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Assessing land potential

The final step in assessing land potential is to introduce the effects of soil unit, phase, texture and slope. The technique for this is similar to the one described on the previous page for introducing the agroclimatic constraints. The difference is that the soil constraints are used to modify the agroclimatic suitability classes, rather than the actual yields: the agroclimatic suitability classes are thus converted into land suitability classes.

The starting point is an assessment of the soil requirements of each crop, in terms of such variables as drainage, fertility, acidity, sodicity (excess sodium), stoniness and slope. These requirements are then matched, for each crop at each input level, with the properties of the soil units in the soil inventory.

The agroclimatic classification is, in effect, put through a series of 'veil gates', each of which may affect the area of land with any particular suitability classification.

An example: how soil conditions affect land suitability

marginally suitable soil causes all land to be downgraded one class

normal phase has no affect on land suitability

rolling land causes one-third of the area to be downgraded one class. one-third to be classed not suitable and one-third unaltered

coarse texture causes all land to be downgraded one class

For example, for each crop the soil units themselves arc, rated as suitable (S1) if the soil has no or only minor limitations to production, marginally suitable (S2) if production is affected markedly, and not suitable if crop production is not possible or very limited. The S1 rating does not affect agroclimatic suitability. A crop grown on S2 soil has its agroclimatic suitability downgraded by one class say from suitable to marginally suitable.

The soil constraints

soil unit rules

suitable soil:

no change

marginally suitable soil:

suitability downgraded one class

unsuitable soil:

all land rated as unsuitable

soil phase rules

normal phase

no change

others specific to crops and soil units

slope rules

level land:

no change

rolling land, low inputs:

one-third not suitable, one-third downgraded one class, one-third no change;

high inputs:

two-thirds not suitable, one-third no change

steep land:

85 percent not suitable, remaining 15 percent rules as for rolling land

texture rules

coarse texture:

suitability downgraded one class for nearly all soils

other textures:

no change

Similar rules affect the other soil gates - those concerned with texture and slope. As the diagram below illustrates, the soil constraints are quite stringent, and can rightly have a major effect on the suitability classifications.

In the example below, only one-quarter of the land has an initial agroclimatic classification of not suitable. But if the soil is only marginally suitable, the phase normal, the land rolling and the texture coarse, more than 90 percent of the land ends up as not suitable, and the remainder is classified only as marginally suitable. As in the rest of the study, the soil gates are applied for both low and high inputs.

The final step converts the land suitability classifications back into the range of potential yield for each crop, in each growing-period zone, for both input levels. An example, that of growing wheat in Africa, is shown below and illustrates how the total available area and potential yield vary with the level of inputs.

Land suitability and potential for wheat in Africa (million ha and tonnes/ha)

Agroclimatic suitability rainfed cassava low level of inputs

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