Amelioration of soil by natural processes

Soil & Tillage Research, 20 ( i991 ) 87 -100 Elsevier Science Publ ishers B.V., A m s t e r d a m

Soil amelioration

87

Amelioration of soil by natural processes

A.R. Dexter* Waite Agricultural Research Institute, The Univers~.ty ~r Ade!aide, Glen Osmond, S,A. 5064

(Australia) (Accepted for publication ! 4 January 1990 )

ABSTP, AC~I"

Dexter, A.R., 1991. Amelioration of soil by natur~t procc.sses. St:if TillageRes., 20: 87-100.

A number o f " n a t u r a l " processes and their roles in soil amelioration are discussed and :3m¢ new results are presented. Drying of soil produces cracks which can provide paths of low resistance t'c; root growth and also can produce fracture of aggregates as the clay matrix shrinks around rigid i nc~,~- ~ions such as sand grains. Rapid wetting of soil can induce micro-cracks which can make the s.~il weaker and more friable and can also in¢.~eas¢ the penetrability of the soil by roots. Ageing ofsoii at'ter disturbance increases its water stability and its ability to resist mechanical stresses. The binding together of soil by roots and fungal hyphae can also induce stability as can the exudates from roots and other ~oil organisms. Roots are particularly important becaus, ~ of the biopores which they leave when they decay. Biopores (comprising root channels and earthworm tunnels ) can provide important pathways for root penetration of subsequent crops, and it is shown that they can restdt in significant yield ~ncrease:. i t is suggested that these "'natural"" processes can be. developed and exploited as "'modified natural" processes, and that a con:iderable potential exists for using them to improve our soils at a modest ¢o:,t.

I N T R O D U C T I O N

The physical condition of ~,~il can deteriorate as a result of a ~.umber of causes, Under agricultural land use these include: decreasing stability in water a~ a consequence of declining organic mag, er ~:o~n*, and c.ornpacfion by vehicular traffic. The visible consequences of these i~hanges are: reduced infiltration ¢f water resulting in increased run-off a~ ' : ~educed ~a te r availability to roots; increased slaking of ~::lods and dispersion G:' clay cesuitin~ in crust formation and increased amounts of readily erodibl¢ ~naterial; reduced soil workability with an increasec~ tendcr~c:y of tillage to ~urn up large, hard ¢~ods instead of producing a fine ~eed-bed; reduced soil aeration resulting in reduced seed geiTaination, ~eedling emergence and crop yield.

Under any given manage~aent, the soil physical condition will tend towards an equilibrium state de~ending on the soil type, the clima'~e and the details of the managemen~ system. Different management systems result in different

* Present address: Silsoe College, Silsoe, Bedford MK45 4DT, U n i t e d Kingdom.

0167-1987/9[ /$03 .50 ~ 1991 - - Elsevier Science Publishers B.V.

g~ A.R. DEXTER

equ i l i b r ium condi t ions , some of which will be "be t t e r " than others. These differences between a:¢¢ . . . . . . . . . . ,~ ,,,,~. ,.,, • m,~na~eme, ~ systems are largely ~t conseq~ e~ce o f the effects tha t they ba r e on a n u m b e r of " 'natura l" processes, the mos t i m p o r t a n t of which are the hydrology and the biological act ivi ty of the soil. A

"number of these effects have been discussed by H e i n o n e n ( 1986 ). A valuable compar i son can be made between these " 'natura l" processes and

" u n n a t u r a l " processes such as tillage. Given enough energy input , a soil in " 'bad" physical condi t ion (e.g. non-aggregated, massive, hard, anaerobic) can be t emporar i ly tu rned into a soil wi th an apparent ly near-perfect s t ructure (e.g. ri seed-be0 o f 1-5 m m d iamete r aggregates overlyi.,3-.g a loosened, well- d ra ined sub-soi l) . However , the s t ructure p roduced in this way may be far f rom being i~i equ i l ib r ium: it may be 1~rechanically ~,nstab~e, and it t~ay collapse when wet to be as bad. i f not worse, :han it was before tillage. In ~=ar~y cases, tillage can accelerate de te r iora t ion of the soil phys~ca| conduction by, for example, accelerat ing decompos i t ion of organic mat te r a~d bv d' lsrupting stable soil particle~, and can result in the. fo rma t ion of z new, undesi rable soil physical condit;.on.

