Suitability of sand amended with carbonized rice husks and goat manure as a growing medium

A laboratory experiment was conducted at Rwanda Agriculture and Animal Resources Development Board (RAB). The study determined a suitable and alternative growing medium to peat moss as an unaffordable medium to Rwandan farmers and soil-based growing media which produce low quality transplants. Growing media were formulated from sand (S) amended with carbonized rice husks (CRH) and goat manure (GM) and peat moss (PM). Nine treatments were formed and applied in a completely randomized design (CRD) with four replications. The growing media were analysed for physical and chemical properties including bulk density (BD) and total porosity (Po), organic matter (OM), organic carbon (OC), total nitrogen (TN), phosphorous, potassium, pH and electrical conductivity (EC). Data obtained were subjected to analysis of variance and means were separated using HSD test at p ≤ 0.05. The results revealed that there were significant differences among the formulated growing media but not in the pH. T 5 , T 6 , T 7 , T 8 and T 9 resulted in good physical and chemical properties as growing media for crop production. Institute, 2010). The statistical model fitted for this experiment was: , where Yij = overall observation, = overall mean, = effect due to treatment, = A random error associated with the response from the j th sample of the i th treatment, i : 1, 2, 3, 4, 5, 6, 7, 8 and 9.


INTRODUCTION
A growing medium is a substance through which plant roots grow, extract water and nutrients (Landis et al., 1990). Growing medium is made from mixtures of components which provide water, air, nutrients and support to plants (Robbins and Evans, 2011). The soilbased growing medium usually results in reduced productivity of the crop (Baiyeri and Mbah, 2006) since natural soils are associated with imbalance of microorganisms, water and air, nutrition, variability and weeds leading to seedling stress exposure (Landis et al., 1990). The soilless media (organic or inorganic) improve crop yields over soil-based because they increase and easily optimize rates of water, nutrient, and oxygen transport (Blok et al., 2017). The use of organic media is advantageous as they are naturally sourced, cost effective in comparison to inorganic substrates and provide additional nutrients to the crop (Raviv et al., 2002). However, they are biologically unstable as they degrade over time leading to compaction and reduced aeration for plant roots (Olle et al., 2012). Common organic substrates include compost, coconut coir, peat moss, bark, rice hulls or sawdust, which are locally available materials that are lightweight, have high waterholding capacity and CEC, and some contain limited quantities of mineral nutrients (Landis et al., 1990). Peat moss is widely used due to its ideal physical and chemical characteristics (Asaduzzaman et al., 2015). However, it is an expensive material to small scale farmers especially in developing countries (Kuisma et al., 2014) and its use has severe environmental implications because it is a non-renewable resource (Maher et al., 2008). In addition, its extraction releases sequestered carbon, destroys natural habitats and degrades the quality of groundwater (Dunn and Freeman, 2011) while sand improves the bulky density and aeration of the growing medium (Raviv et al., 2002). Carbonized rice husks improve soil properties by enhancing soil water and nutrient retention, carbon sequestration, greenhouse gas emission reduction and boosting crop yield (Oladele et al., 2019). Goat manure on the other hand has essential elements required for plant growth including N, P, K, Ca and Mg (Mowa et al., 2018). The use of peat moss is limited because it is expensive and unavailable to most farmers in Rwanda as a developing country (Kuisma et al., 2014). Consequently, most Rwandan farmers use field soils as growing medium for producing transplants and result in poor quality because the natural soils are associated with soil-borne pests and diseases, water, air and nutrients imbalance (Baiyeri and Mbah, 2006). Therefore, the formulation of an alternative growing medium from a mixture of sand amended with carbonized rice husks and goat manure as locally available substrate should be a response of unaffordability of peat moss. The objective of this study was to determine the suitable and alternative growing media to peat moss and soil-based growing media for quality transplants production for Rwandan farmers. We hypothesized that the mixture goat manure and carbonized rice husks with sand substrates will result in growing medium with better levels of chemical and physical properties required for production of quality transplants.

Study site
The study was conducted at Rwanda Agriculture and Animal Resources Development Board (RAB), Rubona Station/ Southern province in Analytical Laboratory for Soil, Plant and Water Analysis. The station is located in Rubona village, Kiruhura Cell, Rusatira Sector, Huye District of Southern province, on longitude of 02946'475''E, latitude of 0229'327''S and at an altitude of 1727 m above sea level. The average annual rainfall and temperature are 1039 mm and 19C, respectively (Ndayambaje et al., 2013).

Preparation of carbonized rice husks
Carbonization of the rice husks was done outdoor with the following materials: a round holed (1 cm diameter) tin of 10L volume and holed (1cm diameter) metal chimney of 25 cm in diameter, dried eucalyptus firewood, shovel, natural water, and watering can, candle and match box. The firewood was filled in the tin and Nkurunziza et al. 11 chimney was fixed to the top of the tin. Fire was set on the woods inside the tin, and the rice husks were piled around the tin until half of the chimney was covered. As the rice husks next to the tin were turning black, they were frequently turned over to prevent them from complete burning to ash until all were carbonized. After carbonization, water was immediately sprinkled over the entire pile using watering can to avoid continuous burning (Sarian, 2008). Thereafter, the CRH was broken up into small pieces by hands to increase the chance of water and nutrient-holding capacity.

