°" ~ SCIENTIA HORTICULTUR~

E LS EV I E R Scientia Horticulturae 60 (1994) 161-166

S h o r t c o m m u n i c a t i o n

Response of onion to soil solarization and organic mulching in semi-arid tropics

I.A. Adetunji* Department of Crop Science, University of Maiduguri, Maiduguri, Nigeria

Accepted 27 April 1994

Abstract

To optimise water use and soil condition during dry season onion production in semi- arid Nigeria, soil solarization with transparent polyethylene film was compared with or- ganic mulches (groundnut shell, millet stover and sawdust) and a no-mulch control. With the exception of sawdust mulch, mulching significantly enhanced vegetative growth and bulb yields of onion. Solarization provides better soil coverage and therefore conserved more soil moisture and more than doubled the concentrations of N H : - N and NO~--N compared to other mulches and the control. Thus with soil solarization the total bulb yield of onion was 80% higher than with no mulching and 25% higher than the best organic mulch (groundnut shell).

Keywords: Allium cepa L.; Growth; Mulching; Solarization; Yield

1. Introduction

Onion (Allium cepa L. ) is a major economic bulb vegetable in semi-arid north eastern Nigeria (Agricultural Extension Research and Liaison Services, 1985 ). It is grown during both wet and dry seasons, but experience has shown that yield and profit are higher when grown under irrigation during the extended dry sea- son. Although mulching with plant residues (organic mulching) has been used to obtain good yield in some vegetables (Quinn, 1975; Adetunji, 1989 ), the scarcity of these plant materials during the dry season made it difficult for vegetable farm- ers to adopt organic mulching. Alternative techniques such as soil solarization could be used to circumvent this problem.

* Present address: Agronomy Department, Ladoke Akintola University of Technology, Ogbomoso, Nigeria.

0304-4238/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD10304-4238 (94) 00695-C

162 LA. Adetunji / Scientia Horticulturae 60 (1994) 161-166

Soil solarization is a mulching technique in which moist soil is covered by transparent polyethylene film through which the soil is heated by solar radiation. Apart from its effectiveness in conserving soil moisture, the hydrothermal effect of solarization stimulates soil biota, favouring release and accumulation of solu- ble plant nutrients beneficial to crop growth. Thus solarized soils usually contain higher levels of soluble mineral nutrients than untreated soils (Baker and Cook, 1974; Jones et al., 1977 ). Soil solarization may be beneficial for vegetable pro- duction in semi-arid Nigeria, since the requirement for irrigation and fertiliza- tion may be drastically reduced. This study evaluated the effect of solarization and plant residue mulching on the growth and yield of onion under the semi-arid tropical conditions of north eastern Nigeria and other similar areas.

2. Materials and methods

2.1. Field experiment

The experiment was conducted during the 1991-1992 dry season at the Teach- ing and Research Farm, University of Maiduguri, Nigeria ( 11 ° 05' N, 13 ° 05' E). The soil is Typic Ustipsamment with sandy loam texture. The plot was under cowpea cultivation for two consecutive planting seasons. There were five mulch- ing treatments using transparent polyethylene film (soil solarization), groundnut shell, millet stover and sawdust plus a no-mulch control. The three plant residue mulches were laid 3.0 cm thick, while the polyethylene film was 0.035 mm thick.

The field was cleared, harrowed, levelled and flood-irrigated to capacity ( 17- 15 cm water). Three hours after irrigation, the mulch materials were laid out following a randomised complete block design. In each of the two planting sea- sons, there were 20 plots, each measuring 4 m × 3 m. The field remained under mulch cover for 5 weeks, after which the materials were removed to air the soil. Immediately following removal of the mulch materials, four random samples of soil core per replication were taken and bulked for determination of soil mineral nutrients. The treatments were then re-applied and 2-week-old onion seedlings were planted through openings in the mulch materials at a spacing of 20 cm × 15 c m .

