B) Quantitative methods
In these methods each factor is rated on a scale from 0
to 100, the actual percentages are multiplied by each other. The resultant index
of productivity/ suitability is set against a scale placing the soil in one or
other productivity and suitability classes.
i) Suitability rating
Rating of the suitability of soil for specific use can be
made from soil survey data by estimating some type of index based on soil characteristics.
Storie Index (Storie, 1978) is one such rating which expresses numerically the
relative degree of suitability of soil for general intensive agriculture. The
rating, being based on soil characteristics only, may not serve as index of land
value. The four important general factors viz.
(i) soil profile characteristics,
(ii) surface texture,
(iii) slope, and
(iv) other factors like drainage, alkalinity etc. are assigned ratings percentage
multiplication of which is used as an index for evaluation. A rating of 100 per
cent expresses most favourable condition and lower per cent indicates less favourable
condition. The rating classes assigned to different rating indices are as follows:
Rating index 80%-100% is assigned Grade 1 i.e. excellent
Rating index 60%-79% is assigned Grade 2 i.e. good
Rating index 40%-59% is assigned Grade 3 i.e. fair
Rating index 20%-39% is assigned Grade 4 i.e. poor;
Rating index 10%-19% is assigned Grade 5 i.e. very poor and
Rating index of less than 10 is assigned Grade 6 i.e. not suitable for agriculture.
Storie Index Rating (SIR) system was developed for the purposes of an appraisal
according to land types. It is based on the product of the factors, even one moderate
factor reduces the value of index considerably. Its use, hence, is limited.
ii) Actual and potential productivity
Requier et al., (1970) have evolved a system of soil appraisal
in terms of actual and potential productivity. It is a modified version of Storie
Index. Nine factors viz. moisture, drainage, depth, texture, base saturation,
soluble salts, organic matter, CEC and mineral reserves are rated on a scale 0-100;
and the percentages are cumulatively multiplied to obtain productivity index (P).
In a similar manner the potentiality index (P?) is calculated after effecting
the management measures. The ratio P?: P indicating the extent to which productivity
can be improved is called the coefficient of improvement.
It is considered that soils with rating index 65-100 are excellent, 35-64 and
20-34 are average 8-19 are poor and below 8 are extremely poor.Maps showing productivity
and potentiality index can also be prepared.It is evident that the land evaluation
system of Requier et al., (op.cit.) does not explain the variability in the yield.
Like Storie index, this system of land evaluation has the limitation in that one
limiting factor reduces the index of productivity. Also, assigning values to factors
like drainage is difficult. Perhaps, factors should be chosen according to the
limitations affecting the crop growth within a particular region to obtain a more
realistic productivity rating.