Tải bản đầy đủ - 0 (trang)
III. Effects of Zero-Tillage on Plant Growth

III. Effects of Zero-Tillage on Plant Growth

Tải bản đầy đủ - 0trang

96



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



On light to medium-textured soils with short, standing stubble and a

friable soil surface, a higher number of emerged plants was observed with

zero-tillage than with conventional tillage (Buhtz et al., 1970; Debruck,

1971 ) . This happened to be the case more often under conditions in which

a lack of available soil moisture restricted seedling emergence on tilled

soils. Straliik (1968) observed more rapid water absorption by seeds on

untilled soil. Therefore, higher rates of seedling emergence can be expected

with zero-tillage than with conventional tillage during warm and dry

periods.

The reverse may be true when the soil is so wet and cold that loosening

it for seedbed preparation results not only in better aeration, but also in

higher soil temperatures. A difference of 1-2OC between tilled and untilled

soil may be decisive for germination and subsequent growth if minimum

temperature requirements are not fulfilled, as can be observed with maize

grown in mulched soils in a temperate climate (Willis et al., 1957).

There are additional effects of zero-tillage which can negatively effect

germination and emergence. Residues of nonselective herbicides can still

be concentrated enough at sowing time to retard or prevent emergence

of zero-tilled plants (Adams et al., 1970). Thick mulches were observed

to smother emerging seedlings (Bakermans and De Wit, 1970), probably

by shading and/or transmitting herbicides such as paraquat. Though these

herbicides must be applied before emergence and should be either inactivated or positionally ineffective, they may be partly absorbed by plant residues and hence still be effective when they come into contact with seeds

and seedlings (Taylor et al., 1966; Schwerdtle, 1971).

Plant residues may contain phytotoxic substances (Boerner, 1960).

Aqueous extracts of corn, sorghum, and cereal straw were toxic for wheat

seedlings in laboratory tests even after several weeks’ exposure of the straw

to field conditions (Guenzi et al., 1967). Microbial decomposition of

wheat straw can result in the development of phytotoxic substances, such

as patulin (Norstadt and McCalla, 1968). Whether soluble plant substances and/or microbially transformed compounds can be leached from

mulch material into the seedbed at such concentrations that germination

and emergence of seedlings can be impeded under field conditions in zerotilled soil is not yet known.

B.



ROOT GROWTH



As measured by root density or weight, the amount of roots observed

at different growth stages and soil layers tended to be lower on zero-tilled

soil (Newbould et al., 1970; Cannel and Ellis, 1972). This difference in

root mass was accompanied be shallower rooting in undisturbed soil, espe-



ZERO-TILLAGE



97



cially during early vegetative growth phases. The extension rate of seminal

root axes was slower, yet lateral branching started earlier in zero-tilled

soil, thus leading to the production of a dense but shallow seminal root

system on undisturbed soil. No differences were found between the effects

of tillage treatments on the length and frequency of adventitious roots of

wheat (J. R. Finney and B. A. G. Knight, personal communication, 1972).

Therefore, the final root weight and pattern of soil depth distribution at

the ripening stage may be similar on tilled and untilled soil (Baeumer et

al., 1971). However, on sandy soil in the Netherlands, shallow rooting

persisted until harvest, probably owing to mechanical impedance of the

soil to root penetration.

Sometimes, the restricted axial growth of roots was compensated for

by greater radial growth and, hence, by a larger diameter of the seminal

root axes of barley (Yueruer, 1972) or greater weight per unit root length

of corn (Barber, 1971). Thesc results probably indicate the effect of increased mechanical resistance of zero-tilled soil to root penetration.

A shallow but intense root system reflects not only increased mechanical

impedance, but also the different pattern of water and nutrient concentration in tilled and untilled soil. No information is yet available about the

function, i.e., the rate of activity in nutrient absorption and water uptake

of roots in differently structured root environments such as tilled and untilled soil. Only such information-in combination with data on root size

and distribution-could provide the explanations for the observed response

of shoot growth to changed soil structure.

