6. Assigning the land suitability classes by matching

6.1 Determining the factor ratings
6.2 Interactions between the factors
6.3 Evaluating 'significance' in combining factor ratings
6.4 Symbols for summarizing land suitability class and subclasses
6.5 An example of the use of the formats
6.6 Incorporating crop yield data and costs

In preparation for this Chapter, the reader is reminded of Steps 3 to 5 already discussed in Chapter 4, namely, the selection of class-determining factors having significance from a physical and economic standpoint, and the specification of critical limits to designate levels of suitability s1, s2, s3, n1 or n2, on a factor by factor basis. These specifications entered on Format 1, including assumptions about the requirements for inputs and land improvements, are the basis for assigning the suitability class and subclasses to each LUT - land unit combination. The specifications guide the land evaluator as to the suitability of a land unit for a LUT, factor by factor, but with no regard for interactions or the relative importance of each factor.

Checklists of land characteristics for describing the land units are given in Chapter 5. This present Chapter describes procedures for matching these land characteristics to the specifications of the LUT. The matching process can proceed through successive adjustments or iterations during both the 'provisionally-irrigable' and 'irrigable' evaluations, the objective being to produce the most practicable and likely project. Typical adjustments during matching could include (i) changes in the LUT (e.g. in cropping, irrigation method or management), (ii) adjustments to inputs (e.g. fertilizer, cultivars, water), and (iii) land improvements (e.g. drainage, land levelling, etc.).

The specifications used in classifying 'provisionally-irrigable' land will normally differ from those used for the succeeding 'irrigable' classification. 'Provisionally-irrigable' land will usually be classified without firm knowledge of the water supply, the seasonal availability of water, and the cost of delivering water to different parts of the project area. Later, at the time of the classification of 'irrigable' land, the evaluation Can be based on firm information about the water supply and land development costs. Recommendations on the economic criteria to use at these successive stages are given in Chapter 7.

In the 'provisionally-irrigable' evaluation, a tentative matching and land classification should be achieved in the field while surveying the land units. Before carrying out the land survey, the land evaluator will have prepared the specifications for each LUT to be evaluated (Format 1) and he should have these with him during the survey. After mapping the land units the steps are:

Step 7: For each land unit, decide which land qualities and land characteristics are 'class-determining' with respect to the requirements and limitations of. the LUT(s). For each land unit, complete Format 2 entering the appropriate values of the land qualities and land characteristics (see Procedures Land Resource Inventory, Chapter 5). Step 8: Match 'critical limits' of each land use requirement or limitation (i.e. from the specifications on Format 1), with the conditions found in the land unit (i.e. Format 2) to obtain a factor rating of s1, s2, s3, n1 or n2 for each combination of LUT and land unit. Enter the factor rating on Format 3. Assumptions about inputs, land improvements and their benefits and costs should also be indicated (see Example 2 in Section 6.5). Step 9: Decide the relative 'Significance' of each 'class-determining' factor (or of a group of interacting factors) by entering Very Important, Moderately Important, Less Important or Not Important, as appropriate, in the column headed 'Significance' (Format 3). This is explained later. Step 10: Combine individual 'class-determining' factor ratings to obtain a tentative land suitability classification for each LUT on each land unit. Interactions between factors (Section 6.2) and 'Significance' (Section 6.3) must be taken into account in this step. Estimates of crop yield and economic benefit/costs, according to the guidelines in Chapter 7, may be needed to assign the classes and subclasses. Enter the tentative land suitability class and subclasses (S1, S2, S3, N1 or N2 etc.) at the bottom of Format 3 or on the map. Step 11: Where necessary adjust the LUT description, or introduce inputs or land improvements, and repeat steps 1-10 until the most practicable cropping, irrigation and management farming system is obtained (the need for such iteration can be entered on Format 3). Step 12-16: Present the classes and subclasses of the 'provisionally-irrigable' or 'irrigable' classification on Formats 4 or 5 as set out in Chapter 3.

