Field Crops Research, 4 (1981) 33--45 33 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands EFFECTS OF DIFFERENT MULCH MATERIALS ON SOIL PROPERTIES AND ON THE ROOT GROWTH AND YIELD OF MAIZE (ZEA MA YS) AND COWPEA ( VIGNA UNG UICULA TA ) P.R. MAURYA and R. LAL International Institute of Tropical Agriculture, Oyo Road, P.M.B. 5320, Ibadan (Nigeria) (Accepted 10 September 1980) ABSTRACT Maurya, P.R. and Lal, R., 1981. Effects of different mulch materials on soil properties and on the root growth and yield of maize (Zea mays) and cowpea (Vigna unguiculata). Field Crops Res., 4: 33--45. The effects of different mulch materials on soil temperature and moisture regimes, root and shoot growth, nutrient uptake, and on yield of maize (Zea mays) and cowpea (Vigna unguiculata) was investigated for a tropical Alfisol. The effect of translucent and transparent polythene sheet mulches was compared with that of rice straw mulch, and with unmulched and ridged soil surfaces. The amplitude of diurnal fluctuation in soil temperature at 5 cm depth was 7, 10, 12, 12, and 15°C for straw mulch, bare soil surface, ridged soil, translucent polythene, and transparent polythene mulch, respectively. Root density was generally high under straw mulch and low under unmulched flat and ridged treatments. The maximum shoot elongation rate was 5.5 cm/day with the straw mulch and 3.0 cm]day with the unmulched ridged treatment. In general, translucent polythene and straw mulch materials yielded more than unmulched ridges and transparent polythene treatments. Soil temperature, as affected by different mulch and seedbed preparation treatments, had a significant effect on root growth, plant vigour, and grain yield. INTRODUCTION Soil temperatures exceeding 30°C at 5 cm depths are commonly observed during the growing season for as long as 3--6 h per day in different agro-eco- logical regions of the tropics (Lal, 1975, Lal, 1976, Prihar et al., 1979). Supra-optimal soil temperatures (in excess of 30°C at 5 cm depth), even if they occur in the seedling stage, can significantly reduce the yield of maize (Zea mays) and soybean (Glycine max) (Lal, 1973, 1974 a,b; Minchin et al., 1976; Harrison-Murray and Lal, 1979; Lal, 1980). The magnitude of yield reduction due to high soil temperature also depends on other factors: soil moisture regime, duration of the supra-optimal soil temperature stress, varietal and inter-species differences and stage of crop growth. Although the mechanisms of yield reduction by sub-optimal soil tempera- ture regimes are adequately known, the effects of supra-optimal tempera- 0378-4290/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company 34 tures are not adequately understood. Root growth and development may be the key mechanisms involved because the yield reduction by high soil tem- perature seems to be associated with impeded nutrient and water absorption and translocation, and/or hormonal imbalances (Russell, 1977). In fact, the interaction between soil temperature and moisture regimes in root growth and development may be an important factor that determines the susceptibil- ity of a plant to soil temperature stress. If root proliferation is rapid, permit- ting timely escape into the sub-soil layers with favourable soil moisture and temperature regimes, the adverse effects of supra-optimal soil temperatures in the surface layer may be minimized. Careful appraisal of the literature concerning the effects of temperature on mulch materials and root growth indicates a divergence of opinion. The effects of mulch on soil temperature and root growth depend on the climatic environments (temperate vs. tropical and summer vs. winter), mode of mulch application, quality and quantity of mulch material, and the rate of its de- composition. Broad generalizations disregarding these factors may lead to erroneous interpretations. Allmaras and Nelson (1971, 