Effect of Degree of Soil Profile Disruption on Plant Growth and Soil Water Extraction1

K. RAKOV AND H. V. EcK2

ABSTRACTModification of slowly permeable soil profiles has been effec-

tive in ameliorating undesirable soil conditions. Various meth-ods and depths of modification have been studied but littleattention has been given to ( i ) the degree of disruption neces-sary to accomplish satisfactory profile modification or ( i i ) therelative merits of topsoil-subsoil mixing and stockpiling andreturning topsoil to the surface after profile modification. Westudied seven degrees of profile disruption (clod size distribu-tions in the B22t and mixing of topsoil with that horizon) insimulated soil profiles in the greenhouse. Grain sorghum (Sor-ghum bicolor) was grown on Pullman clay loam. Disturbance ofthe B22t layer increased yields but once that layer was dis-turbed, degree of disturbance had no further effect on yield.Compared with retaining topsoil on the surface, mixing it withthe B22t did not affect yields.

Additional Index Words: Pullman clay loam, water use,water use efficiency.

MODIFYING SLOWLY permeable soil profiles has effec-tively increased water infiltration rate, changed the

amount and distribution of stored water in the profile,changed plant rooting patterns, and increased yields (1, 2,3, 4, 5, 6, 7). It is also effective for enhancing drainage inpoorly drained soils. Methods of modifying the profile haveincluded excavation and mixing with a backhoe, mixingwith ditching machines, moldboard plowing, disk plowing,slip plowing, vertical mulching, deep chiseling, and others.Different depths of modification have been studied usingsingle methods. Other studies have compared several modi-fication methods in single experiments (4). Little research

has been done, however, to determine (i) the degree of dis-ruption necessary to accomplish satisfactory profile modifi-cation or (ii) the relative merits of topsoil-subsoil mixingand stockpiling and returning topsoil to the surface afterprofile modification.

In this experiment we studied the effects of (i) differentdegrees of profile disruption and (ii) mixing topsoil withsubsoil on plant growth and soil water extraction.

METHODS AND MATERIALSSeven simulated soil profile modification treatments were

studied in a greenhouse experiment. The soil used was Pullmanclay loam. It has a moderately permeable surface horizon over-lying a very slowly permeable B22t horizon. The Pullman seriesis a member of the fine, mixed, thermic family of TorrerticPaleustolls (order Mollisols). A detailed description of the soilis available (9). The profiles were composed of screened top-soil (< 1 cm), undisturbed B22t or various size distributionsof B22t, and undisturbed subsoil (B2bl) from just below theB22 layer. Treatments (Table 1) were based on the degree ofdisturbance of the B22 t layer. We assumed degree of disturb-ance to be directly proportional to the fineness to which the clodswere reduced. Air-dried soil from the B22t horizon was sepa-rated (by screening) into the following clod-size fractions: > 10cm, 7.5 to 10 cm, 5 to 7.5 cm, 2.5 to 5 cm, and < 2.5 cm. Thefractions were then combined to give the size distributions(treatments) listed in Table 1. The soil was screened withoutcrushing, thus screening constituted sorting of clods. Since thehand screening technique used allowed oblong clods to passthrough the screens, some clods with similar short axes variedin length. Dimensions given are those of the shortest axes.

Table 1—Size distributions of soil fractions (of B22t horizon)used in treatments

Treatment

12

>10 cm

3525

7. 5- 10 cm

3525

5.0-7. & cm———— % ———

1020

2. 5-5 cm

1020

<2.5cm

1010

5050

* The difference between treatments 5 and 6 was that In 'treatment 6, topaoil and sub-•oll were mixed.

RAKOV & ECK: EFFECT OF SOIL PROFILE DISRUPTION ON PLANT GROWTH AND SOIL WATER EXTRACTION 745

Table 2—Yields of heads, dry matter, and roots as affected bysoil profile modification treatments

Treatment

1234567

Heads

94. 3at90. 3a

100. 3a92. Oa94. 7a90. 7a70. 8b

Dry matter—— g/pot ———

192. 7a182. 7a197. 4a193. Oa193. 9a172. 2a150. 6b

Roots*

35. 8a34. 3a36. 7a33. 8a36. 7a33. 7a28. Oa

* Root weights are ash-free.t Meana in the same column followed by the same letter are not different from each

other.

Soil containers were 100-cm long cylinders of 20-cm diame-ter steel pipe containing 30 cm of undisturbed subsoil (B2bl)from just below the B22t layer. Cylinders were prepared byhydraulically pushing the pipe sections into the denuded soillayer. Cylinders containing soil were brought into the green-house and brought to uniform weight by removing soil. Alltreatments except treatment 6 first received an 18.3-kg aliquotof a mixture of appropriate size fractions from the B22t horizon(Table 1). Air dry topsoil was then added to each treatment(except 6) in sufficient quantity such that the soil surface wouldbe at approximately the same level in all treatments. Five kilo-grams topsoil were added in treatments 1 to 4 but 6 kg wereadded in treatments 5 and 6. Treatment 6 differed in that thetopsoil and the B22t aliquot were thoroughly mixed before plac-ing it in the cylinder.

For treatment 7, in which undisturbed B22t was studied, pipesections were pushed into that layer. The resultant undisturbedcores were removed from the pipe sections (pipe was openedwith an acetylene torch) trimmed to 30 cm length, and placedin prepared cylinders. To insure contact between the cores andthe undisturbed soil in the cylinders, a small amount of dried,finely divided « 2 mm) B22t soil material was placed betweenthem. To raise the soil level in the cylinders, 3 kg of disturbedB22t (< 2.5 cm) was placed on top of the undisturbed core,then each cylinder received 5 kg of topsoil.