6.1 Determining the factor ratings

Each 'class-determining' factor is first matched individually. Critical limits entered on Format 1 indicate how suitable a land unit is for a given LUT in terms of that factor. For example, if one of the class-determining factors for the LUT 'Irrigated Maize on Small Farms' is 'Rooting' (i.e. the requirements or limitations for root development) and the critical limits are to be represented in terms of 'effective soil depth', the latter is given on Format 1 as ranges of depths corresponding to s1, s2, s3, n1 and n2 suitability levels. The effective soil depths recorded for each land unit will fall within one of these five depth ranges and the appropriate one is selected as the factor rating.

This is illustrated for one factor in the following example:

Example 1

Land utilization type (LUT A):	Irrigated maize on small farms
Class-determining factor: land use requirement land quality represented by the land characteristic(s)	Rooting requirements for rooting (maize) rooting conditions effective soil depth
Critical limits effective soil depth (cm) (i.e. specifications of the LUT)	Factor ratings	Land unit 8 effective soil depth (cm)
over 200	s1	75
100-200	s2
50-100	s3
25-50	n1
0-25	n2

i.e. the factor rating for LUT A on land unit 8 is s3, which is entered on Table 6.

6.2 Interactions between the factors

In combining the factor ratings of several individual factors in order to decide the appropriate land suitability class to assign, the possibility of interactions should be taken into account. In a broad interpretation of the meaning of the word 'interaction' it can be readily appreciated that many factors interact in the resultant productivity index, or economic index which is the integral of their effects. However, the levels of some factors may reduce or accentuate the influence of given levels of other factors, and a straightforward addition or multiplication (even with weightings) may not satisfactorily resolve these interactions.

Examples of two factor interactions where the levels of one factor modify the expression of given levels of the other are given below.

SOME EXAMPLES OF INTERACTIONS

i. The interaction between fertilizer and pesticides in increasing crop yield as found by experiment

	With fertilizer	Without fertilizer
With pesticides	1 t/ha increase	no yield increase
Without pesticides	no yield increase	no yield increase

In this hypothetical example, the crop responds if fertilizer and pesticides are used together, but does not respond to either singly, when used without the other. From this result it can be readily appreciated that a farmer would waste money applying fertilizer, if the crop will be destroyed by pests. Therefore the level of fertilizer and pesticides used should be jointly specified. i

ii. The interaction between water and NPK in terms of crop yield as rated using factor ratings

	Water supply relative to water requirements
NPK supply relative to NPK requirements		s1	s3
	s1	s1	s3
	s3	s3	n1

(Note the parallel with i. above and see a more explicit example of this interaction for nitrogen in Part Two, Figure 14.)

Although impracticable in the course of most land evaluations, the influence of interactions on crop yields, drainability, erodibility, etc. can only be determined quantitatively in the field, e.g. by using factorial experimental designs in which different levels of the various factors are systematically combined. Existing crop yield data that integrate the many influential factors should always be used where available, provided it is relevant for the physical and socio-economic conditions under review.

6.3 Evaluating 'significance' in combining factor ratings

Apart from interactions, some factors will be more important than others in their influence on land productivity, or benefit/cost. Therefore in Step 9, provision is made for grading factors for 'significance' on Format 3, entering Very Important (VI), Moderately Important (MI), Less Important (LI) or Not Important (NI) as appropriate. Whereas factor ratings rate a match in terms of a single factor (or a single interaction) the 'significance' rating indicates the relative importance of the factor in its influence on production or benefit/cost. Single factors are sometimes of over-riding importance, e.g. water quantity, drainage costs, etc. and these are given an appropriate weight using 'significance' levels in a tentative assigning of the land suitability classes, prior to a full economic evaluation.

The 'significance' of individual factors often becomes apparent from the economic analyses described in Chapter 7. However, social, conservation and environmental considerations may not be pin-pointed as having economic relevance. 'Significance' levels remind the land evaluator of their possible importance and variation over the survey area. For example, the problems of a particular irrigation technique may be identified in social terms, or there may be problems of water rights in particular areas. A 'Very Important' rating of 'significance' may be of use in the reiteration of the Steps; this could lead to appropriate changes in the project plan including adjusted cropping, irrigation or management proposals and modified land development plans.