C.



NUTRIENT

ABSORPTION



So far, measurements of nutrient content in leaves or other parts of

plants have been used to detect differences in nutrient absorption of crops

grown on tilled and untilled land. Since the amount of dry matter produced

and the stage of maturity may differ, nutrient content data have to be interpreted with care; total uptake of nutrients by the crop in question may be

a more relevant figure.

P, K, Mg, and Ca contents at various stages have been reported for

corn (Moody et al., 1963; Shear, 1968; Triplett and Van Doren, 1969),

cereals (Kahnt, 1969), and fodder kale (During et al., 1963). In most

cases, the P and K content of plants grown on untilled or mulched soil

was higher or equal to the contents observed on conventionally tilled soil.

Singh et al. (1966), studying uptake of "P-labeled superphosphate by

corn, observed a higher uptake of surface-applied P especially during the

early growth phases, as compared to the uptake from P fertilizer mixed

with the soil by rototilling. Whether this higher uptake is caused by



98



K. BAEUMER AND W. A. P. BAKERMANS



changed P availability due to a different concentration and position of the

applied P compound in zero-tilled soil is not yet known.

Final uptake of P and K by the harvested oats was not significantly

different (Ehlers et al., 1973). It may be concluded that the concentration

and distribution of surface applied P and K do not seriously restrict plant

growth on zero-tilled soil. Rather, different results can be expected with

N, which, in its available form, is more subject to temporarily changing

soil conditions.

Analysis of plant tissue samples from untilled or mulched soil showed

generally higher N content of corn (Moody et al., 1963; Shear, 1968),

cereals (Arnott and Clement, 1966; Kahnt, 1969), and fodder kale (During et al., 1963), as compared to samples from plowed soils. This increase

in N content contradicts the observations on the distribution and concentration of soluble N in undisturbed soil. It can be partly explained if increased N content is accompanied by reduced plant growth, which may

result in increased N content of less mature plant tissue.



f5

L



N uptake



z

0

Total dry matter produced (100 kglha)



FIG.8. Diachronic changes of N content, dry matter produced, and N uptake

of wheat forage grown on tilled and untilled silt loam soil. (From G . Pape and

H. Fleige, personal communication, 1972.)



Our results, presented in Fig. 8, indicate, however, that, independently

from dry matter production, the N content of forage grown on zero-tilled

soil may be higher as compared to plowed treatments. The differences between tillage treatments diminished during later stages of growth. Finally,

at harvest, the lower grain yield and the lower N content of grains resulted

in lower total N uptake on zero-tilled soil, as shown for this (1971; Table

I, winter wheat) and other cases (Debruck, 1971). This time pattern of

N uptake appears to be typical for mulched soil. Parker (1962) compared



99



ZERO-TILLAGE



TABLE I

Total N Uptake by Cereals Grown on Tilled and Untilled SoiP~b

a. Winter wheat, 1971

Nitrogen applied (kg/ha)

Tillage

Conventionally tilled

Zero-tilled

LSD0.05: 5.7



0



50



100



150



Mean



,50.0*



74.5

70.5



103.3

87.5



197.2

113.3



88.8

78.8



44.1



b. Winter barley, 1972

Nitrogen applied (kg/ha)

Tillage

Conventionally tilled

Zero-tilled

1,SDo.o~:6.5



0



80



120



160



Mean



30.7

41.8



72.5

79.2



95.2

93.3



105.2

100.5



75.9

77.2



Silt loam, West Germany.



* Uptake values are expressed as kilograms per hectare.

the effects of buried and mulched corn residues on the N content of corn.

During the first 35 days after planting, he found a higher N content, during

later stages a lower N content of plants grown on mulched soil.

These results do not suggest that N absorption by crops is generally

more restricted on zero-tilled soil. This opinion can be supported by findings such as those presented in Tablc I, which show that barley grown

in 1972 with N fertilization took up more N on untilled soil than on tilled

soil. The causes for this result and the different time pattern in N uptake

are not yet known.

