Effects of Cover Crops on Pore-Size Distribution in a Coastal Plain Soil1

J. F. LUTZ, W. L. NELSON, N. C. BRADY, AND C. E. ScARSBRooK2

W INTER cover crops have been recommended formany years. Beneficial effects such as increased

yields of succeeding crops, soil and water conserva-tion, and an improved physical condition of the soilhave been attributed to them. Experimental datashow conflicting results on the first two contentions,even though most of them are favorable (i, 2, 3, 7,8, io).3 Increased yields usually have been at-tributed to the nitrogen supplied by the cover crop.In some instances this has been substantiated by add-ing commercial nitrogen to the no-cover crop plots.In other cases fertilizer treatments were not as effec-tive as cover crops. Hence, it was generally assumed,without much experimental data, that the responseswere due to the effect of the cover crops upon thephysical condition of the soil. Several investigatorshave studied the effects of crops on aggregation (2,3, 7, io). Relatively little work has-been done on theeffects of crops on the pore-size distribution in soils(I).

It is the purpose of this paper to present some dataon the effects of certain winter cover crops, in tworotations, on the size distribution of pores in the soil.Some penetrometer data also will be given.

MATERIALS AND. METHODSThe cover crop experiment from which soil samples were

taken was located at the Upper Coastal Plain Station, RockyMount, N. C. The soil is predominately Marlboro fine sandyloam. The exchange properties of the soil and available phos-phorus (Truog method) are given in Table I.

TABLE I.—Analysis of soil on experimental area.

Soil

Surface. . . .Subsoil ......

pH

6-55-4

M.E. per 100 grams

Baseex. cap.

4.25-9

Ca

2.482.52

Mg

0-730-45

K

O.I I0.09

P2O5, lbs.per acre

63Trace

The experiment was initiated in 1941. Two rotations,cotton-peanut and cotton-corn, were used with each crop ineach rotation appearing every year. Three winter cover crops,hairy- vetch, Austrian winter peas, and rye grass, were com-pared with no cover in both rotations. In 1945, however, achange was made in the cotton-corn rotation when oats weresubstituted for rye grass.

The cotton, corn, and peanuts were the whole plots andcover crops were split plots on each of these plots. Eachcover crop plot:was 17.5 by 182.4 feet. The cover crop plotswere further subdivided so that three rates of nitrogen wereapplied to the cotton and to the corn.1 The nonlegume covercrops received 9 pounds of nitrogen at planting and 9 poundsin February. In the fall of 1945, however, one half of each

rye grass plot received 27 pounds of nitrogen at planting and27 pounds in February. In the experiment there were threereplications of the cotton-corn and cotton-peanut rotations,although all replications were not' used in making some ofthe measurements. <

The cover crops are usually seeded between the rows ofcotton after the first picking, in the peanut plots after digging,and in the corn plots after the corn is harvested and the landdisked. In 1944 the cover crops were seeded September 22.In 1945 the cover crops after corn and peanuts were seededon September 28, and after cotton on October 24. The firsttwo years of the experiment the cover crops were broadcastby hand and covered lightly with a cultivator. In 1944 a three-disc drill was used and in 1945 a five-disc drill (Van Brunt)was used.

The yields of cover crops were taken just before turning,in the period March 25 to 31. Cover crop samples were takenfrom an area YIO.OOO acre in size, dried, weighed, and analyzedfor total nitrogen. Table 2 gives an outline of sampling datesand treatment on the plots sampled.

The samples were taken in metal sample boxes 2£| inchesin diameter and i9/ie inches deep by means of a special sam-pler (Fig. i) devised by the senior author (6). Pore size

FIG. i.—The sampling apparatus, showing the canin position.

'Contribution from the Department of Agronomy, North Carolina Agricultural Experiment Station, Raleigh, N. C. Publishedwith the approval of the Director as paper No. 253 of the Journal Series.2Professor of Soils, Associate Professor of Agronomy, Research Instructor, and Research Assistant, respectively.

