Use of Isotopic Methods in Soil Science

After reading this article you will learn about the use of isotopic methods in soil science.

Isotopic method can be used for the kinetics of movement of availability of nutrient ions in soils. In addition, this method is also used for the estimation of ions present in the soil solution, soil complex material and the rate at which exchange process occurs. Basically, the radio-tracer label enables the pool of element in the soil which is in equilibrium with those in soil solution to be calculated by the isotopic dilution.

The use of ¹⁴C is also able to estimate the rate of decomposition of organic matter, measurements of mean residence time of organic matter, and older and younger fraction etc. Besides, these, methods of neutron activation analysis are able to analyse or estimate small amounts of plant nutrients particularly those present in soil solutions of flood rice ecosystems.

²⁴Na and ⁴²K are used to detect small amounts of minerals that control the release of cations to plants. The conventional isotopic dilution methods are used to measure the CEC of soils and clays, but simple, accurate, cost effective and easily adaptable routine methods involving the use of ⁴⁵Ca appear to be most elegant.

³²P is also used to study the equilibrium and the kinetics of some chemical transformations of phosphates in soil as well as characterization of labile soil phosphate. Radio tracers namely, ¹⁴C, ³⁵S, ⁵⁹Fe, ⁹⁰Sr etc. are used for the formation and degradation of soil organic complexes and also to trace the fate of metabolites.

Soil organic matter research is recently an important branch of soil science which includes the study of humic substances, their formation and properties, isolation and composition etc.

In this field of research, the use of isotopic techniques helps to study the synthesis and decomposition of organic matter, dynamics of changes in organic matter, in soil, and nutrient availability, effect of organic substances on plant metabolism and nitrogen dynamics in soils etc. ¹⁴C and ¹⁵N tagged substances are usually used to follow the biological transformations vis-a-vis formation of humic substances.

Radiotracers can also be used in soil organic matter turnover which includes losses and gains of organic matter in soil. The availability of radioactive nuclides like ³²P and ³³P which are the two radioisotopes of phosphorus have significant impact on soil physics research in recent years.

These two isotopes (³²P and ³³P) have half-lives of 14.3 and 25.3 days and P- particle maximum energy of 1.71 MeV and 0.25 MeV respectively. Californium—252 (²⁵²Cf), an another useful radioactive isotope, is used for soil physics research.

The use of neutrons has given a novel method of measuring moisture content in the soil. It is based on the physical laws governing the scattering and moderation of neutrons. Despite measurement moisture content in soil, the movement of soil moisture can also be traced by the use of tracer method. Water use efficiency and irrigation practices can also be studied successfully by the use of radiation equipment’s.

Isotopes are helping in elucidating the mechanism involved in ion absorption, accumulation and transport of nutrients in soil-plant systems. The concentration gradients of ions near rhizosphere can also be measured by using ³²P and ³⁵S.

An estimation of available plant nutrients in soils by using tracers is of considerable importance. Among various methods, isotopic methods have given another dimension in the field of soil fertility and fertilizer uses.

However, two laboratory tracers methods namely ‘E’, A’ and ‘L’ values are used to measure the amount of nutrients in the soil at equilibrium, available nutrients in soil and nutrients ill the labile pool respectively.

1. E-value:

This method is a direct application of the isotopic dilution principle. The amount of nutrient in the soil at equilibrium with the same nutrient in the soil solution can be measured with E-value.

Reaction can be depicted as follows:

Surface ³¹P + Solution ³²P D Surface ³²P + Solution ³¹P

At equilibrium,

Surface ³¹P/Surface ³²P = Solution ³¹P/Solution ³²P

In this method, it is possible to measure both capacity and intensity factors of the same sample. The E-value represents labile form of nutrient and represents the total amount of nutrient undergoing isotopic dilution.

2. A-value:

A-value concept was developed by Fried and Dean (1952). When the plant is confronted with two sources of a given nutrient, the plant will absorb from each of these in proportion to the respective amounts available. The amount of available nutrient in soil to be determined in terms of fertilizer standard is known as A’ value.

Mathematically A value can be expressed as:

A = B(1-Y)/Y

where, A = Amount of available nutrient in soil,

B = Amount of applied fertilizer nutrient

Y = Proportion of nutrient in the plant derived from the fertilizer nutrient.

A-value concept is expressed as follows:

‘A’ -Value = [% Pdfs/% Pdff] × rate of P application (kg/ha)

where, % Pdtf = % of total P in the plant derived from fertilizer % Pdfs = (100—% Pdff)

3. L-value:

L-value was first suggested by Larsen (1952). If labeled fertilizer phosphorus is added to a soil at different levels or rates and plants are grown, the specific activity analysis of the plant material gives a constant value which is attributable to the equilibrium between the applied phosphate and exchangeable phosphate in the soil.

The amount so determined is known as L-value and also termed as “Labile form” of phosphorus. Larsen (1952) used the following equation for isotopic dilution to calculate isotopically labile P or L-value.

L-value = [(a₀/a_P) – 1] × Applied fertilizer P

where, a₀ and a_P are specific activities of the applied P and phosphorus (P) in plant respectively.

L-value is the amount of phosphorus in the soil and in the soil solution which is exchangeable with added orthophosphate to the soil as measured by a plant growing in the system.