Introduction

Rice (Oryza sativa L.) was cultivated 7000 years ago in Assam–Meghalaya areas of India and in the mountain regions of Southeast Asia and Southwest China. Nearly 90 percent of the world’s rice is produced and consumed in Asia, which accounts for nine of the top ten rice-producing countries across the globe. India is the largest rice growing country in the world with 42.5 million hectares area; however in production, it is second in the world with 152.6 million tonnes, the first being China with 206.09 million tonnes (FAOSTAT 2021). Rice stands as the predominant cereal crop in the Cauvery deltaic zone (CDZ) of Tamil Nadu, encompassing an area of 1.45 million hectares, equivalent to 11.13 percent of the state's total area.

Rice production in India has increased in past three decades continuously beginning with green revolution but stagnated since 1999. To meet the demand of growing population and maintaining self-sustainability, the present production level needs to be increased to at least 120 million tons by 2030 with an average productivity of 4.03 t ha−1. The increase in production has to be achieved in the backdrop of declining and deteriorating resource based such as land (soil), water, labour, and other inputs without adversely affecting the quality of the environment. This is only possible through intensification of rice cultivation (Biswal et al., 2014). Cropping systems based on climate, soil, and availability of water have to be assessed for realizing the potential production levels through efficient use of available resources. The cropping strategy should ensure ample food provision for households, fodder for livestock, and generate the necessary revenue to cover domestic and agricultural expenses. In Tamil Nadu, the traditional cropping system of rice–rice–pulses demands substantial water for irrigation and nutrient input, resulting in escalated production costs, reduced grain yield, and a lower net income.

The lack of awareness and improper application of nutrient sources, coupled with limited use of organic materials, has led to soil deficiencies not only in essential nutrients but also in deteriorating overall soil health, and the same has been reported in many earlier studies (Babu et al. 2020; Sruthi and Surendran, 2023). Consequently, this has resulted in a decreased crop response to recommended fertilizer doses and diminished positive residual effects on maintaining soil fertility (Surendran et al. 2016). Specifically, cultivating rice–rice–rice crops exacerbates multi-nutrient deficiencies in soils, particularly within rice-based cropping systems. A declining productivity trend has been noted in extensive, long-term experiments across various regions in India involving these kinds of mono-cropping sequences.

Despite its extensive cultivation, the sole production of rice falls short of sustaining farmers' livelihoods within the Cauvery deltaic zone (CDZ). The reasons attributed are low productivity, monsoon failures, pest and disease infestation, labour shortage, etc. Consequently, farmers opt for rice fallow pulse cultivation as a residual crop, requiring minimal additional inputs besides essential agronomic practices like occasional weeding and limited irrigation. This kind of alternate agricultural practice serves as a main source of improved income for farmers during off-seasons or in instances of monsoon failure, bridging economic gaps. Among the various rice-based cropping systems tested, the rice–pulse combination emerges as one of the most prevalent and significant systems within the CDZ, particularly in the coastal ecosystems. TNAU and the Department of Agriculture are promoting such alternate rice based cropping systems to enhance the income of farmers and the cultivation of blackgram pulses within this system has gained a lot of attraction among CDZ farmers as a secondary crop after rice cultivation. Earlier studies showed that selection of blackgram pulses as a residual crop following rice cultivation aids in managing soil fertility levels effectively, especially within specific cropping sequences like rice–pulse (Senthilvalavan and Ravichandran, 2020).

In Pudukkottai district, rice and blackgram are cultivated in an area of 69,143 and 27,067 ha, respectively, and in Tiruchirappalli district 4649 and 3544 ha, respectively. Reasons for low productivity in rice fallow pulses are use of poor quality seeds, poor soil fertility status, low soil moisture availability, non-scientific irrigation management, improper weed management practices, non-availability of water, quality seed materials during the peak season, more incidence of mung bean yellow mosaic virus (MYMV) in the traditional varieties, partial adoption of seed treatment, poor plant population and non-adoption of integrated nutrient management and diammonium phosphate/TNAU pulse wonder and Planofix spray, etc. In a nutshell, many of the recommendations were not being followed by the farmers due to different reasons.