An i m p o r t a n t factor in these processes is "~ae physical chemis t ry of the surfaces of the soil particles. The c~ay particles are d o m i n a n t in this respect because they pro':id,:., h~OSt of the surface area wi th in soils. Soil can be st~actur- ally stable on the macro-scale only if it is s t ructural ly stable on the micro- scale. Thus , stabil i ty is dependen t on the f iocculat ion of the clay particles which, iv, turn , is depende.nt on, amongs t o ther things, the cat ions adsorbed on the cl~w surfaces and on the electrolyte co~cet~=r~*ion in ~he soil solution. Adsorbed ca lc ium ions are hi _gb, i y . desirable because they cause the'. ~.~.~. '~,, ;',a~i-_ cies to ~;'~n':~ staple c o m p o u n d pal l ic ies whereas s o d i u ~ ~ons are "~,tiesirabie becztts~' they cause clay particles to repel and disperse. A high electrelyte concentrat.~on aiso impar t s stability. At t empts to amefiorate a soil by _any means (e i ther '~naturaI" or " u n n a t u r a l " ) will p roduce resu!ts which are at besl tem- porary if the physical chemi.str)" of the soil is causing it to be intrinsically unstable.

The relat ive impor tance o f " n a t u r ~ ~'" and " 'unna tura l" prdcesses can be as- ~cssed f rom the results o f lo~g- term tr ims ( for example, tillage t r ia ls) . It is often found that the year- to-year war~.ation (which is caused by "'natural'" processes) is ~f s imi lar magnit~.~de to the variat;.on between treatraents. "['his in- dicates the signif icant inf luence of the " n a t u r a l " processes, and suggests that it would be wor thwhi le to gain an undersxandi~g of these processes se that they can be util;,z~d to advantage.

S O I L S q ' I C U C T U E E C H A N G E S O N D R Y I N G

When a ~oi! of m e d i u m to high clay cont~:nt dries, it shrinks and verticai ~esiccation cracks form. If the drying is rap;,d, then the cracks will be closely

A M E R L I O R A T I O N O F S O I L BY N, VI U R A L P R O C E S S E S 89

spaced but narrow. If the drying is slow, then the cracks will have grea':er spacings and will bc wider. These cracks can form im?o~tant pathways for rapid ~:ater infiltration, aeration and for deep penetratio~ of roots through soil layers which might otherwise provide mechanical barriers.

When these vertical (p r imary) cracks become wider than about 4 ram, significant convecuon currents of air can occur within them and drying can then occur from the faces of the pr imary cracks. This can result in secondary cracks fon-aing at fight-angles to the pr imary cracks. In some cases, tertiary cracks can form from the surfaces of the seconda :y cracks in the same way.

D ~ i n g of soil aggregates can result in their f ragmentat ion by another mech- anism. Shrinkage of a clay matrix a round rigid particles can lead to tensile stresses arc.~ nd ihe particles. This situation is exactly analogous to the expan- sion of a cavity in a .=~or,..shrink~,ng soil matr ix as descr.;bed by Misra et al. (1986 ), exceW, with a t ransformat ion of coordi,'_ates. A simplified diagram of the situation is shown in Fig. I for the ca~;e of a spherical inclusion at the centre of a spherical soil aggregate. The stresse~ generated adjacent to the inclusion are sufficiem to cause plastic failure from the boundary of the inclusion at :'~ to a radius, rp. In the shell, rp< r <t~, elastic (reversih1.:;) deforma- tions occ,ar. Peak tensile stress occurs at the elastic-p',ast]c b o u n d a ~ (i.e. at r = rp). If this pe~.k tensile stress exceeds the tensile strength of ~he aggregate material, it can cause the aggregate to crack. At low rates of drying, very slow (creep) flow can l imit stress build-up and hence prevent cracking. Max imum cracking, occurs with the rr:ost rapid drying where there is insufficient t ime

( \

/

o~ , Q ', ! \ II /'! / i

~ " - - I - I ~ I / . i l J I I /

¢. t l I

~ .'~a

Fig. 1. Representa t ion o f a spherical aggregate o f r a d ; u m r~ conta in ing a rigid ~tlclu~ion o f radius • i ~ ~IS C ~ F i ~ r e . a n i i n s ~ . ~ e O f i ~ e s o i l matrix a round the inclusion creates radial s=resses, t rr , an~! tangential tensile stress¢~, o,. These ~,~inclpal stresses ca~tse a ~ ,~e of plastic fa'.lure to a radius rp.

