Studies of Clay Skins in a Cecil (Typic Hapludult) Soil:H. Effect on Plant Growth and Nutrient Uptake1

E. M. KHALIFA AND S. W. BuoL2

ABSTRACT

A greenhouse experiment was conducted to study the effectclay skins may have on plant growth and N, P, and K uptake.Wheat (Triticum vulgare, var. Wakeland) plants were grownin uncoated aggregates and in large and small clay skin-coatedpeds from the B22t horizon of a Cecil (Typic Hapludult) soilfor 4 weeks. Data obtained for dry weight and total N, P, andK content of the plants were statistically analyzed. Plantsgrown in the uncoated aggregates had higher dry weights andmore total N, P, and K than those grown in either the large orsmall coated peds. -Most of the plant roots grew outside of thecoated peds with a concentration of fine roots on the surfaceof the clay skin. It is hypothesized that the clay skins reducedplant growth and nutrient uptake (especially P and K) eitherby serving as a barrier for root growth and/or by slowing iondiffusion from the soil peds into the inter-ped soil solution.

Additional Key Words for Indexing: argillans, cutans, rootproliferation, wheat, and diffusion.

UNTIL recent years, interpretation of clay skins in soilshas been largely related to soil genesis and classifica-

tion studies. Only recently have studies on the effect of clayskins on plant growth and nutrient uptake been made. Syn-thetic Fe-kaolinite coatings on illite aggregates have beenfound to reduce both plant growth and K uptake (Soileau,Jackson, and McCracken, 1964). In that study, the growthand K uptake of wheat plants were compared from green-house pots containing coated aggregates with those con-taining uncoated aggregates. They suggested that the Fe-kaolinite coating enveloping the K-rich illite aggregatesmay have restricted normal diffusion of exchanging ionsinto, and K ions from, the aggregate interiors. This wouldreduce the K content of the inter-aggregate soil solution.The authors also indicated the possibility that the inhibitingeffect could have been due to the presence of Fe in thecoating material.

Most studies of clay skins have shown that macro- andmicro-heterogeneities between clay skin or ped surfacematerial and the bulk (whole) soil peds are significant(Mick, 1949; Sherman and Alexander, 1949; Kunze andOakes, 1957; Buol and Hole, 1959 and 1961; Frei, 1964;Rutherford, 1964; Grossman, Odell, and Beavers, 1964;and Heil and Buntley, 1965). Studies have also revealedthat the growth and distribution of plant roots is oftenrestricted to the surfaces of soil structural units (Nie-schmidt, 1934; Weaver and Darland, 1949; Fehrenbacherand Snider, 1954; Fehrenbacher and Rust, 1956; andGrossman et al., 1964). Thus it is possible to theorize that

1 Paper no. 2630 of the Journal Series of the North CarolinaState University Agr. Exp. Sta., Raleigh. Received May 21,1968. Approved July 1, 1968.

2 Graduate Research Assistant and Associate Professor, re-spectively.

clay skins may have an influence on plants which is out ofproportion of their actual quantity. This study was de-signed to determine what effect natural clay skins may haveon plant growth and uptake of N, P, and K.

MATERIALS AND METHODSA Cecil (Typic Hapludult) soil near Raleigh, North Carolina

was found to have well-developed clay skins in the B and Clhorizons. Clay skins separated from the B22t horizon werefound to be different from the bulk samples of that horizon.Physical, chemical, and mineralogical properties of the differenthorizons of the Cecil profile as well as the clay skin material arereported by Khalifa and Buol (1968). Clay skin coated anduncoated aggregates were mixed with acid-washed sand andused as the growth media for wheat plants.