The potential for expanding rice cultivation areas diminishes, because of the pressure on land and hence it becomes imperative to improve the productivity per unit area and complement conventional breeding methods with innovative techniques. The pursuit of self-sufficiency in rice production and the maintenance of price stability are critical political goals in low-income countries. This is due to the pivotal role of rice in ensuring national food security, as well as in creating employment opportunities and generating income for low-income populations (Ghosh et al. 2009). In recent times, concerns have arisen regarding the plateauing yields in rice production and hence researchers are looking for novel initiatives. The System of Rice Intensification (SRI), pioneered by Father Heneri de Laulinie in Madagascar in 1983, has gained significant attention in India and other countries since 1997 as a yield improvement technology. Adoption of this method has the potential to approximately double average rice yields without necessitating changes in cultivars, reduction in water use and purchased inputs (Wang et al. 2003). The integration of new and improved technologies is expected to encourage farmers to discontinue conventional practices and transition to these advanced methodologies (Sharma et al. 2011; Surendran et al. 2021).

To address such challenges and objectives, a participatory field demonstration involving farmers was conducted to investigate the impact of the SRI method on growth, yield, and economic aspects of rice cultivation, aiming to replace conventional practices. The integrated cropping system of green manure–SRI–Pulses presents an alternative approach aimed at enhancing overall system productivity, reducing water demands, and revitalizing soil fertility. This system involves the incorporation of green manure (sunhemp—Crotalaria juncea) directly into the soil, followed by the cultivation of rice using the SRI method. SRI, differing from the conventional flooded rice cultivation, exhibits potential in mitigating challenges related to water scarcity, high energy consumption, and environmental degradation. Subsequently, a distinctive agricultural practice called "rice fallow pulse" is implemented post-Samba rice cultivation, occurring from January to March in the Ponnaniyar sub-basin area. This innovative approach capitalizes on the residual soil moisture available approximately 7–10 days before rice harvest. Under zero-tillage conditions, pulses, notably blackgram, are sown with the aim of optimizing the use of remaining soil moisture for successful crop establishment. Consequently, the current study focuses on the green manure–SRI–rice fallow blackgram cropping system to counteract the declining trend in soil fertility status.

Given these circumstances, it becomes imperative to integrate both organic and inorganic nutrient sources using a scientific approach to yield promising outcomes in rice-based cropping systems. This integrated approach aims to ensure sustained productivity while simultaneously promoting soil health (Senthilvalavan and Ravichandran, 2020). SRI-Rice fallow pulses (RFP) cropping system along with green manure is novel, and this integrated system appears to be holistic and sustainable approach, combining innovative farming techniques to optimize yields, improve soil health, and minimize environmental impacts. The main hypothesis of the farmer participatory demonstration is that SRI method, green manuring and rice fallow pulse will have a positive impact on growth, yield, and economic aspects of rice cultivation, and with these participatory demonstrations, it has been aimed to replace the conventional practices.

Materials and methods

Study area—overview of ponnaniyar

Ponnaniyar sub-basin lies in the south central part of Tamil Nadu (Fig. 1). Ponnaniyar sub-basin comes under the Cauvery river basin. It is bounded by the Bay of Bengal on the east and Palk straight on the south, Karur district on the west, Perambalur, Ariyalur districts on the north-west and Cuddalore district on the north. It has 10,375.94 ha ayacut area which comprises anicut, tanks, and direct ayacut. This sub-basin consists of six blocks, viz. Manapparai, Vaiyampatti, Manikandam, and Tiruverumbur in Tiruchirapalli district and Viralimalai and Annavasal in Pudukkottai district. This basin has 87 non-system and system tanks, 15 channels, and 14 anicuts (Table 1).

Fig. 1
figure 1

Ponnaniyar basin—administrative map

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Methodology

The study was carried out through front line demonstrations (FLD) during kharif,rabi and summer season of 2019–2020 at three blocks, viz. Annavasal, Viralimalai blocks in Pudukkottai district and Manikandam block in Tiruchirappalli district under Ponnaniyar sub-basin of Tamil Nadu of with the active participation of farmers after different extension approaches through regular field visit and interpersonal communication made by the scientists and technical assistant of National Pulses Research Centre, Tamil Nadu Agricultural university, Vamban, Pudukkottai, Tamil Nadu, India.