1973) observed significant differences in root configuration and distribution due to row and interrow variants of tillage and straw mulching. In Sudan, Baumer (1964) reported deeper penetration, increased number, and better distribution of secondary roots with plastic mulch. Similar observations were made by Kawashima and Tanabe {1972). Many researchers have observed a relatively high concentration of roots directly beneath the mulched layer (Chaudhry and Prihar, 1974). Onder- donk and Ketcheson (1973) also reported that mulch suppressed deep root penetration. Lal (1978) observed that the mean and maximum depth of root penetration were generally highest for treatments with mulched inter-row; however, root concentration was found to be significantly greater close to the mulched layer. Research information from field experiments that describe the effects of high soil temperature on root growth is scanty. This report describes the effects of a range of soil temperature and moisture regimes, created under field conditions by diverse mulch materials, on root growth and development and yield of maize and cowpea (Vigna unguiculata) in a tropical environment. MATERIALS AND METHODS These experiments were conducted in four consecutive growing seasons during 1976 and 1977 at the experimental farm of the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria. The texture of the surface layer of the experimental soil is sandy loam changing into clay loam to clay in the sub-surface horizon. There is a distinct gravelly horizon between 30 and 80 cm depth in the soil profile with gravel concentrations ranging from 40 to 80%. The soil is classified as a clayey skeletal, Kaolinitic, isohyperthermic oxic paleustalf. There are two growing seasons in this ecological zone: from April to July and from August to November. There is a long dry season from mid-November to mid-March (Moormann et al., 1975). 35 Soil temperature and moisture regimes were modified by using different mulch materials and methods of seedbed preparation. Mulch materials con- sisted of black polythene {translucent), white polythene (transparent), and rice straw applied at the rate of 6 t /ha. Crop performance on soils under these mulch materials was compared with a bare ploughed soil surface and a ploughed and ridged treatment. All plots were disc ploughed to a depth of 20 cm and harrowed twice. Ridges were constructed manually, they were spaced at 75 cm, 30 cm high and 50 cm wide at the base. Polythene mulches were anchored in the soil by horizontal sticks, located 75 cm apart and pegged firmly in the ground. In addition to making provision for seeding through the plastic, supplementary holes were made for infiltra- t ion of rain water into the soil. Precautions were taken to avoid accumulation of rainwater on the surface of the polythene mulch. Treatments were laid out in a randomized block design with three replica- tions. The plot size was 8 X 5 m. In the first season maize was grown as a sole crop. In the second season plots were split to compare mono-culture maize with maize and cowpea grown as a mixed crop. Crops were seeded around 10 April and 10 August in the first and second season, respectively. The fertilizer was mixed in the ploughed and harrowed soil -- 120 kg N/ha (40 kg at planting and 80 kg at 4 weeks), urea; 13 kg P/ha, single superphos- phate; and 30 kg K/ha, muriate of potash. Mono-culture maize was planted 75 cm between rows and 25 cm within rows. Mixed cropping with cowpea was done by seeding maize and cowpea simultaneously in alternate rows 50 cm apart, with 25 cm spacing in the rows for maize and 15 cm for cowpea. Soil temperature measurements were made at hourly intervals between 08.00 and 20.00 h using a bent-stem soil thermometer set at 5 cm depth. Bi-weekly soil moisture reserves were monitored gravimetrically for the 0--10 cm depth. Periodic observations were made for