Abstract: Effect of rice husk, sawdust, ash and burnt soils as mulches on the growth attributes of onion was studied. Significant variations were observed in plant height, root length, total dry matter accumulation, leaf area index (LAI), crop growth rate (CGR) and net assimilation rate (NAR), whereas relative growth rate (RGR) was found nonsignificant in all stages of growth. Root: shoot ratio was significantly different at the early stages of growth (30 to 60 DAT) and thereafter it was found nonsignificant. All the mulches had positive effect on growth attributes over the control and the use of ash as mulch was found better compared to the other three.
Introduction
Onion (Alllum cepa L.) ranks first among the spice grown in Bangladesh, in production (Anonymous, 1997a), but the average yield was quite low (4.12 t ha–1) compared to that of most other countries (Anonymous, 1997b). Onion is grown in Bangladesh in winter season when short day length and low temperature prevails which are not conducive to produce big size bulbs. Thus more improved variety does not bring about significant yield improvement in onion. So, to minimize the cultivation and production problems emphasis must be given to adopt improved cultural practices. Among different cultural practices mulching has some promising effects on the onion production. Mulching materials with different spectral properties are reported to increase soil temperature and thus enhancing bulb development (Suh et al., 1991; Barkley et al., 1965). Moisture is an important factor that influences the growth attributes of onion. Mulch reduces the water loss by evaporation resulting in more conservation of moisture in soil (Prihar, 1986). Artificial mulching like polythene are reported to increase the growth and yield of onion (Greisenheim, 1952). But research works on onion with indigenous mulching materials like rice husk, sawdust, ash and burnt soils are quite meagre in Bangladesh as well as in the world. Hence, keeping in view the above facts this piece of research work was thus undertaken to study the effect of indigenous cheaply available mulching materials on growth attributes of onion.
Materials and Methods
The experiment was carried out at the Field Laboratory of the Department .of Crop Botany, Bangladesh Agricultural University, Mymensingh, during the rabi season (December, 1995 to April, 1996) with :four mulches viz, rice husk, sawdust, ash and burnt soil each at the rate of 10 t ha–1 on onion (cv. Faridpuri Briati). The experiment was`laid out in Randomized Complete Block Design (RCBD) with four replications. The unit plot size was 5 m x 2 m. The soil belonged to the Old Brahmaputra alluvial tract having sandy ;oam in texture. The plots were fertilized. with cowdung, urea, triple super phosphate (TSP) and muriate of potash (MP) at of 10 t ha–1, 175 kg ha–1, 125 kg ha–1 and 150 kg ha–1, respectively. The whole amount of cowdung and TSP and the 50 percent of urea and MP were applied during final land preparation. The rest 50 percent of urea and MP were applied as top dress at 25 days after transplanting (DAT). About 50 days old seedlings of uniform size were transplanted on 5th January, 1996 in a single row system with 20 cm x 15 cm spacing. The mulches were sprayed over the plots on 29 DAT. Intercultural operations were done as and when necessary.
The first crop sampling was made at 30 DAT and continued at 10 days intervals till physiological maturity at 90 DAT. Prior to harvesting plants m–2 were counted followed by measuring the height of the individual plant by a graduated scale placed from ground level to top of the plant. At the time of each harvest the plant m–2 area were uprooted carefully by a ‘khurpil’ in order to ensure maximum root extraction and carried to the laboratory in polythene bags to prevent transpiration. After cleaning the harvests the root length of each plant of the harvested area (Im2) was measured by graduated scale. The component parts representing root, stem (including bulb) and leaf were separated. Leaf area was measured by an automatic leaf area meter (Model, L1-3000). Then the components were oven dried for 48 hours at 80°C and their corresponding dry weights were determined by a sensitive balance. Different growth estimates such as CGR, RGR and NAR were made following the formulae rendered by Hunt (1978). Data on plant parameters and microclimatic parameters were analyzed statistically and their mean differences were adjusted as per DMRT or LSD.
Results and Discussion
Plant height: Mulching induced significant variation in plant height of onion (Table 1). Maximum plant height (45.38 cm) was recorded in plants mulched with sawdust while control had the least (39.02 cm). The maximum plant height as recorded on 90 DAT under rice husk, ash and burnt soils were 42.98, 42.56 and 39.74 cm, respectively, The increase in plant heights due to various mulches had been reported for onion (Suh et al., 1991). The. increased plant heights due to mulching effects might have resulted from retention of more soil moisture in association with lower soil temperature throughout the growth period whereas the control plants deprived of aforesaid benefits.
| Table 1: | Effect of different mulches on the plant height and root length of onion |
| *DAT indicates days after transplating; Means having similar letter(s) in a column do not differ significantly at 1 % level of probability | |
| Table 2: | Effect of different mulches on the toital dry matter accumulation of onion |
| *DAT indicates days after transplating. Means having similar letter(s) in a column do not differ significantly at 1 % level of probability | |
| Table 3: | Effect of different mulches on the root shoot ratio of onion |
| *DAT indicates days after transplating. Means having similar letter(s) in a column do not differ significantly at 1 % level of probability | |
Rootlength: Root length with ash mulch was significantly higher than control throughout the whole growth period and the root length with rest of the mulches were not significantly different from each other except the growth stage of 50 DAT (Table 1). The irfcreasing root length under ash mulch corrjpared to control might have resulted from low soil moisture content and high soil temperature. The reduced root lengths in all other mulches might have resulted from the retention of high soil moisture and low soil temperature.