90 A.R. DEXTER

for creep to occur. Large inclusions are more effective in inducing cracking than small inclusions.

For aggregates containing multiple inclusions, stress within the elastic de- format ion zones are addi t ive and cracks tend to form in planes containing inclusion centres. Cracks will form first where inclusions are closest together. Kendal l and Moran (1963) discuss the statistics of randomly placed points in a sphere. Integrat ion of their eqn. (2.126) gives the probabili ty, P, of two points (e.g. inclusion centres) placed randomly within a spherical aggregate being closer together than some dimensionless distance, y.

P = 8}, 3 - 9y4+ 2y 6 ( i )

where y=s/2ra and s is the ~eparation of the two points. Clearly, y lies in the range 0_<y_< 1. Equat ion (1) may be developed to examine the joint distri- but ions of nearest distances between inclusions within aggregates containing randomly dis t r ibuted mult iple inclusions.

Drying o f soil occurs not only from the soil surface and from cracks. Plant roots are the most effective way of drying a soil profile dowr: to the depth of rooting. M~ny- plaint species will dry the soil ~:o a matric water potent ia l o f a round Ym = - 1.5 MPa.

Allowing a soil to dry as much as possible (i.e. to - 1.5 MPa or beyond) is one of the most effective forms of soil ameliorat ion. Many fartners, in irriga- t ion areas especially, resist complete drying of the soil because they feel that moist soil is "be t t e r " and because they have the ability to keep !he soil moist. Whils t it is true tha t keeping a soil moist keeps ear thworms and other beneficial organisms alive and active, this does not mean that occasional drying is not advantageous. The cracks and other f ragmentat ion processes which occur on drying can be extremely valuable in generating useful aspects o f soil structure. Also, dry woil can be wetted rapidly, and some benefits can also accrue from this.

R E S P O N S E OF SOIL TO W E T T I N G

When soil becomes wet, it swells and the desiccation cracks close. The rate at which the cracks close varies widely: in some soils this happens almost immediate ly whereas in others, days or weeks can pass before the cracks have finally closed. An impor tan t difference exists between cracks and biopores such as ear thworm tunnels. When soil swells the cracks close, but the biopores do not. Therefore, biopores can cont inue to provide paths for root penetra- t ion even in fully swollen soil.

I f soil is wetted rapidly, for example by an intense rainstorm, some other interesting changes occur to the soil structure. The combined effects of differential swelling and pressure build-up in entrapped air can cause mechanical failure of the soil. In extreme cases, with very rapid wetting, this can result in

AMERLIORATION OF SOIL BY NATURAL PROCESSES 9 |

corr, plete slaking of the soil into separate micro-aggregates typically < 2 5 0 #m. With slower wetting, complete slaking may not occur, but instead there may be partial slaking or mellowing. In this case, arrays of micro-cracks are formed throughout the soil mass which, however, retains ii~ coherence and shape (e.g. Stengei, 1988). These micro-crac!cs make the soi! weaker and more friable ( Dexter et al., 1984). The arrays of micro-cracks produced during rapid wetting may be anisotropic. When differential swelling is the ~rincipal cause of cracking, then *.he cracks will be predominant ly perpendicular to the wetting front. When build-up of entrapped air is the main cause of failure, then the cracks will be predominantly parallel to the wetting front (McKenzie and Dexter, 1985 ).

An example of the effects of rapid wetting on soil structure is shown in Fig. 2 (C.D. Grant , personal communicat ion, 1988). The semi-discs in the figure were produced by the fracturing of whole discs in the indirect tension test for measurement of soil tensile strength. The disc on the left was a control disc which had been moulded at the lower plastic limit, oven-dried, and then fractured. The right-hand disc had been prepared in the same way, oven-dried, wetted from a source of free water from a point en i:s diameter, oven-dried again, and then fractured. The semi-discs were then impregnated in epoxy resin containing white pigment and sectioned. It can be seen that the fight- hand disc contains many micro-cracks which define the boundaries of incip- ient micro-aggregates.

It can also be seen clearly from Fig. 2 that the fracture surface of the left-

Fig. 2. SecTions through half soil discs which had been fractured by indirect tension and embed- ded in white epoxy resin. The discs are con :ol (left) and rapidly wetted and then dried before fiacturivg (right).