D.



CROP



GROWTH



Tillage induced changes in soil environment are effective only in combination with numerous other factors such as weather, weed growth or

diseases and pests. Hence tillage effects can be expected to be highly inconsistent if evaluated over a wide range of ecological conditions (cf. Fig.

12).

Disregarding cases in which lower plant density or less complete weed

control in one or the other tillage system does not allow the comparison

of growth rates, there are situations in which early growth of zero-tilled

crops is either enhanced or retarded by soil conditions.



100



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



In the corn growing region of North America, faster growth of sod-sown

maize seedlings, as judged by plant height, was observed by Moody et al.

(1961), Shanholtz and Lillard (1969), and Jones et al. (1968). This was

explained by an increase in available water and the favorable root zone

environment provided by the mulch and soil structure of undisturbed soil.

Van Doren and Triplett (1969) analyzed the mechanism of this growth

response and concluded that it was directly or indirectly caused by improved soil moisture regime, especially during the first half of the growing

season. With increased available moisture, they observed an increased leaf

area production, which in turn resulted in yield increases if a water shortage during later stages did not restrict carbohydrate synthesis.

More vigorous growth requires additional water withdrawal from soil;

b



a



.



.



15 21



.



29



35



49



64



4-22



5-15



days from planting



6-2



6-23

7-12

sampling date



FIG.9. ( a ) Growth of maize in Iowa on mulched (0-0)

and plowed ( O - - - O )

soil. ( b ) winter wheat in Germany on tilled ( O - - - O )and untilled (0-0)

soil.

(From Parker, 1962; G . Pape and H. Fleige, personal communication, 1972.)

Final Yield of Grain (Dry Matter)

(a) Corn (hu/ncre)



(b) Wheat (tons/ha)



N applied



Mulched



Plowed



TJntilled



Tilled



0

120 Ib/acre

150 kg/ha



120

124



126

130



2.2



3.1



-



-



-



-



3.8



4.5



ZERO-TILLAGE



101



this was calculated by Shanholtz and Lillard (1969). The water use efficiency of maize crops grown on zero-tilled and conventionally tilled soil

was 81% and 57%, respectively. Despite the greater water extraction, visible wilting of maize and sugar beets was delayed for hours or days, which

indicates an enhanced depletion of soil water on undisturbed soil.

In other cases, an early season depression of maize growth was observed

on mulched soil (Moody et d.,1963), as shown in Fig. 9a. Similar effects

were observed with zero-tilled cereals (Fig. 9b, Bosse and Herzog, 1969).

Where N fertilizer had been applied, the difference in dry matter production eventually disappeared. As shown by the N content in Fig. 8 and similar results of Parker (1962), the retardation of early growth cannot be

primarily a consequence of reduced N availability in mulched or zero-tilled

soil, though the time lag of N mobilization, sometimes observed on undisturbed soil, may take part in this effect.

Burrows and Larson ( 1962) and Moody et al. (1963) found lower soil

temperatures under mulch to be the causative factor in retarding early

growth of corn. Where soil temperatures are high enough to answer the

requirements for optimum growth, as shown by Van Wijk et al. (1959),

the depression of soil temperature by zero-tillage methods should be without noticeable effects on the early growth of corn. Whether the depressed

early growth of zero-tilled cereals is caused by lowered soil temperatures

has not been yet investigated.

E.



CHANCES

I N WEEDPOPULATIONS



Leaving an arable soil undisturbed prevents deeply buried but viable

weed seeds from germinating. This results in diminishing rates of emerging

annual weeds if the replenishment of the weed seed population is curtailed

by preventing seed shedding of weeds as accomplished with effective weed

control (Roberts and Dawkins, 1967). Debruck (1971) and Schwerdtle

(1971) verified this effect, which is shown in Fig. 10. Incomplete herbicidal control resulted in increased annual weed growth, especially of gramineous annual weeds.