Natural soil peds were collected from the B22t horizon be-tween the depths of 66 and 91 cm. The collected clods wereallowed to air dry. With the help of a binocular microscope, thecompletely clay skin-coated peds were separated from the par-tially coated ones. Completely coated peds were separated intotwo groups according to their size. Large coated peds, having adiameter of 2.6 to 3.8 cm, were separated from the small coatedones (1.3 to 1.9 cm in diameter). To obtain soil aggregates thatwere mostly uncoated the partially coated soil peds were brokennormal to the ped surfaces while observing the process under abinocular microscope. Thus, clods having a diameter of 2.6 to3.8 cm, now referred to as uncoated aggregates, were selected.About 10 kg of each of the three kinds of aggregates wereprepared.

Quartz sand was soaked in a 6N HC1 solution for 48 hours.The excess acid was separated from the sand by flushing thesand first with tap water and then with distilled water until theleachate was chloride-free. The sand was spread on clean sheetsof paper and allowed to air dry.

Eight treatments, replicated three times, were prepared foreach of the three types of aggregates. Thus, there were 24 treat-ment combinations consisting of all factorial combinations ofthe treatments and aggregates. For every replicate, 300 g of soilaggregates were mixed in 950 g of acid-washed sand. This mix-ture was placed into 1 liter waxed-paper pots. Each experimentwas laid out according to a randomized block design in thegreenhouse. A reference pot containing the same amount ofsand and soil and having a hole in the bottom was used to cal-culate the amount of water needed to reach field capacity.

Eight nutrient solutions were prepared and used on each ofthe three types of aggregates. The nutrient solutions were pre-pared as described by Meyer, Anderson, and Swanson (1955).The kind and amount of the salt solutions used in the differenttreatments are given in Table 1.

Fifty wheat seeds (Triticum vulgare, var. Wakeland) werespread uniformly in every replicate as well as the reference pot

Table 1—Kind and amount of 1M salt solutions used in thedifferent nutrient solution treatments

ml/liter of nutrient solutionTreat- —————————ment KNO, KH,PO4 Ca(NOj), MgSo( KC1 CaCl, NaHjPO4 NaNOj

Com-plete 2 2

-N 2-K-P 2-NP-NK-PK-NPK

3 22

3 23 2

22

3 22

2 3

24 3

3

3

2 2

22

102

KHALIFA AND BUOL: STUDIES OF CLAY SKINS IN A CECIL SOIL: II. 103

and covered with 50 g of acid-washed sand. Distilled water wasadded to the reference pot until water came out of the hole inthe bottom of the pot. Water was added to each pot until itequalled the weight of the reference pot. Seeds germinated in 1week and all the replicates were thinned to 40 plants/pot. Start-ing with the second week, nutrient solutions were added at therate of 50 ml/pot every 2 days. This amount was increased to100 ml/pot the third week and to 200 ml/pot the fourth week.After the addition of equal amounts of nutrient solution distilledwater was used to irrigate the pots to the weight of the referencepot.

The pots were harvested after 28 days and the roots andshoots in each pot were separated. Care was taken in separat-ing the roots from the soil aggregates. Observations on the dis-tribution of the roots were recorded while harvesting. The rootswere quickly washed with distilled water and dried on clean tis-sue paper. The harvested roots and shoots were then oven driedat 60 to SOC and weighed.

After grinding the roots and shoots, samples were taken forN, P, and K analyses. Only total plant values are reported; how-ever, separate root and shoot values are reported by Khalifa.3

Potassium and P were determined following the procedure de-scribed by Schuffelen, Muller, and Van Shouwenburg (1961).Nitrogen was determined using an automated Micro-Dumasapparatus (Coleman Nitrogen Analyzer, Model 29A) as de-scribed by Stewart, Porter, and Beard (1964).

In accordance with the factorial nature of the experiment,analyses of variance were performed on the data for the N, P,and K contents as well as the total dry weight of the wheatplants. In these analyses the total variability was divided intoparts corresponding to the variance sources. Statistical tests ofsignificance involved comparing the calculated mean square foreach variance source with that of the experimental error. Thecomputations followed the standard procedures presented inSteel and Torrie (1960) and Cochran and Cox (1960).

