3	69.1	65.9	58.2	60.6	61.6
Mean	143.35	130.6	149	140.8	64.8	61.78	61.4	62.7	59.9	54.09	56.6	56.9
SEm	8.67	8.07	11.50	5.38	5.08	2.97	4.51	2.43	5.22	2.95	2.10	2.07
CD(0.05)	25.8	23.3	34.1	15.61	15.09	8.83	13.4	7.01	15.52	8.77	6.24	6.06
CV %	10.48	10.7	13.40	11.73	13.58	9.31	12.71	11.83	15.09	9.46	6.42	11.18

Growth parameters

The cumulative effect of in situ incorporation of different legume green manures and inorganic fertilizers showed that the growth components viz., plant height, number of grains per cob, cob length of maize varied significantly with different treatment combinations (Table 3). Significantly higher growth components viz., plant height, number of grains per cob, cob length were recorded in the treatment with in situ incorporation of dhaincha +100% RDF (222.2, 385 and 16.2 cm, respectively) which was followed by in situ incorporation of sunhemp +100% RDF and in situ incorporation of green gram +100% RDF and was found at par with in situ incorporation of dhaincha +75% RDF and significantly higher than sole crop of maize +100% RDF (193.2 cm, 341 cm and 15.3 cm respectively). The probable reason for recording maximum growth attributes may be due to better availability of major and micronutrients in the plots treated with biomass incorporation, integrated with higher doses of inorganic fertilizers in balanced form during the vegetative phase (Makinde 2007; Rajeshwari et al. 2007). Besides, the biomass incorporated and decomposed might have helped in mobilization of the nutrients from unavailable form to available form as reported by Balyan et al. (2006); Verma, Nepalia, and Kanthaliya (2006); Balai et al. (2011). The other reason might be due to the possibility of more number of leaves per plant in these treatments which led to more leaf area which is a measure of size of the assimilatory system of plant and is a product of leaf length and width which is important for the accumulation and partitioning of photosynthates to the economic parts of the plant (Shambhu Sharan Kumar et al. 2017). The results so obtained in performances may probably be due to the nutrients which are responsible for increased cell division, cell enlargement, growth, photosynthesis and protein synthesis for the quantitative increase in plant growth (Panwar 2008). These findings are also in accordance of those reported by Ghafoor and Akhtar (1991), Balyan et al. (2006) and Ayoola and Makinde (2009) who stated that application of high doses of nutrients in a balanced form integrated with the incorporation of green manures has a significant effect on growth and development of maize.

Yield parameters

Grain yield is the end result of morphological and physiological processes occurring during the growth and development of any crop, including maize. The yields have also shown a similar trend with significantly higher grain and straw yields (Table 4) recorded in the treatment with in situ incorporation of dhaincha +100% RDF (65.2 q/ha and 74.0 q/ha, respectively) which was followed by in situ incorporation of sunhemp +100% RDF (63.2 q/ha and 71.4 q/ha respectively) and in situ incorporation of green gram +100% RDF and was found at par with in situ incorporation of dhaincha +75% RDF (61.6 q/ha and 67.2 q/ha respectively). Yield reduction in maize was observed at 50% RDF fertilizers application in spite of incorporating green manures (dhaincha/sunhemp/green gram) (grain yield of 51.1, 48.5 and 46.3 q/ha respectively) when compared to sole crop of maize with 100% RDF fertilizers application alone without incorporating any green manure (55.8 q/ha). However, dhaincha green manure incorporation at 75% RDF fertilizers recorded significantly higher grain yield (61.6 q/ha) over to that of 100% RDF fertilizers alone (55.8 q/ha). The increase in the grain yield may be due to green manure biomass incorporation in combination with optimal fertilizer treatments which resulted in more number of grains per cob and better grain development due to adequate nutrient supply (Balai et al. 2011; Tasneem et al. 2004).The higher growth attributing traits could also be attributed to sustained availability of macro and micro nutrients which may have taken part in nourishment of the crop and also enhanced the activity of meristematic cells and cell elongation where incorporated green manures providing micronutrients have favorable effect on metabolic process, enzymatic activity and growth hormones which in turn lead to better vegetative growth in terms of higher plant height and number of leaves which eventually contributed to increased dry matter production. Similar results were reported by Shanmugam and Veeraputhram (2000); Iman, Sharanappa, and Yayabharath (2002); Ranjbar and Bahmaniar (2007).

