Effect of Tillage on Soil-Water Movement During Corn Growth1

A. B. WEATHERLY AND J. H. DANE*

ABSTRACTThe occurrence of drought during the growing season often

limits the amount of soil water available for plant growth.The availability of soil water can further be limited if ahard layer exists not too far below the soil surface. Fieldexperiments were therefore conducted on a Cahaba sandy loamsoil (Typic Hapludult) to determine the effect of tillage prac-tices on soil water movement during corn (Zea mays L.) growth.Four tillage practices, viz., conventional-tUlage-without-subsoil-ing, conventional-tillage-with-subsoiling, no-tillage-without-sub-soiling, and no-tillage-with-subsoiling, were studied on a soilwith a plow plan. Soil water contents and soil water pres-sure heads were measured with a neutron probe and tensio-meters, respectively. Water extraction by roots was determinedby application of the integral form of the general transportequation for soil water. Soil water movement and water uptakewere less for the conventional-tillage-without-subsoiling treat-ment than for the other treatments. The two subsoiled treat-ments and the no-tillage-without-subsoiling treatment indicatedroot penetration and soil-water uptake below 50 cm.

When soil water uptake patterns were compared with cornyield on a weight-per-ear basis, there was a direct, positive cor-relation between the amount of water taken up by the cornplants and corn yield.

Additional Index Words: hard pan, drought stress, watermovement.

Weatherly, A. B., and J. H. Dane. 1979. Effect of tillage onsoil-water movement during corn growth. Soil Sci. Soc. Am. J.43:1222-1225.

THE FREQUENT OCCURRENCE of droughts in Alabamais often the major limiting factor for high crop

yields (16). The available soil water is further limitedbecause of compacted zones, usually induced by heavyfarm machinery (1, 2, 6, 10). Often, soil compactionoccurs in a layer immediately below the depth of cul-tivation (8) and has been shown to have a detrimentaleffect on crop yields (1, 11, 13, 14).

Soil water flow theory has been applied in variousfield experiments to analyze soil water movement.Van Bavel et al. (15), in a field experiment with uni-form soil, determined the rate of water extraction byroots, rz, in cm3 H2O/cm8 soil/day by applying thegeneral water transport equation

3F [1]

where 0 is the volumetric water content, t is time,Q is the Darcy flux, and z is vertical distance. Whenrz is integrated over chosen limits of z, the accumu-lated extraction by roots plus evaporation can be de-termined.

The Darcy flux is determined by the hydraulic gra-dient and conductivity. These hydraulic properties

1 Contribution from the Auburn University Agric. Exp. Stn.,Auburn, AL 36830. Received 11 May 1979. Approved 20 July1979.

2 Former Graduate Research Assistant and Assistant Professorof Soil Physics, respectively, Dep. of Agronomy and Soils, Au-burn University, Auburn, AL 36830.

are, however, affected by the geometry and distribu-tion of soil particles and thus by soil compaction.Croney and Coleman (4) found that the amount ofwater held at high suctions increases with increasingsoil compaction. Warkentin (17) reported similar re-sults but noted that the compaction effect is less forclay soils than for coarse-textured soils. The alterationin the water retention characteristics of the soil withcompaction is due to the decrease in the number oflarge pores (6, 17). Warkentin (17) further statedthat, since the larger pores provide ready access forroots and allow high water permeability, compactionshould restrict root growth, water movement, andwater uptake. Generally, an increase in bulk densitywith compaction alters the flow patterns of water tothe rooting zone (18). Soil compaction also increasessoil strength (13, 14), which is highly dependent onthe water content and increases with soil drying (2, 3).

Although water flow behavior in the field and wateruptake in the root zone have been described, few fieldstudies have attempted to measure water movement to-wards plant roots in soils with a compacted layer. Sincethe general flow equation is too complex to be solvedanalytically, estimates of soil water flow and wateruptake by plant roots require in situ measurementsof hydraulic heads and water contents for a numericalsolution of the equation. The objective of this studywas to determine soil water movement during activeroot growth in a soil with a compacted layer, as in-fluenced by cultivation and subsoiling.

