Introduction

Silicon (Si) and NPK are important nutrients for rice production (Ma et al. 2001). Adequate available Si uptake enhances the growth of rice by increasing rice tolerance to both abiotic and biotic stresses (Liang et al. 2007; Guntzer et al. 2012). Si availability is key to sustainable rice production, thus inadequate Si can limit rice yield production (Klotzbücher et al. 2014). Generally, tropical soils have low available nutrients including Si due to soil degradation such as erosion, sediment transportation and leaching process (Meena et al. 2014; Li et al. 2014;). Intensive rice crop production can deplete soil available Si (Cornelis and Delvaux 2016) due to high uptake by rice plants (Ma et al. 2001; Makabe et al. 2009). Soil acidification in rice paddies can cause Si deficiency since pH affects Si availability in the soil (Tavakkoli et al. 2011).

A sawah lowland farming, which has a small-scale irrigation scheme adapted for an integrated watershed management, was suggested as a promising strategy to tackle soil erosion and sediment transportation in inland valleys (Hirose and Wakatsuki 2002; Hayashi and Wakatsuki 2002). Sawah is an Indo-Malaysian word for paddy. It is a lowland rice management system that involves bunding, puddling, leveling of soil to ensure good water management via irrigation and drainage (Wakatsuki et al. 2005). Sawah system ensures that certain water level is maintained in field plots during the plant growing period, restores and replenishes soil nutrients through geological fertilization and soil erosion control. The mechanisms of nutrient replenishments in sawah system encourage rice growth and enhance microbial populations, which improve biological nitrogen fixation.

Study has shown that rice-mill wastes can serve as source of Si fertilizer to mitigate Si deficiency in rice-based agriculture (Song et al. 2014b). In southeastern Nigeria, chemical fertilizers are scarce and when available are often very costly for most resource-poor farmers. The use of chemical fertilizers to replenish lost nutrients in cultivated soil is negligible because most low-income farmers cannot afford the costs (Agbede and Kalu 1995).

Application of organic waste is recommended due to the potentials of organic waste to increase soil pH and nutrients and thus reduce heavy mineral fertilizer application to agricultural soils (Unagwu et al. 2013; Unagwu 2014, 2019; Nessa et al. 2016; Joardar and Rahman 2018). Under intensive agriculture, continuous chemical fertilizer application is reported to increase soil acidity which creates nutrient imbalance in the soil (Ojenyi 2000) and consequently reduces crop yield. The possible alternative for smallholder farmers to increase crop yield and achieve sustainable soil productivity is via organic amendment application (Okenmuo et al. 2018; Unagwu et al. 2019). Crop residues improve soil nutrient availability, crop yields (Singh and Singer 2006), improve the overall ecological balance of the crop production (Song et al. 2014b) and enhance soil–water relations of degraded soils (Nwite and Okolo 2016). Lal (2003) observed increased crop yield following organic waste application. Application of crop residues such as rice-mill wastes to agricultural soils improves soil fertility (Lehmann and Joseph 2009), nutrient use efficiency, nutrient cycling, and increases crop yield in rain-fed lowland rice systems (Seng et al. 2004; Song et al. 2014a). Rice-mill wastes can be applied to degraded soils to improve their fertility status (Nwite et al. 2012).

Ebonyi state, southeastern Nigeria, has numerous rice milling industries. These mills produced large quantities of fresh and partially burnt rice-mill wastes (bio-wastes). Despite the huge rice-mill wastes generated and the potential negative effects these wastes can have on the environment, the only means of disposing these wastes is by burning at the various dump sites (Nwite et al. 2012). Thus, this work evaluated the possibility of using the available rice-mill wastes to increase rice production, evaluated the effects of rice-mill wastes on some soil chemical properties and the relationship between the Si nutrient uptake and grain yield of rice.