Cost economics

Economic analysis of maize (Table 5) indicated significantly higher net returns (Rs. 61819) and higher B:C ratio in the treatments receiving 75% RDF and incorporation of either dhaincha (3.24) or sunhemp (3.15) when compared to sole crop of maize with 100% RDF fertilizers (2.88) to maize crop, indicating cost reduction of Rs. 995 through saving of 25% RDF fertilizers and additional net returns of Rs. 8,995 over sole crop of Maize+ 100% RDF. Similar findings were also observed by Singh et al. (2005); Godhawale and Dahipale (2007).

Soil physico-chemical properties

Soil pH and EC (Table 6) showed no significant difference between the treatments, whereas the soil organic carbon was significantly high in the treatment with in situ incorporation of dhaincha +100% RDF (0.52) when compared with 100% RDF (0.41) and was at par with all the other treatments incorporated with any of the green manure crop. The buildup of organic content was found high in the treatments incorporated with green manures when compared to 100% RDF. But the increase is more in dhaincha incorporated soil when compared to sunhemp and green gram. The organic manure incorporated has the properties of enhanced soil quality by the way of increasing soil organic carbon and microbial activities (Jat et al. 2013). The similar effects of different organic and inorganic sources of nutrients on the organic status of soil after harvest of the crop were revealed by Tetarwal, Ram, and Meena (2011) in rainfed maize.

Table 5. Effect of green manure incorporation on Cost economics of maize (pooled).

Treatments	Grain yield (q/ha)	Gross Returns (Rs)	Cost of cultivation (Rs)	Net Returns (Rs)	B:C ratio
T1	55.8	80854	28030	52824	2.88
T2	48.5	70394	27289	43105	2.58
T3	51.1	74031	26004	48027	2.85
T4	46.3	67090	25839	41251	2.60
T5	59.3	85944	27289	58655	3.15
T6	61.6	89364	27545	61819	3.24
T7	56.1	81389	27380	54009	2.97
T8	63.2	91623	30380	61243	3.02
T9	65.2	94611	29095	65516	3.25
T10	61.6	89268	28930	60338	3.09

Table 6. Effect of green manure incorporation on physico-chemical properties of soil.

Treatments	pH				EC (dS/m)				OC (%)
	2015–16	2016–17	2017–18	Mean	2015–16	2016–17	2017–18	Mean	2015–16	2016–17	2017–18	Mean
T1	6.97	6.94	6.67	6.86	0.08	0.28	0.13	0.15	0.40	0.42	0.43	0.41
T2	6.82	6.91	6.61	6.78	0.09	0.25	0.12	0.16	0.45	0.44	0.46	0.45
T3	6.70	6.70	6.55	6.65	0.09	0.23	0.11	0.15	0.51	0.48	0.49	0.50
T4	6.88	6.94	6.59	6.80	0.10	0.18	0.12	0.16	0.45	0.44	0.44	0.45
T5	6.83	6.95	6.57	6.78	0.09	0.20	0.11	0.13	0.49	0.47	0.48	0.48
T6	6.97	6.90	6.53	6.80	0.10	0.23	0.13	0.14	0.52	0.52	0.50	0.51
T7	6.87	6.75	6.58	6.73	0.09	0.25	0.14	0.15	0.51	0.48	0.46	0.48
T8	6.65	6.55	6.56	6.59	0.10	0.22	0.14	0.16	0.53	0.54	0.47	0.51
T9	6.87	6.71	6.54	6.71	0.10	0.21	0.15	0.16	0.52	0.53	0.50	0.52
T10	6.84	6.75	6.57	6.72	0.09	0.28	0.14	0.15	0.47	0.48	0.46	0.47
Mean	6.84	6.81	6.58	6.74	0.09	0.23	0.13	0.15	0.48	0.48	0.47	0.48
SEm	0.10	0.09	0.04	0.05	0.01	0.02	0.01	0.01	0.05	0.03	0.01	0.02
CD (0.05)	NS	0.26	0.10	NS	NS	NS	NS	NS	NS	NS	0.04	0.06
CV %	2.57	2.20	0.94	2.27	14.35	14.62	12.64	14.07	16.55	12.05	5.04	12.28
Initial values	6.72	7.28	6.69	6.89	0.08	0.28	0.10	0.15	0.46	0.45	0.43	0.45

Table 7. Effect of green manure incorporation on soil available macronutrients.

