DETERMINATION OF TOTAL AVAILABLE MOISTURE IN Sol LS*

Transcription

1 134 Poceedings of The South Afica11 Suga Technologists' AssociationMach 1965 DETERMINATION OF TOTAL AVAILABLE MOISTURE IN Sol LS* by J. N. S. HILL Abstact In this pape, oistue chaacteistics of nine Natal soils ae pesented. Fo the field capacities deteined diectly in the field, and fo 15ba oistue pecentages obtained on undistubzd soil coes, tlie total available oistue popety has been evaluated fo each soil. It has bzen shown that b~lk density influences oistue content ove the entie angz of atic suction 0.1 to 15.0 bas. Futheoe, it has b:en deonstated that bulk density exets a siila effect on the wilting point deteined by the sunflowe technique. It has b:en concluded that undistubed soil coes ae essential fo eliabl: deteinations of the wilting point, and coesponding total availabl: oistue evaluations. Intoduction The plantavailable oistue angz in agicultual soils is populaly bzlieved to b2 that wate held bztween the upps liit o field capacity and the lowe liit o pzanent wilting pzcentagz. Today, with oe infoation on the wate equieents and consuptive use of plants, it is geneally accepted that gavitational wate oving though the oot zone is available to and is taken up by plants. Howeve, the field capacity is still consideed to bs the uppz liit of ost ipotance since the conlibdtion by gavitational wate to the total seasonal wate equieent of a cop is sall. At the lowe end of the ange, two factos coe into play. Fistly, in this tension ang: (noally designated as b:ing bztween 10 and 20 bas, atic potential) oistue content chang:s in ost soits ae slight with chang2s in tension. Secondly, a plant facto ust coe in hee, owing to vaiation in osotic elations of diffeent plants, which pobsbly accounts fo the width of the angz. It is usual to accept the 15ba pzcentagz as an indication of the lowe liit of available oistue in soils. Total available oistue is, theefoe, that wate held bztween the field capacity and the 15ba pecentage. Field capacity is eithe deteined diectly in the field (fo dainagz elease cuves) o is estiated fo soil oistue etention data obtained in the laboatoy, eploying soe factional value of Iba atic suction (0.05 to 0.33 bas). Infoation on the oistue holding popeties of soils is consideed to bz ipotant in iigation planning. Tog:the with a knowledg: of the ooting chaacteistics of the cop, oistue holding chaacteistics of the soils enable wate dxties to b: calculated, bzaing in ind the peakpziod consuptive use. To date, thee exists vey little published infoation egading the oistue chaacteistics and *Postgaduate ateial by the wite fo the Depatent of Soil Science, Univesity of Natal. total available oistue pe foot, fo the vaious soil seies occuing in the sugabslt of Natal. With inceasing inteest b2ing shown in ovehead iigation, such infoation is ugcntly equied. Thus a study has b:en ade at Tongaat of the pocedue fo deteining total available oistue in local soils and esults ae detailed b2low. Mateials and Methods (a) Mateials Fo this study topsoil saples of nine ajo soil seies occuing at Tongaat wee col1:cted. In Table I, a bief desciption of each soil is given, tog2the with the echanical analysis? and doinant clay ineals.:: Respxtive soil nub:s appea in the figdes pesented in the text. (b) Methods Field capacity oistue content of the vaious soils was deteined in the field by daily sapling fo gavietic oistue deteinations. An aea of abt 100 squae feet was thooug:ily soaked with wate each day fo one week, afte which evapoation was pevented by eans of plastic sheeting, and sapling coenced. Fo oistue etention data, the 1 ba ceaic plate extacto, 15ba ceaic plate extacto and pessue ebane appaatus wee used. Ten undist.ub:d coes (14 ijiches diaete and 1 inch deep) wee collected fo each of eight equilibiu pessue do,teinations, at the sae site as used fo diect easueent of the field capacity. Sufficient undistubed soil fo investigating the influence of bulk dtnsity on oistue etention was also collected. The pocedue involved with the us? of this eqilipent has b:en given by Richads (1954). Finally, a copaison of pzanent wilting pecentages bztween loose and copacted soil by the laboatoy technique and the sunflowe ethod (adapted fo Salte and Hawoth, 1961) was caied out. Results and Discussion Moistue chaacteistics d2teined on undistub2d soil coes ae pesented in Figde 1, in wluch the field capacity is indicated. Natual field capacities have b:en obtained fo the dainagz cuves d:picted in Figue 2. As can b: sten, field capacity is not a standad tension bxt vaies bztween 0.1 bs fo the sands, to abxt 0.33 bs fo the heavie soils. Total available oistue contents have b:en calculated by expessing soil oistue on the volue basis. The easued bulk densities of the undistubzd coes wee used fo this pupose, and in Table 2 field tdeteined by the hydoete ethod. $identified by eans of Xay diffaction and DTA in the Soil Science Laboatoies, Univesity of Natal.