Collection and preparation of peat moss, sand and goat manure
Peatmoss was purchased from Holland Greentech-Rwanda. Goat manure was prepared from goat droppings collected from loafing shed located around the Rwanda-Israel Horticulture Centre of Excellence (HC o E). Thereafter, they were air-dried until a constant weight was reached and then ground using a mortar. Sand was collected from Rusine river and sieved with 2mm sieve to get medium to coarse sand. Thereafter, the obtained sand was washed to flush out any salt content and then air-dried to remove any water that remained. Topsoil was collected in a field located around the experimental site using diagonal sampling method to obtain a composite sample. Thereafter, the sample was air-dried for one week and then subjected to oven sterilization.

Physical characterization
Bulky density was determined by inserting into the medium a 5 cm diameter thin-sheet metal tube of a known weight (W1) and volume (V) of 5 cm 3 . The medium was excavated around the tube and the medium beneath the tube bottom cut. Excess medium substrates from the tube ends were removed using a knife and then dried at 105 o C for 2 days and weighed to get W2. Bulk density was calculated using the formula (g/cm 3 ) = (W2 g -W1 g)/ V (cm 3 ) as described by Anderson and Ingram (1993). The total porosity was measured using the formula: where qb and qs are bulky density and particle density in percentage respectively (Baruah and Barthakur, 1998).

Chemical characterization
pH was measured on 2.5:1 soil water suspension while EC of the saturated paste extract was measured to determine the level of salinity as described by (Okalebo et al., 2002). Total nitrogen was calculated using the formula: % N in soil sample = (A-B) × 0.1×v×100 / 1000 × W × al (2) where A = volume of the titre HCl for the blank, B = volume of the titre HCl for the sample, V = final volume of the digestion, W = weight of the sample taken and al = aliquot of the solution taken for analysis (Okalebo et al., 2002). Total phosphorus was determined using ascorbic acid as described by (Okalebo et al., 2002) with the following formula: where C = the corrected concentration of P in the sample, V = volume of the digest, f = dilution factor, W = weight of the sample. The potassium content of the growing medium was determined using the procedure of flame photometry as described by Okalebo et al. (2002). Organic matter (OM) was calculated after ash content determination using the formula: ash (%) = [(W3 -W1) / (W2 -W1)] x 100 and organic matter (%) = 100 -ash% (4) where W1 = the weight of the empty, dry crucible containing growing medium, W2 = the weight of the dry crucible containing growing medium and W3 = the weight of the dry crucible containing growing medium following ignition. W3 -W1 = weight of the ash (Okalebo et al., 2002). The organic carbon (OC) was calculated using the formula; OC (%) = T × 0.2 × 0.3/sample weight (g) (5) where T is the titre (Okalebo et al., 2002).

Data analysis
To determine the effects of the substrate mixtures on physical and chemical properties of the formulated growing media, analysis of variance (ANOVA) was carried out; and the means for significantly different treatments were separated using Tukey's honestly significant difference test at p ≤ 0.05. The data analysis was carried out using the Statistical Analysis System package, SAS software version 9.2 (SAS Institute, 2010). The statistical model fitted for this experiment was: , where Yij= overall observation, = overall mean, = effect due to treatment, = A random error associated with the response from the jth sample of the ith treatment, i: 1, 2, 3, 4, 5, 6, 7, 8 and 9.

Physical characterization
The substrate mixtures used in this study significantly (p≤ 0.05) influenced the physical properties of the formulated growing media. Peat moss alone indicated the highest porosity of 90% followed by T 3 , T 6 and T 7 . However, its bulky density value of 0.09g/cm3 was the lowest followed by T 3 , T 4 , T 5 , T 6 , T 7 , T 8 and T 9 which were not significantly different. Sand alone showed the highest bulky density of 1.53g/cm3 and the lowest value of porosity (42.42%) followed by T 6 (48.49%) and T 4 (56.28%). While treatments T 3 , T 5 and T 7 were similar in the physical properties, Treatments T 8 and T 9 showed no significant difference in both bulky density and total porosity parameters (Figure 1).

Chemical characterization
Substrates used revealed significant differences in chemical properties of the growing media formulated from them. Peat moss had the lowest pH value of 5.8 while the values of the other treatments were not significantly different. The highest EC was observed in T 4 (4.78mS/cm) followed by T 3 (3.92mS/cm). Sand alone showed the lowest value of 0.33 mS/cm followed by T 9 (0.71mS/cm). Peatmoss showed the highest contents of N, P and K of 0.18%, 600Ppm and 7.18meq/100g while sand alone recorded the lowest content of these nutrients compared to other treatments. The highest values of organic matter and carbon were obtained in peat moss 100% with 89.70 and 52.03% respectively followed by T 4 and T 3 . Sand alone medium also recorded the lowest OM and OC contents compared to the other formulated media ( Figure  2).