The selected cropping sequence consists of Green manure (sunhemp), Rice (SRI), and Blackgram (RFP). Initially, sunhemp, a green manure, is cultivated and subsequently incorporated into the soil. Following this, rice is cultivated utilizing the System of Rice Intensification (SRI) method. Prior to the rice harvest, the subsequent crop of blackgram is cultivated. Mean maximum and mean minimum temperatures registered in the years were 38.30 °C and 27.60 °C; 39.60 °C and 28.50C of Pudukkottai and Tiruchirappalli districts, respectively. Totally 639.7 mm rainfall and 616.4 mm rainfall were received in Pudukkottai and Tiruchirappalli district, respectively, during the cropping period. The soil of the experimental site was normal to slightly sodic in reaction (pH 8.35 to 8.61), (EC 0.20 to 0.37dSm−1), clay loamy texture with low to medium in nitrogen ( 128.75 to 350.00 kg ha−1), low to high in phosphorus (10.00 to 38.75 kg ha−1), and low to medium in potassium (193.75–398.44 kg ha−1) contents. The demonstration was conducted in an area of 50 hectares with Ninety-two (92) farmers. The farmer’s practices involved conventional method of rice transplanting of 25-days-old seedlings with (15 × 10) cm spacing at the rate of 2–3 seedlings per hill, 2–4 cm standing water, chemical weeding by application of Butachlor. The improved practice of SRI included transplanting 12–14-days-old seedlings in (25 × 25) cm spacing at the rate of one seedling per hill, weeding by cono weeder with alternate wetting and drying-based irrigation and nitrogen application based on leaf colour chart (LCC). SRI results in reduced use of irrigation water and seed expenses and increase in the grain yield and maintenance of soil health. The crop was transplanted during 1st week September and harvested during 2nd week of January during the study period. Observations on different yield parameters and final crop yield (grain &straw) were taken, and economic analysis was performed by calculating cost of cultivation, gross return, net return, and cost–benefit ratio on the basis of prevailing market price of inputs and minimum support price of the produce.

Apart from SRI, two other improved practices, viz. Green manuring and Pulses, were also demonstrated. Green manure application involved the use of high-quality sunhemp (Crotalaria juncea) seeds at a rate of 50 kg ha−1. These seeds were sown between June and July and incorporated into the soil at approximately 40–45 days after sowing (DAS) as part of the system of rice intensification (SRI).

Nursery management

The high-yielding rice variety TKM 13 was utilized with a seed rate of 7.5 kgha−1, sown in a nursery area covering 2.5 cent ha−1. The seeds were treated with Pseudomonas at a rate of 10 g per kg of seeds, while Azospirillum and Phosphobacteria were applied at 30 g each per kg of seed. To initiate the growth process, pre-sprouted seeds were planted on raised nursery beds, meticulously prepared to resemble garden crop beds. A fine layer of vermicompost, approximately 50 kgs, was applied, creating a base upon which the sprouted seeds were thinly distributed. Another layer of manure was then used to cover the seeds, followed by a mulch of paddy straw. Careful watering was performed using a rose cane to ensure optimal moisture content. Finally, banana leaf sheaths were employed to facilitate the easy lifting and transportation of the seedlings.

Main field preparation

Land preparation is similar to that of irrigated rice cultivation. However, levelling was done carefully so that water can be applied very evenly followed by formation of channels at every 3 m distance to facilitate drainage. Spacing followed was at intervals of 25 × 25 cm, and lines were marked in both directions, using a marker, creating a grid-like pattern. The seedlings were transplanted at the points where these lines intersect. The transplanting process involves the utilization of 14-days-old seedlings, which are transplanted systematically in a square planting pattern of 25 × 25 cm using a rice transplanter. Besides, it has been ensured that the optimal plant population is maintained, with approximately 16 plants per square meter.

Irrigation and water management

The purpose of irrigation is to wet the soil and saturate the soil with moisture, Subsequent irrigation is only when soil develops hair line cracks. The irrigation is followed as per the standard procedure of SRI-based rice cultivation. This kind of alternate wetting and drying of soil results in increased microbial activity in the soil and easy availability of nutrients to the plants.

Weed management

The absence of standing water in the system of rice intensification (SRI) tends to result in more weed growth. To address this issue, a cono weeder was employed at specific intervals: 10, 20, 30, and 40 days after transplanting. While weeding, the practice involved incorporating the weeds into the soil by manoeuvring the weeder between the rows. Weeds situated near the rice hills and tillers were removed by hand manually to ensure their optimal growth. Nitrogen application was carried out using the standard practice of LCC method.

For the cultivation of rice fallow pulse, specifically the Mungbean yellow mosaic virus-resistant Blackgram Var. VBN 8, seeds were utilized at a rate of 20 kg per hectare. These seeds were sown approximately 7 to 10 days before the rice harvesting period, when the soil was having enough moisture condition (waxy condition). This sowing timeframe spanned from the 15th of January to the 3rd week of February across all locations. The maintenance of an optimal plant population was ensured, targeting 32 plants per square meter. Before seeding, the seeds were treated with Pseudomonas at a rate of 10 g per kg of seeds, along with the application of bio-fertilizers such as rhizobium and phosphobacteria, each at 30 g per kg of seeds. During the growth stages, a 1% TNAU Pulse wonder solution was utilized for foliar spraying when the crop reached 50% flower initiation; it is a crop booster sprayed for enhancing the growth and more flower production in blackgram. Pest management strategies were implemented, employing pheromone traps at a rate of 12 traps per hectare against Spodoptera and applying Dimethoate at 500 ml per hectare to combat MYMV and leaf crinkle disease. At the harvesting stage, the seeds were dried until they reached below 10% moisture content and subsequently stored in gunny bags for safekeeping.

