Tebru¨gge and Du¨ring (1999) also documented increased soil
aggregation and earthworm population as a result of increased
residue cover on soil and the consequent reduction of soil bulk
density at the upper layer (0–10). The residue cover provided
suitable habitats for soil organisms such as earthworms and
arthropods, which burrow the soil creating better aeration and
infiltration within the soil. These observations clearly support the
adoption of no-till with residue cover on soil surface.
3.2. Soil porosity
Primary and secondary tillage operations pulverize the soil,
break clods and loosen the soil, and often result to increased soil
macropores and total porosity.Rahman et al. (2008)related soil
physical, chemical and microbiological properties of an Andosol to
land use and tillage and confirmed that total porosity was
significantly greater in conventional tillage (CT) compared to
paddy soil in which rice was grown with puddling (PD).Glab and
Kulig (2008)also documented the influence of mulch and tillage on
soil porosity. These researchers reported that mulch addition
increased the total porosity in more compacted soil under reduced
tillage. Elder and Lal (2008)observed a general trend of total
porosity of soil under mouldboard plough (MB)>Bare soil
(B)>No-till (NT) as presented inTable 3. However, no significant
difference was observed in total porosity up to 4 weeks after tillage
operation. Air-filled porosity (fa) was reported to be significantly
more in MB than in NT treatment, indicating that MB created
macropores compared to NT. Tilled soils had a greater proportion
of macropores (>15mm), relative to NT soils.
Lowest transmission pores content (0.078 cm
3
cm
3
) at the 0–
10 cm soil layer in reduced tillage without mulch (RZ) was
reported byGlab and Kulig (2008). The volume of transmission was
significantly higher in conventional tillage treatment (Table 4).
Reduced tillage without mouldboard plough (RM) reduced volume
of transmission pores. Mulching was reported to ameliorate the
soil condition in the upper layer (0–10 cm) and increased
transmission pores content. Organic material presence in soil
increases the transmission pores and consequently enhance root
penetration and water movement.
3.3. Penetration resistance of soil
The penetration resistance of soil is a function of its compaction
level as well as the volume of voids present within the soil
compartment.Osunbitan et al. (2005)reported significant variations in soil penetration resistance among tillage some tested
treatments. These authors recorded highest resistance of
0.61 kg cm
2
under the more compacted NT soil and least value
of 0.15 kg cm
2
under intensively manipulated plough and
harrowed (PH) plot. However, they observed no significant
difference among the soil penetration resistance on all the
treatments at the 0–5 cm depth. There were significant departures
from this finding at the 5–10 cm and 10–15 cm depths, the NT
treatment recording a significantly and consistently high penetration resistance. These authors also confirmed no significant
difference in resistance between manually tilled soil and
conventionally tilled soil (PP and PH).
They also documented increase in penetration resistance with
time after tillage under manually tilled soil (MT), plough–plough
(PP) and plough–harrowed (PH) plots but also observed slight
decrease under NT treatment. Penetration resistance had a
consistent relationship with yield of cassava over different soils
(Baver et al., 1972; Vine and Ahmad, 1987), and largely determine
yield when soil available water and air were significant. Soil air less
than 12 ml air per 100 ml soil will limit cassava growth (Vine and
Ahmad, 1987). Arvidsson (1998) reported small differences
between the mouldboard-ploughed and chiseled soil in the tilled
part of the soil, whereas the penetration was higher in untilled soil.
The mouldboard plough was confirmed more efficient than chisel
implement in soil loosening. Similar observation was made by
Carter (1996). The soil pulverized with mouldboard plough
however is susceptible to erosion.
4. Tillage effect on cassava yield
Effect of various tillage treatments on cassava yield depends
mainly on soil type, the site history as well as the climate
conditions during preparation and planting (Howeler et al., 1993).
