Write the cation exchange capacity of the sandy loam soil.
Answers
Explanation:
Key Points
Cation exchange capacity (CEC) is the total capacity of a soil to hold exchangeable cations.
CEC is an inherent soil characteristic and is difficult to alter significantly.
It influences the soil’s ability to hold onto essential nutrients and provides a buffer against soil acidification.
Soils with a higher clay fraction tend to have a higher CEC.
Organic matter has a very high CEC.
Sandy soils rely heavily on the high CEC of organic matter for the retention of nutrients in the topsoil.
Background
Cation exchange capacity (CEC) is a measure of the soil’s ability to hold positively charged ions. It is a very important soil property influencing soil structure stability, nutrient availability, soil pH and the soil’s reaction to fertilisers and other ameliorants (Hazleton and Murphy 2007).
What are exchangeable cations?
The clay mineral and organic matter components of soil have negatively charged sites on their surfaces which adsorb and hold positively charged ions (cations) by electrostatic force. This electrical charge is critical to the supply of nutrients to plants because many nutrients exist as cations (e.g. magnesium, potassium and calcium). In general terms, soils with large quantities of negative charge are more fertile because they retain more cations (McKenzie et al. 2004) however, productive crops and pastures can be grown on low CEC soils.
The main ions associated with CEC in soils are the exchangeable cations calcium (Ca2+), magnesium (Mg2+), sodium (Na+) and potassium (K+) (Rayment and Higginson 1992), and are generally referred to as the base cations. In most cases, summing the analysed base cations gives an adequate measure of CEC (‘CEC by bases’). However, as soils become more acidic these cations are replaced by H+, Al3+ and Mn2+, and common methods will produce CEC values much higher than what occurs in the field (McKenzie et al. 2004). This ‘exchange acidity’ needs to be included when summing the base cations and this measurement is referred to as effective CEC (ECEC).
Measuring CEC
Different laboratories use various methods to measure CEC, and can return contrasting results depending on the fraction of the soil measured. In Australia, some laboratories measure CEC directly and others calculate it as CEC by bases. Cation exchange capacity is commonly measured on the fine earth fraction (soil particles less than 2 mm in size). In gravelly soils the effective CEC of the soil as a whole is diluted, and if only the fine (clay) fraction is analysed, reported CEC values will be higher than actual field values.
Measuring CEC involves washing the soil to remove excess salts and using an ‘index ion’ to determine the total positive charge in relation to original soil mass. This involves bringing the soil to a predetermined pH before analysis. Methods, including pre-treatment, for measuring CEC and exchangeable cations are presented by Rengasamy and Churchman (1999) and described in detail by Rayment and Higginson (1992).
Units
CEC is conventionally expressed in meq/100 g (Rengasamy and Churchman 1999) which is numerically equal to centimoles of charge per kilogram of exchanger (cmol(+)/kg).
Answer:
Soils are made up of sand, organic matter, silt and clay particles. Soils with high sand content have low holding capacity for cations compared to clayey and silty soils. Clay and silt particles have negatively charged sites which enable them adsorb and hold on to cations. Sandy soils have CEC between 1-5meq/100g
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