'Figures in parenthesis refer to "Literature Cited", p. 46.4The cotton and the corn received adequate amounts of phosphorus and potash directly. The peanuts received gypsum.

43

44 SOIL SCIENCE SOCIETY PROCEEDINGS 1946

TABLE 2.—An outline of the sampling dates and treatment on the plots sampled for porosity measurements.

Date

Aug. 3, I 9 4 5 - - - - . . . . . . . . . . . . . . . . . . . . . . .Aug.,3, 1945..... . . . . . . . . . . . . . . . . . . . . . .Sept. 27, 1945. . . . . . . . . . . . . . . . . . . . . . . . . .Oct. 19, 1945,. . . . . . . . . . . . . . . . . . . . . . . . . .Oct. 19, 1945 . . . . . . . . . . . . . . . . . . . . . . . . . .Aug. 13, 1946. . . . . . . . . . . . . . . . . . . . . . . . . .Aug. 13, 1946... . . . . . . . . . . . . . . . . . . . . . . .

Sept. 17, 1946 . . . . . . . . . . . . . . . . . . . . . . . . .

Rotation*

Peanuts (cotton)

Cotton (corn)

Cover crops

Check, oats, peas, vetch

Number ofreplications

sampled

i

2333333

Number ofsamplesper plot

44455444444

*The crop listed without parenthesis was the one growing at the time of sampling; the one listed in parenthesis was the preceding crop.tCheck means no winter cover crop. JPeas refers to Austrian winter peas.

distribution was determined by the Learner-Shaw method(5), using plate glass instead of metal. The volume per-centage of pores drained by tensions of 10, 20, 30, 40, 50, and60 cm water columns was determined. The samples wereoven dried and the total porosity calculated as per cent byvolume.

Penetrometer measure-ments were made with aplunger type apparatus sup-plied by the Rototiller Com-pany (Fig. 2). A metalplunger, with a tapered point,is released by a trigger andallowed to fall a given dis-tance. The plunger and a slitin the side of the tubethrough which it falls arecalibrated in inches and canbe read in sixteenths. Meas-urements were made in 1946on the check and vetch plotsas follows:Peanuts (cotton) and cotton

(peanuts)August 13,22—Eight read-

ings per plot,- three rep-lications

September 1 7 — S i x t e e nreadings per plot, threereplications

Cotton (corn)September 1 7 — S i x t e e n

readings per plot, threereplications

EXPERIMENTAL DATACOVER CROP YIELDS

The cover crops maderelatively poor growth, inthe years prior to thewinter of 1944-45. Inthat year and again inthe winter of 1945-46good growth was ob-tained. The yields forthose two years are givenin Table 3.

crements of water loss at the various tensions, it wasfound that the greatest increment was lost at the 10-cm tension. In fact, the loss at 10 cm was greater insome instances than the sum of the increments fromthe 10- to 6o-cm tension. Furthermore, there was noconsistent relation among the losses between the 10-and 6o-cm tensions. Therefore, only the ib- and 50-cm losses and the total porosity are plotted. It isevident in Figs. 3 and 4 that the changes in thepercentage of large pores (10 cm) are largely respon-sible for the differences in the percentage drained at60 cm and in the total porosity.

Fig. 3 shows the porosity data from the samplingsin 1945. On August 3 samples were taken from three

w0.

TOTAL

FIG. 2.—The penetrometer.

PORE-SIZE DISTRIBUTION

Upon plotting the in-

40

35

30

J 0

9

5 I0

! 5

0

7.5

5.0

2.5

CORN PEANUTS PEANUTS COTTON(COTTON) (COTTON) (COTTON) (CORN)

8-3-45 — 9-27-45 — 10-19-45

FIG. 3.—Percentage by volume of pores drained at the 10-and 6o-cm tensions, and the total porosity, in 1945. (P =Austrian winter peas; V = Vetch; R = Rye grass; O =Oats, as cover crops.)

