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II. Comparison of Environmental Conditions in Tilled and Untilled Soils

II. Comparison of Environmental Conditions in Tilled and Untilled Soils

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ZERO-TILLAGE



81



depends on the amount of mulch present, the activity of soil animals and

the prevailing weather conditions. Where such a top layer of biologically

stabilized crumb structure has been established, slaking of silt material and,

consequently, formation of a dense crust are rarely observed on zero-tilled

silty loams.

The structure and size of deeper layers depend mainly on their soil texture and on the texture dependent reaction to changes in soil moisture

and temperature. In 'hit soils, zero-tillage induces a platy structure (Bulfin,

1967). This type of frost structure is unstable in tilled soils owing to excessive water in the top layers during the thawing process; however, it normally remains visible throughout the year in zero-tilled silty loam soils.

A polyhedric structure is typical for soils with a high clay content, low

capillary water conductivity and distinct swelling and shrinking properties.

Since swelling of the clay after remoistening will close every cleavage again

if the soil has not been previously mechanically loosened, tilth induced

by frost or drought may be a transient phenomenon in zero-tilled clay soils

(Czeratzki, 1971).



B. SOIL FLORA

AND FAUNA

Changes in soil flora and fauna can be expected when zero-tillage practices are introduced. Suitable information is lacking, especially with regard

to reactions of microorganisms to zero-tillage effects per se. Indirect evidence that zero-tillage changes microbial activity is derived from tests of

cellulose decomposition under field conditions. On zero-tilled soils, higher

decomposition rates than on plowed soils were observed by Herzog et al.

(1969), whereas Bender and Adamczewski (1970) found the reverse.

However, these results reflect more the prevailing soil conditions than possible changes in microbial populations.

In two field experiments with continuous wheat in England (Corbett

and Webb, 1970), the total number of nematodes was sometimes larger

with zero-tillage, while migratory parasitic nematodes were usually less numerous on untilled soils. Small-sized species of nematodes seemed to be

favored on naturally compacted soils; inadequate observations do not allow

further conclusions.

Although earthworms form the most conspicious group of soil-inhabiting

animals, little information is available about the changes in weight and

number of earthworms upon introduction of a no-tillage system. For sampling, all investigators used vermifuges, which do not allow complete recovery of existing worms; therefore, only relative values can be reported.

In West Germany, Schwerdtle (1969) found on the average a 12-fold increase in number and a 16-fold increase in weight of earthworms collected



82



K. BAEUMER AND W. A. P. BAKERMANS



on zero-tilled plots after three years’ cropping with corn. In England, Wilkinson (1967) reported less spectacular increases, e.g., on cereal stubble

fields a mere 1.6-fold weight increase. On former leys and permanent pastures, he did not observe any difference in the weight of earthworm populations resulting from tillage treatments. On three sites of former permanent

pastures in the Netherlands, we found about half as many earthworms on

tilled as compared to untilled plots after seven years.

More than in any changes in abundance of earthworm populations, the

agronomist is interested in their activities which alter the ecological conditions of naturally compacted soils. On loamy sand in West Germany, Graff

(1969) measured the rate of casting from September to May over a period

of three years. On untilled, mulched plots, between 2 and 4.5 kg dry matter

per m2, which is within the range of values encountered normally on old

pastures (Evans and Guild, 1948), were deposited on the soil surface.

Graff (1969) observed 20- to 25-fold increases in rate of casting on untilled plots as compared to turnplowed barley stubble. Normally, most

earthworms deposit their castings in the soil, not on top of it. In compacted

soils, however, most castings are deposited on the soil surface. Still, even

on zero-tilled soils, considerable soil mixing is to be expected.

Earthworm tunnels which open to the soil surface may influence the

rate of water infiltration. Since small tunnels are difficult to distinguish

from soil cracks, the number of earthworm “mittens” may serve as a first

approximation. On untilled cereal stubble fields, we found an average of

55 mittens per m2 soil surface. One centimeter below the soil surface, an

average of 68 tunnels (diameter 2-10 mm) was observed on no-tilled stubble as compared to 15 on plowed stubble (W. Ehlers, personal communication, 1972).