92 A.R. DEXTER

TABLE 1

Percentages of roots of lucerne and wheat able to penetrate control (slowly welled) anO rapidly wetted artificial hardpans (al ter Ward, 1986)

Plant species Roots penetrating (%)

Control Rapidly wetted

Lucerne (Medicago saliva cv. 'Hunte r River ~ ) 81 Wheat ( Trit icum aest ivum cv. "Kite' ) 33

93 86

hand (control) disc is smooth whereas the fracture surface of the right-hand (rapidly wetted) disc i~ much rougher because it runs around the micro-aggregates in its path. Fracture surface analysis is proving to be a valuable tech- nique for rapid quantification of soil structure (Gran t and Dexter, 1986; Dexter and Hhkansson, 1989; Dexter and Horn, 1988).

Soil mellowing by the inducement of micro-cracks does not occur if the soil ; . . : . - . ; , ; ~ , , . . . . . . ,~-r than a matric water potential of around I MPa (Sato, g i l l it I l l It L J ¢dI, AA $ v v ~. / '1. . 'l.~m,,- A

1969; Grant and Dexter, 1986, 1989). Soil which has been wetted rapidly appearg tG offer less resistance to root

penetrat ion (Whiteley et al., 1981 ). Experiments have been done to test the effectiveness of a rapid wetting treatment on the ability of roots of lucerne znd wheat to penetrate a~ificial hardpans (Ward, 1986 ). A control t reatment did not have rapid wetting. Both treatments were brought slowly to a fixed water potential ( - 10 kPa) before root growth com~aenced. The results are shown in Table 1. It can be seen that the slaking of the compacted layer (hardpan ) caused by the rapid wetting made the soil more penetrable by roots.

INCREASES G 7 STABILITY WITI t TIME

Soil is thixotropic. That is, soil which has been sheared or moulded (for example by tillage implements or beneath vehicle wheels) is weaker than undisturbed soil at the same density and water content. This effect occurs largely because cementing bonds between soil particles become broken by mechanical disturbance and partly because the clay particles become displaced from their equilibrium positions to positions of higher free energy.

Soil which has been disturbed mechanically is therefore more susceptible to erosion and compaction than undisturbed soil. If soil is left at constant water content and constant density after disturbance, then its water stability and strength are gradually regained with time. This regain is caused partly by the re-arrangement of the clay particles to new positions of lower free ertergv (the true thixotropy effect), and partly by the re-formation of cementing bonds

A M E R L I O R A T I O N O F SOIL BY N.a~TU.RAL PROCESSES 93

between the soil particles (Utomo and Dexter, 1981; Kempcr and Rosenau, 984; Molope et al., 1985; Kemper et al., 1987; Dexter et al., 1988 ). The re-arrangement of the clay particles at constant water content leads co

a change in pore-size distribution and hence to a change in matric water potential. The progress of these changes can be followed by tensiometers, and some results for ? South Australian topsoil are shown in Fig. 3. It can be seen that the water potential becomes more negative with ageing time. The potential varies approximate!y linearly with the logarithm of ageing time at least over the range 0 .3-30 days. This change in water potential, ~,, will increase the shear strenth, %, of the soil through its effect on the effective stress. Shear strength is given approximately by

zs = C + [ a n - ( 1 - E ) ~ , ] t a n O, (2)

where: Cis the soil shear cohesion; 0 is the angle of internal friction (probably around 40 ° ); ~, is the externally applied normal stress acti'ag on the shear plane; and E is the air-fille6 porosity (Greacen, 1960). The ( 1 - -E) term gives the proportion of the area of the shear plane over which the water potential, ~,, acts, and the minus sign before the ( l - - E ) term allows for the fact that ~, is negative. Cementat ion between soil particles increases the C term in eqn. (2).

An experimental method for distinguishing between the cementation and particle re-arrangement mechanisms of age-hardeaing has recently been pro- posed by Dexter et al. (1988). Practical applicat;ons of this effect include periods of rest (of perhaps 5-7 days) for age-hardening of the soil after any disturbance and before other management practices, such as traffic or irrigation, are applied.