Vacuum gauge tensiometers were installed at a soil depth of 15 cm 3 weeks after planting to monitor the soil water potential under each treatment. Plots were ir- rigated to field capacity whenever soil moisture potential fell to 56% of field ca- pacity. At the field capacity of the soil used, the tensiometer readings are between 0.015 and 0.018 MPa. Therefore, 0.038 MPa was taken as 50% field capacity for this study. For 14 consecutive days in the middle of the growing season, soil tem- peratures at a depth of 15 cm under each treatment were monitored at 07:00, l 0:00, 16:00 and 19:00 h using a Wakster thermometer model NA-160 attached to a soil probe. The intensity of weed infestation was assessed by visual scoring (Drinjiff and Kerkhoveng, 1979 ), while the degree of crust formation on the soil surface was measured with a penetrometer. Thirty plants per plot were carefully

LA. Adetunji / Scientia Horticulturae 60 (1994) 161-166 163

uprooted, washed and measured for growth and yield characteristics at maturity ( 14 weeks after planting).

2.2. Soil analysis for nutrient determination

Soil NOj--N and NH4 + -N concentrations were determined by extraction with 1 N KC1 and the extracts analyzed by the Carlson (1978) method. Extractions for exchangeable bases were made by saturation of the soil with neutral NH4OAc. An atomic absorption spectrophotometer was used to determine the K, Ca, Mg and Mn in the extracts. Phosphorous was measured by the colorimetric method (Jackson, 1958 ).

2.3. Plant tissue analysis

For bulbs and leaves, chemical analyses were carded out 9 weeks after planting. The plants were separated into leaves and bulbs, which were dried to constant weight at 95 °C. Total P and K in milled samples were determined with an atomic absorption spectrophotometer after digestion with a perchloric-nitric-sulphuric acid mixture. Nitrogen was determined by the Kjeldahl method. Nutrient up- takes by bulbs and leaves were estimated from the total bulb and leave yields and mineral composition.

Statistical analysis of the data involved a two-way analysis of variance from which the least significant differences (at the 5% level) were calculated.

3. Results

3.1. Soil condition

Solarization and other mulch treatments maintained a high soil water potential regime of between -0.01 and -0.03 MPa during irrigation cycles as compared with bare plots where soil moisture rapidly depleted to -0.075 MPa (Fig. 1 ). The morning temperature of the soil was not significantly affected by solarization and other mulch treatments, but in the afternoon the increase in temperature was retarded by sawdust, millet stover and groundnut shell mulches. Thus, maximum temperatures under these treatments were significantly lower than in control and solarized plots (Fig. 2 ). Soil solarization and other mulch treatments increased soil penetrance by an average of 48% and weed control by 56% over the un- mulched treatment.

Soil solarization significantly increased the concentration of soil N ( N H 4 + N O 3- N) by 83, 48, 46 and 55% above those of sawdust, groundnut shell, millet stover mulches and unmulched treatment, respectively. Similar trends in response were recorded in P concentration, while levels of K + and Ca2++ Mg 2+ +Mn 2+ and pH were not significantly different.

164 LA. Adetunji /Scientia Horticulturae 60 (1994) 161-166

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DAYS AFTER WATERING TO FIELD CAPACITY

Fig. 1. Time course of soil water potential in fields with solarization ( • ), sawdust mulch ( O ), millet chaff mulch ( • ) , groundnut shell mulch ( [ ] ) or no mulch (control) ( • ) . Vertical lines represent LSD at P< 0.05.

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Fig. 2. Variations in average soil temperature of fields with solarization ( • ) , sawdust mulch ( © ), millet chaff mulch ( • ), groundnut shell mulch ( [] ), or no mulch (control) ( • ). Vertical lines rep- resent LSD at P< 0.05.

3.2. Growth characteristics

Soil solarization significantly enhanced the growth of onion, but sawdust mulch depressed growth. Root number, root fresh and dry weights, leaf number, and leaf fresh and dry weights per plant under solarized conditions show a consistent superiority over those of other mulched and unmulched treatments.

LA. Adetunji / Scientia Horticulturae 60 (1994) 161-166 165

Table 1 Effect of soil solarization and organic mulching on yield and yield components of onion at final harvest

Yield and yield components Soil treatments LSD (P<0.05)

Solarized Sawdust Groundnut Millet Control soil mulch shell stover (bare soil )

mulch mulch

Bulb diameter per plant (mm) 68.5 38.4 57.1 49.7 41.2 10.42 Bulb fresh weight per plant (g) 124.3 46.4 79.6 85.2 47.3 15.26 Bulb dry weight per plant (g) 16.6 6.2 9.1 10.4 5.7 2.56 Total bulb yields (t ha-~ ) 8.1 3.5 6.5 6.3 4.5 1.02 Marketable bulbs (%) 87 48 74 73 64 2.51

Data in percentages were analysed after angular transformation.