Treatments	Soil available N (kg/ha)				Soil available P2O5 (kg/ha)				Soil available K2O (kg/ha)
	2015–16	2016–17	2017–18	Mean	2015–16	2016–17	2017–18	Mean	2015–16	2016–17	2017–18	Mean
T1	189	171	163	175	64	53	60	59	296	286	209	263
T2	187	146	226	187	62	52	72	62	244	311	226	261
T3	173	134	218	175	61	50	76	62	258	272	236	255
T4	132	129	226	162	62	44	67	57	215	263	226	235
T5	202	197	226	208	69	59	74	67	288	335	243	289
T6	207	176	247	210	68	57	79	68	310	322	241	291
T7	194	163	222	193	65	52	70	62	271	310	236	272
T8	220	234	230	228	75	67	78	73	336	369	250	318
T9	258	226	205	230	72	63	84	72	342	344	248	311
T10	214	213	230	219	69	57	75	67	294	332	238	288
Mean	198	179	219	199	67	55	74	65.14	285	314	235	278
SEm	19.47	16.18	13.43	9.34	3.38	4.04	4.13	2.26	25.04	18.67	11.03	11.2
CD (0.05)	57.85	48.08	39.9	27.1	10.05	12.01	12.2	6.33	74.40	55.47	32.7	31.3
CV %	17.06	15.66	10.62	14.43	8.81	12.64	9.73	10.28	15.20	10.28	8.12	11.9
Initial Values	215	188	176	193	63	56	64	61	238	294	265	266

Table 8. Effect of green manure incorporation on soil available micronutrients.

Treatments	Soil available Zn (ppm)				Soil available Fe (ppm)				Soil available Mn (ppm)				Soil available Cu (ppm)
	2015–16	2016–17	2017–18	Mean	2015–16	2016–17	2017–18	Mean	2015–16	2016–17	2017–18	Mean	2015–16	2016–17	2017–18	Mean
T1	1.38	0.99	1.24	1.20	10.28	10.34	8.98	9.87	11.17	9.86	8.25	9.76	1.42	2.60	1.71	1.91
T2	1.33	1.07	1.27	1.22	10.40	10.72	9.11	10.08	10.06	11.79	8.56	10.14	1.41	2.55	1.69	1.88
T3	1.29	1.04	1.23	1.19	10.09	12.30	9.05	10.48	10.24	10.34	8.10	9.56	1.59	2.29	1.66	1.85
T4	1.21	0.95	1.14	1.10	11.54	9.92	9.04	10.16	9.74	9.94	7.70	9.13	1.52	2.04	1.52	1.70
T5	1.28	1.11	1.27	1.22	10.17	11.88	9.42	10.49	11.24	12.07	9.12	10.81	1.49	2.48	1.70	1.89
T6	1.27	1.01	1.21	1.16	10.24	13.01	9.36	10.87	12.28	10.66	8.97	10.64	1.56	2.35	1.67	1.86
T7	1.24	0.95	1.17	1.12	9.83	11.55	9.38	10.25	11.22	9.82	8.26	9.77	1.47	2.31	1.62	1.80
T8	1.53	1.27	1.49	1.43	13.26	14.56	9.54	12.45	13.40	13.42	10.48	12.43	1.86	3.26	2.19	2.44
T9	1.43	1.19	1.38	1.33	13.25	15.96	9.73	12.98	12.51	12.57	9.84	11.64	1.64	3.04	2.00	2.23
T10	1.32	1.07	1.27	1.22	10.42	14.18	9.67	11.42	11.05	11.27	8.78	10.37	1.61	2.24	1.64	1.83
Mean	1.33	1.06	1.27	1.22	10.95	12.44	9.33	10.91	11.29	11.17	8.81	10.42	1.56	2.51	1.74	1.94
SEm	0.12	0.059	0.08	0.053	1.20	1.16	0.44	0.565	1.05	0.618	0.53	0.437	0.17	0.180	0.10	0.086
CD (0.05)	NS	NS	0.25	NS	NS	3.46	1.29	1.63	3.12	1.84	1.59	1.26	NS	NS	0.31	0.25
CV %	15.54	9.62	11.42	12.87	19.03	16.20	8.08	15.86	16.12	9.59	10.52	12.77	18.47	12.41	10.40	13.75
Initial values	1.26	1.07	0.96	1.10	9.34	10.67	10.12	10.04	10.46	11.68	9.30	10.48	1.38	2.14	1.25	1.59

Table 9. Plant macronutrient uptake of Maize as influenced by incorporation of legume green manures.