leaf colour, plant height, and leaf area index. Leaf nutrient status was determined on ear leaves sam- pled at initial silk stage. Grain yield and yield components were recorded at maturi ty. Root growth and development were also investigated. Root density was measured by taking soil cores (7.5 cm diameter and 7.5 cm deep) at 20 cm lateral distance from the rows. Root density was also measured at a 40 cm depth by core sampling at 10 cm intervals. Measurements on root growth were made at different growth stages. Root elongation was measured in situ through 60 X 60 cm glass plates installed across the rows at a 10 cm lateral distance from the plants (B~hm, 1976). RESULTS AND DISCUSSION Soil moisture The effects of mulch treatments on soil moisture reserves in the first and second season of 1976 are depicted in Fig. 1. The results for 1977 are not presented. During the first 6 weeks, the three mulched treatments had a 36 greater soil moisture reserve in the surface layer than the unmulched ridged and bare flat soil surfaces {Fig. 1). In the second season, the maximum soil moisture was observed under the black polythene mulch and the minimum with ridged treatments. During the first season of 1976 (with many periods of frequent and adequate rains) the differences in soil moisture reserve due to mulching were not conspicuous. Differences in soil moisture due to treat- ments were significant during periods of dry spells in the second season. On the average, mulched treatments had 2--3% more soil moisture than un- mulched plots. Compared with the ridged treatment, the fiat soil had a 0 .5 -1% greater soil moisture reserve in the 0--10 cm surface layer. 15 ~ CROP ~ BLACK POLYTHENE o - - -o WHITE POLYTHENE e - - - - , o S T R A W A_ - -z~ RIDGE -- - B A R E _ ~ _ . . [] RAINFALL . " - - ~Io A._~ "~ " " " >" z -"" Z 0 15 ~ | e _ • z :~ o ~ .._1 W ~ ,~ . CRO~ ~ b - 1.1.1 ° ~ Z b J t,m IO o m m ~/ m m o TIME ( WEEKS AFTER SEEDING) Fig. 1. Effect of mulch material and method of seedbed preparation on soil moisture content in 1976. (Rainfall data provided by courtesy of Dr. T.L. Lawson) . 37 Soil temperature Mean weekly records of soil temperature at 5 cm depths for the first 3 weeks in the first growing season are shown in Fig. 2. There were differences in soil temperature with respect to mulching, time of day, and methods of seedbed preparation. Compared with the flat bare ground surface, straw mulch reduced the maximum soil temperature and clear plastic increased it (both by about 5°C). The amplitude of diurnal fluctuation in the soil tem- perature was 7, 10, 12, 12 and 15°C for straw mulch, bare soil surface, 4 0 x - - -x BLACK POLYTI-iENE WHITE POLYTHENE STRAW o---.o RIDGE ~ --,~. 8ARE A ? 35 OC ~E bJ l-- ..J m 30 \ X i s/ ~ I I ~ SOIL 2.'~ I I i i i i 8 I0 12 14 16 18 TIME (HOUR) I 20 Fig. 2. Mean w e e k l y soil temperature measurements at 5 cm depth on a sunny day in the first growing season of 1976 . ridged soil, black polythene, and white polythene, respectively. In the first season, soil temperature during the seedling stage may have been supra-opti- mal (above 30°C) for a varying duration in a day depending on the soil mois- ture regime and the prevailing weather conditions (Fig. 2). Although soil tem. perature decreased drastically with increase in canopy cover, maximum tem- perature was generally supra-optimal during part of the day throughout the growing season for ridge and polythene treatments. 38 Root growth and development Detailed measurements of root development at different growth stages were made during the first growing season. Mulching significantly increased mean root density in the 0--40 cm layer. Mean root density in the mulched treatments was 8 mg/cm 3 compared with a mean of 4 mg/cm 3 for ridged and bare flat treatments. Treatment effects on root dry weight were not significant in the first 3 weeks after seeding (Fig. 3). Nevertheless, total root weight was generally high under straw mulch and low under bare flat ground surface. p.. z .J a. Gn I - 0 I- ! 