RESULTS AND DISCUSSION

During the growth period, one difference was observedin the growth of the above ground portions of the wheatplants. In the -N treatments N deficiency symptoms ap-peared on the plants growing in the uncoated aggregates4 days earlier than on those growing in the coated peds.Marked differences in total root growth and pattern of rootproliferation were clearly observed at the time of harvest.In the uncoated aggregate treatments roots were distributeduniformly throughout the pots, both in the sand and the soilaggregates. Main roots as well as fine roots penetrated allthe way through the soil aggregates. In the large coatedpeds the plant roots were concentrated in the sand aroundthe peds. Many roots were also found around the walls andon the bottom of the pots. The fine roots were adhering tothe surface of the peds without any observable penetration.In the small coated peds the main plant roots frequentlypenetrated through the peds but the fine roots were concen-trated in the sand around the soil peds.

Mean values of the total dry weight and N, P, and Kcontents of the plants grown in the different treatmentsare plotted in Fig. 1, 2, 3, and 4. Analysis of variancevalues are given in Table 2.

In all treatments greater dry weight was obtained fromthe plants grown in the uncoated aggregates than from

•^E. M. Khalifa, 1968. Properties of clay skins in a Cecil(Typic Hapludult) soil. Ph.D. Thesis. North Carolina StateUniversity, Raleigh.

I5%

(b)

2.6

2.4

2.2

2.0

1.6

0 1.4

1.2

1.0

0.8

1.0

0.8-K

_w^^_ • Uncoated aggregates——— • Snail Coated aggregates*...*..• • Large Coated aggregates

Fig. 1—Total dry weights of wheat plants grown in three typesof soil aggregates at various NPK treatments, 4-week growth.

those grown in either the small or the large coated peds.Dry weight differences were larger for the roots than forthe shoots.4 Total contents of N, P, and K were significantlygreater in plants grown in the uncoated aggregates than inthe plants grown in the coated peds except in the case ofthe —N treatments where there were no significant differ-ences in the total N content. Plants grown on the smallcoated peds always had more P and K and producedgreater dry weight than those grown on the large coatedpeds. This was also true with respect to total N contentwhere N was added in the nutrient solution. These differ-ences were slightly significant in most cases.

The nonsignificant differences between the three aggre-gate treatments in the total N content of the plants grown inthe —N treatments suggest that the wheat plants were able

Table 2—Significant interaction sources of variation from theanalysis of variance of the four characteristics

Characteristic

TotalDry Weight

TotalNitrogenContent

TotalPotassium

Content

TotalPhosphorus

Content

Source of VariationN x uncoated vs. coatedN x large coated vs. small coatedNPK x uncoated vs. coatedNPK x large coated vs. small coated

N x uncoated vs. coatedN x large coated vs. small coated

N x uncoated vs. coatedN x large coated vs. small coatedNK x uncoated vs. coatedNK x large coated vs. small coatedNPK x uncoated vs. coatedNPK x large coated vs. small coated

P x uncoated vs. coatedP x large coated vs. small coated 'N x uncoated vs. coatedN x large coated vs. small coatedK x uncoated vs. coatedK x large coated vs. small coatedNK x uncoated vs. coatedNK x large coated vs. small coatedNP x uncoated vs. coatedNP x large coated vs. small coatedPK x uncoated vs. coatedPK x large coated vs. small coatedNPK x uncoated vs. coatedNPK x large coated vs. small coated

F+8.2075 (**)0. 0000 (NS)6.0566 (»)1.5943 (NS)

211.9333 (*»)36.2173 ('«)

50.8690 (**)12. 3986 (»«)10.8922 (»«)21.0892 (**)14.8319 (>*)11.5874 {««)

17.9104 (**)1.4925 (NS)8.9552 (»»)

22. 3880 (»*)23.8805 (»*)1.4925 (NS)

28.3582 (**)22.3880 (*«)7.4626 (»*)8.9552 (**)

13.4328 (**)20. 8955 (**)44.7761 («*)11.9402 (**)

i(**)= significant at 1% probability.significant.

