Ground water level and soil moisture content
During the growth period, the ground water level initiated about 120 cm below the soil surface at planting time. Then, the ground water level dropped to 145 cm below the soil surface and came up to 90 cm again below the soil surface (Fig 2). Soil moisture content (SMC) at 15-30 cm depth presented in the available ranges (between FC and PWP) at planting time to 35 DAP (day after planting). Thereafter, SMC dropped below PWP to 140 DAP and came up above PWP until harvest (Fig 3). This can indicate that SMC adequate for plant growth during the early growth stage. Then, SMC showed below during PWP level at 30 cm soil depth. Cassava roots may uptake restricted soil moisture at 30 cm soil depth. However, SMC below 30 cm depth may be presented in available ranges and roots located beyond such depth are able to uptake adequate soil moisture.
Fig 2: Ground water level during the growth period.
Nutrient concentration of leaf
Fig 3: Soil moisture content (% w) at 0-15 cm (--l--) and 15-30 cm (--š--) depth during the growth period.
Nutrient management methods (NMM) had no significant effect on P and K concentration of leaf at 3 and 5 MOA, but a significant difference was observed in N concentration (Table 1). The highest N was achieved in CFR+PGPR treatment both at 3 and 5 MOA. This was associated with the presence of rhizobacteria responsible for fixing atmospheric N, solubilizing K and phosphate, which enhance uptake by plants (Chauhan et al., 2015; Pii et al., 2015).
PGPR belongs to the genus Bacillus that can produce IAA and thereby chlorophyll biosynthesis as reported by K.C. et al. (2020)
Growth, yield and starch content
Table 1: Nutrients concentration (g kg-1) in leaf at 3 and 5 months of age of cassava grown after rice under rainfed conditions.
NMM was significantly different (p≤0.05) in above-ground fresh weight, single storage root fresh weight and storage root yield of cassava, but no significant difference in storage root number and starch percentage (Table 2). CFR+PGPR treatment produced the maximum of above-ground fresh weight, single storage root fresh weight and storage root yield of cassava. CFR+PGPR gave higher storage root yield than that of application CFR alone by 14%. This can be due to improvement of plant development related to the presence of rhizobacteria treated the cassava stakes. Application of PGPR can result in the nutrient acquisition and the synthesis of phytohormones (Compant et al., 2010).
It is known that PGPR is the rhizobacteria that can fix N and positively affect plant growth (Bashan and de-Bashan, 2010; Duca et al., 2014).
Similar to N, the uptake of K and P may be mediated by PGPR when interacting with their host plant (Lugtenberg and Kamilava, 2009; Richardson and Simpson, 2011)
. Besides improved plant nutrition, the biosynthesis of phytohormones is also considered to directly stimulate plant growth. Auxin, gibberellin, cytokinin, ethylene and abscisic acid are phytohormones produced and released (Spence and Bais, 2015)
, Application of CFR+PGPR gave significantly higher single storage root fresh weight than that of CFR alone. This may be associated with auxin produced in meristematic areas and linked to cell elongation. Alteration in root morphology and development in plants inoculated with PGPR auxin producer was reported by Spence and Bais (2015)
. Cassava stakes treated with PGPR (Bacillus
sp. Strain CaSUT007) increased root and shoot lengths by more than 30% and increased fresh and dry weights by more than 25% compared to distilled water control, auxin to be the primary compounds of phytohormone (Buensanteai et al., 2013).
Regarding starch content, different nutrients management methods gave a similar effect on the starch percentage of storage root (Table 2).
Net return from different nutrient management methods
Table 2: Growth, yield components, root yield and starch content at harvest of cassava grown after rice under rainfed conditions.
The economics of the different nutrient management methods are presented in Table 3. The highest net return over materials cost was achieved by the treatment of CFR+PGPR (619 USD ha-1
). In the present study, production costs did not include labour. Although the smallholder farmers normally used household labour for cassava grown after rice., a sufficient net return for them. In this concern, Polthanee et al., (2014)
reported that cassava grown after rice provides a net income of about 556-1
,104 USD ha-1
, depending on different paddy fields.
Table 3: Yield, production cost, gross income and net income of cassava grown after rice under rainfed conditions.