0 I- 0.4 0.2 A 0 . 8 ~ . . . _ " - - h Fig. 3. Total dry weight of roots of maize seedling up to 3 weeks after seeding in the first season of 1 9 7 6. The effect of treatments on root growth was noticeable 30 days after seeding. Four weeks after seeding, root density of maize at a 15--20 cm depth was high in the bare flat treatment compared with other treatments (Fig. 4). However, maximum root density in the surface layer (2.0 mg/cm 3) 39 Block polythene 2.0~ ! E LO E z o.q ~, White polythene < q i ~ , Strow mulch I c ~ Ridge A V 2O Bare o I ;20 15 Fig. 4. R o o t dens i ty o f maize 4 weeks af ter seeding in the first season o f 1976. was observed in the straw-mulch treatment. Root density in the surface layer of the unmulched treatments was noticeably lower than with the mulched treatments (Fig. 4). Similar trends in root density were observed 10 weeks after seeding (Fig. 5). However, maximum root density at the 40 cm depth was observed under a black polythene mulch. Under favourable conditions of soil moisture and temperature, a root elongation rate of 4 cm/day was observed under the straw mulch treatment. Elongation rate was low in the seedling stage and reached a maximum value 3--4 weeks after seeding. In general, elongation rate was inversely propor- tional to the maximum soil temperature recorded at 5 cm depth (Fig. 6). A significant negative linear correlation was observed between maximum soil temperature during the 3rd and 4th weeks after seeding and rate of root elongation. The regression equations derived for different treatments are: E = --0.63T + 29.0, r = --0.96** white polythene E = --1.13T + 39.4, r = --0.89** straw mulch E = --0.68T + 28.5, r = -0 .92** black polythene E = - 0 . 4 7 T + 19.7, r -- --0.91"* ridges and bare ground surface where E is the elongation rate in cm/day and T is the mean maximum soil temperature in ° C. The maximum rate of root elongation of cowpea was 3.2 cm/day under the straw mulch treatment. Mean elongation rates were 3.2, 3.0, 2.4, 2.2, and 2.0 cm/day (with an LSD 0.05 of 0.11) for rice straw, black polythene, 40 i >- )-. Z IJJ I ,- 0 0 n.- B l o c k polythene White polythene uich Ridge B a r e , 0 5 30 '40YZ515 Fig. 5. Root density of maize 10 weeks after seeding in the first season of 1976. white polythene, bare flat, and ridged treatments, respectively. In general, the rate of elongation of cowpea roots was smaller than that of maize. Plant vigour Mean shoot elongation rate was higher in mulched than with unmulched treatments. Maximum shoot elongation rate was 5.5 cm/day for the straw mulch treatment and 3.0 cm/day for the ridged treatment (Fig. 7). Similar trends were observed in the development of canopy cover as measured by leaf area index {Table I). Leaf area index and shoot elongation rate were statistically similar for straw and black polythene mulch treatments. Leaf nutrient concentration Chlorotie symptoms and inter-veinal chlorosis were commonly observed on the young leaves of clear polythene and ridged treatments. These differ- ences were more pronounced during the first 2 weeks after emergence. Colours of the emerging leaves, monitored by Munsell colour charts for plants, were 7.5 GY 5/6, 7.5 GY 4/6, 5 GY 5/6, 5 GY 6/8 and 5 GY 7/8 for 41 I E o . . . . . 0 T V l B P E I I A T U i l [ C~ • I I K ~ T E L O N G A T I O N L - . . . . . ~ . . . . . . " 0 - . ~ . BLACK POLYTHENE O, e - • . . . . . . . . . . . . . e.. 2 ........ : WHITE POLYTHENE 0 4 i~ . .o I STRAW MULCH 0 4 3 2 ~ . . . . . I RIDGE 0 4 2 I BARE 0 a 13 i I il 2 4 5 TIME (WEEK AFTER SEEDING) 30 2o 40 30 w 20 ~ 30 _ 0 if) zo ~ t ~o ,~ ~E 3O z 20 ~. w 4o ~ 3O 2O '6 Fig. 6. Ef fec t o f soil t e m p e r a t u r e o n r o o t e l o n g a t i o n ra te o f ma ize fo r t he f i rs t 6 weeks a f t e r seed ing in t he f i rs t s e a s o n o f 1976. black polythene, straw, bare flat, white polythene, and ridged treatments respectively. Tissue concentrat ion of N and P was not significantly affected by treat- ments {Table II). In the first season, however, leaf contents of K and Ca were significantly lower with ridged and white polythene than with the straw mulch treatment. Grain yield and yield components Grain yield was significantly affected by mulching treatments and grow- ing seasons (Table III). Maize grain yield was generally higher in the first 42 f tratv Mulch I 0 2 Ridgo ~: JE Bare iI i I I I I I i I I 0 I 2 3 4 5 6 7 6 9 I0 II IZ 13 14 15 16 TIME ( DAYS AFTER SEEDING) Fig. 7. Rate of shoot elongation of maize with different mulch treatments in the first season of 1976. T A B L E I Leaf Area I n d e x of m a i ze as in f luenced by var ious t r e a t m e n t s ( l s t season, 1976) T r e a t m e n t s Days a f te r seeding 10 15 20 25 30 35 40 45 Black p o l y t h e n e 0 .039 0 .109 0 .217 0 .512 0 .9 1 7 2 .506 3 .367 3.77~ White p o l y t h e n e 0 .026 0 . 064 0 .129 0 .251 0 .406 1 .188 2 .176 2.82~ S t r aw m u l c h 0 .031 0 .120 0 . 243 0 .543 0 .807 2 .359 3 .372 3 .840 Ridge 0 .021 0 .056 0 .099 0 . 225 0 . 3 7 5 1 .268 2 .153 3 .324 Bare 0 .032 0 . 096 0 . 210 0 . 444 0 .7 7 3 2 .020 2 .782 3 .400 F (values) 2 . 8 1 4 " 6 . 646** 1 3 . 7 2 1 " * * 1 5 . 8 4 2 " * * 1 7 . 9 9 7 " * 2 .728*** 9 . 6 0 7 * * * 4 .040 L.S.D. (0 .05) 0 .013 0 .035 0 . 054 0 .121 0 .191 0 .418 0 .6 3 4 0 .6 6 2 *, **, * ** Signif icant a t t he 0 .10 , 0 .05 and 0 .01 level, respect ive ly . TABLE II Plant nutrient uptake as affected by various mulches (a) First crop 1976 43 T r e a t m e n t N% P% K% Ca% Black po ly thene 2.58 0.160 0.73 0.30 White po ly thene 2.55 0.173 0.63 0.24 St raw mulch 2.59 0.176 0.86 0.34 Ridge 2.48 0.166 0.66 0.27 Bare 2.48 0.163 0.77 0.28 L.S.D. (0.05) NS NS 0.19 0.07 (b) Third crop 1977 Black po ly thene 2.62 0.18 1.69 0.39 White po ly thene 3.11 0.18 1.69 0.42 Paddy s t raw 2.87 0.21 1.55 0.34 Ridge 3.05 0.19 1.72 0.35 Bare 2.65 0.19 1.70 0.39 L.S.D. (0 .05) NS NS NS NS NS, Non.Signif icant . TABLE In Effects of mulch treatment on grain yield (t/ha) at 15% moisture content 1976 1976 1977 1977 Treatment (Ist Season) (2nd Season) (Ist Season) (2nd Season) Maize Maize Maize Cowpea Maize Maize Maize Cowpea mono- mixed mixed mono- mixed mixed -culture cropped cropped -culture cropped cropped Black polythene 5.6 3.1 2.2 0.20 6.0 5.7 2.1 0.14 White polythene 5.1 2.7 2.0 0.16 2.4 3.0 1.2 0.10 Straw mulch 5.1 3.7 2.9 0.12 5.5 4.5 1.5 0.08 Ridges 4.2 1.2 0.6 0.04 3.1 0.9 0.4 0.05 Flat baxe 4.8 1.4 1.4 0.06 4.7 4.1 1.5 0.05 L.S.D. (0.05) 0.7 0.6 1.0 0.05 0.6 1.0 0.7 0.04 season than in the second because in the latter the rains ended abruptly, reducing the length of the growing period. Maize yield was suppressed by growing in association with cowpea although this decrease was proportional to the decrease in plant populations under mixed cropping. For the same season and cropping system, maize and cowpea grain yields were signifi- cantly affected by mulching and seedbed preparation treatments. In general, black polythene and straw mulching produced more yield than other treat- ments. The lowest yield was generally obtained on ridged plots. Unit grain weight of maize was also significantly affected by treatments. In the second season of 1976, the weight of 500 maize grains was 120, 116, 116, 104 and 83 g for black polythene, white polythene, straw mulch, bare flat, and ridged treatments respectively. The least siginificant difference at 44 the 5% level of probabili ty was 25 g. The percentage of unfilled grain was higher in ridged and unmulched flat than with mulched treatments. Mixed cropping with cowpea did not influence the percentage of unfilled grains or unit grain weight. CONCLUSIONS The results indicate a