MATERIALS AND METHODSTo study the effect of different tillage practices, an investiga-

tion was conducted on four plots of a more comprehensive fourtreatment, six replication experiment with corn (Zea mays L.)on a Cahaba sandy loam (Typic Hapludult) near Tallassee,Alabama. The soil had a compacted layer abruptly beginningat 15 cm (depth of disking operation) and extending to a depthof 32 cm (which was about 10 cm below the depth of plowing).

The four tillage treatments were conventional-tillage-without-subsoiling, conventional-tillage-with-subsoiling, no-tillage-with-out-subsoiling, and no-tillage-with-subsoiling. The conventional-tillage-without-subsoiling consistent of turning the soil to adepth of 20 cm, followed by disking to a 15-cm depth to forma suitable seedbed. The corn (Pioneer Brand 3369A) was plantedon 24 April 1978 in 15-m rows spaced 90 cm apart, and resultedin an average population of 35,000 plants/ha. Fertilizer andherbicides were applied according to recommendations. Theconventional-tillage-without-subsoiling and the conventional-tillage-with-subsoiling plots were cultivated twice during thegrowing season. Rainfall was measured throughout the growingseason. The soil-particle size distribution, determined by thehydrometer method, is presented in Table 1.

Table 1—Particle-size distribution by depths ofCahaba sandy loam.

Soil depth

0-2222-4747-6767-8787-107

107-127

Sand(>50/0

655040393746

Silt (2-50 /t)

302635384337

Clay(<2/»)

52425232017

1222

WEATHERLY & DANE: EFFECT OF TILLAGE ON SOIL-WATER MOVEMENT DURING CORN GROWTH 1223

Soil water pressures were obtained at 2-day intervals withtensiometers, inserted in duplicate on each treatment plot, atdepths of 15, 40, 60, 80, 100, and 120 cm. Nylon tubing con-nected the tensiometers to mercury manometers placed incabinets.

Neutron scatter equipment (Troxler 104A and 2601 rate-meter) was used every other day to determine soil water con-tents at 20-cm intervals from 20- to 140-cm depth at two loca-tions within a plot. The curve relating relative count rate tovolumetric water content was calibrated in situ.

Water release curves, hydraulic conductivity functions, andbulk density values were determined at 20-cm intervals betweendepths of 20 and 120 cm on undisturbed soil cores. Samplingwas accomplished by inserting brass cylinders (5.35 cm i.d. and3.0 or 6.0-cm high) vertically into the soil. Replicate cores weretaken for each depth increment. The 3.0-cm high soil sampleswere placed in Tempe pressure cells (12), saturated with water,and then drained at air pressures of 0.01, 0.02, 0.05, 0.10, 0.30,and 0.80 bars. Air pressures of 3.0 and 13.0 bars were applied tosoil samples, which were passed through a 2-mm sieve. Resultsof four replicate samples were averaged to form the water re-lease curve for each depth increment of each tillage treatment.

Saturated hydraulic conductivities at each depth were deter-mined on three replicate soil cores of 6.0-cm height. The soilcores were placed m Tempe pressure cells that had the porousplate removed. The sample was connected to a water sourcethat maintained a constant hydraulic head difference of 6.0 cm.Steady-state, saturated flow was maintained, and the saturatedhydraulic conductivity, K,, was calculated from the Darcyequation.

The relationships between hydraulic conductivity and watercontent were calculated by method II of Green and Corey (7),which is a modified Millington and Quirk (9) procedure. Thenecessary matching factors were obtained from the ratio ofmeasured and calculated K, values.

Water uptake by corn at a particular depth was computed fromEq. [I]. In order to calculate soil water uptake by roots forone of the five depth intervals (0-30, 30-50, 50-70, 70-90, 90-110 cm), Eq. [1] was modified to an integral form. After mak-ing appropriate assumptions, the following equation results:

Table 2—Bulk density values and their standard deviations ofthe different treatments as a function of depth.

P r(z)dz = ••[2]

The term on the left side of Eq. [2] represents total uptake ofwater by roots from a layer with thickness zntl— zn over a timeinterval At. The first term on the right side can be determinedfrom volumetric water content profiles. The other two termson the right side of Eq. [2] were determined from the Darcyequation. These determinations required knowledge of hydrau-lic head gradients and the relationships between hydraulic con-ductivity and water content. The hydraulic head values, re-quired for the solution of the Darcy equation, were obtainedfrom tensiometer readings throughout the first half of the grow-ing season and from the water retention curves when the ten-siometers no longer functioned because of dry soil conditions.