Moles are predators of earthworms and increase in number when fields

are left undisturbed for a prolonged period. In Switzerland, Vez (1969)

counted 10 to 12 molehills per are on zero-tilled plots as compared to

1 to 2 on conventionally tilled plots. Similar differences can be observed

in burrows of voles and mice; these have not yet been quantified.



C. SOILORGANIC

MATTER

With zero-tillage, plant residues remain on the soil surface. This is essential when soil erosion limits successful farming. Where erosion presents

no problem, a mulch cover may be desirable to create a favorable soil

tilth. The highest permissible level of mulch is determined by conditions

governing the effective performance of drilling and weed control operations. In the Netherlands, 3000 to 4000 kg straw (dry matter) per hectare

can remain on the ground only when it is chopped up into small pieces



ZERO-TILLAGE



83



and evenly distributed. For dryland farming, an amount of 3500 kg straw

per hectare at harvest is considered to provide adequate soil protection

from erosion without presenting problems with seeding, weed control and

soil fertility.

Brown and Dickey (1970) determined losses of wheat straw buried in

the soil, placed on the soil surface and exposed above the soil surface to

simulate a standing stubble. At two locations in Montana, they found that

the rate of dissipation increased with greater contact between soil and plant

material and decreased when rising amounts of straw were applied. Lower

mean annual temperature, though in combination with higher precipitation,

retarded straw decompositibn.

During the first 3 months of exposure, the weight of above-surface and

on-surface straw increased by as much as 1 3 % due to an accumulation

of soil particles inside the hollow straws. Some soil probably is moved by

wind or raindrop splash and, near the ground, by the activities of soil-inhabiting animals.

The experiment of Brown and Dickey was begun in May. After 18

months’ exposure, only 22 to 40% losses were measured on and above

soil as compared to 93 to 98% buried in the soil. In the more humid climate of Germany, higher rates of wheat straw decomposition were observed, e.g., 40% during the period September through July (K. W.

Becker, personal communication, 1972). The weight of adhering soil particles equaled the amount of straw remaining after 1 1 months’ exposures.

These figures indicate that a slow surface accumulation of straw residue

can occur if the above-mentioned process of incorporation into the soil

is not effective.

Plant residues with higher N content decompose more rapidly. Cornstalks applied to the surface of a cornfield in Iowa in May lost 50% of

the initial weight after 20 weeks’ exposure (Parker, 1962). Sugar beet tops

left on the ground in Western Europe are completely disintegrated by July.

Hence, leafy and succulent plant material presents no problems in mulch

management.

In Fig. 1, some results of trials on former grassland and recently plowed

soils concerning the distribution of organic matter in soil are summarized.

No differences in organic matter concentration were found in regularly

plowed soil layers. In undisturbed soils, the concentration was highest near

the soil surface and declined steadily to subsoil values below those on conventionally tilled soils. The gradient of organic matter content was more

pronounced on former grassland soils, where zero-tillage presumably preserved the original distribution of organic matter.

No thorough investigation has yet been published as to whether zerotillage results in an accumulation of total soil organic matter. Available



84



K. B A E U M E R A N D W. A. P. B A K E R M A N S

organic matter ( % . d r y s o i l )

1



30i



2



3



4 5



arable l a n d

(16)



6



7



N ( '/. )



8 9 1011



grassland



(22)



arable land



(16)



FIG.1. Average distribution of organic matter and N in tilled (-)

and untilled

(- - - ) soil. (From Bakermans and De Wit, 1970, grassland; Buhtz et al., 1970,

arable land.)



data from long-term trials (Moschler et al., 1972; Buhtz el al., 1970) suggest that zero-tillage increases the total organic matter of the soil.

