The factors that determine how suitable a land unit is for a given LUT or farming system, are called 'class-determining' factors. Table 12, in Chapter 4 lists individual factors that may be selected as 'class-determining' according to whether they affect:
A. The crop (i.e. agronomic factors effecting yields or production)
C. Land development or land improvements
D. Conservation and the environment
E. Socio-economic conditions
Each of the 32 factors listed in Table 12, and their interactions, are discussed serially in Part Two, in five sections headed alphabetically with the titles as above. The 32 factors retain their alphabetical prefix as in Table 12, e.g. B begins with number 14, and C with 19, etc.
Part Two is intended to help in decisions needed for Steps 3 - 5 of the Guide to Procedures (Chapter 3), namely;
i. Which factors are 'class-determining'?
ii. What critical limits best specify, for individual class-determining factors, the requirements and limitations of a LUT, for s1, s2, s3, n1 and n2 levels of suitability? These critical limits are the specifications entered on Format 1 as described in Steps 4 and 5.
It is recommended that critical limits are first specified factor by factor. The number of class-determining factors under review will be progressively shortened during the evaluation; or their influences will be aggregated into estimates of crop yields, production, or as costs and benefits. As explained in Chapter 6, it is essential, after establishing factor ratings for individual factors or groups of factors, to estimate the interactions (see Section 6.2), and the 'significance' (Section 6.3) of each factor or interaction. These should always contribute to a decision on which land suitability class (S1, S2, S3, N1 or N2) is appropriate for a given land unit - LUT combination. Hence the critical limits of all previously listed class-determining factors do not necessarily constitute the specifications of land suitability classes. Some factors may prove of over-riding importance, and others can be relegated to the status of not being 'class-determining'. Thus S1 land should have factor ratings of s1 for the most important factor(s), but it can include lower factor ratings (s2, s3 or even n1) if these factors are deemed as not being 'class-determining' in the final stage of the evaluation.
In the early stages of the evaluation, it is important that critical limits are thought out in terms of the requirements and limitations of the cropping, irrigation and management systems under consideration. Crops require radiation, suitable temperatures, a continuous supply of water and nutrients, a suitable environment for root growth, suitable conditions for irrigation or for harvesting, mechanization, post-harvest ripening, etc. Conversely, crops are variously limited by their susceptibilities or tolerances to excess water, excess salts or toxicities, deficiencies, pests, frost, storms, etc. Similarly, irrigation methods, such as surface, sprinkler or drip, have their different requirements and limitations, as do management systems (e.g. manual vs. mechanized). It is important to decide which of the land use requirements and limitations are 'class-determining'.
An approach via requirements and limitations will encourage the land evaluator to think about land characteristics and the need for inputs or land improvements with a view to achieving the best match between the conditions of the land and the land use. The most relevant and influential land characteristics, inputs and land improvements can then be pinpointed in terms of their impact on the land use requirements and limitations. For simplicity, the process can later be short-circuited by listing Class specifications which are both rational and practicable in terms of the physical or economic measure of land suitability class.
In developing critical limits of each factor leading to Class specifications, it is important to bear in mind the implications of translating physical productivity, inputs and land improvements into economic terms. Thus, where possible, benefits and costs associated with individual factors should be worked out in detail. In practice, during the early stages of an evaluation, factor ratings s1, s2, s3, n1 and n2 may be used to indicate levels of production and development benefits and costs. These may be improved as quantitative data becomes available during the course of the investigations.
Note the definitions of the terms s1, s2, s3, n1 and n2 given in Table 13.
The list of 32 factors discussed hereunder, while comprehensive, is not intended to be exclusive, and the land evaluator should feel free to modify or regroup the factors as is convenient for any given evaluation. Care should be taken in so doing, not to double count the same aspect twice under different headings. The divisions and grouping of factors in Part Two are for convenience only and, in practice, interactions may prove of great importance. These should be recognized and handled appropriately for the given evaluation.