significant effect of soil temperature and moisture regimes on root growth and grain yield of maize and cowpea. Since the soil moisture regime under white and black polythene mulch was identical, the differences in grain yield associated with the black polythene mulch are at- t r ibuted to differences in soil temperature regime. The maximum soil tem- perature with black polythene mulch was 3--4°C lower than that with white polythene mulch. The low grain yield with ridges is at t r ibuted to a combina- t ion of high soil temperature and low soil moisture. Even though the maxi- mum soil temperature was lower on ridges than with the white polythene, a significant reduction in grain yield on ridges was caused by soil moisture stress. Differences in grain yield of ridged versus black polythene mulch also are at tr ibuted to low soil moisture in ridges since the maximum temperature in both treatments was not greatly different. Chlorotic symptoms of nutritional disorder on young maize seedlings grown on ridges and with white polythene mulch are at tr ibuted to high soil temperature. Similar symptoms on maize seedlings grown in controlled green- house and growth chamber studies were reported by Lal (1974 a,b) and Harrison-Murray and Lal (1979) when seedlings were grown at root tempera- tures exceeding 35 ° C. In addition to chlorotic symptoms, the leaf tips of the seedlings emerging from white polythene mulch were scorched due to a "greenhouse effect". Chlorotic symptoms, however, disappeared with the supplemental application of N 4 weeks after seeding. Nevertheless, the tissue contents of Ca and K were significantly reduced in those treatments with high maximum soil temperature. Additional research is needed on the mech- anisms of nutrient absorption and translocation under high soil temperature. The high root density directly beneath the straw mulch layer is at tr ibuted to favourable soil moisture and temperature regimes. High soil temperatures suppressed the rate of root elongation and decreased root density in the sur- face layer of the unmulched fiat and ridged treatments. High soil temperatures observed in the vicinity of Ibadan for crops planted on ridges or mounds in March--April can have a detrimental effect on grain yields of sensitive crops such as maize. It is not advisable to delay planting since this will reduce the probabili ty of a good second crop. The choice is limited either to varieties that may be tolerant of high soil temperatures or to the cultivation of crops with an adequate quanti ty of crop residue mulch. Progress has been made in the temperate regions to adapt varieties to cooler soil temperatures in the early spring. Similar breeding work is needed to se- lect varieties tolerant of the high soil temperatures of the tropics. 45 Methods of seedbed preparation may provide additional flexibility. A no- tillage system with crop residue mulch has been shown to reduce the maxi- mum soil temperature by 4--6°C and to improve soil moisture by increasing infiltration and decreasing water runoff and soil-water evaporation (Lal, 1974a, 1975). REFERENCES Allmaras, R.R. and Nelson, W.W., 1971. Corn. (Zea mays L.) root configuration as in- fluenced by some row--interrow variants of tillage and straw mulch management. Soil. Sci. Soc. Am. Proc., 35: 974--980. Allmaras, R.R. and Nelson, W.W., 1973. Corn root configuration response to soil tem- perature and matric suction. Agron. J., 65: 725--730. Baumer, M., 1964. Plastic mulch, nitrogen fixation and root development. Sudan J. Vet. Sci. Anim. Husb., 5: 38--39. B6hm. W., 1976. In-situ measurement of root length natural soil profiles. J. Agric. Sci. (Cambridge), 87 : 365--368. Chaudhry, M.R. and Prihar, S.S., 1974. Root development and growth response of corn following mulching, cultivation, and inter-row compaction. Agron. J., 66: 350--355. 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