effects of petroleum mulch on soil water content and soil temperature1

4
Effects of Petroleum Mulch on Soil Water Content and Soil Temperature 1 AHANG KOWSAR, L. BOERSMA ANDG. D. JARMAN S ABSTRACT Changes in soil temperature and soil water content of mulched and unmulched soil, subjected to the same radiation load, were measured at 2-hour intervals under controlled labor- atory conditions. Soil was packed into boxes with inside dimen- sions of 40.0 by 4.0 by 50.0 cm at a pre-determined water content. Part of the soil surface was covered with petroleum mulch. The soil was subjected to a diurnal temperature cycle by varying the output of infrared heat lamps. At a depth of 1 cm the mulch-covered soil was 5C warmer than the bare soil at the time the soil temperatures attained their maximum value. At all other times the temperature dif- ference between mulched and bare soil was smaller. The bare soil rapidly lost water in the upper 4 cm. The mulch-covered soil lost water in the upper 1 cm of soil but gained water at depths below this zone. This gain in water in the zone where seedlings are normally placed, indicates that the beneficial effect of petroleum mulch on germination and seedling growth must be attributed to improved soil water conditions as well as to improved soil temperature conditions. Additional Key Words for Indexing: seed, germination. W HEN sprayed onto the soil in a band over a row of planted seeds, petroleum mulch forms a thin skin of resins over the soil particles. Seeds covered in this manner may germinate several days earlier and develop more rap- idly than seeds not covered. Germination is more uniform as well. Usually the beneficial effect of petroleum mulch is attributed to increased soil temperatures (1, 6). A wide range of temperature increases resulting from mulch appli- cations have been reported. Temperature differences of up to 11.2C at a depth of 15 cm have been observed (13). Temperature increases ranging from 6C to 12C at seedling depth have been claimed in commercial literature. Mulch- ing increased germination, earliness and the total yield of cucumbers by 25%, while an increase of 1.2C in soil temperature at a soil depth of 5 cm was observed (7). Earlier maturity, larger fruit size, and some increase in yield was noted where petroleum mulch was applied on cucumbers, sweet corn, bush beans, and pole beans (8). Petroleum mulch increased the earliness and uniformity of emergence of some vegetable crops (11). Some research reports mention an effect of petroleum mulch on changes in soil water content (4). Earlier growth of crops and increased yields have been attributed to a reduction of evaporation caused by sealing of the soil sur- face, an increase in soil temperature and a reduction of leaching of minerals from the root zone. In one investiga- tion (5) an increase in soil water in the 0-3 cm soil layer was noted while no changes in soil temperature at the 2 cm depth were observed. Two days after application of the mulch the soil water content was 1 to 2% higher under the mulch than under the bare soil. Four days after application of the mulch the water content was 2 to 4% higher under

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Page 1: Effects of Petroleum Mulch on Soil Water Content and Soil Temperature1

Effects of Petroleum Mulch on Soil Water Content and Soil Temperature1

AHANG KOWSAR, L. BOERSMA AND G. D. JARMANS

ABSTRACTChanges in soil temperature and soil water content of

mulched and unmulched soil, subjected to the same radiationload, were measured at 2-hour intervals under controlled labor-atory conditions. Soil was packed into boxes with inside dimen-sions of 40.0 by 4.0 by 50.0 cm at a pre-determined watercontent. Part of the soil surface was covered with petroleummulch. The soil was subjected to a diurnal temperature cycleby varying the output of infrared heat lamps.

At a depth of 1 cm the mulch-covered soil was 5C warmerthan the bare soil at the time the soil temperatures attainedtheir maximum value. At all other times the temperature dif-ference between mulched and bare soil was smaller. The baresoil rapidly lost water in the upper 4 cm. The mulch-coveredsoil lost water in the upper 1 cm of soil but gained water atdepths below this zone. This gain in water in the zone whereseedlings are normally placed, indicates that the beneficialeffect of petroleum mulch on germination and seedling growthmust be attributed to improved soil water conditions as wellas to improved soil temperature conditions.

Additional Key Words for Indexing: seed, germination.

WHEN sprayed onto the soil in a band over a row ofplanted seeds, petroleum mulch forms a thin skin of

resins over the soil particles. Seeds covered in this mannermay germinate several days earlier and develop more rap-idly than seeds not covered. Germination is more uniformas well.

Usually the beneficial effect of petroleum mulch isattributed to increased soil temperatures (1, 6). A wide

range of temperature increases resulting from mulch appli-cations have been reported. Temperature differences ofup to 11.2C at a depth of 15 cm have been observed (13).Temperature increases ranging from 6C to 12C at seedlingdepth have been claimed in commercial literature. Mulch-ing increased germination, earliness and the total yield ofcucumbers by 25%, while an increase of 1.2C in soiltemperature at a soil depth of 5 cm was observed (7).Earlier maturity, larger fruit size, and some increase inyield was noted where petroleum mulch was applied oncucumbers, sweet corn, bush beans, and pole beans (8).Petroleum mulch increased the earliness and uniformityof emergence of some vegetable crops (11).