6.4 Symbols for summarizing land suitability class and subclasses

The FAO Framework symbols to indicate land suitability orders, classes and subclasses, are given in Table 1 and are explained in Appendix 1. For the purposes of mapping, the land evaluator may need to give additional explanatory information using his own notation, designed for a particular survey. Such notation should be consistent within any one survey and devised to avoid misinterpretation. An illustration of the type of symbolization that might be appropriate is as follows:

The FAO Framework symbols in the above are S3d, indicating that the land is only Marginally Suitable for LUT A because of a drainage requirement or limitation. The additional symbols given above are the LUT A below the line, and the accented d. The accented letter, e.g. d', may be used to indicate that a land development cost is involved in the assigning of land class S3. The magnitude of the land development cost could be indicated by using one, two or three accents as in d', d' ', or d' ' '. In Table 17, a list of letter suffixes indicating land suitability subclasses (i.e. denoting the major class-determining factor or factors) is suggested. In this list the land development cost factors are all accented as described above. Conservational and environmental factors may be given a different accent, i.e. ê. These may also be subclasses. The use of brackets with additional information should be used at the discretion of the land evaluator based on his own rules. In this illustration, the symbol (ê) could indicate that a long-term erosion hazard should be borne in mind.

The evaluator may require an additional symbol after mapping and classifying the 'irrigable' lands. The boundaries of aggregated land units may be defined and it may be necessary to indicate the economic viability of these using V1, V2, V3, N1 and N2 instead of the suitability class symbols used for the 'provisionally-irrigable' land and for reconnaissance evaluations.

Table 17 LETTER SUFFIXES FOR INDICATING LAND SUITABILITY CLASSES AND SUBCLASSES

	CLASS-DETERMINING FACTOR	Letter Suffix
A.	Crop (agronomic) factors
1.	Growing period	b
2.	Radiation	j
3.	Temperature	c
4.	Rooting	r
5.	Aeration	d
6.	Water quantity	m
7.	Nutrition (NPK)	n
8.	Water quality	q
9.	Salinity	x
10.	Sodicity	y
11.	pH, micronutrients and toxicities.	z
12.	Pest, disease, weeds	p
13.	Flood, storm, wind, frost	u
B.	Management factors
14.	Location	l
15.	Water application management	w
16.	Pre-harvest farm management	v
17.	Harvest and post-harvest	h
18.	Mechanization	k
C.	Land development cost factors
19.	Land clearing	c'
20.	Flood protection	f'
21.	Drainage	d'
22.	Land grading and levelling (topography)	t'
23.	Amendments (physical, chemical, organic)	a'
24.	Leaching	x'
25.	Reclamation period	r'
26.	Irrigation engineering (construction)	i'
D.	Conservation, environmental factors
27.	Salinity/sodicity hazard
28.	Ground or surface water hazard
29.	Long-term erosion hazard
30.	Environmental hazard
E.	Social and economic factors
31.	Farmers' attitudes to irrigation	f
32.	Others if class-determining

6.5 An example of the use of the formats

An illustration of the use of the Formats for evaluating a land utilization type in a saline arid area, to be reclaimed, is given in Example 2. This illustrates the use of factor ratings where several factors are class-determining, the interactions to be considered, and the application of 'significance' in assigning the land suitability classes and subclasses. It also shows the need for refinements in successive iterations, and the importance of translating from physical into economic criteria, as further explained in Chapter 7.

EXAMPLE 2

Land utilization type (LUT B):	Winter cropping (wheat, beans, clover) Summer cropping (maize, sunflower) in an arid area to be reclaimed and farmed in 2.5 ha smallholdings Land productivity index = 1.0 for the top of s1 land
Class-determining factor 9: 1/	Salinity (control of)
land use limitation:	Tolerance of the above crops to salinity
land quality:	Potential soil salinity hazard after reclamation and its effect on the land productivity index
represented by land characteristics:	- soil permeability (0-75 cm depth) - depth to barrier (cm) - hydraulic resistance to vertical flow through barrier (C) - land elevation above mean sea level (m amsl)
Class-determining factor 21:	Drainage
land use requirement:	Drainage requirements (design)
land quality:	Drainability
represented by:	Cost of drainage (nil, low, medium, high or $/ha)
Class-determining factor 24:	Leaching requirements
land use requirement:	Leaching requirements (cm of water) for reclaiming the land
land quality:	Conditions for leaching
represented by:	- present soil salinity (0-75 cm depth) EC of saturation extract (dS/m) - water depth in metres of water of quality EC 1-2 dS/m - cost of leaching ($/ha)
Class-determining factor 25:	Reclamation period
land use requirement:	Reclamation period (years)
land quality:	Not relevant
represented by:	Value of production foregone ($/ha)

(Note the interdependence of factors, especially between salinity control and drainage.)