LUTZ, ET AL.: PORE-SIZE DISTRIBUTION

TABLE 3.— Yields of cover crops*

Cover crop

Peas... . . . . . . . . . . . . . . . . . . . . .V e t c h . . . . . . . . . . . . . . . . . . . . . . .

Cotton 1946(corn 1945)

1945

2,3435,9435,467

1946

3,7674,0934,198

Corn 1946(cotton 1945)

1945

2,6073,6273,783

1946

1,4271,9602,137

Cotton 1946(peanuts 1945)

1945

2,9024,6833,833

1946

5,i872,2402,583

Peanuts 1946(cotton 1945)

1945

2,4573,5i33,797

1946

3,HO2,2872,007

*Pounds of dry matter per acre in tops at the time of turning.

PEANUTS COTTON , CORN COTTON(COTTON)(PEANUTS) (CORN) (COTTON)(PEANUTS) (CORN)

8-13-46 — 8-22-48 — 9-17-46

FIG. 4.—Percentage by volume of pores drained at the 10-and 6o-cm tensions, and the total porosity, in 1946. *Sig-nificant differences; **highly significant differences. (P =Austrian winter peas; V = Vetch; R = Rye grass; O =Oats, as winter cover crops.)

cover crop treatments (check, Austrian winter peas,and vetch) from two rotations (corn following cot-ton and peanuts following cotton). There were fairlylarge differences between treatments on the corn plotsbut because of the limited number of samples, statisti-cal analysis showed that they were not significant. Onthe peanut plots the cover crops showed little effect.The reason for the difference between the two rota-

, tions is the time of cultivation in relation to time ofsampling. The peanut plots had been cultivated about10 days before sampling, while it had been 6 weekssince the corn plots were cultivated. This effect ofcultivation on the large pores is still further shownby the decrease on the peanut plots from August 3to September 27. The percentage drained at 60 cmhad increased at the expense of the larger ones (10cm). At the later date relatively little difference be-tween treatments was evident except in total porosity.

The data from the October 19 sampling of the cot-ton plots show several interesting points. First, thatthe increase in large pores and total porosity resultedfrom turning under the cover crops; second, that thedifferences at the 6o-cm level and in total porositywere due to the differences at the lo-cm tension; and,third, that there was a greater percentage of large

• pores and total porosity in this rotation, where cornwas the preceding crop, than in the others. This is

more clearly shown by the 1946 data on samplestaken from different rotations on the same dates.

Fig. 4 shows the data from three samplings in1946. The two groups of bars on the left are peanuts(following cotton) and cotton (following peanuts),sampled on August 13. As indicated by the asteriskson the graph, there was a significant increase in thepercentage of large pores (10 cm) due to the covercrops as compared with the check. At the 6o-cmtension and in total porosity there was a highly sig-nificant increase". There were no significant differ-ences between the three cover crops or between thetwo rotations (peanuts-cotton or coftow-peanuts).

The two groups of bars in the middle of Fig. 4show the data from the cotton (corn) and corn (cot-ton) rotation on August 22. There was no significantdifference between any of the treatments at the 10-cm tension on the cotton (corn) plots, but at the6ocm tension and in total porosity there was a highlysignificant increase of the three cover crops over thecheck. On the corn (cotton) plots the peas gave ahighly significant increase over the others at the 10-and 6o-cm tensions and a significant increase in totalporosity. A very interesting fact is that the checkplot on the cotton (corn) plots contained a greaterpercentage, of large and total pores than did the covercrop plots on the cotton (peanut) rotation on August13. This confirmed the 1945 data, but in both yearssampling of the two rotations was at different dates.Therefore, it was decided to sample the cotton (corn)and the cotton (peanut) plots on the same date. Thetwo groups of bars on 'the right of Fig. 4 show theresults.

At the lo-cm tension there was a highly signifi-cant difference between all of the cover crops on bothsets of plots. At the 6o-cm tension the cover cropsgave a significant increase on the cotton (peanut)plots, but not on the cotton (corn) plots. There wereno significant differences in total porosity due to•cover crops.