DISCUSSION
The physical and chemical characterization of formulated growing media from different mixtures of substrates such as sand, carbonized rice husks and goat manure has never been investigated in Rwanda. The results obtained revealed that as goat manure increased in the medium as the medium had low bulky density and high porosity. These results agree with those of Mupondi et al. (2010) who reported that goat manure reduced bulky density and increased porosity of growing media. Supportively, Seguel et al. (2013) got reduced bulky density of soils when mixed with goat manure. In addition, Carbonized rice husks also reduced bulky density and improved porosity of the growing media which could be attributed to their low bulky density and high porosity as natural substrates (Varela et al., 2013). This is also supported by Jeon et al. (2010) who reported that CRH improved porosity of soils and Orge and Abon (2012) who indicated that incorporating them in the soils reduce bulky density of those soils. Peat moss (T1) has low bulky density and high porosity by nature (Aendekerk, 2000) while sand (T2) had high bulky density due to its heaviness (El-Hamed et al., 2011) and improves aeration of the media due to its porosity (Gungor and Yildirim, 2013). The significant differences in chemical properties of the formulated media could be attributed to differences in natural chemical composition of the substrates used. The results indicated that the ratios of 50 and 40% of GM in a Figure 1. Physical properties of different formulated growing media. T1: Peat moss 100%, T2: Sand 100%, T3: Top soil + Goat manure 70%: 30%, T4: Sand + Goat manure 50%: 50%, T5: Sand + Goat manure + Carbonized rice husk 50%: 40%: 10%, T6: Sand+ Goat manure + Carbonized rice husk 50%: 30%: 20%, T7: Sand+ Goat manure + Carbonized rice husk 50%: 20%: 30%, T8: Sand+ Goat manure + Carbonized rice husk 50%: 10%: 40% and T9: Sand + Goat manure + Carbonized rice husk 50%: 0%: 50%. Different letters above the bars indicate significant difference according to Tukey's test (p ≤ 0.05) and the bars stand for standard deviations (SD). growing medium revealed in high concentration of OM, OC and major elements (N, P and K) compared to other treatments. This could be explained by that GM has essential elements such as nitrogen, phosphorus, potassium, with general concentrations of 2.2-3.4, 0.3-0.7 and 1.5-2.5 respectively and increases the organic matter content in the medium (Azeez and Averbeke, 2010). However, the increase in GM ratios generally increased EC levels in the media. This resulted are in agreement with Mowa et al. (2017) who showed that GM increased the EC levels of a growing medium. CRH also improved concentration of essential elements and organic matter because as their ratios were increasing as the concentrations were increasing. These resulted are supported by the reason that CRH contain phosphorous, calcium, potassium, magnesium and they act as organic fertilizer ingredient (Orge and Abon, 2012). As CRH ratio increases as the EC levels of a growing medium were reducing. These results might be due to the fact that CRH alone had lower levels of EC and high levels of K which thereafter provided K+ and improved physical properties which resulted in reduced Na+ and hold a lower exchangeable sodium percentage level and higher K+/Na+ ratio in the medium (Phuong et al., 2020). On another, sand alone showed low concentration levels of chemical properties which may be due to its nature of having very low concentration of these major elements while PM resulted in the best chemical properties compared to formulated media due to that it normally contains more organic matter, nitrogen and phosphorus (Atif et al., 2016).

Conclusion
The mixture of sand, goat manure and carbonized rice husks as growing medium substrates at different ratios have significantly influenced the physical and chemical properties of formulated media. The significant differences observed in the study are affected by differences in physical and chemical properties concentrated in the substrates and their ratios in a given growing medium. An increase in the ratios of CRH and GM resulted in reduced bulky density, enhanced porosity and nutritional status of a medium. The mixture of GM and CRH with sand substrates improves levels of OM, OC, N, P and K of the medium. An increase in levels of CRH while reducing the levels of GM resulted in reduces EC levels in a medium and vice-versa. The treatments T 5 , T 6 , T 7 , T 8 and T 9 can be used for crop production to search the best alternative growing medium because they revealed good physical and chemical properties to other treatments compared to peat moss. Further studies should identify other local and available substrates which may result in the best physical and chemical properties to search the best alternative growing medium to peat moss for production of Figure 2. Chemical properties of different formulated growing media. T1: Peat moss 100%, T2: Sand 100%, T3: Top soil + Goat manure 70%: 30%, T4: Sand + Goat manure 50%: 50%, T5: Sand + Goat manure + Carbonized rice husk 50%: 40%: 10%, T6: Sand+ Goat manure + Carbonized rice husk 50%: 30%: 20%, T7: Sand+ Goat manure + Carbonized rice husk 50%: 20%: 30%, T8: Sand+ Goat manure + Carbonized rice husk 50%: 10%: 40% and T9: Sand + Goat manure + Carbonized rice husk 50%: 0%: 50%. Different letters above the bars indicate significant difference according to Tukey's test (p ≤ 0.05) and the bars stand for standard deviations (SD). quality transplants.

CONFLICT OF INTERESTS
The authors have not declared any conflict of interests.