3.3. Yield and yield components

Yield and yield components of onion were significantly higher in solarized soil than in other mulch treatments and the control, whereas sawdust mulch consis- tently depressed all yields and yield components more than any other treatment (Table 1 ). At final harvest, the total bulb yields per hectare from solarized soil were 130, 25, 29 and 80% higher than those obtained from sawdust, groundnut shell and millet stover mulches and the control, respectively. Similar trends were observed in the percentage of marketable bulbs.

3.4. Nutrient uptake

Solarization significantly increased N, P and K levels in leaves and bulbs. There were, however, no significant differences in the uptake of the three minerals be- tween groundnut shell and millet stover mulches. The least nutrient uptake was observed in onions grown under sawdust mulch.

4. Discussion

The results of this study dearly show that solarization increased onion growth and yield through increased soil nutrients, effective weed control and conserva- tion of soil moisture without the application of fertilizer and herbicides but with supplementary irrigation.

The higher concentration of N and P in solarized soil may be attributed to the hydrothermal effect of solarization, which probably killed and degraded native microbiota, thereby creating microaerobic conditions favouring release and ac- cumulation of soluble N and P. Other mulch material could not generate enough heat to conserve enough moisture to release soluble nutrients at a rate compara- ble to that of the solarized soil. A similar result was reported in soil treated by

166 I.A. Adetunji / Scientia Horticulturae 60 (1994) 161-166

solarization in California (Stapleton et al., 1985 ) and in Israel (Chen and Katan, 1980).

Solarization-type mulching and sawdust mulch conserved more moisture and drastically reduced weed populations because polyethylene film and finely di- vided sawdust provided better soil coverage, thereby reducing soil aeration, which in turn prevented germination of weed seeds and smothered those which had germinated. Sawdust mulch depressed onion growth and yield, either because of some growth retardant present in the unidentified and untreated timber from which the sawdust was collected, or because of immobilisation of soil N as re- flected in the low concentration of soluble N under sawdust mulch. A similar result was reported by Bollen ( 1959 ).

The phenomenon of solarization as observed in this study has great potential in the agriculture of arid regions in the developing countries. A major limitation to this method is the high cost of polyethylene film, but this is greatly outweighed by the increase in bulb yield of over 40% without any other input except water under solarization as compared with the unmulched treatment. However, be- cause this is a preliminary study, further investigations are needed, especially on leafy and fruit vegetables, to confirm or refute the results presented here.

References

Adetunji, I.A., 1989. Growth and yield response of fluted pumpkin to mulching in semi-arid regions. J. Arid Agric., 2: 49-55.

Agricultural Extension Research and Liaison Services (AERLS), 1985. Production of onions. Insti- tute of Agricultural Research, Ahmadu Bello University, Zaria. Extension Guide No. 2.

Baker, K.F. and Cook, R.J., 1974. Biological Control of Plant Pathogens. Freeman, San Francisco, CA.

Bollen, W.B., 1959. As quoted by Allison, F.E., 1973, pp. 500-517. In: Development in Soil Science. Vol. 3. Elsevier, Amsterdam.

Carlson, R.M., 1978. Automated separation and conductimetric determination of ammonia and dis- solved carbon dioxide. Anal. Chem., 50:1528-1531.

Chen, Y. and Katan, J., 1980. Effect of solar heating of soils by transparent polyethylene mulching on their chemical properties. Soil Sci., 103: 271-277.

Drinjiff, A.H. and Kerkhoveng, J., 1979. Effect of efficient weeding on yields of irrigated cotton in Eastern Kenya. PANS (Pest Artic. News Summ.), 16: 596-605.

Jackson, M.L., 1958. Soil Chemical Analysis. Prentice-Hall, Englewood Cliffs, NJ. Jones, T.L., Jones, V.S. and Ezell, D.O., 1977. Effect of nitrogen and plastic mulch on properties of

Troup loamy sand and on yield of'Walter' tomatoes. J. Am. Soc. Hortic. Sci., 102: 273-275. Quinn, J.G., 1975. A further assessment of mulching and staking on a rain-fed tomato crop in north-

ern Nigerian states. Hortic. Res., 15: 31-39. Stapleton, J.J., Quick, J. and DeVay, J.E., 1985. Soil solarization: Effect on soil properties, crop fer-

tilization, and plant growth. Soil Biol. Biochem., 17: 369-373.