Observations recorded

Observations on growth and yield parameters for green manure (sunhemp)-rice (SRI)-blackgram (RFP) cropping system were recorded in kharif, rabi and summer seasons of 2019–20. For green manure biomass kg.m−2 at 45 days after sowing, for rice number of plants m−2, plant height (cm) at harvest, number of productive tillers hill−1, number of filled grains plant−1, 1000 grain weight (g) and grain yield (kg.ha−1) and for rice fallow blackgram plant height at harvest, number of branches per plant and pod length (cm) and yield parameters, viz. number of pods per plant, 100 grain weight and grain yield were recorded at maturity stage as per the standard procedures. The economic indicators of various treatments were worked out by using the current market price of inputs and blackgram yield.

Rice equivalent yield (REY)

Since various types of crops are included in cropping system, it becomes very difficult to compare to economic produce form a sequence with the other of a different nature. Efforts have been made to convert the yields of different crops into equivalent yield of anyone crop and in this case, rice being the major crop, rice is considered and computed the rice equivalent yield. Yield and yield parameters of rice and other crops in the cropping system were recorded. Economic yields of component crops were converted into rice equivalent yield (REY), taking into account the prevailing market prices of different crops in the cropping sequences. The REY values were computed as per the following formula given by Verma and Modgal (1983).

$${\text{Rice equivalent yield }}\left( {kg.ha^{ - 1} } \right) = \frac{{\left( {YCC \, \times \, MPCC} \right) + {\text{Yield of main crop }}\left( {kg.ha^{ - 1} } \right)}}{{{\text{Price of main crop }}\left( {Rs.ha^{ - 1} } \right)}}$$
(1)

where YCC = yield of component crop (kg ha−1) and MPCC = market price of component crop (Rs. ha−1).

The initial and post-harvest soil EC, pH, NPK were analysed using the following standard procedures, the available nitrogen by Subbiah and Asija 1956, available phosphorus by Olsen et al. 1954 and available potassium by Jackson 1973) .

The cost of cultivation was calculated each crop by taking account of prevailing market price of inputs. Local market price was used for grain of rice and blackgram and biomass of green manure (sunhemp) for calculating the gross return and net return. The cost of green manure biomass (sunhemp) is Rs. 0.50 kg−1, rice grains Rs.19 kg−1, and blackgram grains 50 Rs.kg−1 as the prevailing market rate.

Water productivity (Rs.ha−1 mm) was calculated as the ratio of gross income (Rs. ha−1) and total water used. Extension gap was calculated by the formula suggested by Samui, et al. (2000).

$${\text{Extension gap}} = {\text{Demonstration yield}} - {\text{Farmers yield}}$$
(2)

Results and discussion

The technology demonstrations on three crop sequence of green manure–system of rice intensification (SRI)–rice fallow pulses were implemented to increase the soil fertility and also crop productivity and farmers’ income, and the results obtained were discussed.

Growth and yield parameters

Under improved production techniques (IPT), the sunhemp plants exhibited a significant increase and height was observed as 147 cm and this might be due to the enhanced growth and maintenance of an optimal plant population. Similarly, employing the System of Rice Intensification (SRI) led to a greater plant height of 73 cm, an increased number of productive tillers averaging 38.3, and a 1000 grain weight of 23.5 g in rice (Table 2). These favourable outcomes could be attributed to various factors, including seed treatment with bio-fertilizers such as Azospirillum and phosphobacteria, along with bio-inoculants like Pseudomonas fluorescence. Additionally, optimal fertilizer application, cono weeding practices, and efficient nitrogen management guided by the LCC contributed significantly to these improved results.