The general water-uptake formula was solved for the follow-ing conditions:

1) The flux between two layers was determined by using theaverage hydraulic conductivity between the soil layers.

2) The flux between soil layers was determined by using theminimum hydraulic conductivity between the two layers.

Soil water measurements commenced when corn plants wereabout 20 cm high (15 May 1978) and ended 60 days later, whenwater use patterns indicated very little uptake of soil water bythe corn roots. At the end of the growing season, pits weredug in each plot, and the distribution of roots in the soil profilewas observecf.

RESULTS AND DISCUSSIONThe lower bulk density values at the 20-cm depth

for the two subsoiling treatments demonstrate theeffect of subsoiling on compacted soil (Table 2).

Similar effects have been reported by Croney andColeman (4) and Warkentin (17). Bulk densities insubsoiled treatments were lower because of the dis-ruption of the compacted layer.

TreatmentSoil

depthBulk

densitytStandarddeviation

g/cm' g/cm'Conventional-tillage- without-

subsoilingConventional-tillage- with-

subsoilingNo- tillage- without-subsoilingNo- tillage- with-subsoilingAll treatmentsAll treatmentsAll treatmentsAll treatmentsAU treatments

20

202020406080

100120

1.57

1.411.571.431.631.651.541.491.45

0.03

0.020.030.030.020.020.030.020.02

t Average of 3.0 and 6.0-cm thick undisturbed core samples.

The hydraulic-conductivity-matching factors, usedin the calculations of hydraulic conductivity as a func-tion of water content by the modified Millington andQuirk procedure, ranged in value from 0.151 to 6.895.The range of matching-factor values arises from thedifficulty in precisely determining pore-size distribu-tions from the water retention curves.

Use of average hydraulic conductivity values or theminimum hydraulic conductivity value between twoconsecutive layers resulted in very similar water up-take patterns as calculated from Eq. [2]. Water up-take values will therefore be reported as the averagevalues calculated by the two methods. The calcula-tions also indicated that for all treatments, water fluxbetween layers was small compared to the total wateruptake by the roots from each layer.

Soil water uptake by roots as a function of depthand treatment by 20-day intervals shows that plants ineach treatment used about the same amount of waterin the top 30 cm of soil over each of the three time in-tervals (Fig. 1A-1D). After 40 days, most of the waterin the surface layer was depleted, and rainfall amountswere not sufficient to allow appreciable uptake in the0- to 30-cm depth layer. The conventional-tillage-without-subsoiling treatment (Fig. 1A) indicated verylittle soil water uptake below 70 cm. For the entiregrowing season, only about 12% of the total soilwater uptake (22.6 cm) in this treatment occurredbelow 50 cm. Since rainfall was deficient for thegrowing season and because little water uptake oc-curred below 50 cm, the conventional-tillage-without-subsoiling treatment was subjected to severe droughtstress. Drought stress was evidenced by the fact thatthe corn on the conventional-tillage-without-subsoilingplots went through extended periods of wilting andalso because plant height was less on these plots thanon the others.

Total soil water uptake (25.4 cm) for the conven-tional-tillage-with-subsoiling treatment (Fig. IB) wasgreater than for the conventional-tillage-without-sub-soiling (22.6 cm). This was mainly due to water up-take from below 50 cm, which was about 22% of thetotal soil water uptake. In addition, corn on the con-ventional-tillage-with-subsoiling plots showed lesswilting and plants were taller than for the corn onthe conventional-tillage-without-subsoiling plots.

About the same total water uptake (25.9 cm) oc-curred in the no-tillage-without-subsoiling treatment(Fig. 1C), as in the conventional-tillage-with-subsoiling

1224 SOIL SCI. SOC. AM. J., VOL. 43, 1979

SOIL WATER UPTAKE, CM2 4 6 8

SOIL WATER UPTAKE, CM2 4 6 8 10

CONVENTIONAL TILLAGEWITHOUT SUBSOILING

SOIL WATER UPTAKE, CM4 8 12 16

130

Fig. 1—Soil water uptake by corn as a function of depth andtime for four different tillage treatments.

treatment (25.4 cm, Fig. IB). However, corn on theno-tillage-without-subsoiling plot used more waterfrom below the 50-cm depth (28% of total wateruptake) than corn on the conventional-tillage-with-subsoiling (22% of total water uptake).