Total dry matter accumulation: Dry matter accumulation was significantly higher for mulched plots compared to the control (Table 2). The highest DM (181.84 g m–2) was recorded from the plants with ash mulch while control had the lowest (153.81 g m–2). The plant DM of ash mulch was greater than that of sawdust (171.01 g m–2) followed by rice husk (164,92 g m–2) and burnt soil (160.36 g m 21. Increase in DM production depends on the photosynthetic efficiency (source), sink strength and translocation capacity of crops (Evans, 1975). The increasing Lals with their greater photosynthetic ability resulted in increased dry matter accumulation (Pandey, 1980; Watson, 1947).
Root shoot ratio: The effect of mulches were statistically significant during the early growth stages (30-50 DAT) and thereafter it was insignificant (Table 3).
| Table 4: | Effect of different mulch on Leaf Area Index (LAI) and Crop Growth Rate (CGR) of onion at different dates after transplanting |
| In a column figures followed by no common letter(s) are significantly different at 1 % level. DAT = Days After Transplanting | |
| Table 5: | Effect of different mulch on Relative Growth Rate (RGR) and Net Assimilation Rate (NAR) of onion at different dates after transplanting |
| In a column figures followed by no common letter(s) are significantly different at 1 °/0 level. DAT --= Days After Transplanting | |
The unmulched plants had the maximum root shoot ratio while sawdust had the least. At the vegetative stage, more photosynthates were diverted to produce a sound root mass but with the transition of bulbing most of the photosynthates were consumed by the developing bulbs. The resultant effect was a decrease in the root shoot ratio.
Leaf area index (LAI): Mulching significantly increased LAI at all stages of growth. After establishment of the transplants the LAI sharply increased from a lower value of about 0.1 at 30 DAT to around 0.9 at 70 DAT. After attaining the maximum the LAI sharply declined with the approach of maturity. The highest LAI (nearly 0.88) was, however, observed in plants mulched with sawdust and the lowest (0.67) in the control (Table 4). Mulches in potato increased LAI significantly at different stages of growth (Roy et al., 1990).
Crop growth rate (CGR): All mulches significantly influenced SGR at all stages of development except 70 DAT and at final harvest on 90 DAT. Except ash CGR values in all mulches attained their peak at 70 DAT while in ash it had the maximum at 80 DAT. Plants grown under ash attained highest CGR (4.27 g m–2 d–1) followed by sawdust (4.16 g m–2 d–1) and burnt soil (4.11 g m–2 d–1). The CGR in control and rick husk was almost identical (Table 4).
CGR in onion decreased during 31-45 DAT, after which it increased gradually upto 75 DAT and then decreased gradually (Rao, 1988). CGR is positively correlated with LAI (Watson, 1947). Thus CGR increased with the increase in LAIs (Table 4) in all the treatments and the lower values of CGR at the initial stages of growth was the result of lower LAIs.
Relative growth rate (RGR): Mulching increased RGR in most stages of growth but the values were statistically insignificant (Table 5). The increase in RGR in potato was also insignificant (Roy et al., 1990). The maximum RGR at 60 DAT correspondent to the rapidly developing bulbs. The decline in RGR after reaching the maxima was due to leaf abscission and crop age (Pandey et al., 1978; Katiyar, 1980).
Net assimilation rate (NAR): NAR values were statistically significant at all stages of growth except 60 and 90 DAT. There was a high increment of NAR at 40 DAT followed by a lag period around 50 DAT and thereafter, it attained maximum at 60 DAT. After attaining the maximum, a gradual decrease was apparent till 80 DAT (Table 5). NAR had higher values during the early vegetative growth but declined with age in chickpea (Haloi and Baldev, 1986) and this was possibly due to mutual leaf shading and increased number of old leaves (Pandey et al., 1978). The maximum values of NAR at 60 DAT were related to the rapidly developing sinks (Haloi and Baldev, 1986).
Considering the different growth parameters in relation to changing soil environment, mulching practice showed better performances than the control. All the mulches are readily available at cheaper rates under the socio-economic conditions of Bangladesh and these mulches are recommended in onion cultivation. The use of ash as mulch was found better compared to the other three.