Materials and methods

Description of the study area

A field study was conducted in lowland and undulating area of Ishiagu, Ebonyi State, Southeastern Nigeria, in 2014 and 2015 cropping seasons. The study area is within latitude 05°56′N and longitude 07°41′E of the derived savannah zone of the Southeastern Nigeria. The area has a wet and dry climate, with a mean annual temperature of about 30 °C and rainfall of 1350 mm, which spans from April to October while the dry season spans from November to March each year. The geology of the study area comprises sandy shales and has fine-grained micaceous sandstones and belongs to the Asu River Group (Ezeh and Chukwu 2011). The soils are Aeric Tropoaquent (USDA 1998) or Gleyic Cambisol (FAO 1988) and have moderate soil organic carbon (SOC), low pH and cation exchange capacity (CEC).

Field preparation

The experimental field was divided into two different main plots. Two rice-mill wastes [rice husk ash (RHA) and rice husk dust (RHD)] constituted the main plots. The main plots were demarcated by a 0.6-m raised bund. Six different levels (0, 2.5, 5.0, 7.5, 10.0 and 12.5 t/ha) of RHA and RHD constituted the sub-plots. The treatments were replicated three times. Poultry dropping (PD) was applied as basal application at 10 t/ha to provide additional N since N content of rice-mill wastes was insufficient for adequate rice growth and yield performance (Nwite et al. 2011). Each sub-plot (6 m × 6 m) was demarcated with a raised 0.6-m bunds, plowed, puddled, leveled, and bunded with inlet and outlet channels for irrigation and drainage. Water was supplied, controlled and maintained via the inlets and outlets channels from 2 weeks after transplanting to the grains ripening stage. RHA and RHD were incorporated manually in the sub-plots at 20 cm soil depth, 2 weeks before transplanting. Oryza sativa var FARO 52 (WITA 4), a high-tillering rice variety, was used as the test crop in this study. The rice seedlings were first raised in the nursery. After 3 weeks, the seedlings were transplanted to the plots (spacing distance was 20 cm × 20 cm) at two seedlings per row. At maturity, rice grains were harvested dry and the grain yield was determined by weight.

Soil sampling and laboratory analyses

Bulk soil samples were collected from 0 to 20 cm depth before treatment application for pre-planting soil analysis (Hence, the reason why PD was added as basal application.). Similarly, after each cropping season (after harvest), soil samples were taken from the amended plots, air-dried and sieved through 2.0-mm mesh and taken to laboratory for chemical analyses. The soil pH was determined in distilled water and 0.1 N KCl solution using a soil solution of 1:2.5 (Mclean 1982). SOC was determined by wet oxidation method (Walkley and Black 1934) as modified by Nelson and Sommers (1982). Total nitrogen (TN) was determined using semi-micro-Kjeldahl methods (Bremner and Mulvaney 1982). Soil available Si was determined in a 1 mol L−1 acetate buffer (pH 4.0) at a 1:10 ratio for 5 h at 40 °C while shaking, occasionally (Imaizumi and Yoshida 1958). Bray II method was used in determining available P (Bray and Kurtz 1945). More so, the nutrient contents of the rice-mill wastes were determined following the above methods.

Statistical analysis

Data generated from the study were analyzed using GENSTAT 3 7.2 Edition. Significant treatment means were separated and compared using least significant difference (LSD) at 5% probability level.

Results and discussion

Soil analysis

Some basic physical and chemical properties of the soil used in the present study as the medium for plant growth before rice-mill waste application are shown in Table 1. The test soil was sandy clay and was associated with low pH (4.8), SOC (11.4 g kg−1) and available Si (31 mg kg−1). The low nutrient status observed for the test soil is attributed to continuous cropping, increased land use intensity and inadequate application of organic amendments.

Table 1 Physico-chemical properties of the soil (0–20 cm) before rice-mill waste application
Full size table

Organic amendment analysis

The composition of rice-mill waste and poultry manure is presented in Table 2. The RHD had the highest SOC, followed by RHA, while PD recorded the lowest SOC. The TN was higher in poultry dropping than for RHA (Table 2). Except for available Ca, the result showed that RHA had higher available K, Mg and P compared with PD and RHD, respectively.

Table 2 Properties of the organic amendment used in the study
Full size table

Further, the C:N in PD was lower than that of RHD and RHA. The high C:N associated with RHD and RHA can negatively affect soil N availability for plant uptake due to N immobilization by the soil microbes. Hence, the reason why PD was added as basal application.

