Once annual cropping patterns or crop combinations for each agro-ecological cell in each LGP-Pattern have been formulated, Part III of the crop productivity model (Figure 2.1) formulates crop rotation options. This is done by taking into account crop combination restrictions in space and time of cropping patterns, and fallow requirements of crop combinations that have been selected to participate in the annual cropping patterns.
These two model variables provide for sustainability of production in the longer term. Additionally, they contribute (together with yield limits imposed in the selection of single crops in Part I of the model, and the production stability parameter imposed on the selection of rotations in Part IV) to the overall stability of production system.
It is necessary to impose certain crop combination restrictions (as a model variable) for dryland cropping patterns to avoid continuous monocropping. It is also advisable to make a provision for biological nitrogen fixation in the cropping patterns at the low level of inputs circumstance.
At this stage of the model development, the reference crop combination restrictions are that no crop combination should occupy more than two-thirds share of the total cropping area during any year (i.e. total cropping hectare-days available in the agro-ecological cell). The remaining one-third of the annual cropping share of the total hectare-days be occupied by another crop combination according to the following formula.
Where the crop combination (i.e. cropping pattern) occupying the two-thirds of the cropping area is made up of non-legume crops, the remaining two-thirds of the cropping area should be occupied by a crop combination comprising of legume crops under the low inputs situations. Under intermediate and high inputs circumstances, the latter crop combinations should comprise of non-cereal crops if the former cropping pattern comprise of cereal crops; or non-legume crops if the former are legume crops; or non-tuber crops if the former are tuber crops.
For wetland rice, the above restrictions are not imposed and all of the annual cropping time of an area could be considered for occupation by monocultural cropping pattern, if required. Similarly, the restrictions are not applied to cassava and the perennial crops of banana, sugarcane and oil palm.
The above restrictions for annual and perennial crops may be modified if it is desired that certain proportion of the area of an agro-ecological cell or a group of agro-ecological cells be set aside for specific crops, or where the demand parameter in the objective function imposes a restriction on the types of products that are required.
In their natural state, many soils cannot be continuously cultivated without undergoing degradation. Such degradation is marked by a decrease in crop yields and a deterioration in soil structure, nutrient status and other physical, chemical and biological attributes.
Under traditional farming systems, this deterioration is kept in check by alternating some years of cultivation with periods of fallow. The intensity of the necessary fallow is dependent on level of inputs, soil and climate conditions and crops. However the prime reason for incorporating fallows into crop rotations is to enhance sustainability of production through maintenance of soil nutrient fertility.
Nutrient fertility of soils (i.e. the ability of soils to supply nutrients to crops) under traditional subsistence farming (corresponding to LUTs with low inputs) depends mainly on the soil organic matter present in the humus form.
The amount of humus organic matter in soils depends on the relative rates of addition of organic residue and their subsequent breakdown. The relative rates are related to the type, extent and duration of growth of vegetation (natural or crop) and activity of soil organisms, all of which are influenced by soil and climatic conditions.
Maintenance of nutrient fertility of land, cultivated with subsistance low inputs LUTs, is achieved through natural bush or grass fallow as a means of soil fertility regeneration. With intermediate inputs LUTs, providing higher inputs to soils, means of maintaining soil fertility is through fallow which may include for a portion of the time a grass or grass-legume ley or a green manure crop.
Factors affecting changes in soil organic matter are reviewed in Nye and Greenland (1960) and in Kowal and Kassam (1978). They include temperature, rainfall, soil moisture and drainage, soil parent material, and cultivation practices.
The most common used equations for describing changes in the soil humus content with time under fallow and under cropping are:
| dHf/dt = Af - kf Hf | (6.1) |
| dHc/dt = Ac - kc Hc | (6.2) |
| where: | Hf | = | humus content of soil under fallow |
| Hc | = | humus content of soil under cropping | |
| Af | = | addition of humus to soil per unit time under fallow | |
| Ac | = | addition of humus to soil per unit time under cropping | |
| kf | = | decomposition constant under fallow | |
| kc | = | decomposition constant under cropping | |
| t | = | time. |
Equation 6.1 states that after a period of time under fallow (tf) the humus content of soil will approach the value Af/kf asymptotically. Integrating the equation 6.1 gives:
| Hf = Af kf - (Af/kf - Ho) e-kftf | (6.3) |
where Ho is the initial humus content of the soil and Af/kf is the equilibrium level of humus in soil (Hef).
