Defining and Assessing Soil Quality

Accurate estimation of groundwater recharge is extremely important for proper management of groundwater systems. Many different approaches exist for estimating recharge. This paper presents a review of methods that are based on groundwater-level data. The water-table fluctuation method may be the most widely used technique for estimating recharge; it requires knowledge of specific yield and changes in water levels over time. Advantages of this approach include its simplicity and an insensitivity to the mechanism by which water moves through the unsaturated zone. Uncertainty in estimates generated by this method relate to the limited accuracy with which specific yield can be determined and to the extent to which assumptions inherent in the method are valid. Other methods that use water levels (mostly based on the Darcy equation) are also described. The theory underlying the methods is explained. Examples from the literature are used to illustrate applications of the different methods.

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Using groundwater levels to estimate recharge

Degradation of agricultural land and the resulting loss of soil biodiversity and productivity are of great concern. Land-use management practices can be used to ameliorate such degradation. The soil bacterial communities at three separate arable farms in eastern England, with different farm management practices, were investigated by using a polyphasic approach combining traditional soil analyses, physiological analysis, and nucleic acid profiling. Organic farming did not necessarily result in elevated organic matter levels; instead, a strong association with increased nitrate availability was apparent. Ordination of the physiological (BIOLOG) data separated the soil bacterial communities into two clusters, determined by soil type. Denaturing gradient gel electrophoresis and terminal restriction fragment length polymorphism analyses of 16S ribosomal DNA identified three bacterial communities largely on the basis of soil type but with discrimination for pea cropping. Five fields from geographically distinct soils, with different cropping regimens, produced highly similar profiles. The active communities (16S rRNA) were further discriminated by farm location and, to some degree, by land-use practices. The results of this investigation indicated that soil type was the key factor determining bacterial community composition in these arable soils. Leguminous crops on particular soil types had a positive effect upon organic matter levels and resulted in small changes in the active bacterial population. The active population was therefore more indicative of short-term management changes.

Soil type is the primary determinant of the composition of the total and active bacterial communities in arable soils

Nitrous oxide emission by nitrification and denitrification in different soil types and at different soil moisture contents and temperatures

Soil physical attributes, i. e., soil water potential, soil oxygen, and soil strength, directly affect plant growth. The least limiting water range (LLWR) is the range of soil water content within which plant growth is least limited by water potential, aeration and mechanical resistance and has not been evaluated in tropical soils. The objective of this research was to characterize the LLWR of a Typic Hapludox cropped to corn under no-tillage. Seventy-two undisturbed soil samples were collected at the crop row and interrow. The soil water retention curve and the soil resistance curve were evaluated in order to compute the LLWR. The results indicated that the LLWR varied positively with bulk density up to 1.1 Mg m-3 and negatively for higher densities. The LLWR ranged from 0.0073 to 0.1252 m3 m-3. Soil resistance to penetration was the limiting factor at the dry end of the LLWR in 85% of the samples while field capacity was the limiting factor at the wet end in 97%. Further studies on the LLWR in tropical soils would be relevant under a wide range in texture and management.

http://www.scielo.br/pdf/rbcs/v22n4/02.pdf

D. R. Nielsen

Papers

Field Capacity and Available Water Capacity