Each treatment was replicated three times. Cylinders werearranged in a completely randomized design. Each cylinder wasgiven sufficient water to bring the soil to approximately 30%moisture. In each case, the final 1.35 liter of water containedenough N and P to equal a fertilizer application of 500 kg ofN/ha and 100 kg of P/ha (on an area basis).

After the soil drained, four grain sorghum (Sorghum bicolor(L.) Moench) hybrid DeKalb E-59 seeds were placed on thesoil surface of each cylinder and covered with 288 g of dry top-soil on 18 May. After emergence, the stand was thinned to twoplants per cylinder. Small quantities of water were occasionallyadded to the soil surface to ensure plant establishment. From8 June to maturity the experiment was irrigated each time plantsin any of the disturbed soils (any treatment except 7) showedsymptoms of drought (seven irrigations). Cylinders wereweighed, cylinder bottoms were sealed (sheets of rubber wereplaced under each cylinder), and water was added to each cyl-inder periodically to keep water on the surface. The soil waswetted from the surface and from the sides (through shrinkagecracks that formed between soil and cylinders as the soil dried).After 6 hours of flooding, excess water was removed from the

Table 3—Water used and water use efficiency as affected bysoil profile modification treatments

Treatment

1234567

Waterused*

kg28. 2abt26. Sab27. Sab27. Sab29. 3a25. 9b20. Ic

Water use efficiencyHeads

301. la300. 8a276. 4a300. 8a309. 5a285. 9a285. 5a

Dry matter0/g —————

146a147a140a143a151a152a134a

* Total water used between 8 June and 6 Sept.t Means In the same column foUowed by the same letter are not significantly different

from each other.

Fig. 1—Profiles of treatments 2, 3, and 7. Plants were at bloomstage. Note that profiles maintained differing physical proper-ties after five wetting and drying cycles.

soil surface, bottoms were opened, and the soils were allowedto drain. Cylinders were reweighed after 20 to 48 hours. Thefinal irrigation occurred on 7 Sept. and plants were harvestedon 10 Oct. By 10 Sept., most plants had reached physiologicalmaturity.

We measured water used, between 8 June and 6 Sept., andyields of heads, dry matter (heads plus forage), and roots(Table 2). For root weights, cylinders were opened with anacetylene torch and soil was washed from the roots. Roots weredried, weighed, and then ashed (root weights are reported onan ash-free basis).

RESULTS AND DISCUSSIONProfiles of treatments 2, 3, and 7 are shown in Fig. 1.

The cylinders were opened in late July after five wetting anddrying cycles. Note that although the soil settled, the clodsretained their size and shape, maintaining differences inphysical properties caused by the treatments.

Both head and dry matter yield data show that disturb-ance of the B22t layer increased yield but once that layerwas disturbed, the degree of disturbance had no furthereffect on yields (Table 2). Although root yields were notsignificantly affected by any treatments, yields tended to belower on the undisturbed soil.

Compared with retaining topsoil on the surface, mixingit with the B22t layer (treatments 5 and 6) did not affectyields. The trend towards lower dry matter yield when top-soil and subsoil were mixed was due to low yield on onecylinder (data not shown). Apparently, when this soil pro-

746 SOIL SCI. SOC. AMER. PROC., VOL. 39, 1975

30 -

"̂

I 20

ic. 10XI5o

Fig. 3—Roots washed from soil of treatments 2, 3, and 7. Rootsare from profiles shown in Fig. 1.

file is modified, it is not necessary to keep the topsoil onthe surface.

Water use between 9 June and 6 Sept. (Table 3) showsthat significantly less water was used on the undisturbedtreatment than on the other treatments. However, water useefficiency data show that the amount of water used pergram of heads or dry matter produced was the same forall treatments.

Data on water use between individual irrigations (Fig. 2)showed that in every period between irrigations, plants onthe undisturbed treatment used less water than those ondisturbed soils. This was true even though cylinders withthe undisturbed soil contained more soil than the others,thus, held more total water. Also, plants in the undisturbedtreatments showed drought symptoms before those in thedisturbed treatments. Examination of rooting patterns ofdisturbed and undisturbed treatments indicated that com-pared with the disturbed treatments, less water was used onthe undisturbed soil because root growth and root explora-tion in the B22t layer and in the undisturbed subsoil corebelow the B22t layer was much less in the undisturbed treat-ment (Fig. 3). Roots in the undisturbed B22t layer weresmall and distributed around the soil-cylinder interface.Although they became more plentiful below the layer, theywere not as numerous as those in cylinders with disturbedB22t layers. Although soil moisture was not measured whenthe roots were washed from the soil, when the cylinders

6/21 7/5 7/19 8/3

Date Measured

8/20

Fig. 2—Water used between 8 June and 6 Sept. on disturbed(treatments 1, 5) and undisturbed (treatment 7) profiles.

were opened, those with the undisturbed B22t layer werenoticeably wetter in that layer and below it than those withthe disturbed B22t layer.

The disruption treatments studied in the greenhouse werelimited by the size of the cylinders, however, the leastdrastic (largest clods) treatment was as good as the mostdrastic. We do not have comparable studies in the field,however, field studies on the soil on plowing depths (7, 8)have shown that disruption into (but not through) the B22tlayer was optimum for use under graded furrow irrigation.Hauser and Taylor (4) found that chiseling on 2 m centersdid not have a lasting effect on water infiltration into thissoil while disk plowing did. Perhaps some treatment moredrastic than deep chiseling on 2 m centers and less drasticthan moldboard plowing would be sufficient. Apparently,to be lasting, the treatment should be drastic enough torearrange the soil aggregates.