The observed reduction of annual weed may be caused, at least for some

species, by less favorable conditions for germination and/or by destructive

effects of herbicides applied on viable, yet dormant weed seeds. Paraquat,

for instance, drastically reduced the viability of grass seeds (Evans, 1961;

Schwerdtle, 1971). Deep plowing and cultivation serve to keep many

perennial species at least in check; with zero-tillage, some of these weeds

remain almost undisturbed. Therefore, large populations of perennial

weeds can build up in untilled soil, if either adequate control measures are

neglected or available herbicides are ineffective or perhaps not applicable.



102



K. BAEUMER AND W. A. P. BAKERMANS



zero-tillage



-



persistent weeds



I



looE

+-.-.-.d

1



2



3



year



FIG.10. Changes in weed population with time on continuously tilled (O---O)

and untilled soil. (From Debruck, 1971.)



In addition, on old sods, some species of well established and adapted

grassland vegetation may be able to resist most control measures feasible

in a zero-tillage system (Hood, 1965). Peters (1972) compiled a list of

the most problematic weed species in reduced tillage in the United States.

Quite often it can be observed that zero-tillage methods result in higher

amounts of volunteer plants from previous crops, especially where cereals,

maize, or sorghum are grown continuously. Although yields may not be

lowered seriously, the value of the crop is depreciated at least by lack of

purity.



F. DISEASESAND PESTS

Takeall (Ophiobolus graminis) and eyespot (Cercosporella herpotrichoides) are soil-borne fungi which cause serious diseases, especially of

wheat, when cereals are grown continuously in the same field. It has been

frequently observed that cereal crops were less severely attacked by these

pathogens on zero-tilled soil than on plowed soil (Hood, 1965; Schwerdtle,

1971). Brooks and Dawson (1968) found a reduced rate of spread for

Ophiobolus on untilled soil, especially when soil temperatures began to

rise and plant growth became rapid.

The occurrence of pests is also influenced by zero-tillage. In North

America, several species of the corn rootworm endanger the production

of maize. Musick and Collins (1971) found in Ohio that the number of

eggs oviposited by the Northern corn rootworm (Diabrotica longicornis)

increased with the amount of ground cover; hence, there were more eggs

in zero-tilled soil. Since root damage was significantly lower for zero-tilled

corn plants, it was suggested that zero-tillage impeded survival of eggs or

larvae. Insect populations in old grassland are often very large and may



ZERO-TILLAGE



103



pose problems for sod-seeded crops. Therefore, it is necessary to protect

seeds and seedlings with suitable insecticides.

A soil cover of crop residues attracts and protects animals which can

especially damage the emerging seedlings. Slugs, mice, voles, hares, and

birds were observed to feed preferentially on zero-tilled crops. Some of

the reported damage, of course, may have been caused by unsatisfactory

sowing methods. In dryland farming, some rodents may increase to such

an extent that zero-tillage methods are rendered impracticable if control

methods are inadequate or too expensive.



IV.



Crop Husbandry



Zero-tillage is still passing through its first stage of trial and error; a

vast and reliable body of knowledge about the applicability of methods

and implements, such as that gained in conventional systems, has not yet

been accumulated.

A.



SOWINGMETHODS



It appears to be rather difficult to insert seeds into the soil at the proper

depth or at equidistant intervals where drilling performance is hampered

by a layer of surface trash and where seed bed preparation is to be reduced

to the least possible soil disturbance. In contrast to traditional tillage, in

which the process of sowing requires several separate manipulations, often

with a time lapse between each, zero-tillage machinery should adequately

accomplish three tasks in one operation: it should open the soil for seed

insertion, place the seed properly, and sufficiently cover the seed.

Available equipment for opening the soil falls into two categories: “direct drought” and “power operated.” Working area assumed to be equal,

power operated implements such as rotavators and powered harrows generally cause greater soil disturbance than “direct draught” implements such

as flat sweeps, rolling or moldboard coulters, and tines.