= significant at 5% probability. (NS> = not

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

to completely use the available N from the different aggre-gates. This means that the total availability of N over the28-day growing period was unaffected by the clay skins.It is suggested that the earlier appearance of N deficiencysymptoms in the uncoated aggregate treatments was causedby early root growth into the uncoated aggregates, thusrapidly depleting the available N in the —N treatments.The coated aggregates restricted root growth but withtime N was able to move from the peds to the roots on theped surface. Differences in the growth and proliferationof plant roots between the three aggregates are also thoughtto be responsible for the differences in the N, P, and K con-tents between the different treatments where these nutrientelements were supplied. The greater growth of the roots inthe uncoated than in the coated aggregate treatments re-sulted in more uptake and accumulation of N, P, and Kfrom the nutrient solution in the uncoated than in thecoated treatments.

The lesser uptake and accumulation of P and K byplants from the coated than from the uncoated aggregatesis believed to be the effect of clay skins. These clay skinsprobably act in two different ways:

1) Clay skins around the soil peds are thought to actas a micro-clay pan and retard or restrict root penetrationand proliferation. This results in smaller root volume andlower dry weight. In effect, plant roots were not able toget to the exchange sites of K and PO4 ions.

2) The continuity of clay skins, with their high contentof clay and continuous orientation of their fabric, aroundthe soil peds may have decreased or restricted the normaldiffusion of exchanging ions into, and K and PO4 ionsfrom, the aggregate interiors. This effect would lower theconcentration of these ions in the soil solution and decreasetheir uptake and accumulation by the wheat plants.

The above mentioned possibilities can also be used toexplain the differences in the total P and K content be-tween the plants grown in the small and the large coated

6.0^ 5.5

LSDI I

1% 5%

0

IcQ>4J

gUaaOH

4.5 '

U.O3.5

3.0

2.52.0

1.5 '

1.0 'O.S

/ '•'//

/ /•'-

///S

/•'V

-N +N

i = Uncoated aggregates— _ —. — = Small coated aggregates. . . . . . . . . » Large coated aggregates

Fig. 2—Total nitrogen content of wheat plants grown in threetypes of soil aggregates with and without nitrogen fertiliza-tion, 4-week growth.

peds. Wheat plants grown in the small coated peds wereable to accumulate more P and K and produced greaterdry weight than those grown in the large coated peds whenthe soil material was the only source for P and K. Thegreater P and K contents may have resulted from the factthat a few plant roots penetrated the small coated pedswhereas plant roots were not observed to penetrate thelarge coated peds. Also, the rate of diffusion of K and PO4ions from the small coated peds is expected to be greaterthan from the large coated peds because of their smallerdiameter. This difference in diameter, thus greater surfacearea per unit weight, could be responsible for the greaterK and P uptake by plants grown in the small coated pedsthan those grown in the large coated peds.

2.0

1.8

1.2

0.9

0.3

2.0

•S 1.5

+PUncoated aggregates •Small coated aggregatesLarge coated aggregates

Fig. 3—Total potassium content of wheat plants grown in threetypes of soil aggregates at various NPK treatments, 4-weekgrowth.

I I

(b)

-N +N-K +K

-———- - Uncoated aggregates*•— ^ —* " Small coated aggregates......... . Urge coated aggregates

Fig. 4—Total phosphorus content of wheat plants grown inthree types of soil aggregates at various NPK treatments,4-week growth.

TABATABAI AND HANWAY: POTASSIUM SUPPLYING POWER OF IOWA SOILS 105

Although at this time it is only possible to postulatemechanisms involved in nutrient uptake restriction by clayskins in the Cecil soil studied, it seems clear that clay skinsdo have a significant role in soil-plant root relationships.It seems evident that any evaluation of nutrient availabilityin argillic horizons with nearly continuous clay skins shouldbe done with structurally undisturbed peds rather thanground soil material.

ACKNOWLEDGEMENTSAppreciation is expressed to Dr. L. A. Nelson, Associate

Professor of the Experimental Statistics Department, N. C.State University, for his assistance in the statistical analysis ofthe greenhouse data.