Cereal and legume sequences: test a crop plan
A crop plan can contain several different names and still repeat the same important biological relationship. It can also look sensible on one plot while exceeding the farm’s labour limit when every plot is considered together. Testing a plan means checking its rules systematically, rather than praising variety in general.
The crop codes, suitability windows and resource units below are fictional classroom premises. Seasons S1, S2 and S3 are labels, not Kenyan planting months. The exercise supplies no real crop calendar, fertiliser recommendation or promise of disease control.
Sequence and rotation answer different questions
A sequence describes the order of crops over the stated periods. A rotation repeats a defined order. A three-season record does not prove that its writer intends to repeat it. Once repetition is proposed, the last crop and the first crop of the next cycle become neighbours and must also satisfy the rules.
FAO’s explanation of crop sequences distinguishes flexible sequences from repeated rotations. It also explains that outcomes can depend on disease relationships, water, nitrogen and other conditions. The paper model uses explicit constraints so that a conclusion can be checked without guessing those conditions.
Read the four crop cards
| Crop code | Family group | Supplied relationship to problem H | Allowed season labels | Labour units per plot in its season | Water-demand units per plot |
|---|---|---|---|---|---|
| C1 | Cereal; grass family | Host | S1 or S2 | 2 | 2 |
| C2 | Cereal; grass family | Host | S3 only | 2 | 3 |
| L1 | Legume family | Non-host | S1, S2 or S3 | 1 | 1 |
| L2 | Legume family | Non-host | S1 only | 1 | 2 |
Problem H is an unnamed fictional host-dependent problem. The host labels are supplied facts, not diagnoses inferred from family names. In real crop systems, different species in one family may have different host relationships, and unrelated plants can sometimes share a problem. Use the card, not a general shortcut.
The task’s rules are: plant one crop on each plot in each season; use only an allowed season; do not place two H-host crops consecutively on the same plot; include at least one legume on each plot within the three-season horizon; meet the fictional buyer requirement for exactly one cereal plot each season; use no more than three labour units across both plots in any season; and keep total water-demand units within budgets of four in S1, three in S2 and four in S3.
All plots are stipulated suitable for the listed cards. Labour and water-demand units are abstract classroom quantities, not worker hours, litres or irrigation instructions. Passing these rules establishes feasibility within the exercise only. It does not establish that the plan is optimal in a real location.
Worked case 1: audit a feasible three-season sequence
| Plot or check | S1 | S2 | S3 |
|---|---|---|---|
| East plot | C1 | L1 | C2 |
| West plot | L2 | C1 | L1 |
| Number of cereal plots | 1 | 1 | 1 |
| Total labour units | 2 + 1 = 3 | 1 + 2 = 3 | 2 + 1 = 3 |
| Total water-demand units | 2 + 2 = 4 | 1 + 2 = 3 | 3 + 1 = 4 |
Start along the East row: host C1 is followed by non-host L1, then host C2. Neither adjacent pair contains two hosts. The West row is non-host, host, non-host. Each row includes a legume. Every crop also falls within its allowed season window.
Next read down the columns. Each season meets the buyer’s one-cereal-plot requirement. Labour totals three units, and water-demand totals four, three and four units, meeting their respective ceilings. Checking only the East plot would not establish those whole-farm requirements. The supported conclusion is that Plan A satisfies every listed rule over S1–S3.
The conclusion is deliberately bounded. No crop yields, prices or cash requirements are supplied. Calling the plan “most profitable” would therefore go beyond the evidence. Nor do three compliant seasons establish that H has disappeared; the host-gap rule is a planning condition, not a measured disease outcome.
Worked case 2: different cereal names still share a host role
Plan B changes East to L1 → C1 → C2 and West to C1 → L1 → L1. Its author says that East changes crop name at every step, so its sequence provides a break from H. That reasoning fails at the S2-to-S3 transition: both C1 and C2 are stipulated hosts.
The plan passes the allowed-season, legume, cereal-per-season, labour and water checks. Passing those other rules does not cancel the host-pair violation. The precise rejection is “East has consecutive H-host crops in S2 and S3”, not the vaguer “there are too many cereals”.
A repair must be checked against all constraints again. Swapping only the S2 assignments would give West consecutive C1 crops in S1 and S2, moving the violation rather than solving it. Swapping the S1 assignments as well gives East C1 → L1 → C2 and West L1 → C1 → L1. This revised plan passes all seven rules; its S1 water total is three units, below the four-unit budget. Local changes can solve one problem while creating another, so retest every constraint.
