CHAPTER I INTRODUCTION
1.1 Background
Dryland is a piece of land with limited water sources throughout the year and never in a
flooded condition. Dryland areas are widely used as moorland, fields, mixed gardens and
pastures, but there are still many lands that are left abandoned. The main obstacle in
managing drylands for agriculture is the low level of soil fertility caused by chemical
constraints that limit plant growth, so it is necessary to improve the quality and level of soil
fertility.
Each soil has different physicochemical properties. The physicochemical properties of
soil include zero point charge, permanent charge and variable charge. (Bohn et al 2017)
explains that zero point charge is a certain pH when the surface charge is electrically neutral
or zero. Zero point charge status is a good indicator of soil management (Sposito, 2008). If
the zero point charge value of pH is lower than the actual pH, then soil colloids tend to have
high activity to absorb cations such as potassium. The colloidal charge can be determined by
the difference in soil pH0-pH values. If the pH value is lower than the pH0 value. Then the
soil surface is net positively charged, otherwise if the soil pH is greater than pH0 then the
soil surface is net negatively charged.
Some of the main constraints in dryland farming systems in the wet tropics include acid
soil reaction, low organic C and N content, low CEC and base saturation, high phosphate
fixation, erosion problems and water availability. Gilman (1984) states that soils in tropical
climates are classified as soils with variable charge system conditions and have low clay
activity. Characteristic of low clay activity is the dominance of variable charged colloids (pH
dependent charge) in the clay fraction, such as kaoliite, oxides and hydroxides of Fe and Al.
Soils on the earth's surface such as Oxisol, Alfisol, Spodosol and Andisol are variable
charged soils (Theng 1980). Variable charged soils have diverse morphological, mineralogy,
chemical, physical and biological characteristics with a wide range, however, there is a
common property that the magnitude and sign of the surface charge depends on pH, ionic
composition and ionic concentration.