Some research reports mention an effect of petroleummulch on changes in soil water content (4). Earlier growthof crops and increased yields have been attributed to areduction of evaporation caused by sealing of the soil sur-face, an increase in soil temperature and a reduction ofleaching of minerals from the root zone. In one investiga-tion (5) an increase in soil water in the 0-3 cm soil layerwas noted while no changes in soil temperature at the 2cm depth were observed. Two days after application of themulch the soil water content was 1 to 2% higher under themulch than under the bare soil. Four days after applicationof the mulch the water content was 2 to 4% higher under

Page 2: Effects of Petroleum Mulch on Soil Water Content and Soil Temperature1

784 SOIL SCI. SOC. AMER. PROC., VOL. 33, 1969

the mulch and after 10 days the mulched and the bare soilhad about equal amounts of soil water. An application ofpetroleum mulch increased the soil temperature 2.0C ata depth of 5 cm and the soil water content 1.5% in thetop of 8 cm of soil (14).

The purpose of the present investigation was to establishwhether the effect of an application of petroleum mulch onseed germination and early plant growth should be attrib-uted to changes in the soil water content, to changes in soiltemperature, or to both.

PROCEDURE

Changes in soil water content and soil temperature resultingfrom diurnal temperature cycles imposed on a slab of soil weremeasured. Part of the soil surface was covered with petroleummulch. Measurements were made at several depths below themulch covered surface as well as below the bare surface.

The dimensions of the soil slab were chosen to represent asituation where a 20-cm wide band of mulch was appliedover rows spaced 80 cm apart. For these conditions lines Aand B shown in Figure 1 represent lines of symmetry and con-stitute the boundaries of the problem to be studied. Thewidth of the slab was therefore 40 cm. The depth of the slabwas set at 50 cm after it was established that under the experi-mental conditions used no temperature changes occurred atthis depth. The thickness of the slab was set at 4 cm to allowthe measurement of changes in soil water content with gammaattenuation using an 241Am source. The soil was contained inboxes made of J/4-inch plywood, sealed on the inside withepoxy paint and covered on the outside with thermal insulat-ing material.

Chehalis silty clay loam containing 6.5, 65.6, and 27.9%sand, silt, and clay, respectively, was used for the experiments.An initially uniform soil water content was obtained by pack-ing the boxes with a mixture of screened frozen soil and icein a cold room maintained at a temperature slightly belowfreezing. The amount of crushed and screened ice mixed withthe soil was measured to give a gravimetric water contentof 28%. The density of the soil was 1.0 g/cm3. Shortly beforethe start of an experiment the mulch was applied accordingto the geometry shown in Fig. 1.

Heat was applied to the soil slabs with three, 250-watt,infrared lamps, mounted 56 cm above the surface of the soil.A collimating device was inserted between the soil surfaceand the lamps. A diurnal cycle simulating the daily course ofthe sun was obtained by changing the light intensity with avariable transformer at hourly intervals. The total heat input

Fig. 1—Schematic diagram showing the dimensions of the soilslab used in the experiments in relation to the field situationsimulated.

was adjusted to obtain temperature amplitudes correspondingto those observed in field experiments involving the use ofmulch near Corvallis, Oregon.

Changes in soil water content were monitored with a colli-mated gamma beam. Readings were taken at predeterminedpoints at regular intervals. The soil container was placed on alathe bed. A bolt was threaded through a nut underneath oneof the guides allowing the soil container to be easily movedback and forth for positioning in the gamma beam. The gam-ma source and radiation detector were mounted on a platform,movable in the vertical direction, for positioning at the desiredsoil depth.

Soil temperatures were measured with individually cali-brated Fenwal GB 41 P8 glass probe thermistors. These wereinserted into the soil through holes penetrating the thermalinsulating material and plywood wall of the boxes. Measure-ments were recorded at 2-hour intervals with a single channelstrip chart recorder. Thermistors were switched in sequenceinto a wheatstone bridge measuring circuit using a timer androtary stepping switch. The bridge voltage output was re-corded as a series of steps, each representing a thermistor. Foreach thermistor a table was prepared for the conversion ofchart readings to actual temperatures.