The incorporation of green manure within the crop sequence played a crucial role in preventing nutrient loss from the soil. Green manure plants have the capacity to draw nutrients from the soil and store them within their biomass. Upon decomposition and mixing into the soil, these plants release the stored nutrients, thus facilitating nutrient recycling and serving as a vital source of nutrients and energy for numerous beneficial soil organisms. Previous studies have demonstrated the release of nitrogen, phosphorus, and potassium in the soil through the decomposition of Crotalaria juncea L (Sinha et al. 2009). Ultimately, this might have helped by providing plant available nutrients and enhancing the growth parameters. However, it is noteworthy that the rate of nutrient release is significantly influenced by climatic factors like temperature and moisture (Sinha et al. 2009), as well as the types of plants (Saria et al. 2018). These findings align with similar conclusions presented by researchers such as Hosaain et al. (2003) and Gupta and Sharma (1991).

Number of pods per plant 39, number of seeds per pod 7, and 100 grain weight 4.6 g (Table 2) were observed in rice fallow blackgram in comparison with conventional method. The reason might be the seed treatment with rhizobium and phosphobacteria and bio inoculants, i.e. Pseudomonas fluorescens, proper irrigation during critical stages, especially flowering, pod formation, and development stages, foliar application of micronutrient and growth regulators and IPM practices for controlling of pest and diseases. The increase in grain and straw yields owing to vermicompost application could be ascribed to better mineralization, leading thereby to higher availability of nutrients, higher occurrence of beneficial micro-organisms, growth-promoting hormones, antibiotics as well as enzymes (Banik et al., 2006). Continuous supply of nutrients in balanced quantity throughout the growth stages enables the plants to assimilate sufficient photosynthetic product and thus increased dry matter accumulation. Therefore, implementation of improved production technologies produced more panicles and grains/panicle with increased test weight, resulting in higher grain yield (Sudhakar et al. 2006).

Table 1 Basic information of the Ponnaniyar sub-basin in Pudukkottai and Tiruchirappalli districts
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Biomass and grain yield

The rice and blackgram yield showed 26% and 25% increase in improved production technologies on GM–SRI–Rice fallow pulses system over the farmer’s practices. In the Green manure–SRI–Rice fallow pulses system, the mean highest biomass yield of sunhemp 21,094 kg ha−1, rice grain yield of 8480 kg ha−1 through SRI method, and  690 kg ha−1 of blackgram yield (Table 3) were obtained and it was higher than the conventional method of cultivation. Incorporation of sunhemp as crop residue enhances both crop productivity and enrichment of soil fertility as well as healthy environment by activating microbes, thus playing a major role in the sustainability of cropping systems. Incorporation of sunhemp can significantly increase rice yield and subsequent crop yield by substituting a part of inorganic fertilizers, while the increase of yield gains and reduction of fertilizer application may be location specific and improved agronomic practices (Shan Huang et al. 2013). This might be due to the production of higher number of effective tillers per plant and higher number of grains per panicle and grain yield which was in conformity with Maiti et al. (2003). Sesbania aculeata can be grown as green manure crop and helps in improving physical properties of soil including infiltration rate and soil organic carbon (Boparai et al. 1992). Most of these green manures, acknowledged for its capacity to form root nodules, fix nitrogen, and thrive in challenging conditions like waterlogged or drought-affected environments, has emerged as a yield enhancement strategy in sustainable agriculture (Ambika et al. 2015). Sunhemp stands out as the primary contributor among green manures in furnishing biomass and nitrogen to agricultural fields (Dubey et al. 2015). Moreover, Crotalaria juncea, commonly known as sunhemp or Tropic sun, is native to India and extensively cultivated in various countries. Often grown specifically for its role as green manure, it is recommended to plough sunhemp before reaching its full flowering stage. As a leguminous plant, Crotalaria is renowned for enriching the soil with nitrogen content for subsequent plants. However, studies indicate variations in nitrogen input when considering the addition of shoot, root, or both to the soil. Research demonstrated that the recovery rate of nitrogen was notably higher in subsequent plants when solely the root was incorporated compared to those with both shoot and root (Choi et al. 2008). The incorporation of Crotalaria juncea as a green manure crop not only enhances yield but also facilitates substantial nitrogen fixation, potentially offering ample nitrogen content to rice plants. Consequently, it stands as a viable alternative for rice farmers facing resource limitations (Dickmann et al. 1996).