Whether the observed differences in accumulated organic matter are

caused by restricted decomposition andlor higher production of organic

matter on zero-tilled soils is not yet known. As compared to a tilled

chernozem soil in East Germany, concentrations of CO, in the atmosphere

near the soil surface were lower on untilled soil (Buhtz, 1972). These

observations suggest a reduced rate of mineralization in naturally compacted soils.



D. PARAMETERS

OF SOIL STRUCTURE

With zero-tillage, soils are loosened only locally and superficially; yet

they have to bear the normal load of traffic in the field. Hence natural

consolidation and mechanical compaction will cause a denser packing of

zero-tilled topsoils. The average decrease in total porosity was found to

vary between 0 and 6% (v/v) (Czeratzki and Ruhm, 1971; Herzog and

Bosse, 1969; Vez and Vullioud, 1971a,b). A few exceptions were noted

on heavy river clay rich in organic matter (Van Ouwerkerk and Boone,

1970), on two sites with silt loam-chernozem soil (Buhtz et al., 1970)

and on silty clay (Bachthaler, 1971) where values of total porosity were

lower on tilled than on zero-tilled plots, probably as a result of compaction

caused by tillage operations.

In general, the differences in total porosity were greatest in the soil layer

which is loosened by plowing, but not compacted by seedbed preparation

and cultivation ( 10-1 8 cm) . In deeper soil layers, the differences .tended

to diminish. Near the soil surface, they varied with the effects of tillage

operations, weather, and biological activities.

Mean values of total pore space average over sampling dates, crops,



85



ZERO-TILLAGE



and locations, eliminate extreme values, which may be decisive for plant

growth and farming operations in critical situations. The lowest sampling

means of the porosity data published were found with values near 35 and

38% (v/v) on untilled plots on sandy soil (22-27 cm) and clay soil

(15-20 cm), respectively (Czeratzki and Ruhm, 1971). The very high

density of the soil layer 22-27 cm on sand merits special attention, as it

is probably induced by mechanical compaction and perhaps by downward

displacement of finer soil particles, which by turnplowing are redistributed

to upper soil layers. This could be proved by particle size and pore size

distribution analysis, but no information is yet available.

The observed minimum values did not mark the final stage of soil density on zero-tilled soils. At subsequent dates, porosity increased again, especially on stable soils with medium to high clay content. The above-stated

lower levels of pore space were reached within two to three years of zerotillage, after which time seasonal fluctuations of total porosity tended to

be smaller as compared with conventionally tilled soils (Van Ouwerkerk

and Boone, 1970).

Similar results are shown in Fig. 2, which contains a time series of

porosity measurements in the top 2-6 cm layer of arable silt loam soil derived from loess (Ehlers, 1973). Zero-tillage resulted in a smaller total

porosity but also in reduced variability of- the sampling means; consetotal

porosity



40



40

20

1s

h



>



10



$ 5



- 0



10



5



medium

pores



0



3



15

10



- 30rm



sma II

pores

0.2 - 3.0p m



5



0

10

5



0

oats



-*



un ti lied



radish



rotabated



cultivated



tilled



FIG. 2. Changes of total pore space and pore size distribution with time at

a depth of 2-6 cm on tilled and untilled silt loam soil. (From Ehlers, 1973.)



86



K. BAEUMER AND W. A. P. BAKERMANS



quently, homogeneity in time increased in naturally compacted soils. The

remaining fluctuations of total porosity presumably result from the combined effect of seasonal changes in climate and soil cover on the activities

of soil flora and fauna.

The example in Fig. 2 shows further that changes in total porosity were

accompanied by concomitant changes in other pore size fractions. It can

be concluded, therefore, that mechanical loosening effects mainly the fraction of large pores and that dense parts of the soil remain more or less unchanged. As compared to the fraction of large pores, the other pore size

fractions fluctuated to a smaller extent.



5



Y



30

..