2 Poceelings of The South Afican Suga Technologists' AssociationMach 1965

3 136 Poceediligs of The South Afica11 Suga Technologists' Associatio~lMac capacities, I 5ba pecentages, aveage bulk densities, and total available oistue in inches pe foot ae shown fo the diffeent soils. As can be seen, total available oistue vaies only fo 0.99 to 2.29 inches pe foot fo the Fenwood sand and Rydalvale clay loa espectively. The ode of diffeence is not as geat as expected, and it will be seen that bulk density of the saple accounts fo this anoaly. Copaisons wee ade of oistue etention at each of the selected tensions, between 100s: soil duped into the containe ings and siila soil copacted in the etaine ings to diffeent bulk densities. The influence of copaction on the atic potential o oistue etention at low tensions has been investigated by Box (1961) and Jaison (1956). These wokes found that inceasing the bulk density of a soil inceased eithe the atic potential at constant oistue content, o vicevesa. The wite has extended these investigations into the high tension ange, and has found the effect still to be opeative at the 15ba pecentage. It thus occued to hi that the phenoenon could akedly affect total available oistue deteinations. In Table 3, coelation () and egession () coefficients between bulk density and oistue content (pecentage by weight).fo atic suction values 0.1, 0.3, 1.0, 2.0, 10.0, 15.0'bas ae given fo the diffeent soils. It is of inteest to note that whilst the effect diinishes with inceasing tension in the case of the sandy soils, it pesists and even inceases in soe cases, with the heavy soils. This phenoenon is pehaps best illustated by eans of 3diensional diagas. In Figue 3, the x, y and z odinates ae epesented by atic suction (bas subdivided on log scale), oistue content pecen (weight by weight) and bulk density (g./cc.) espect tively. Fo this figue it can be seen that the use of the distubed soil saple in the deteination of the 15ba pecentage esults in the case of clay soils, in toolow values fo the wilting point, whilst the sae effect is visible at the field capacity. At the uppe liit of available wate, howeve, this effect is oe ponounced in the sandy soils, but vanishes towads the wilting point. Befoe any conclusions egading the effect of this phenoenon on calculates of available soil oistue wee dawn, howeve, a check was ade using Helianthus anuus plants gowing in loose and copacted soils. Results of the peanent wilting pecentage obtained by this ethod ae shown in Table 4, togethe with coesponding 15ba pecentage oistue contents. As can be seen, the sae pinciple applies, viz. the highe the bulk density, the highe the wilting point, whilst in ost cases the 15ba pecentage copaes favouably with the wilting point obtained using sunflowe plants. Conclusions The effect of bulk density on oistue etention at constant atic potential is an inteesting one. It influences both the field capacity and the 15ba pecentage oistue contents of the soil. In the case of sands, the effect deceases with inceasing tension, becoing negligible afte ost of the available oistue has been withdawn. In clays, howeve, the effect p~ysists, soeties inceasing, with incease in atic suction. In the latte instance, the phenoenon has a aked effect on calculates of total available oistue. A check against the standad physiological ethod fo deteining the peanent wilting point show that the sae pinciple applies. It is concluded, theefoe, that undistibuted soil coes ae essential fo eliable and value estiates of total available oistue in soils. It is also of inteest to note that, in the case of clay soils, total available oistue calculations should be based eithe on distubed saples thoughout, o on undistubed coes fo both field capacity and 15ba pecentages. Refeences Box, J. E. (1961). The influence of soil copaction upon the theodynaics of soil oistue. Ph.D, thesis, Utah State Univesity. Jaison, V. C. (1956). Petinent factos govening the availability of soil oistue to plants. Soil Sci. 81: Richads, L. A. (1954). Ed. The diagnosis and ipoveent of saline and alkali soils. U.S.D.A. Handbook 60. Salte, P. J. and Hawoth, F. (1961). The available wate capacity of a sandy loa soil. 1 A citical copaison of ethods of deteining the oistue content of soil at field capacity and at the peanent wilting pecentage. J. Soil Sc.. 12: 326.