Equation 6.2 states that after a period of years under crop (tc), the humus content of the soil will approach some equilibrium level defined by the asymptote Ac/kc. Integrating the equation 6.2 gives:
| Hc = Ac/kc - (Ac/kc - Ho) e-kctc | (6.4) |
The equilibrium levels and the rates at which they are reached therefore depend on the rates of addition and the rates of decomposition of humus.
It is possible to relate the level of humus that will eventually establish at any cropping intensity within a cropping-fallow rotation cycle (Hcf) to the level that is established after a long rest fallow (Hef). Also, when equilibrium is reached within a particular cropping-fallow rotation cycle, there is no overall net change in humus content for the rotation cycle. It can be demonstrated that the relation between Hcf and Hef is given by:
 | (6.5) |
Using equation 6.5, the length of fallow period (tf) required within the cropping-fallow rotation can be determined for desired ratios of Hcf/Hef, if kf and kc are known.
In Part III of the productivity model (Figure 2.1), fallow requirements of crops and cropping patterns have been defined through equation 6.5 for Hcf/Hef ratio maintained at or greater than 0.5. The value of kc and kf assumed in the model are given in Table 6.1.
The fallow requirements have been derived for the inventoried environmental conditions for four main groups of crops: cereals, legumes, roots and tubers, and banana and sugarcane. The environmental frame used consists of individual soil units, thermal regime, represented by thermal zone T1 (Tmean > 25%), T2 and T3 (Tmean 20–25°C), T4 and T5 (Tmean 15–20°C) and T6, T7 and T8 (Tmean 5–15°C), and moisture regime, represented by length of growing period zones 60–89, 90–119, 120–179, 180–269 and 270–365 days.
Basic values of fallow requirements (F), expressed as percentage of time during the cropping-fallow cycle (i.e. tf/(tc+tf)x100) the land must be put under fallow, for the low inputs LUTs were first calculated, as presented in Table 6.2.
These reference values were modified depending on the particular crop, and Fertility Capability Classification (Fcc) of the soil (Sanchez, Couto and Buol 1982) (Table 6.3).
Reference fallow period for LGP > 270 days is 50% greater compared with those for LGP 120–269 days due to additional problems with weeds, pests and diseases, and leaching and erosion. For LGP 90–119 days fallow requirements are greater by 25% due to additional problems with fallow establishment from dry conditions and degradation hazards. For LGP 60–89 days, fallow requirements are greater by 50% due to problems with fallow establishment, degradation hazards and need to conserve moisture.
TABLE 6.1
Decomposition constant (%) under cropping (kc) and fallow (kf)
| Thermal zone | Length of growing period (days) | |||||
| 60–89 | 90–119 | 120–179 | 180–269 | 270–365 | ||
| T1 | kc | 1.50 | 1.50 | 2.00 | 4.00 | 6.00 |
| kf | 0.75 | 0.75 | 1.00 | 2.00 | 3.00 | |
| T2 | kc | 1.50 | 1.50 | 2.00 | 4.00 | 6.00 |
| kf | 0.75 | 0.75 | 1.00 | 2.00 | 3.00 | |
| T3 | kc | 1.50 | 1.50 | 2.00 | 4.00 | 6.00 |
| kf | 0.75 | 0.75 | 1.00 | 2.00 | 3.00 | |
| T4 | kc | 1.00 | 1.00 | 1.50 | 3.00 | 4.50 |
| kf | 0.50 | 0.50 | 0.75 | 1.50 | 2.25 | |
| T5 | kc | 1.00 | 1.00 | 1.50 | 3.00 | 3.00 |
| kf | 0.50 | 0.50 | 0.75 | 1.50 | 1.50 | |
| T6 | kc | 1.00 | 1.00 | 1.00 | 2.00 | 3.00 |
| kf | 0.50 | 0.50 | 0.50 | 1.00 | 1.50 | |
| T7 | kc | 1.00 | 1.00 | 1.00 | 2.00 | 3.00 |
| kf | 0.50 | 0.50 | 0.50 | 1.00 | 1.50 | |
| T8 | kc | 1.00 | 1.00 | 1.00 | 2.00 | 3.00 |
| kf | 0.50 | 0.50 | 0.50 | 1.00 | 1.50 | |
TABLE 6.2
Reference fallow factor (F) (%)
| Thermal zone | Length of growing period (days) | ||||
| 60–89 | 90–119 | 120–179 | 180–269 | 270–365 | |
| T1 (>25°C) | 75 | 70 | 65 | 65 | 75 |
| T2 and T3 (20–25°C) | 70 | 65 | 60 | 60 | 70 |
| T4 and T5 (15–20°C) | 70 | 65 | 60 | 60 | 70 |
| T6,T7and T8 (5–15°C) | 75 | 70 | 65 | 65 | 75 |
For moderately warm and moderately cool temperature regimes (T2, T3, T4, T4 and T5 zones) all reference values are decreased by 25% due to lower pest and disease problems and better fallow establishment conditions. For cool temperature regime (T6, T7, and T8 zones), reference values remain unchanged because temperature constraints on the rate of fallow establishment is considered to outweigh any advantage from lower pest and disease infestation.