Corn on the no-tillage-with-subsoiling plot (Fig.ID) made considerable root penetration (as observedat the end of the growing season) into the subsoil andused more water than in any other treatment. Totalwater uptake was 27.0 cm of which 28% was frombelow 50 cm. This treatment resulted in the tallestplants and showed less wilting than with any othertreatment.

Maximum use of subsoil water occurred from the 70-to 90-cm depth (Fig. IB-ID). Figure 2 provides acomparison of soil-water uptake for all treatmentsand illustrates the decline in use of subsoil waterbelow the 70- to 90-cm depth.

Average evapotranspiration rates for the 60-day in-terval in which soil water measurements were maderanged from 3.8 mm/day in the conventional-tillage-without-subsoiling treatment to 4.5 mm/day in theno-tillage-with-subsoiling treatment. These values in-dicate close agreement with corn studies conducted byWright et al. (19) and Doss et al. (5).

The manner in which soil water data were takennecessitated the comparison of water-use data withyield on an ear weight basis. The tensiometers and

130Fig. 2—Total water uptake by corn roots as a function of depth

for all four treatments: (1) conventional-tillage-without-sub-soiling, (2) conventional-tillage-with-subsoiling, (J) no-tillage-without-subsoiling, and (4) no-tillage-with-subsoiling.

neutron-probe access tubes were installed directly be-tween healthy corn plants. Therefore, measurementsof soil water uptake by roots were made on a smallplant-population basis and not on a plot basis. Theyields for the conventional-tillage-without-subsoiling,the conventional-tillage-with-subsoiling, the no-tillage-without-subsoiling, and the no-tillage-with-subsoilingwere 0.126, 0.167, 0.151, and 0.183 kg/ear, respectively.The no-tillage-with-subsoiling treatment, which re-sulted in the highest yield on a kg/ear basis, also re-sulted in the greatest water uptake (27.0 cm) of anytreatment. The conventional-tillage-with-subsoilingand the no-tillage-without-subsoiling treatments re-sulted in similar kg/ear yields and almost identicalwater uptake values (25.4 and 25.9, respectively). Theconventional-tillage-without-subsoiling treatment re-sulted in the lowest yield on a kg/ear basis and thelowest water uptake (22.6 cm).

Stand on the conventional-tillage-without-subsoilingplot was higher than on the other treatment plots.Therefore, competition among plants might have af-fected the yield on a weight-per-ear basis. However,no significant correlation between mean ear numberper row, which was a direct indication of stand, andmean weight-per-ear existed for the four treatments.Furthermore, any correlation that existed was posi-tive rather than negative. If competition was a factorin the corn plots, a negative correlation would be ex-pected between stand and yield per ear. It is thusreasoned that any difference in yield between treat-ments was due to treatment and not competition.

CONCLUSIONS

For the particular soil studied, the results of thisexperiment suggest the following conclusions: (z) sub-soiling permitted greater root penetration into the50- to 110-cm soil layers and thus allowed greater wateruptake at these depths (2.71 cm of water for the con-ventional-tillage-without-subsoiling vs. 5.59 cm of wa-ter for the conventional-tillage-with-subsoiling treat-

TSIPORI & SHIMSHi: ...EFFECT OF TRICKLER LINE SPACING ON YIELD OF TOMATOES 1225

ment; and 7.25 cm of water for the no-tillage-without-subsoiling vs. 7.56 cm of water for the no-tillage-with-subsoiling treatment); (if) for a relatively dry grow-ing season, similar to the one in which this study wasconducted, vertical soil-water flux in the soil profilecan be expected to be small compared to water up-take by roots; (Hi) no-tillage-with-subsoiling was themost effective tillage method in permitting water up-take by the roots from the lower soil layers; and (iv)as the amount of rainfall during a growing season in-creases, it is expected that the beneficial effect of sub-soiling would be reduced.