Treatments	N uptake (kg/ha)			P uptake (kg/ha)			K uptake (kg/ha)
	2015–16	2016–17	Mean	2015–16	2016–17	Mean	2015–16	2016–17	Mean
T1	77.7	80.3	78.9	20.6	22.5	21.6	65.8	59.8	62.8
T2	60.9	65.4	63.1	18.4	29.1	23.7	58.0	48.8	53.4
T3	68.0	64.0	65.9	17.9	25.0	21.4	58.7	50.6	54.7
T4	64.1	51.8	57.9	14.8	17.9	16.4	54.9	38.1	46.5
T5	89.5	87.3	88.4	27.7	38.1	32.9	72.4	67.1	69.7
T6	91.4	89.1	90.3	27.4	31.9	29.6	75.4	70.0	72.7
T7	77.3	71.8	74.6	23.2	24.6	23.9	66.4	59.5	62.9
T8	102.0	107.6	104.8	34.8	43.4	39.1	82.7	75.5	79.1
T9	106.8	113.2	110.0	34.9	38.7	36.8	86.8	88.1	87.5
T10	96.6	92.2	94.4	31.1	26.9	29.0	77.2	70.1	73.7
Mean	83.4	82.3	82.5	25.1	29.8	27.4	69.8	62.8	66.3
SEm	6.63	3.95	3.72	2.88	2.62	2.89	4.99	4.46	3.25
CD (0.05)	19.7	11.74	11.1	8.55	7.8	5.58	14.8	13.3	9.6
CV %	13.8	8.32	11.4	19.8	15.2	17.4	12.4	12.3	12.4

Soil available macronutrients

Increase in the availability of N, P, K nutrients (Table 7) was observed due to incorporation of green manures (sunhemp/dhaincha/green gram). This increase was much pronounced at 100% RDF fertilizers when compared to 50% and 75% RDF fertilizers. Among different green manures, dhaincha incorporation proved superior in improving the availability of nutrients in soils and this is followed by sunhemp and green gram. Soil available nitrogen was found significantly high in the treatment with in situ incorporation of dhaincha +100% RDF (230 kg/ha) when compared with 100% RDF (175 kg/ha) and was found at par with the treatments of in situ incorporation of sunhemp and greengram combined with 100% RDF (228 kg/ha and 219 kg/ha respectively), in situ incorporation of dhaincha +75% RDF (210 kg/ha) and in situ incorporation of sunhemp +75% RDF (208 kg/ha). The increase in soil available N might be due to the direct addition of full or partial N through decomposed green manures and greater multiplication of soil microbes, which could convert organically bound N to inorganic form (Panwar 2008). Similar results have also been demonstrated by Thorup-Kristensen, Magid, and Jensen (2003) and Fowler, Condron, and McLenaghen (2004) that green manures can be used to improve the retention and supply of nitrogen (N). Whereas the soil available Phosphorus and Potassium were found significantly high in the treatment with in situ incorporation of sunhemp +100% RDF (73 kg/ha and 318 kg/ha, respectively) which was at par with in situ incorporation of dhaincha +100% RDF (72 kg/ha and 311 kg/ha respectively) followed by in situ incorporation of green gram +100% RDF, in situ incorporation of dhaincha +75% RDF and in situ incorporation of sunhemp +75% RDF when compared to sole crop of maize with 100% RDF (59 kg/ha and 263 kg/ha respectively). In organically managed or integrated soil systems soil organic P mineralization has been shown to make a significant contribution to plant P requirements (Oehl et al. 2001a), while the inclusion of green manure crops enhances biological P cycling in soil and improves the dissolution and bioavailability of sparingly-soluble phosphates (Cavigelli and Thien 2003; Kamh et al. 1999). Re-mineralization also takes place in the soil due to recycling of microbial P during microbial death and predation and signifies mineralization of recently synthesized organic P (Mary and Recous 1994). The availability of K might be due to addition of K to the available pool of the soil, besides reduction of K fixation and release of K due to interaction of decomposed green manure biomass with clay (Das et al. 2004).