.6o[ - 5 0

A_

~l ~ - ,~.0

~ - 3 0

-10

0 -0.5

@

- - ~ w = 1.05PL

._ I | I . - 0 0 . 5 1 .0 1 . 5

Iog,o(Ume, days]~

Fig. 3. Measured changes in matfic water potential with logarithm of time (days) after mould- ing by hand of Mintaro sandy loam. The soil was moulded and maintained at three different water contents, w, expressed as proportions of the lower plastic limit, PL, of the soil.

94 A.R. DEXTER

E F F E C T S OF R O O T S

Roots have a profound effect on soil. A rcot tip penetrat ing into soil with no pre-existing macro-structure produces a biopore. The mean soil density remains constant and tne volume of the roots is accommodated by the loss of pore space from the surrounding soil, Thus, the soil around a root can be compacted to some extent for a distance of the order o f one root d iameter beyond the surface of the root.

When the root decomposes, which is usually within about one year from the non-lignified tissue of annual crops, a biopore remains. The amounts of roots and hence biopores are quite impressive. For wheat crops in South Aus- tralia there may be typically 15 km of roots in the soil profile per m 2 of soil surface. Root density, Lv (length of root per unit volume of soil) , is highest in the top 0.1 m of soil with typically 100 km m-3; at 0.3 m depth, Lv may be 10 km m-~; and by 1.2 m dcpth, i~. may have dropped to i km m - 3 (Jakobsen and Dexter, 1988). The biopores produced deeper in the profile last much longer than those nearer the surface. Near to the surface, the actions of rain, wind, vehicular and animal traffic all combine to fill or to collapse the tunnels.

Roots play a role both in the format ion of an aggregated structure and in the stabilization of this structure. The aggregation process occurs more strongly when the~e i~ a hir:~ ampl i tude and a high frequency of wetting and drying cycles in the soil (Horn and Dexter, 1988). The former is positively corre- lated with root density, and the latter can be increased by the use of periodic irrigations rather than by ke,~ping the soil moist. Similarly, the well-known increase in aggregation undex grass-based pastures (e.g. Low, 1955 ) may be largely a consequence of the influence of the high densities of grass roots on the soil hydrology.

Stabi!izatio~ of soil aggrega.tion by roots can occur in a number of ways. Firstly, the roots themselves tend to bind large aggregates together. This state- ment can be put into perspective by realizing that a root density of Lv = 100 km m - 3 corresponds to a mean spacing between root axes of only 3 mm (Bar- ley, 1970a). Perhaps even more impor tant for tile binding of aggregates are the fungal hyphae which are associated with the roots of many plant species. Fungal hyphae form strands of 1-10/zm diameter and may be more than 10 mm long. The lengths o f hyphae per unit volume of soil are extremely large, and may be up to L~= 104 or 10 s km m -3 (Tisdall and Oades, 1979). On this basis, the fungal hyphae in 10 ~3 of such top soil, if placed end-to-end, would reach all the way to the moon!

Exudates from roots are impor tant in stabilizing micro-aggregates ( < 250 pro) . Other exudates come from microbes, such as bacteria and fungi, in- volved in the decomposi ton of root material .

The biopores formed by one crop can provide channels for deep rooting of a following crop. This has been shown unequivocally by Elkins et al. ( 1977 )

AMERLIOR ~,TION OF SOIL BY NATURAL PROCESSES 9 5

0 0 0

o

0 0 0

Fig. 4. Possible evidence for a tendenc-, o f roots in an anaerobic soil to grow preferent ia l ly towards air-filled pores. This pea root has grown almost complete ly a round a hole (af ter Dexter, 1986c).

for the case of cotton (Gossypium hirsutum) following bahia grass {Pas- palum notatum). A hard-pan in the soil which they used could not be penetrated readily by the roots of cotton, but could be penetrated by the roots of bahia grass. Apparemly, the penetrat ion ability of bahia grass is associated with a fibrous sheath beneath the epidermis. The bahia grass crop increased the number of biopores larger than l m m diameter, and these resulted in increased rooting densities at the depth of the following cotton crop. It may be wo_~.hwhilc t~ screen ~i~ore plant species for their ability to penetrate strong soil. This is a good exr~mple of"biological tillage".

There is some evidence that roots may grow preferentially towards biopores under conditions of poor aeration (Dexter, 1986c). This is probably a response to oxygen concentrat ion gradients around the biopores. The effect is illustrated in Fig. 4. There was no evidence of this effect in well-aerated soils, and further work is needed to quantify the effect in relation to soil aeration status. Different plant species would probably respond differently.