A comparison of the two rotations shows that thecotton (corn) rotation gave a highly significant in-crease at the lo-cm tension and a significant increaseat the 6o-cm tension and in total porosity. The checkplot following corn had porosity values of the samemagnitude as the cover crop plots following peanuts.Also, the cover crops apparently were less effectivefollowing corn. This would 'be expected because ofthe large mass of material (tops and roots) added bycorn in contrast to the almost complete removal (topsand roots) of. peanuts.

46 SOIL SCIENCE SOCIETY PROCEEDINGS 1946

PENETROMETER DATA

Penetrometer measurements were made on thecheck and vetch plots on the same dates in 1946 thatthe porosity samples were taken. As shown by the.August 13, data in Fig. 5, the vetch produced a sig-nificant increase in the depth of penetration on thepeanut plots but not on the cotton plots. On August22 there was a highly significant increase due to vetchon both crops. However, both sets of values werelower than the corresponding ones a week earlier.The difference apparently was due to the differencein soil moisture content. Oh August 13 the soil con-tained approximately 20% water and on August 22approximately 1.3%. About the same magnitude ofincrease due to vetch existed in depth of penetrationon September 17. It was highly significant on thepeanut plots, significant on the cotton (peanut) plots,and slightly less than significant on the cotton (corn)plots. An interesting fact is the greater penetrationon the cotton (corn) plots than on .the others. Thiswas expected in light of the porosity data.

The depth of penetration on August 13 was cor-related with the porosity data for the same date andthe following values found:

.r Regressionvalues . coefficient

Penetration and 10 cm porosity .78** 2.09Penetration and 60 cm porosity .88** 4.57Penetration and total porosity .73* 5.25

RELATION OF POROSITY TO CROP YIELDS

During the first 3 years of the experiment, thecover crops produced no significant effect on theyields of cotton or corn, other than from nitrogen,and no effect on peanuts. In 1945, large increases incorn yields resulted from turning under the legumi-nous covers, but they were no greater than thoseobtained with commercial nitrogen. The increases ob-tained in cotton yields likewise were no greater thanthose obtained with commercial nitrogen. Thus, it

35eZ 5O54a.bJzuiOLU.O

igo

8-13 8-22 9-17 8-13 8-22 9-17

n9-17

PEANUTS(COTTON) COTTON(PEANUTS) (CORN)

FIG. 5.—Depth of penetration. All measurements on the cropridge or row. *Significant differences; **highly significantHiffprenres. (C. — Oipi-k • V — V*>trli ^\iiugc ui luw. 'oignmcani ainerences;differences. (C = Check; V = Vetch.)

appears that for cotton and corn in that one year, thedifferences in the physical soil properties were out-weighed by the chemical properties, especially nitro-gen. However, the yield of peanuts was increasedover 400 pounds per acre as an average of all covers,irrespective of the amount of commercial nitrogenadded. The peanut yields are not yet available for1946, but the cotton yields just barely missed show-ing a significant correlation with porosity. The cornyields were not affected by cover crops more thancould be explained by the nitrogen supplied by thelegumes.

SUMMARYPore-size distribution measurements were made in

1945 and in 1946 and penetrometer readings takenin 1946 on a series of cover crop plots on the UpperCoastal Plain Experiment Station at Rocky Mount,N. C. Two rotations of major crops were involved,vis., cotton-peanuts and cotton-corn, with each cropoccurring in three replications each year.

In some instances significant and highly significantincreases were produced-by cover crops in the per-centage of pores drained at 10- and 6ocm tensions,in* total porosity, and in the depth of penetration.(See Figs. 4 and 5.)

The check plot in the cotton (corn) rotation hadapproximately the same percentage of large pores andtotal porosity as the cover crop plots in the cotton(peanut) rotation. The greater porosity followingcorn is attributed to the large amount of plant mate-rial (tops and roots) left by the corn.

Highly significant correlations were found betweenthe percentage of large pores and the depth ofpenetration.