4 Poceedings of The South Afican Suga Technologists' AssociationMach 1965 Soil oistue content % wlw Tie afte satuation (hous) FIGURE 2 : Dainage cuves of soe Natal soils: field capacity indicated*

5 Poceedings of The South Afica11 Suga Tech~ologists' AssociationMacli 1965 TABLE 1.Soe cheical and physical popeties of nine Natal soils SOIL SERIES CARTREF.. (7) WILLIAMSON. (5) WALDENE.. (4) WIERMERE (3) AVOCA... (6) SHORTLAS (2) RYDALVALE. (1) CLANSTHAL. (8) FERNWOOD. (9) DESCRIPTION OF SOIL IN THE NATURAL STATE Pinkishgey loay coase sand loose... Geybown sandy loacopact..... Dak geybown sandy loacopact... Dak gey clay loacopact Geybown loay sand vey copact... Red loay clay fiable..... Black clay loa fiable Red sand loose Gey sand loose..... MECHANICAL COMPOSITION % Sand Silt Clay DOMINANT CLAY MINERALS KI MI KIM M [ K 1 K MK K K K = kaolinite I = illite M = ontoillouite TABLE 2.Field capacities, 15ba pecentages, natual bulk densities and total available oistue data fo soe Natal soils. CARTREF.. WILLIAMSON WALDENE.. WIERMERE AVOCA.. SHORTLAS RYDALVALE. CLANSTHAL. FERNWOOD.

6 Poceedings of The South Afican Suga Technologists' AssociationMach 1965 Soil oistue 5 Matic Suction Bas Soil oistue 70 Matic Suction Bas FIGURE 3 : Inteelationship between soil bulk density, oistue etention, and atic posentia1.,

7 140 Poceedings of The South Ajican Suga Technologists' AssociationMac TABLE 3.Coelation () and egession () coefficients showing the influence of soil bulk density on oistue etention fo the ange of atic potential 0.1 to 15.0 bas. CARTREF... WILLIAMSON... COEFFICIENT^ : : MATRIC POTENTIAL BARS O : : : : : * : WALDENE WIERMERE..... AVOCA..... SHORTLAS... RYDALVALE. CLANSTHAL... FERNWOOD....,: : : : : : : : : : : : : : : : DN : : : : : : $ : : t25 degees of feedo. 'significant at 5 % level. :significant at 1 % level. not deteined. TABLE 4.Copaison between peanent wilting points obtained by the sunflowe ethod, and 15ba oistue contents nine Natal soils at vaious bulk densities. CARTREF I 1.; 3.11 WILLIAMSON WALDENE I ::E I 1: I 1:; I 18.D WIERMERE AVOCA I : I I? : I SHORTLAS RYDALVALE CLANSTHAL ;: FERN OOD