In the modern days of agricultural science, crop rotation and green manuring (GM) offer a technology to achieve sustainable production efficiently. Since continuous intensive and conventional cultivation leads to a decline in soil organic matter content, soil nutrients and water holding capacity are affected severely, resulting in a loss of soil sustainability. One of the options to maintain sustainability in agriculture by restoring soil quality (especially in tropical soils) and reclaiming degraded soil is to increase soil organic matter content by green manuring (Kumar et al. 2010, 2014; Chimouriya et al. 2018) because this practice is eco-friendly, non-polluting, and non-hazardous to soil, water, and air (Yang et al. 2018). Therefore, for sufficient production of quality food grains and to feed ever growing population, green manuring practices are promising tools for the better sustainability of agriculture in the longer run and with almost no hazards to the environment (Kumar et al. 2014). Moreover, due to the continuous exertion of soil and incompetent methods of soil management like excessive tillage and burning of vegetative residues, exposure to climatic changes, the accelerated soil degradation has led to reduced crop production (Florentín et al. 2010). The soil management practices to increase fertility and productivity should include an increase in biomass along with reducing its decomposition (Bunch 2012). Green manuring, on the other hand, being a practice in which the undecomposed plant material is turned under the soil to provide organic material and nutrients to the soil, can be considered as a suitable alternative to positively affect the physical, chemical, and biological properties of the soil (Fageria 2007). Giving proper attention and gaining knowledge regarding the composition, rates and placement of green manure crops, the application of fertilizers can be reduced drastically (Schröder 2005) and in most of the cases fertilizers can be saved up to 30%. The potential of green manures is not limited to only improvement of soil quality and increasing the yields of subsequent crops, rather pest management and weed control has also been reported when green manuring practices are followed appropriately (Boydston and Hang 1995; Al-Khatib et al. 1997). However, the increased production cost of planting green manure led to minimize farmers’ acceptance to this approach, which is showing a lack of support for its sustainability (Ntakirutimana et al. 2019).

Delay planting/sowing due to irregular onset of monsoon and non-availability of quality seed of suitable variety cause yield reduction in paddy and blackgram. Injudicious application of fertilizers and hand weeding by the farmers also cause the lower yield in rice (Samant 2015). However, in the current study, improved production technology resulted in higher yield. This corroborates the findings of the Central Rice Research Institute (CRRI) that hybrid Rajalaxmi are superior over Lalat (CRRI 2006). Thus, the large level demonstration might have a positive impact on farming community for increase the production in the district over conventional method of cultivation of rice and blackgram. Similar results were also reported by Mondal et al. (2005) in rice with different cropping systems.

Table 2 Influence on improved production techniques on green manure–SRI–rice fallow blackgram of plant population, yield parameters, and water consumed (pooled mean of 92 locations)
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Table 3 Influence on improved production techniques on green manure–SRI–rice fallow blackgram minimum, maximum and mean yield (pooled mean)
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Soil fertility status

Post-harvest soil pH, EC, available nitrogen, phosphorus, and potassium contents varied with all 92 locations of the GM–SRI–RFP cropping system. Overall mean of the post-harvest soil EC 0.29 dSm−1 and pH 8.02 were changed, when compared to initial soil EC of 0.26 dSm−1 and pH of 8.51. Highest soil available nitrogen and phosphorus and potassium was recorded with GM–SRI–RFP involving cropping systems such as sunhemp–rice–blackgram of 223.14, 27.76, and 312.11 kg ha−1 of N, P, and K, respectively, compared to initial soil available NPK status (212.02, 23.24, and 300.46 kg ha−1 of N, P, and K, respectively) (Table 4). Post-harvest available soil N, P, and K status was high with sunhemp–rice–rice fallow blackgram in the cropping system, even after the removal of adequate quantity of N by the succeeding rice. This might be due to considerable amount of N added through incorporation of biomass of sunhemp as a green manure and N fixed by the rhizobial nodules by rice fallow blackgram resulting in higher post-harvest available soil NPK. Moreover, large quantity of biomass added, its decomposition, synthesis and subsequent mineralization leads to improvement in the organic carbon status of the soil. Post-harvest available soil N, P, and K status was high with sunhemp–rice–rice fallow blackgram in the cropping system, even after the removal of adequate quantity of N by the succeeding rice. This could be ascribed to the increase in the available N, P, and K contents in soil resulting from the increased availability of nutrients which ultimately increased nutrient content in the plant tissues and greater biomass production at higher rates of fertilizer application. Since the uptake of nutrient is a function of dry matter and nutrients content, the increased grain and straw yields together with higher NPK content resulted in greater uptake of these elements (Balasubramaniyan, 2004; Surendran et al. 2016; Mishra et al. 2021). The release of organic acids has increased the soil phosphorus content, which in turn increased the availability of P2O5. The extensive root system of legume improved the physical condition of the soil and the liberated CO2 and organic acids which might have helped in dissolving native potassium in soil and thereby increasing the availability of potassium (Shridhara et al. 2016). Soil available nutrient status was low due to exhaustive nature of the crop and non-contribution to soil nutrition with rice–rice cropping system (Mukundam et al. 2012). Similarly Porpavai et al., (2011) also reported that legumes were potentially important to diversify cereal based mono-cropping into cereal-legume sequences which had nutrient cycling advantages. An increased uptake of nitrogen, phosphorus, and potassium by rice might be due to constant release of nutrients that satisfied the demand of the hybrid rice at every stage of the crop (phenophase) as opined by Sudhakar and Kuppuswamy (2007). This could be attributed to the comparatively lower C/N ratio of vermicompost, which resulted in faster decomposition and release of nutrients as compared to farm yard manure (Pareek and Yadav 2011).