Lal and Dinkins (1979)andEzumah (1983)reported higher yield of
cassava in untilled soil than in tilled soil in an Oxisol in Zaire. These
researchers confirmed NT resulted in low root density, low dry
matter and N accumulation in leaves stems and roots. In contras
Tebru¨gge and Du¨ring (1999) also documented increased soilaggregation and earthworm population as a result of increasedresidue cover on soil and the consequent reduction of soil bulkdensity at the upper layer (0–10). The residue cover providedsuitable habitats for soil organisms such as earthworms andarthropods, which burrow the soil creating better aeration andinfiltration within the soil. These observations clearly support theadoption of no-till with residue cover on soil surface.3.2. Soil porosityPrimary and secondary tillage operations pulverize the soil,break clods and loosen the soil, and often result to increased soilmacropores and total porosity.Rahman et al. (2008)related soilphysical, chemical and microbiological properties of an Andosol toland use and tillage and confirmed that total porosity wassignificantly greater in conventional tillage (CT) compared topaddy soil in which rice was grown with puddling (PD).Glab andKulig (2008)also documented the influence of mulch and tillage onsoil porosity. These researchers reported that mulch additionincreased the total porosity in more compacted soil under reducedtillage. Elder and Lal (2008)observed a general trend of totalporosity of soil under mouldboard plough (MB)>Bare soil(B)>No-till (NT) as presented inTable 3. However, no significantdifference was observed in total porosity up to 4 weeks after tillageoperation. Air-filled porosity (fa) was reported to be significantlymore in MB than in NT treatment, indicating that MB createdmacropores compared to NT. Tilled soils had a greater proportionof macropores (>15mm), relative to NT soils.Lowest transmission pores content (0.078 cm3cm3) at the 0–10 cm soil layer in reduced tillage without mulch (RZ) wasreported byGlab and Kulig (2008). The volume of transmission wassignificantly higher in conventional tillage treatment (Table 4).Reduced tillage without mouldboard plough (RM) reduced volumeof transmission pores. Mulching was reported to ameliorate thesoil condition in the upper layer (0–10 cm) and increasedtransmission pores content. Organic material presence in soilincreases the transmission pores and consequently enhance rootpenetration and water movement.3.3. Penetration resistance of soilThe penetration resistance of soil is a function of its compactionlevel as well as the volume of voids present within the soilcompartment.Osunbitan et al. (2005)reported significant variations in soil penetration resistance among tillage some testedtreatments. These authors recorded highest resistance of0.61 kg cm2under the more compacted NT soil and least valueof 0.15 kg cm2under intensively manipulated plough andharrowed (PH) plot. However, they observed no significantdifference among the soil penetration resistance on all thetreatments at the 0–5 cm depth. There were significant departuresfrom this finding at the 5–10 cm and 10–15 cm depths, the NTtreatment recording a significantly and consistently high penetration resistance. These authors also confirmed no significantdifference in resistance between manually tilled soil andconventionally tilled soil (PP and PH).They also documented increase in penetration resistance withtime after tillage under manually tilled soil (MT), plough–plough(PP) and plough–harrowed (PH) plots but also observed slightdecrease under NT treatment. Penetration resistance had aconsistent relationship with yield of cassava over different soils(Baver et al., 1972; Vine and Ahmad, 1987), and largely determineyield when soil available water and air were significant. Soil air lessthan 12 ml air per 100 ml soil will limit cassava growth (Vine andAhmad, 1987). Arvidsson (1998) reported small differencesbetween the mouldboard-ploughed and chiseled soil in the tilledpart of the soil, whereas the penetration was higher in untilled soil.The mouldboard plough was confirmed more efficient than chiselimplement in soil loosening. Similar observation was made byCarter (1996). The soil pulverized with mouldboard ploughhowever is susceptible to erosion.4. Tillage effect on cassava yieldEffect of various tillage treatments on cassava yield dependsmainly on soil type, the site history as well as the climateconditions during preparation and planting (Howeler et al., 1993).Lal and Dinkins (1979)andEzumah (1983)reported higher yield ofcassava in untilled soil than in tilled soil in an Oxisol in Zaire. Theseresearchers confirmed NT resulted in low root density, low drymatter and N accumulation in leaves stems and roots. In contras
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