Effects of bio-waste and application rates on soil chemical properties

Soil pH

The soil pH was significantly affected following rice-mill waste application across the cropping seasons (Table 3). In both 2014 and 2015 cropping seasons RHD and RHA had significantly higher soil pH compared with the unamended treatment (0 t ha−1) The amended plots had 2.1–23.2% increase in soil pH when compared with the initial soil pH (Table 1). The high soil pH in the amended plots is attributed to high Ca, Mg and K associated with the rice-mill waste whose effect on the soil can be likened to that of lime (Lickaz 2002). Mbah et al. (2017) reported that soil pH increases following organic waste application.

Table 3 Effect of treatments and application rates on soil pH and soil organic carbon
Full size table

Soil organic carbon (SOC)

Overall, there was significant effect on the SOC with rice-mill waste application. In 2014 and 2015 cropping seasons, SOC in RHA ranged from 6.27–8.20 to 7.38–10.1 g kg−1 and from 8.27–10.4 to 7.63–10.3 g kg−1 for RHD, respectively. The RHA and RHD treatments did not significantly (p < 0.05) affect their SOC content in both cropping seasons (Table 3). Increase in treatment application rates significantly increased the SOC, across both cropping seasons (Table 3). This is due to the high SOC associated with rice-mill waste.

Total nitrogen (TN)

All the amended plots significantly improved the TN relative to the unamended plot. Across treatment application rate, RHA and RHD did not significantly (p > 0.05) differ in their TN content in the first year of application (2014 cropping season), but significantly improved the soil TN in 2015 (Table 4). As anticipated, increase in RHD and RHA application rates significantly (p > 0.05) increased the TN across both cropping seasons (Table 4). The TN in the amended plots was higher than the initial TN (Table 1). This is due to organic amendment applied and due to the sawah system mechanisms that encourage replenishment of nutrients for rice growth, as well as enhance microbial population help in nitrogen fixation (Nwite et al. 2016a). In 2015 cropping season, increase in treatment application rates beyond 10 t ha−1 significantly reduced the TN by 22% and 63%, for RHA and RHD, respectively.

Table 4 Effect of treatments and application rates on total nitrogen, available P and soil available Si
Full size table

Available P

In both 2014 and 2015 cropping seasons, RHA and RHD application significantly affected available P. Relative to the initial available P (5.6 mg kg−1) associated with the test soil (Table 1), in 2014, RHA and RHD treatments increased the mean available P from 5.6 to 11.9–10.7 mg kg−1, respectively, and 14.6–15.4 mg kg−1 in 2015 (Table 4). Across the treatments and application rates, RHA and RHD had significantly higher available P than the unamended plot. The significant increases in available P associated with RHA and RHD compared with the unamended plot is due to high available P in the rice-mill wastes applied. The unamended plots (Table 4) record higher available P content relative to the initial available P (Table 1). This is linked to the contributions of sawah system in improving soil fertility status in lowland rice field. This is because pre-farm activities such as bunding, surface field leveling, irrigation and drainage modifications in the sawah system help in preventing erosion (Nwite et al. 2016b). Sawah system has been reported to have positive effect in restoring/replenishing lowland with nutrients due to geological fertilization (Wakatsuki et al. 2005).

Soil available Si

RHA and RHD treatments and their application rates had significant effect on soil available Si (Table 4). Soil available Si in the amended treatments was not significantly different in the first cropping year of study but differed significantly (p < 0.05) in the second cropping year. RHA treatment mean for soil available Si was significantly higher compared with RHD treatment (Table 4). As anticipated, soil available Si recorded for RHA and RHD in 2014 and 2015 cropping seasons was higher than the initial soil available Si (31.0 mg kg−1). The present results corroborate the findings of Magale et al. (2011) and Lima et al. (2011) who reported that application of increasing amounts of Si-rich sources enhanced greater availability of Si in the soil. Across both cropping seasons, increase in treatment application rates significantly increased soil available Si (Table 4). Soil available Si range was 30.7–88.3 mg kg−1 and 25.7–90.7 mg kg−1 for RHA and RHD amended plots, respectively. RHA and RHD treatments applied beyond 10 t ha−1 significantly reduced the soil available Si. This result suggests that the optimum application level of RHA and RHD treatments required to enhance the soil available Si level in the study area is about 10 t ha−1. The increase in soil available Si from 31 to 192 mg kg−1 following bio-waste application is comparable to the work of Song et al. (2014a) who reported a significant increase in soil available Si (130–270 mg kg−1) after 10 years of manure treatment (rice crop residues) application in Eastern China. When incorporated into the soil, rice husk contains approximately 86% of Si which could be taken up by rice plants (Klotzbücher et al. 2015). Thus, RHA and RHD are potential alternative resources for improving soil available Si.