20



10



0



p o r e space



('I-vlv)



60



10



20



40



30



10



;



20



0



30

1



pore

size



A



-F



3



P



N

I

W



w



W



0



I



I



w



0)



o



O



"



W



m o



o z



F



3



=



z

A



-F



3



W



R



W



I



I



W



O

I



w



m



0



0



:!

0--.



-Fn

3



FIG. 3. Changes of total pore space and pore size distribution with depth on

tilled (right panel) and untilled (left panel) silt loam soil. (From W. Ehlers,

personal communication, 1972.)



Figure 3 shows the vertical pore size distribution of a silt loam soil (W.

Ehlers, personal communication, 1972). Although in this case the total

porosity and the fraction of large pores did not differ much between zerotilled and conventionally tilled soils, the pattern of porosity reveals an important difference: on undisturbed soil, the relative space occupied by each

pore size fraction varied less than on the ploughed soil, where the layers

at 0 to 15 cm and 25 to 30 cm were compacted as compared to the layer

at 15 to 20 cm. The compaction at 25-30 cm is presumably caused by

pressure and smearing actions during plowing. It resulted not only in an

absolute reduction in large and medium pores-based on volume as well

also in a relative increase in small and very small

as on weight-but

pores-based on volume only, as discussed by Ehlers (1973). Untilled

soil, though generally denser, may also exhibit more structural homogeneity in space as compared to conventionally tilled soils.

A relatively higher amount of smaller pores, but greater homogeneity

in time as well as in space are thus the dominant changes in porosity when



ZERO-TILLAGE



87



a soil remains untilled for a long period. Another feature may be connected

with the continuity of pores. Since earthworm tunnels can be regarded as

primarily continuous pores, an estimate of the relative pore space occupied

by them may serve as a first approximation. Figure 4 shows that the space

occupied by large pores with presumably uninterrupted connections to the

atmosphere is more than doubled near the soil surface and in the top 20

cm of zero-tilled soil as compared to plowed soil (W. Ehlers, personal

communication, 1972) .

p o r e space (% v/v)



o



0.2



0.4



0.6



ae



1.0



FIG. 4. Pore space occupied by

rainworm tunnels on tilled (O---O)

and untilled (0-0) silt loam soil

(From W. Ehlers, personal communications, 1972.)



701

ao



Other composite parameters of soil structure are resistance to penetration and shear stress, which are highly dependent on texture, soil moisture,

and porosity. In general, larger resistance to a cone-shaped probe forced

into the soil was observed on zero-tilled soil (Buhtz et al., 1970). On sand

soil, J. M. Houben (personal communication, 1972) found no rooting

when penetrometer resistance exceeded 40 kg/cm2. In one case, we observed that continuous zero-tillage on sandy soil produced a comparable

compaction in layers between 5 and 30 cm.



E.



AERATION

AND SOILMOISTURE



Soil aeration depends on porosity and water content. Hence when a soil

is water saturated to field capacity (soil moisture tension: 0.1 bar A p F

2), the extent of the remaining pore space filled with air (air capacity)

may be critical for the maintenance of soil aeration. A minimum volume

of 10% is thought to be necessary for adequate gas exchange between

the soil air and the free atmosphere.

Though zero-tillage generally causes a decrease of large, mostly air filled

pores (diameter > 30 pm) and thus reduced aeration, air capacity at p F



88



K. BAEUMER AND W. A. P. BAKERMANS



2 was observed only on medium- to heavy-textured soil to be below 10%

(v/v) (Van Ouwerkerk and Boone, 1970; Czeratzki and Ruhm, 1971;

W. Ehlers, personal communication, 1972). Impeded aeration, if caused

by zero-tillage, may provide a serious objection to the application of this

system on heavy soil in humid regions. With regard to this point, an evaluation of large, continuous pores, such as earthworm tunnels, would be of

interest.