Table 4 Influence on improved production techniques on green manure–SRI–rice fallow blackgram system on initial and post-harvest soil fertility status (pooled mean of 92 locations)
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Organic anions arising from decomposition of organic matter form stable complexes with Fe3+, Al3+ and prevent their reaction with phosphates ions and result in significantly higher available P due to organic sources. Soil exhibited significantly higher available K status under higher rates of organic sources as compared to medium and lower quantities. Most of the K in plants remains in organic form, and the decomposing organic matter may have a solubilizing effect on native soil potassium. These findings are in done conformity with Kharub and Chander (2008). However, N mineralization from GM plants is significantly influenced by soil characteristics such as pH and texture, management practices, climate, age of the plants, and their C/N ratio (Nagarajah et al. 1989; Singh et al. 1992; Kumar and Power 2018). P availability may be increased in soils of high P fixing capacity because P adsorption sites may be masked by organic compounds that are synthesized during GM decomposition (Easterwood and Sartain 1990). Moreover, organic compounds may also release P from adsorbed sites through anion exchange phenomenon (Kafkaf et al. 1988). Therefore, in general GM decreases soil P sorption (Singh and Jones 1976; Bumaya and Naylor 1988). This might be due to subsequent P sorption on soil and more utilization of available P by the soil microorganisms (White and Ayoub 1983).

Rice equivalent yield (REY)

The mean higher rice equivalent yield of 2766 kg ha−1 was obtained when green manure–rice–rice fallow pulses cropping system than the conventional system (Table 5). The highest REY was registered in Annavasal and Viralimalai blocks than Manikandam block, whereas minimum rice equivalent yield of 2008 kg ha−1 in N K pattu village of Manikandam block was noticed with rice–blackgram system. This might be due to supply of nutrients through in situ incorporation of sunhemp, higher production potential of SRI–rice fallow–blackgram along with the good market price of rice and blackgram that yielded better grain yield than conventional method. The higher rice equivalent yield indicates that the residual advantage of a sunhemp crop on the succeeding rice–blackgram besides contribution in total system productivity. These results are in conformity with finding of Sharma et al., (2008), who reported that inclusion of legume during summer/rabi in rice-based cropping system resulted in an increased in productivity and profitability (Shridhara et al. 2016).

Total water productivity

Among the eleven villages of Pudukkottai and Tiruchirappalli districts, the higher water productivity was observed in Punganur followed by N.K.pattu villages of Manikandam block in Tiruchirappalli district and least water productivity was recorded Kothirapatti village of Annavasal block in Pudukkottai district under IPT. The mean higher water productivity of 0.7087 kg ha−1 m−3 (Table 5) was observed under improved production techniques in GM–SRI–RFP system than conventional method (0.2512 kg ha−1 m−3). Similar kind of improved production technology that results in improved water productivity is reported by Surendran et al. 2021).

Extension gap

The extension gap (Table 5) was higher (1105 kg ha−1) in improved production techniques of green manure–system of rice intensification (SRI)–rice fallow blackgram. The results confirmed that if improved production technologies are adopted, it will subsequently change this alarming trend of galloping extension gap (Samant 2014). The new improved technologies will eventually lead to the farmers to change the attitude of the farmers avoid the use of old varieties and to adopt new variety apart from the adoption of improved production technologies. Similar results were reported by Sharma et al. (2011).

Table 5 Effect of green manure–SRI–RFP demonstration on mean biomass yield, grain yield of rice and blackgram, rice equivalent yield, total water productivity and extension gap
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Economics

The higher mean gross return of Rs.10,200, 151,283, and 48,134 ha−1 of sunhemp as Green manure, rice SRI, and rice fallow blackgram was obtained, respectively (Table 6), in improved production techniques over conventional method, i.e. farmer’s practice. A higher cost–benefit ratio (1.66, 4.23 and 2.91 of green manure–rice–blackgram, respectively) was found in improved practice due to higher net return as compared to the conventional method (0,2.87 and 2.38) attributed to more grain production. The variation in net return and benefit–cost ratio may be attributed to the improvement in the yield and variation in the price of agri-inputs and produce. This might be due to the higher productivity of rice and blackgram grains. Many earlier studies also showed that gross returns were higher in SRI (Bharathy 2005).