1/ The numbers refer to the list of factors in Format 1.

These four factors would be given as 'specifications' of the LUT B (Format 1) using critical limits as in Example 2, Format 1.

Example 2 FORMAT 1: SPECIFICATIONS FOR LUT B (Winter & Summer Cropping) 1/

CLASS-DETERMINING FACTOR	REPRESENTED BY: LAND CHARACTERISTIC OR COST OF IMPROVEMENT	CRITICAL LIMITS FOR:
		s1	s2	s3	n1	n2
9. Salinity control (i.e. hazard remaining after reclamation and effect on crop yields)	- soil permeability (0-75 cm depth) m/hr 2/	>1.0	1.0-0.5	0.5-0.1	0.1-0.05	<0.05
	- depth to barrier (m)	>5	5-3	3-2	2-1	<1
	- hydraulic resistance to vertical flow through barrier (C) 3/	<50	50-100	100-200	200-250	>250
	- land elevation (m)	>20	20-15	15-10	10-3	<3
21. Drainage	- drainage design requirements in terms of cost 4/	nil	moderate	moderate -high	high	very high
	- actual costs ($/ha)	to be determined
24. Leaching requirement	- present soil salinity (0-75 cm depth) EC of saturation extract (dS/m)	<10	10-30	30-60	60-75	>75
	- water depth (m) (water quality EC 1-2 dS/m assumed)	<0.5	0.5-1	1-1.5	1.5-2	>2
	- cost of leaching	nil	moderate	moderate -high	high	very high
25. Reclamation period	- period to reduce salinity (0-75 cm depth) to < 4 dS/m EC (sat. extr.) (years) 4/	<0.5	0.5-1	1-2	2-4	>4
	- value of production foregone 4/	nil	moderate	moderate -high	high	very high

1/ Note that this is only illustrative; the evaluator must produce his own specifications for each LUT (see guidelines in Part 2).

2/ Later on, where proved appropriate, a soil textural class may be substituted if this correlates satisfactorily with soil permeability.

3/ See Part Two, C.21.

4/ Special studies may be necessary to determine the drainage costs and value of production foregone during reclamation; these should be included in later reiterations.

The land evaluator takes the above specifications for LOT B with him as he surveys and maps the land units in the field. On any given land unit he might record the following data for the land qualities or characteristics on Format 2:

Example 2 FORMAT 2: LAND QUALITIES AND CHARACTERISTICS OF LAND UNIT NO WITH AN ASSESSMENT OF INPUTS AND LAND IMPROVEMENTS REQUIRED

FORMAT 2: Land Unit 130 NAME OF EVALUATOR: T. Spade DESCRIPTION: Banypoh land system, Facet 1
CLASS DETERMINING FACTORS: - land qualities or characteristics, inputs or improvements	UNIT OF MEASUREMENT	LAND CHARACTERISTIC OR QUALITY VALUE		INPUTS & IMPROVEMENTS ASSUMED FOR LUT B
		PRESENT	FUTURE
9. Salinity hazard after rehabilitation
- soil texture (0-75 cm)		scl	scl
- soil permeability (0-75 cm)	m/ha	>1.0	>1.0
- depth to barrier	m	2.5	2.5
- hydraulic resistance to vertical flow through barrier	C	75	75
- land elevation	m	>20	>20
21. Drainage
- drainage design requirement				500 m of 50 mm perforated pipe per ha at 1.5 m depth
- preliminary estimate of drainage cost				moderate
- revised estimate of drainage cost	$/ha			not yet determined
24. Leaching requirement during the reclamation period
- present soil salinity (0-75 cm depth) EC of saturation extract	dS/m	variable from 10-30	4
- water depth needed to reclaim (water quality EC = 1-2 dS/m assumed)	m			1.2 m of water
- preliminary cost of leaching				low
- revised estimate of cost	$/ha			not yet available
25. Reclamation period
- period to reduce salinity (0-75 cm depth) to 4 dS/m EC (saturation extract)	years			<0.5 years
- value of production foregone	$/ha			to be calculated