The observed relative increase in the amount of medium to small pores

caused by zero-tillage has consequences for the water-holding capacity of

the soil. Plowing up grassland results in the redistribution of organic matter

( Fig. 1 ) ; zero-tilled sod retains its original accumulation of organic matter

near the soil surface. Water-holding capacity is related to organic matter

content, especially on sandy soils; this was confirmed by Van Ouwerkerk

and Boone (1970), who found that water content at p F 2 changed more

in conjunction with organic matter content than with soil porosity. Hence,

in the top 6 cm of the zero-tilled soil, a higher water content at p F 2

was found than in the plowed soil, whereas the reverse was true in the

layer at 11-1 6 cm.

Thus, beginning with a permanent pasture, changes in soil behavior

caused by different tillage systems cannot be ascribed solely to differences

in porosity. On arable land, the situation is less complicated since waterholding capacity generally increases with increasing pore space of an

equivalent diameter <30 pm. On zero-tilled soils, therefore, a relatively

larger part of the pore space ( % v/v) was found to be water filled, though

this is of no consequence for the amount of available water, i.e., the difference in moisture content in % (w/w) at p F 4.2 and p F 2.0 (Van

Ouwerkerk and Boone, 1970; W. Ehlers, personal communication, 1972).

Information about the energy associated with soil water is essential for

understanding its movement in soil and its availability for plant growth.

W. Ehlers (personal communications, 1972) investigated changes in gravimetric water content and matric potential in time and down the profile

of a silt loam derived from loess. Differences between tilled and untilled

soils are shown for four situations (Fig. 5, a-d). These are characterized

by beginning and advanced stages of either depletion (4-8 and 6-7, respectively) or recharge of soil moisture (6-10 and 6-21, respectively).

Most remarkable is the observation that soil water tensions were effected

by the tillage system down to a soil depth of 220 cm. This demonstrates

the consequences of changing the structure of one soil layer only for the

moisture regime of a whole profile. Differences in soil water content between tilled and untilled soils were relatively small and inconsistent compared to differences in soil water tension and hydraulic potential. Zerotilled soil with a similar water content generally had a lower soil water



89



ZERO-TILLAGE

matric potential



JI



hydrauiic potential



(cm water column)



-400 -200



0



I""""



Q+



-...



100



200



9



water content



(cm water column)



*200



9 6 - 21 -1971

i

P



t



FIG.5 . Matric potential, hydraulic potential, and water content of tilled (O---O)

and untilled (0-0)

silt loam soil during phases of depletion (a, b ) and recharge of soil moisture (c, d ) . Arrows indicate situations where gradients of hydraulic potential are zero. (From W. Ehlers, personal communication, 1972.)



tension, which indicates a smaller resistance to water uptake by plant roots

and a higher conductivity of soil water.

The largest differences between tilled and untilled soils were observed

during the rewetting phase (Fig. 5 , c, d ) . The rain water infiltrated rapidly

into the plowed layer, but slowly into the subsoil of the tilled plots. Thus,

soil water tensions were reduced to near zero in the upper soil layer, but

continued almost unchanged at greater depth. Yet, in the untilled soil, the

soil water tensions remained at a higher level near the soil surface and

decreased rapidly in the subsoil.



90



K. BAEUMER AND W. A. P. BAKERMANS



This behavior indicates a lower resistance of the zero-tilled soil to infiltration, which may be typical for light to medium loam soils, where earthworms or other soil animals construct a continuous set of large pores connecting the soil surface with the subsoil. Decaying roots which remain

undisturbed in place could provide ready avenues for water infiltration into

the soil profile as well (Barley, 1954).

Mulch physically absorbs raindrop impact energy. Thus, slaking and

sealing of the soil surface is prevented or at least retarded. Therefore,

zero-tillage generally reduces surface runoff. On silt loams with an 8-10%

slope planted with row crops, reduction ranged from one-half to one-sixth

of the amount observed on clean tilled land (Harrold et al., 1967; Shanholtz and Lillard, 1969; Jones et al., 1969).