Seeds and fertilizers are inserted into the soil with hollow chisels or

tines, fluted spear point openers, or double-disk seeders. When employed

with a front disk, this device is known as a triple-disk seeder. Seeding furrows or slots are closed by devices such as covering chains, dividing knives

or concave disks. Under dry conditions, seed-firming press wheels (which

press seeds into the soil before any soil covers the seed), and press wheels

(which press from the soil surface, so that seeds are covered by a compacted layer of soil) may help to close the furrows and improve the soil

water supply to the seeds.



104



K. BAEUMER AND W. A. P. BAKERMANS



Equipment usage has been described and tested by Stickler and Fairbanks (1965) and Taylor et al. (1969). Insufficient establishment of

stands was caused mainly by failure to control sowing depths and to provide enough cover. When surface covering trash has to be cut by rolling

coulters, difficulties may be encountered with fresh straw, which sometimes

is too tough to be cut sufficiently.

Sowing methods, by means of which either a strip or the total soil surface is cleared of existing vegetation or remaining plant residues and loosened thoroughly, as by rotavating or listing, are intended to minimize the

risk of establishing an adequate stand. If sowing depth and covering are

left uncontrolled, no great improvement can be expected. Although problems encountered with a living sod or a thick mulch layer are solved more

easily by rotavating or listing, the specific advantages of zero-tillage soil

structure may be lost. Thus, such methods are useful only for row crops

and in renovating pastures.

B.



WEED CONTROL



With zero-tillage, consistently satisfactory performance of herbicides is

imperative as cultivation cannot be used to destroy vegetation that escapes

the herbicide. As compared to conventional tillage, herbicide functions are

extended. Before sowing, the vegetation initially present must be completely destroyed. This task calls for broad spectrum, nonselective herbicides with relatively short residual effects, e.g., paraquat, dalapon, or amitrole. During germination and subsequent growth of zero-tilled crops,

potential weed competition has to be sufficiently suppressed. Here, highly

selective and persistent herbicides are needed, herbicides which are not

injurious to the crops grown, e.g., atrazine or simazine. At present, there

is no herbicide available which possesses the qualities that would meet both

demands equally well. Hence, without a panacea in sight, complicated systems of weed control must be developed if zero-tillage is to be used

continuously.

The first element is the herbicide system. Split application of one or

several herbicides in combination is one method to increase the effectiveness of an herbicide system in some zero-tillage situations. Thus, at the

transition from a sod crop, either old grassland or a cover crop to a zerotilled crop, a combination of translocated herbicides, applied as soil and/or

foliage treatment, can be used as a first step to kill gramineous and broadleaved perennials. In a prolonged fallow, e.g., in humid areas during the

winter, more persistent herbicides can be applied to kill perennial weeds.

The timing of these treatments is determined by the time when the existent

vegetation can be controlled and by the cropping program. If a susceptible



ZERO-TILLAGE



105



crop is to follow, the application has to be timed in accordance with the

persistence of the herbicide at different locations.

The determining factors are the dose needed to kill existent vegetation,

soil moisture and temperature, frequency and amount of precipitation,

which all influence the rate of inactivation or percolation of the herbicide

used. If the amount of mulch present is not relevant to the success of zerotillage, doses of most herbicides can be kept low when the existent vegetation is reduced by mechanical means, e.g., by shredding, grazing, or cutting

for conservation. On the other hand, foliage treatment with translocated

herbicides, e.g., dalapon or “hormone” weed killers requires lush vegetation with a large leaf area in order to be effective.

As a second step before sowing, a quick-acting but nonpersisting herbicide, such as paraquat or diquat, can be used to destroy the still living

unwanted vegetation. During the third stage, the growing crop is treated

with chemicals which selectively control all weeds escaping from previous

herbicide treatments.

Starting from a stubble situation, the system of split herbicide applications begins at stage 2, i.e., paraquat application followed by a selective

weed killer, which is tolerated by the following crop.

