CHAPTER 6
 NUTRIENT MOVEMENT IN UNDISTURBED SOIL COLUMNS

 Introduction


 The efficiency of field-applied fertilizers is dependent on their

availability to the plant roots. In the high-rainfall tropics,

efficient management of applied fertilizers involves practices that

maintain the fertilizer nutrients in the active crop root zone (Sanchez

et al., 1982a). The availability of a plant nutrient in a root-zone is

affected in turn by the interactions of the specific nutrient with the

soil and by the transport mechanisms involved in bringing the nutrient

to the root surface. Quantification of nutrient availability is

complicated by the technology employed to assess the root/soil

environment. The quantification of nutrient availability in soils of

the tropics is further complicated by the limited local access to

scientific infrastructure (trained scientists and equipped laboratories)

(Lipton, 1987; Vallaeys et al., 1987).

 The soil column is a frequently used tool in soil-science research

 because it allows the soil to be moved intact from the field to

 laboratories which have controlled environments and sophisticated

 equipment. It is generally assumed that the chemical and physical

 characteristics, as well as the combined dynamic behavior of a soil,

 simulate the behavior of the soil in its natural field setting.