E A R T H W O R M S

Earthworms can have a profound effect on soil structure, both through the casts which they excrete and through their tunnels. There are numerous species of earthworms which behave ir~ different ways. However, for simplicity, this section will include only a few general ideas.

Worm numbers are greatest in clayey soils in humid regions and are vir- tually zero in sands in arid regions. In old pasture at the Waite Institute, there were 250-750 worms m -2, whereas there were only about 20 worms m '~ in a wheat-fallow rotation (Barley, 1970b). Tillage has a rather drastic effect

96 A.R. DEXTER

on ear thworm nun-~bers typically reducing populat ion dens i t i e s o about 0.15 of their values in non-ti l led soil (Low, 1972 ).

Worms can move around in the soil either by exerting pressure to push soil aside or by ingesting the soil and hence making tunnels. Recent measure- ments have found that ear thworms can exert mean max imum axial pressures of 73 kPa and mean max imum radial pressures of 230 kFa (McKenzie and Dexter, 1988a,b). Ingested soil is moulded in the ear thworm's gut at very low pressures ( ~ 260 Pa) , and the casts when excreted have much lower bulk densities ( ~ 1 .15 t m -3) than the soil in which the worms tunnel ( ~ ! .5-1 .6 t m -3) (McKenzie an_d Dexter, 1987).

The ability of ear thworms to tunnel through soil does not appear to be in- fluenced by soil strength at least up to penet rometer strengths of Q= 3.5 MPa (Dexter, 1978). In contrast , the rate of root elongation would typically be reduced to 0.2 of maxim, ,m at this soil strength (Dexter , 1987 ).

Eart.hworm tunnels pI ovide pathways of low mechanical resistance for root penetrat ion, and extensive rooting in the tunnels is often observed (e.g. Ed- wards and Lofty, 1978; Ehlers et al., 1983; Wang et al., 1986). The densities of ear thworm tunnels are very small compared with the root densities given in the pre vious section. In old pasture at the Waite Institute, densities of tunnels ( > 1.5 mm diameter ) of L,,= 1.3 km m -3 and 0.09 km m -3 have been measured at depths cjf i00 mm and 200 mm, respectively (B.M. McKenzie, personal communica t ion , 1988 ). Densities are even lower at greater depths. Several roots are often observed growing in the same tunnel.

I N F L U E N C E O F C R A C K S A N D B I O P O R E S O N C R O P S

When meetivg the sui-face o f a hardpan or compact soil layer a root tip may a t tempt to penetrate it or may be deflected, depending on the strength rela- t ions o f the soil. Roots growing throug~h large voids or through soft goil are easily deflected. Dexter (1986a) found that the propo~ion , Pp, o fwhea t roots penetrat ing a horizontal hardpan below seedbeds of different structures could be described by

Pp = e x p ( - - ~r~, (3)

where Q is the penet rometer resistance of the hardpan in MPa, and where a ranged from 0.3 for a seedbed of fine aggregates to 0.6 for a very coarse seedbed. The horizontal distance that a root axis has to grow before meeting a crack or biopore is a mat te r of chance. Dexter (1986b,c) gave distr ibutions of the distances for different pat terns of cracks and for biopores in random arrays. In the case of desiccation cracks, forming hexagons with the distance, L, between parallel sides, the distr ibution of distances can be described approximately by

A M E R L I O R A T I O N O F SOIL BY N A T U R A L PROCESSES 97

Dr =0 .8 LPc( 1.0+ 0.4P¢ s) (4)

where Pc is the propor t ion of root axes that will meet a crack within the distance De. Using the results of Dexter (1986b) the chance, Pec, of a deflected wheat root entering a crack which it has met can be described by

Pe~ = 1 . 0 - 1.54exp( - 1.23w) (5)

where w is the crack width in ram. Accolding to eqn. (5) , deflected whea~ roots will not enter cracks less than 0.35 mm wide and 45% will miss a crack 1 mm wide. Roots which are not deflected, but which penetrate the hardpan may subsequently meet and take advantage of cracks that are much narrower.