8 Poceedings of The South Afican Suga Technologists' Association i.i cs M. R. A. Wood: Thee ae soe wide vaiations in field capacity etention values. Is this due to diffeences in clay ineals? The Rydalvale seies contains both ontoillonite and kaolinite wheeas the Shotlands soil is pedoinantly kaolinitic. We find that the bulk density figue fo Dwyka soils is usually between 1.6 and 1.8 glee. on undistubed coes. M. Hill's figue is uch lowe. M. Hill: The total available oistue values pesented in this pape wee deteined on saples of a paticula soil seies fo a paticula site, the cheical and physical pobeties pesented thus epesent these paticula saples only. I do not clai that these values epesent the condition of the seies thoughout the suga belt. Hee, the ethod.involved in deteining total available oistue is ipotant, athe than the figues. The lowe tension at which field capacity is epesented in the Rydalvale seies is indiectly elated to the high ontoillonite content of this soil. In the Rydalvale seies the ontoillonite clay is esponsible fo the foation of any sall but stable peds. Thus, in bulk, thee is a fai popotion of noncapillay and lage poes between these peds esulting in a lowe tension at field capacity oistue content. Clay ineal types can be tied in with oistue etention studies. D. Maud, fo instance, has shown an exteely good coelation between the silt plus clay faction of kaolinitic soils and thei oistue etention at vaious suctions. I have also noticed this elationship. Howeve, D. Maud expessed a elationship between total available oistue and textual analysis which I have not obtained. I think that uch.9 of the eo in D. Maud's figues aises fo, fistly, the geneal acceptance of 0.3 bas as the field capacity suction wheeas, in fact, it vaies between soils; secondly, anoalous values of bulk density in soe soils, and, also, I believe, that the bulk density effect deonstated in this pape contibuted towads his eos. Regading the bulk density values fo Williason and Waldene seies, I have neve encounteed as high a figue as M. Wood suggests. Howeve, it is possible that M. Wood's saple is oe sandy o copact than the soil used in this expeient. My figue is the aveage of eighty undistubed coes taken at appoxiately the idpoint of the Ahoizon. D. Dodds: Will M. Hill please elaboate on the sunflowe ethod fo deteining the peanent wilting point? M. Hill: A single sunflowe seed is planted in a can of soil having a specific volue in this case it was 630 ccs. The soil was copacted to the bulk densities shown. The plant is wateed until it has i gown about fou pais of leaves. The can is then soaked with wate afte which evapoation is pevented by sealing the soil suface with a plastic ateial which is tied ound the ste of the plant and the outh of the can. The plant is placed out in the open whee it $ tanspies eadily until the fist syptos of wilt occu. All leaves, with the exception of the uppeost pai, ae then eoved. The plant and can ae placed in a dak, satuated atosphee fo 24 hous to see if it ecoves tugo. If it does, it is bought out into the sunshine again. The pocess is epeated until the plant does not ecove fo wilt in a dak, satuated atosphee. This, then, is taken as the point whee the plant has extacted all the available oistue fo the soil and is called the peanent wilting point. At the peanent wilting point the plant will die, even in a satuated atosphee, if wate is not added to the soil. The peanent wilting pecentage is not a constant tension; it vaies widely between about 10 and 20 bas. This is because in any soils oistue content changes between 10 and 20 bas ae slight with changes in tension, and also the osotic elations of diffeent plants vay. Nevetheless, a faily close ageeent has been obtained by any wokes between the 15ba pecentage and the peanent wilting point. M. Wilson (in the chai): M. Hill entions dealing with eighty saples fo each soil seies. How was the sapling done? M. Hill: A 14 inch diaete 1 inch deep coe saple was used. Bulk density values obtained with this size osaple wee not significantly diffeent fo those found using the 4 inch saple. M. G. D. Thopson: It would appea fo M. Hill's data that the way the wilting point is deteined has an ipotant effect on the value of total available oistue, apat fo the effective ooting depth. M. von de Meden: We have also found that total available oistue inceases as the clay content of a soil inceases but only up to a cetain point. Afte this, an incease in clay content sees to esult in a decease in total available oistue although the oistue contents at wilting point and field capacity ae both highe. Ou bulk densities ae based on 4 inch coes and, consequently, give an aveage value ove a geate depth. The salle coes M. Hill is using give a point value fo a uch salle aea. Saples of Waldene soils fo Chaka's Kaal have had an aveage bulk density of 1.5 g./cc., but in the top eight inches it has gone as high as 1.9 g.lcc. This, again, was with the lage coe. With the salle coe, one would pesuably find a fai aount of vaiation. M. Hill: The fact that inceasing clay content beyond a cetain level sees to decease the total available oistue in a saple is explicable in tes of bulk density effects. Bulk densities tend to decease with inceasing clay content and also the effect of bulk density on oistue etention at high atic suctions has been shown in this pape to be ipotant in the case of clayey soils. Thus, in the fist case, although field capacity and wilting point oistue content incease with inceasing clay content of a saple, the bulk density deceases and, consequently, total available oistue pe unit depth of soil vaies with bulk density. This can be seen fo the equation used fo deteining total available oistue.

9 142 Poceedings of The South Afican S~cga Technologists' AssociationMac M. Gosnell: In Figue 2 the field capacity vaies fo 48 to 96 hous in the vaious soil types. At the edian point of 72 hous on evey one of the soil types we would be vey close to the finally deteined field capacity. M. Hill: I agee. M. von de Meden: In soils like Waldene, which ae vey badly dained, would it not take longe than 72 hous fo the second foot to coe to equilibiu? M. Hill: The dainage tie does incease the deepe one goes into the soil but the effective ooting depth in any of ou soils is faily close to the suface, especially in the copact soils. And, of couse, the plant takes up gavitational wate, so that any wate ove and above field capacity afte 72 hous would pobably be used by the plant. M. Landsbeg: Have you attepted to calculate the vaiations between you saples? M. Hill: This was done pio to deciding on a cetain nube of saples. I think one should take ten saples fo gavietic deteination of field capacity. It is inteesting to note the vey low standad eo of the wilting point deteined by the sunflowe technique. It was lowe than the standad eo deteined fo the laboatoy technique.