The second element of weed control in long-term zero-tillage systems

consists of the proper choice of suitable main and catch crops as well as

a sophisticated use of cropping sequences. Crops with slow seedling growth

or a prolonged ripening phase can be grown only if protected by persistent

and highly selective herbicide treatments. Otherwise, fast-growing and

therefore weed-suppressing crops, which do not slacken in competitive

power during later growth stages, are to be preferred. If water conservation

during fallow periods is not an essential part of a cropping system, green

manure or mulch crops should be grown whenever possible. The competitive power of such fast growing crops must be strengthened by using high

seed rates and liberal amounts of fertilizers.

As a third element of weed control, the effectiveness with which normal

husbandry operations are performed is even more important in zero-tillage

than in conventional tillage. Irregular stands resulting from imperfect sowing methods, malnutrition of crops, which may cause either poor growth

or severe lodging, lack of plant protection against pests and diseases, and

ineffective timing and dosing of herbicide applications, often encourage

weed growth. Extra efforts are necessary in zero-tillage systems with regard

to early identification and prevention of a beginning invasion of rhizomatous perennial weeds, especially from field borders.

Herbicide systems have been developed and tested for zero-tillage situations by Triplett (1966, maize), Bakermans and De Wit (1970, cereals

and other crops in mixed farming), Phillips ( 1972, wheat-fallow-wheat



106



K. BAEUMER AND W. A. P. BAKERMANS



rotation), Wicks (1972, maize), Kincade (1972, soybeans), Kirby (1972,

grain sorghum), and Peters (1972, maize and sorghum).

In zero-tillage, problems are posed mainly by gramineous weeds. In continuous no tillage maize cropping in North America, crabgrass (Digitaria

sanguinalis, Digitaria ischaemum ) and fall panicum (Panicum dichotorniflorurn) may become prevalent. Peters (1972) proposed to solve this

problem with split applications of herbicides, especially of residual selective herbicides. It has yet to be proved whether such a measure suffices

to stabilize the system. Changes of the cropping system which would allow

the use of different herbicide-crop combinations for weed control would

probably be more effective.

D. Tiedau (personal communication, 1972) found that a sequence

which completely controlled quackgrass or couchgrass (Agropyron repens)

was horseradish (Raphanus sativus) sown in autumn into the cereal stubble followed by maize in the spring. Both crops are fairly competitive in

relation to couchgrass, radish owing to its fast growth during early stages

and maize after it closes its leaf canopy. They allow the repeated application of relatively small, yet still effective doses of herbicides, e.g., for the

green manure crop the sequence dalapon (preemergence)-paraquat (preemergence)-dalapon

(postemergence), and for maize paraquat (preemergence)-atrazine (postemergence) . The combined effects of competitive crops and suitable herbicides applied at times when the weeds involved

are most susceptible have to be used in a system of small steps in order

to solve the problems with perennial weeds under continuous zero-tillage.

C.



PASTURERENOVATION



Since it is feasible to destroy or temporarily suppress the existing vegetation of unproductive grassland with herbicides and to establish herbage

stands with sod seeding equipment, the concept of zero-tillage has been

adapted for pasture renovation over a wide range of situations. It is a

promising alternative to conventional methods, in which topographic, climatic, and edaphic conditions do not allow the use of the plow or other

deep tillage implements (Charles, 1962).

Renovation methods differ according to whether a complete or partial

replacement of the existing vegetation is intended. For complete renewal,

herbicides and sowing methods have to be applied, as described for the

transition of old grassland to arable land. Some regrowth of weeds and

indigenous species which, in a renovated sward, is difficult to control with

herbicides alone, seems to be inevitable (Allen, 1968). Hence, success in

reseeding a permanent pasture will finally depend on the subsequent management of the newly established sward.



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

III. Effects of Zero-Tillage on Plant Growth

Tải bản đầy đủ ngay(0 tr)

×