Rice grain yield (t ha−1) as affected by organic treatments and application rates

Rice-mill wastes and their application rates had significant (p < 0.05) effect on rice grain yield in the 2014 and 2015 cropping seasons (Table 5). The grain yield mean values ranged from 5.05–6.17 to 5.80–6.96 t ha−1 in the 2014 and 2015 cropping seasons, respectively. The amended plots had 89.1–107.4% increased grain yield compared with the unamended plot. Overall, rice yield obtained in the two cropping seasons was above the usual < 2 t ha−1 often obtained by the local smallholder farmers under non-sawah farming system. The higher yield obtained can be attributed to improvement in available Si and soil fertility index following the application of RHA and RHD treatments. The results obtained corroborated to that of Datta and Shinde (1985), who reported that under upland and water-logged conditions, application of Si in rice field increased the rice grain yield. Further, Mukhtar et al. (2012) observed that Si nutrition significantly affected crop growth, physiological attributes and yield parameters.

Table 5 Effect of treatments and application rates on rice grain yield (t ha−1)
Full size table

In a similar study, Nwite et al. (2016a) reported that sawah tillage environments significantly increased rice grain yield than the farmers’ growing environment in a 3-year study.

In 2014 cropping season, RHA applied at 10.0 t ha−1 had a significantly higher (6.62 t ha−1) rice grain yield compared with the other RHA rates applied. Beyond 10 t/ha RHA application rates, there was significant reduction in rice yield. In 2015, the RHA applied at 7.5 t ha−1 had highest (p < 0.05) yield (6.93 t ha−1). There was significant reduction in rice yield at increasing RHA application rates. For RHD, the highest yield (7.61 t ha−1) was obtained at the application of 7.5 t ha−1 in 2014 and 5 t/ha in 2015. The yields obtained at these rates did not vary significantly with increasing RHD application rates. The regression analysis between the rice grain yield and soil available Si was significant at p < 0.05 (Fig. 1). Rice grain yields exhibited quadratic relationship with soil available Si. This result could suggest that the rates below 10 t ha−1 could be sufficient, although further long-time investigation is needed. The observed variations in the soil available Si in relation to rice grain yield across the two seasons following RHA and RHD application suggest that the treatments differ in their mineralization rate which resulted in the observed varied effect of rice grain yield (Klotzbücher et al. 2011). In addition to increasing soil NPK content, this study demonstrates that increasing the available Si of lowland soils can significantly improve rice yield.

Fig. 1
figure 1

Correlation between grain yield and soil bioavailable Si as affected by a RHA and b RHD at cropping seasons. A˜ and B˜ = 2014; A* and B* = 2015 cropping season

Full size image

Conclusion

The study evaluated the effects of RHA and RHD treatments on soil Si availability and other fertility indices on the growth and yield of rice in a sawah-based rice cropping system. Overall, RHA and RHD treatments had significantly higher yield compared with the unamended plot. For RHA, 5–7.5 t ha−1 application rate gave the highest rice grain yield while the highest yield from RHD was obtained between 10 and 12.5 t ha−1 application rates across the cropping seasons. The results obtained indicated that RHA and RHD application increased rice grain yield. These amendments can be a potential source of soil available Si and which can be replace mineral Si fertilizers which are costly and unaffordable to local smallholder rice farmers. Our investigations show that continuous rice-mill waste application can increase the soil available Si, other fertility index and consequently increase rice yield. The effect of rice-mill wastes on rice performance merits long-term studies to further understand the underlying mechanisms associated with Si release, mineralization and its subsequent uptake by rice plant in a sawah managed rice field and to ascertain the optimum bio-waste application rates to increase rice yield.