TABLE 6.3
Effect of soil fertility on reference fallow factor (F)
| Thermal zone | Soil fertility capability classes1 | Length of growing period (days) | ||||
| 60–89 | 90–119 | 120–179 | 180–269 | 270–365 | ||
| T1 | I | 75 | 70 | 65 | 65 | 75 |
| (>25°C) | II | 80 | 75 | 70 | 70 | 80 |
| III | 85 | 80 | 75 | 75 | 85 | |
| IV | 90 | 85 | 80 | 80 | 90 | |
| T2 and T3 | I | 70 | 65 | 60 | 60 | 70 |
| (20–25°C) | II | 75 | 70 | 65 | 65 | 75 |
| III | 80 | 75 | 70 | 70 | 80 | |
| IV | 85 | 80 | 75 | 75 | 85 | |
| T4 and T5 | I | 70 | 65 | 60 | 60 | 70 |
| (15–20°C) | II | 75 | 70 | 65 | 65 | 75 |
| III | 80 | 75 | 70 | 70 | 80 | |
| IV | 85 | 80 | 75 | 75 | 85 | |
| T6,T7 and T8 | I | 75 | 70 | 65 | 65 | 75 |
| (5–15°C) | II | 80 | 75 | 70 | 70 | 80 |
| III | 85 | 80 | 75 | 75 | 85 | |
| IV | 90 | 85 | 80 | 80 | 90 | |
Fallow requirements for Fluvisols and Gleysols are set lower because of their special moisture and fertility conditions.
Fallow requirements (F) for all suitable soil units are presented in the Appendix in Table A6.1 for the low level of inputs situations for cereals, legumes, roots and tubers, and banana and sugarcane.
Fee Class:
Fallow requirements at the intermediate level of inputs are taken as one third of those at the low level. At the high level of inputs, fallow requirements are set at 10%.
For wetland rice on Fluvisols, fallow requirements are assumed to be 10% for all the three levels of inputs. For Gleysols, fallow requirements are 40% at the low and intermediate levels, and 10% at the high level.
For long-term perennials (i.e. oil palm, coffee, tea, sisal), fallow requirements are assumed to be nil. For short-term perennials (i.e. pineapple, pyrethrum) and cotton fallow requirements are similar to those for cereal crops.
The fallow factors have been verified against available published data and similar work done earlier by Young and Wright (1980) in the context of FAO's regional assessments.
In Part III of the productivity model (Figure 2.1), crop rotation options are formulated for each agro-ecological cell for each cropping pattern option generated in Part III of the productivity model. This is accomplished in two steps. Firstly the appropriate crop combination restrictions are applied to rule out risky or undesired crop combinations in space and time, and secondly to incorporate the appropriate fallow requirements for each suitable cropping pattern.
With cropping patterns comprising of more than one crop, average fallow requirements for the crops concerned are applied to define the rotations.
At the same time, Part III of the productivity model defines the extent of fallow land and therefore the portion of biomass that can be used for livestock production in Part V of the model (Technical Annex 5).