Soil available micronutrients

The soil available micronutrients (Table 8) were also significantly influenced by the cumulative effect of in situ legume green manure incorporation and inorganic fertilizers. It was observed that the soil available Fe was found high in the treatment with in situ incorporation of dhaincha +100% RDF (13.0 ppm), whereas the soil available Mn was found high in the treatment within situ incorporation of sunhemp +100% RDF (12.4 ppm), but both the treatments were at par with each other and were significantly different from all the other treatments. Whereas the soil available Zn and Cu showed a similar trend numerically but no significant difference was observed between the treatments statistically. This can be attributed to the fact that green manures incorporation influence the micronutrient availability in soil through changes in oxidation reduction capacity and releasing micronutrients during decomposition. The results are in accordance with Dubey, Dubey, and Jain (2015) reporting that green manures improve soil structure, letting more air into the soil and improving drainage. The increased availability of Zn due to green manuring might be due to the mining of sub soil Zn. The behavior of Cu in a green manured soil differs from that of Zn because there exists some competition for Cu between the adsorption on the oxides and soluble organic matter presumably with chelating ability. Hence, Copper is present in soil solution at a higher concentration than Zn.

Plant nutrient uptake

Analysis of plant nutrient contents of maize (Figure 1) showed that N and K contents were found high in the treatment with in situ incorporation of dhaincha with 100% RDF (1.61% and 1.46%), whereas plant P content was found high in in situ incorporation of sunhemp with 100% RDF (0.59%) which were however significantly higher than sole crop of maize with 100% RDF (1.37%, 1.25% and 0.37% respectively). Moreover, a similar trend was also observed in the uptake studies. The N and P uptake by maize plant was found highest in the treatment with in situ incorporation of dhaincha with 100% RDF (110.0 kg/ha and 87.5 kg/ha) and uptake of P was found highest in the treatment with in situ incorporation of sunhemp with 100% RDF (39.1 kg/ha), but both the treatments are at par with each other in uptake of N, P and K nutrients (Table 9) and significantly higher than sole crop of maize with 100%RDF. The increase in N uptake could be ascribed to slow and continuous supply of the nutrients, coupled with reduced N losses via denitrification or leaching, in green manure incorporated plots which might have improved the synchrony between plant N demand and supply from the soil (Haile, Dechassa, and Ayana 2012; Tilahun et al. 2013). Available P in soil significantly enhanced due to incorporation of green manures (Mohana Rao et al. 2016) and the increased availability of P resulted in more uptake of P by the maize plant. Yadav, Aslam, and Kushwaha (2005) also reported maximum P uptake when 25% N was substituted by green leaf (Sesbania) manure. Duhan et al. (2001) observed that incorporation of green manure, in general, increased the K uptake in rice grain and straw. Tiwari, Dwivedi, and Dikshit (2002) noticed faster rate of mineralization and greater utilization of nutrients through the use of green manuring in combination with chemical fertilizer. Sarwar et al. 2003 also observed increased uptake of N, P and K by various crops when inorganic and organic sources of nutrients in the form of chemical fertilizer, FYM, green manuring and compost were applied to the soil.

Figure 1. N, P and K contents of maize plant as influenced by incorporation of legume green manures.

Conclusion

The cumulative effect of in situ incorporation of legume green manures along with inorganic fertilizers showed that in situ incorporation of dhaincha with 75% RDF can be considered the best treatment with high B:C ratio which was at par with in situ incorporation of dhaincha with 100% RDF and in situ incorporation of sunhemp with 100% RDF over sole crop of maize along with 100% RDF. Moreover, the soil available nutrients, plant nutrient contents and uptakes were also found highest in the same treatments. Hence, it can be concluded that in situ incorporation of legume green manure crops results in increase in the population of soil microorganisms from the degradation of plant material that aid in decomposition of this fresh material. The increased percentage of organic matter (biomass) improves the soil physical properties viz., water infiltration, retention, aeration and other soil characteristics. Further incorporation of such leguminous cover crops into the soil allows the nutrients held within the green manures to be released and are made available to the succeeding crops.

Acknowledgements

The authors are thankful to Acharya N.G. Ranga Agricultural University, Guntur, Andhra Pradesh, for providing of funds and necessary facilities for conducting this research.

Disclosure statement

No potential conflict of interest was reported by the authors.