RESULTS AND DISCUSSIONSoil Temperature Measurements

The temperature at a depth of 1 cm below the mulch(Fig. 2) was always higher than the temperature at thesame depth below the bare soil. The maximum temperaturedifference between the two points was almost 5.0C duringthe first day. Larger as well as smaller temperature differ-ences between mulched and bare soil have been reportedin the literature. The temperature difference varied duringthe day. Time of observation is rarely reported in the litera-ture and this omission makes a comparison of experimentalresults impossible. Other variables, not considered here,such as initial soil water content, maximum amplitude, andsoil type can cause variations in the temperature differences.

The maximum temperature difference at a depth of 1 cmdecreased from the first to the fourth day; the differencewas 4.9C, 4.1C, 2,9C, and 2,3C for the first, second,third, and fourth day, respectively. The maximum tempera-ture under the mulch remained nearly constant and rangedfrom 39.2C to 39.8C during the course of the experiment.The maximum temperature under the bare soil increasednearly linearly from 34.3C to 37.5C. The increase in maxi-mum temperature under the bare soil is the result of agradual water loss which decreased the specific heat. Nonet water loss occurred under the mulch-covered soil andconsequently, little change in the temperature regime wasobserved. At a depth of 3 cm (Fig. 3), the maximum tem-perature increased about 1C from the first to the secondday under both the mulch-covered and bare soil. From thesecond day on the temperature regime remained the sameat this depth. The temperature was always highest underthe mulch-covered soil. The maximum temperature dif-ference at 3 cm was about 3.0C.

Soil Water Content Measurements

Changes in soil water content at depths of 1.5 and 2.5cm are shown in Fig. 4 and 5, for mulch-covered and baresoil. At depths of 1.5 and 2.5 cm below the mulched sur-

Page 3: Effects of Petroleum Mulch on Soil Water Content and Soil Temperature1

KOWSAR ET AL.: EFFECTS OF PETROLEUM MULCH ON SOIL WATER AND TEMPERATURE 785

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Page 4: Effects of Petroleum Mulch on Soil Water Content and Soil Temperature1

786 SOIL SCI. SOC. AMER. PROC., VOL. 33, 1969

Table 1—Gravimetric soil water content and corresponding soilwater suctions at a depth of 1.5 cm below mulched

and bare soil surfaces at different timesduring the experiment

Day

112

Time

hr8:00

20:0020:00

Water

Mulch

%28.033.531.6

content

No mulch

%28.021.816.5

Soil watt

Mulch

bars

1.30.50.8

;r suction

No mulch

bars

1.33.4

11.0

face the water content increased. Most of the gain in waterat these depths occurred during the first day. The distribu-tion of water with depth below the mulched surface 16hours after the initiation of the experiment is shown inFig. 6. It indicates that the gain in water is the result ofa downward redistribution. Equations presented by Gary(2, 3) for thermally induced liquid and vapor flow wereused to calculate the redistribution of water for conditionsof this experiment. Results of these calculations are alsoshown in Fig. 6 and appear to be in reasonable agreementwith the experimetal data. At a depth of 1.5 cm belowthe bare soil the water content decreased. The greatestdrop in water content occurred during the first day whenthe soil lost 10% water. At a depth of 2.5 cm under thebare soil, the water content also decreased. Although therewas a slight increase in water content at the early hours ofthe day, each cycle had a lower water content than hadbeen noted at the same time of the preceding cycle.

CONCLUSIONSThe application of petroleum mulch caused a slight

increase in soil temperature and a considerable change insoil water conditions. The mulch prevented loss of watervapor and the soil atmosphere at the position where seedsnormally would be placed remained humid.

The changes in soil water content can be very importantfor the germination of seeds. Even slight changes in soilwater content bring about appreciable changes in soil watersuction (Table 1). The importance of differences in soilwater suction with respect to germination and seedlinggrowth has been discussed by several authors. Medicagotribuloides Desr. shows a distinct reaction to soil watersuction in the germination process (12). After 24 hours,80% of the seeds germinated at a suction of 0.001 barwhile no seed germinated at a suction of 0.20 bar. Thisphenomenon was attributed to the area of contact betweenwater and seed. At higher moisture suctions, there is lesscontact between the seed surface and the water. The avail-ability of water (solvent) in a liquid or gaseous form isessential for the process of imbibition which causes thesolution of colloidal particles, swelling of seed and thesubsequent breaking of the seed coat (9). An intensivestudy of wheat germination showed that the percent ofgermination at lower suctions is much higher than at highersuctions (10).

While the bare soil lost water rapidly during the first dayand gradually for the rest of the experiment, below themulched surface the water content remained almost con-stant for the duration of the experiment. A redistribution

DEPTH—CMFig. 6—Changes in volumetric soil water content below a

mulch covered surface as a function of depth, 16 hours afterinitiation of the experiment (solid line). The broken lineshows calculated changes in soil water content at the sametime, using steady-state equations.

of the water below the mulch was observed which seemsto improve germinating conditons.