The above (Format 2) records:

i. The observations of the land evaluator as he characterizes the land unit in the field.

ii. His evaluation of the observed land characteristics' in terms of estimates of inputs and land improvements (i.e. drainage requirements, and reclamation leaching). Hence, for example, the soil will be leached after the drains are installed to reduce the soil salinity from its present level (10-30 dS/m) to an acceptable level (4 dS/m).

iii. Cost estimates based on the drainage design (i.e. on the lengths, sizes, spacing and depths of pipe or channel) plus the cost of leaching, plus the value of production foregone while the land is being drained and leached. The period during which the land is being drained and leached may displace any existing agriculture, therefore the reclamation period must be stated together with the value of production foregone during this period. In the early field evaluation only rough indications of the costs (low, medium or high) need be provided; further refinements can he included on the format at a later date using actual costs.

The land evaluator next proceeds to Step 8, the matching of the land characteristics plus inputs and land improvements (Format 2) against the specifications of LUT B (Format 1) for the selection of the appropriate factor ratings. These are entered on Format 3 as follows:

Example 2 FORMAT 3: FACTOR RATINGS (LUT B with land unit 130)

Format 3: FACTOR RATINGS LUT B: Winter and summer cropping NAME OF LAND EVALUATOR: T. Spade LAND UNIT: 130
CLASS-DETERMING FACTORS	FACTOR RATING		SIGNIFICANCE	COMMENTS
9. Salinity hazard after reclamation	s1	s2	Very Important
- soil permeability	s3
- depth to barrier
- hydraulic resistance to vertical flow through barrier	s2
- land elevation	s1
21. Drainage cost			Very Important
- preliminary estimate	s2
- revised estimate		n.a.
24. Leaching requirement			Not Important
during reclamation period
- cost of leaching	s1
- revised estimate
25. Reclamation period			Less Important
- period to reduce salinity	s1
- value of production foregone		n.a.
TENTATIVE LAND SUITABILITY CLASS AND SUBCLASS
FINAL LAND SUITABILITY CLASS AND SUBCLASS

n.a. Not yet available

It should be noted that a representative set of land units could first be studied in this detail, then the entries simplified.

The tentative field assignment of the land suitability class and subclass is:

which indicates a salinity limitation after reclamation which is expected to reduce yields on land unit 130, and a drainage cost which is medium. The combination of the two lowers the class to S3, although each of the two limitations received a factor rating of only s2. The land is only marginally suitable, and further estimates of drainage and other costs will be required; these may confirm the class or may result in a downgrading to N1.

6.6 Incorporating crop yield data and costs

Physical data on crop yields and land improvements and inputs must be produced by the land evaluator for the economic evaluation, leading to a revision of land suitability classes if necessary.

It is helpful to indicate the ceiling or highest expected yields on the S1 land in the descriptions of LUTs. Yields relative to this ceiling can then be set for each land suitability class as a land productivity index (e.g. s1 = 1.0-0.8, s2 = 0.8-0.7, s3 = 0.7-0.6, etc. but not necessarily these values). Note that factor ratings may be used for relative yields where further economic evaluation is required to translate these into economic terms.

In Example 2, the only 'class-determining' factor listed that would affect yield is the salinity of the land after reclamation (the other factors listed are all limitations because of costs). Some land units are expected to have patches of salinity, or to prove more difficult to manage for salinity control than others. Their future yields are therefore expected to be depressed relative to other land units. In the Format 3 example above, a factor rating of s2 for the factor 'Salinity' might, say, be taken as 75% of the yield anticipated for the ceiling of s1. Ceiling s1 yields for each of the crops (wheat, beans, clover, maize, sunflower) of LUT B would be listed in a table.

Similarly, the physical inputs (fertilizer, pesticides, etc.) and land development costs (engineering costs for area-specific development) should be first described in physical terms and then costed. This process produces the information necessary for the economic evaluation of land suitability, first for 'provisionally-irrigable' land and later, for 'irrigable' land, as is elaborated in the next Chapter.