Table 6 Effect of frontline demonstration on cost of cultivation, gross return, net return, BCR of green manure, SRI-RFP cropping system in Ponnaniyar sub-basin
Full size table

Lesson learned by farmers and the impact of the study

Traditionally, farmers had been employing a rice–rice–pulses cropping system, which demanded higher water and nutrient resources and suffered from inadequate irrigation and weed management practices. However, with the introduction of improved production technology (IPT) focusing on green manure–SRI–pulses, farmers gained substantial knowledge regarding the cultivation method of green manuring, particularly sunhemp, and learned effective techniques for its incorporation into the soil. Additionally, they received guidance on proper soil sample collection methods and the impact of these technologies on soil health, when the results of post-harvest soil nutrient status were shared with them (photos attached as supplementary file).

Incorporating the system of rice intensification (SRI) led to reduced input usage, such as lower seed rates and the implementation of alternate wetting and drying practices, along with nitrogen application guided by LCC assessments. Furthermore, farmers acquired extensive knowledge regarding the utilization of high-yielding blackgram varieties resistant to MYMV, along with appropriate seed rates and seed treatment procedures involving Imidacloprid, Pseudomonas, Rhizobium, and phosphobacteria. They also learned the application of TNAU Pulse Wonder as a foliar spray for optimal crop growth (Table 5).

Through the implementation of improved production techniques aimed at enhancing soil fertility with the incorporation of sunhemp, improved methods for rice cultivation (SRI), and the production of rice fallow blackgram, farmers have gained invaluable technical knowledge that has significantly altered their approach to adopting appropriate production technologies. According to the survey conducted across 50 hectares involving 92 farmers in the demonstration, the impact has transferred to an area of 170 hectares, engaging 250 farmers, specifically concerning the practices of green manure–SRI–rice fallow pulses.

Many farmers expressed their initial reluctance to adopt treated seeds, maintain appropriate plant densities, and cultivate high-yielding rice varieties resistant to blast disease and MYMV in blackgram. Additionally, they were unaware of the precise quantity of spray fluid required for both growth regulators and pesticides. However, they now possess a comprehensive understanding of these technologies and their substantial impact on crop yields. As a result, they have observed reductions in water usage and seed costs, while experiencing enhanced grain yields and improved maintenance of soil fertility, thereby rendering their agricultural practices economically viable.

Conclusion

The decline in rice production in Tamil Nadu has been a concern in recent times. Addressing these challenges requires concerted efforts involving water management strategies, implementing climate-resilient agricultural practices, providing education and support to farmers regarding modern techniques, encouraging sustainable farming practices, etc. With this as background, improved production technologies were demonstrated in farmers' fields. The demonstration results showed that the prevailing farming practices among farmers can be effectively replaced by adopting the green manure–SRI–rice fallow system, especially in irrigated conditions, to attain elevated productivity per unit area and increased income. Farmers, upon receiving training about GM–SRI–rice fallow pulses, reported a remarkable 26% and 25% surge in rice and blackgram productivity, respectively, compared to the conventional method of rice–rice–pulses cultivation. Consequently, this led to a substantial increase in net income. The extension gap also has been identified and can be addressed by adopting improved production techniques, focusing on high-yielding varieties. Newly released high-yielding and disease-resistant varieties are anticipated to substantially increase production and profits. This includes the utilization of existing high-yielding rice variety TKM13 and blackgram variety VBN8. Moreover, soil physico-chemical properties and the availability of macro nutrients in the soil after the harvest were improved by the implementation of the sunhemp–SRI–rice fallow blackgram system compared to the conventional cropping system of rice–rice–rice fallow pulses. The highest soil available nitrogen, phosphorus, and potassium levels were observed with the GM–SRI–RFP system, specifically with the sunhemp–rice–blackgram cropping sequence, recording increments of 4.98%, 16.28%, and 3.73% for N, P, and K, respectively, compared to the initial soil available NPK status.

In a nutshell, integrating green manuring into the SRI method alongside pulse cultivation, farmers can experience improved yields due to enhanced soil fertility, better nutrient availability, weed control, and improved soil structure and moisture retention, leading to overall improved crop productivity and sustainability.