On a silt loam in Ohio, Triplett et al. (1968) found an increase of both

the infiltration rate and total infiltration with increasing soil cover by cornstalks, the zero-tillage normal residue treatment resulting in higher values

than the conventional tillage treatment. Partition of the mulch effect due

to physical protection and structural stability of the soil showed that in

this case the accumulation of soil stability was more effective than physical

protection of the soil surface.

The purely protective effect of residue cover may influence the rate of

soil water evaporation too. During the initial constant rate of evaporation,

when rates depend solely on the saturated hydraulic conductivity of the

soil and the evaporative demand of the atmosphere, Bond and Willis

( 1969) observed decreased evaporation with increasing residue rates. During the following stage of falling rates of evaporation, when the soil surface

also dried underneath a mulch cover, no differences in evaporation rate

between bare and mulched soil were found.

Most of the evaporation losses in row cropped soil should occur before

the closing plant canopy reduces the incident radiation and thereby evaporation at the soil surface. Hence differential gains in soil moisture content

by means of zero-tillage as induced by mulch protection are to be expected

mainly during the early growth stages of row crops, when evaporation rates

of the more or less saturated soil are high.

Generally, zero-tillage resulted in higher mean volumetric moisture content in the upper 30-60 cm soil layer than conventional tillage on soils

situated in the subhumid regions of North America on gently sloped silt

loams planted with row crops (Harrold et al., 1967; Amemiya, 1968;

Blevins et al., 1971; Jones et al., 1969). The greatest differences in total

available water always occurred early in the growing season (Shanholtz

and Lillard, 1969; Van Doren and Triplett, 1969).

Zero-tillage methods could be expected to increase water conservation

in dry-farming regions. As compared to conventional stubble mulch fallow,



ZERO-TILLAGE



91



complete chemical fallow resulted in lower moisture conservation at one

location in the semiarid Great Plains (Black and Power, 1965), whereas

water storage gains were observed at two other locations (Smika and

Wicks, 1968; Army et al., 1961). These contradictory results could perhaps be explained in part by the differential reduction of evaporation by

a mulch cover during constant and falling rate drying phases. After prolonged falling rate drying, the cumulative evaporation from a mulched or

bare soil was nearly equalized (Bond and Willis, 1969).

Greater soil water storage may be attained with chemical fallow only

under the following conditions: first, when cumulative evaporation during

drying intervals is less with an undisturbed upright standing stubble than

with a residue cover knocked down by subtillage; second, when more frequent rains are prevalent, since mulches are of little value for water conservation during extended dry periods.

F.



SOIL TEMPERATURE



Soil temperature depends on the thermal conductivity and volumetric

heat capacity of a soil, and on the amount of heat that enters or leaves

the soil surface. Hence the amount of soil cover and the water and air

content of the various soil layers are decisive factors for the temperature

regime of soil.

Van Duin (1956), who used the results of a theoretical investigation,

showed that loosening the upper soil layer by mechanical means should

increase the diurnal temperature variation near the surface of a clean-tilled

soil, but decrease the amplitude in the deeper, undisturbed layers. Thus,

the loosened zone acts as an insulating layer. During phases of rising soil

temperature, a tilled soil should be warmer near the surface but cooler

in the subsoil than in undisturbed soil. The reverse is true during periods

of falling temperature.

Since the effect of a mulch cover is similar to that of a loosened soil

layer, differences in soil temperature between conventionally tilled and

zero-tilled soil will become larger with increasing amounts of cover.

During the growing season, untilled or merely mulched soil were observed to be cooler than clean tilled soil (Van Wijk et al., 1959; Parker

and Larson, 1962; Shanholtz and Lillard, 1969). At a depth of 10 cm,

the average difference in maximum soil temperature ranged from l o to

3 O C , which was greater than differences in minimum temperature, which

were less than 1OC.

During cool periods in May, Moody et al. (1963) observed higher soil

temperatures on mulched soil, confirming the postulated reversal of differences during periods of falling temperatures (Van Duin, 1956). Similar



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