The horizontal distance to a biopore can be described by

Db = --0.8 l d In ( I.O--Pb)/H (6)

where Pb is the propor t ion of deflected axes which meet a biopore within a Eistance Db, d is the pore diameter, and H is the fraction of the soil volume occupied by biopores. The chance of a deflected wheat root entering a biopore which it has met may be writ ten as

Peb = 1.0-- 0.36 ( d + 0.2 )(-0.77) (7)

from the data given in Dexter (1986c) . These possibilities for roots entering cracks and biopores can be integrated

into models for crop growth. This has been done for cracks by Jakobsen and Dexter ( 1987 ) and for the case of biopores by Jakobsen and Dexter (1988 ). Some results for biopores are shown in Fig. 5. It can be seen that for op t imum grain yield, biopore densities should be in excess of 600 m -2 at the Waite

s r

_ 4

q) °~

.E

(0

/ 2

0 I O 1 0 0 0

W a i t e I n s t i t u t . ~

M i rm ipa

I I 2 0 0 0 3 0 0 0

Biopore density, m -2

Fig. 5. Predicted yields of wheat as functions of the density of vertical biopores of 2 mm diameter for two sites in South Australia (after Jakobsen and Dexter, 1988).

98 ~,.rt DEXTER

Insti tute ( 336 mm rainfall in the growing season ) and 1000 m -2 at Minnipa (196 mm rainfall in the growing season ). Actual biopore densities fall far short of these values since, even in old pasture at the Waite Institute, the density of tunnels > 1.5 mm diameter does net exceed 450 m -2 (B.M. Mc- Kenzie, personal communica t ion , 1988 ).

The decline in grain yie!d with biopore densities > 1200 m -2 at Minnipa, as shown in Fig. 5, appears te be a real effect. The increased rooting and increased crop t ranspirat ion at such high biopore densities leaves insufficient water in the soil for grain filling with a con:~equent reduction in grain yield. This effect is often observed in practice in such dry and marginal areas where sub-soiling can result in increased dry mat te r product ion but reduced grain yields for the same reason.

C O N C L U S I O N S

The preceding sections have shown that the natural processes of wetting and drying of the soil and the activities of ola~.,: roots and soil fauna can have profound effects on the structure and physical 0roperties ofsoil . These effects are usually beneficial in the sense of making the soil more suitable for agricultural use.

The importance of these effects is not only that they exist, but that they can be adapted and to some extent nlodified and contro| led. They are then, of course, no longer completely "na tu ra l " processes, but perhaps "modif ied natural" processes. For examples even with the absence of irrigation facilities, the hydrology of the soil can be modif ied because the drying of the soil can be modif ied by cropping, by the use of mulches, and by surface tillage treatments. The extent of drying produced modifies both the crack pat tern produced and the response of the soil to subsequent wetting.

Ageing of soil after disturbance increases its stability and resistance to further disruption. Increased stability through direc'~ binding by roots and fungal hyphae and through bonding by mucilages from plant roots and soil fauna can also be important .

Biological tillage aims to create biopores for use by subsequent crops. Bio- pores may be created by the roots of plants able to penetrate strong (e.g. compacted) soils or by soil fauna such as earthworms. There seems to be considerable scope for screening large numbers of plant species for their ability to penetrate strong soils, and for incorporat ing the better of these into crop ro- tations. Also, there is considerable scope for introducing additional, exotic species of ear thworms which are more active under Austral ian conditions, than the introduced, European species which are struggling to survive in Aus- tral ian agricultural soils at present. Suitable species are most likely to exist in Africa or on the Indian sub-continent.

The development and use of " 'modified na tura l" processes may enable the physical status of many soils to be raised to new, high equ~!ibrium levels at

AMERLiORATION OF SOIL BY NATURAL PROCESSES 99

r e l a t i v e l y l o w cost . I n c o n t r a s t , m o s t a t t e m p t s to m a k e p e r m a n e n t i m p r o v e -

m e a t s to so i l s t h r o u g h t h e " u n n a t u r a l " p r o c e s s o f i n t e n s i v e t i l l a g e a r e p r o b a -

b l y fu t i l e . H o w e v e r , a m u c h m o r e d e t a i l e d u n d e r s t a n d i n g o f t h e b a s i c m e c h - a n i s m s i n v o l v e d in n a t u r a l p r o c e s s e s o f so i l a m e l i o r a t i o n is r e q u i r e d b e f o r e t h e y c a n b e fu l l y e x p l o i t e d .

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100 A.R. DEXTER

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Amelioration of soil by natural processes