Page 80 - Enzymes in Tropical Soils

68 Enzymes In Tropical Soils

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Different from the soil bulk density which ranges from 1.30 to 1.35 g cm ,
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the values of the soil particle density are higher, ranging from 2.6 to 2.7 g cm . The
soil bulk density is always lower than the soil particle density; the bigger the
difference between  b and  s , the more porous the soil is. For soil with a porosity
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of 50%,  b = 0.5  s . Sandy soils may have  b about 1.6 g cm . Loam soils with good
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aggregation and clay soils may have  b as low as 1.1 g cm . Low value of  b is
caused by high soil porosity. Conversely, soil without porosity or massive soils may
have  b = s .
The bulk density and porosity of soils may change due to several processes
such as soil compaction and soil tillage (Afandi et al., 1997b; Zhang et al., 1997;
Gregorich et al., 2011). Soil compaction may increase the soil bulk density, and
conversely, the soil tillage will decrease the soil bulk density (Fig. 5.3.).
Structurally, the decrease in soil volume or the shrinkage of soil porosity by
compaction is caused by a more closely packed soil particles in soil aggregates, and
hence, causes more limited rooms between soil particles or between soil
aggregates. With this process,  b in C condition (Fig.5.3) is higher than that in B
condition and that in B condition is higher than that in A condition.
Therefore, by compaction, the bulk density will increase from the common
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value of 1.32 g cm to a very high value equals to that of the soil particle density
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( s ) of 2.7 g cm when all soil pores dissappear or the soil becomes massive. This
process will eventually decrease the soil porosity and limit the diffusion of O 2 into
the soil system. Consequently, it will inhibit the decay (respiration) of organic
matter by soil microorganisms.
Conversely, soil tillage will decrease the soil bulk density. This process will
increase the soil porosity and the easiness of O 2 diffusion into the soil system.
Hence, soil tillage may increase the populations and activities of soil
microorganisms. This suggests that soil tillage is of great importance when organic
matter is incoorporated into the soil system. The availability of O 2 in soils may
enable the soil microorganisms to obtain the highest energy from the respiration of
organic matters. This will increase the populations and activities of soil
microorganisms. One of their activities is the production of soil enzymes. Herrick
and Lal (1995) reported that the introduction of sheep residues into soils enhanced
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the soil porosity from 13 to 21%, the sorptivity from 34 to 71 mm h , and also
decreased the soil compaction.

Abdul Kadir Salam - 2014