Worked case 3: test the join before calling it a rotation
Ignore the earlier season windows for this separate abstract exercise. Its cards explicitly permit either a host cereal C or non-host legume L in every period, and the only rule is no consecutive H-host crops, including across repetition. A proposed repeated cycle is C → L → C.
Within one displayed cycle, the two transitions pass. At the join, however, the final C is followed by the first C of the next cycle. That pair fails. The sequence C, L, C is acceptable over three isolated periods under this rule, but its repetition is not.
Text alternative: a corrected four-phase abstract cycle is C → L → C → L → back to C. Every host C is followed by non-host L, including at the join. This solves the separate abstract rule. It is not an extension of Plan A’s crop-specific windows, which would require fresh suitability evidence for any later season.
Qualify claims about legumes and nitrogen
A legume in a sequence does not justify a fixed numerical nitrogen credit to its successor. Relevant evidence includes effective fixation, the amount grown, material removed and the conditions affecting the next crop. A record saying “legume present” contains none of those quantities.
For example, two fictional legume plots might have the same crop name but different biomass and residue-removal records. It is reasonable to investigate whether their carry-over effects differ; it is not reasonable to assign identical benefits automatically. A potential biological mechanism and a measured amount are different kinds of evidence.
Similarly, a non-host interval may reduce continuity for the specified problem without controlling every pest or disease. A plan that addresses H might have other water, soil, market or labour constraints outside this exercise. State which goal a rule serves and which outcomes remain unmeasured.
Three solved practice tasks
Task 1: find a whole-plan violation
Two equal plots are assigned C1 in S1. All other checks are postponed. Which immediate constraints fail? Solution: there are two cereal plots instead of exactly one, and labour is 2 + 2 = 4 units, above the three-unit ceiling. A valid crop card on each plot is not enough.
Task 2: distinguish names from supplied host evidence
An extra card X belongs to a different family but is explicitly labelled an H-host. Can it automatically follow C1 under the no-consecutive-host rule? Solution: no. Family difference does not override the given host relationship. C1 followed by X would still make two consecutive hosts.
Task 3: assess a nitrogen claim
A report says, “L1 guarantees the next cereal a fixed nitrogen benefit”, with no fixation, biomass or removal record. Solution: the amount is unsupported. Request the missing evidence and describe a possible contribution conditionally. Do not insert a nutrient rate from memory.
Frequently asked questions
Is any crop sequence a rotation?
No. A rotation repeats a defined order; a sequence need not repeat.
Why check the last-to-first transition?
Repetition makes those crops consecutive, even though they appear at opposite ends of the written cycle.
Does a different crop name guarantee a host break?
No. Use verified host information for the particular problem rather than names alone.
Why check each season across all plots?
Shared labour and output requirements can be violated even when individual plot sequences look acceptable.
Does including a legume guarantee a fixed nitrogen gain?
No. Fixation, biomass, removals and other conditions affect what remains available.
Does a passing plan prove that H is eliminated?
No. It meets an exercise constraint; no disease outcome has been measured.
Can S1, S2 and S3 be read as real planting dates?
No. They are fictional order labels with supplied suitability rules, not an agronomic calendar.
Compare production of maize and common dry beans
The earlier C/L cards test the order of crops across seasons. Here the named crops are maize, a cereal, and common dry beans, a legume. Both can supply food and saleable grain, but their harvested parts, handling and nutrient contributions differ. Beans contribute dietary protein; maize grain is widely used as an energy-rich staple. Legume nitrogen fixation depends on effective symbiosis and conditions; it does not mean beans need no soil-fertility planning or guarantee a nitrogen credit after grain removal.
| Stage | Maize for dry grain | Common beans for dry seed | Record to retain |
|---|---|---|---|
| Plan | Choose a locally suitable maize variety and dry-grain market/use | Choose a suitable bean variety and its growth habit; this case uses a bush type | Site assessment, previous crop, seed identity, intended use |
| Prepare and sow | Prepare the assessed seedbed; sow maize seed using the locally approved placement and fertility plan | Prepare the assessed seedbed; sow bean seed using its own approved placement and fertility plan | Actual preparation, sowing date, emergence and gaps; no copied maize spacing |
| Manage establishment and growth | Check stand, weed competition, water availability and crop-specific pest observations | Check stand and weeds, with attention to flowers/pods; use the bush habit stated here rather than inventing climbing supports | Dates, observations, approved actions, follow-up results |
| Judge harvest | Assess dry-grain maturity, including kernel black-layer evidence in the supplied assessment | Assess filled seed and pod maturity; avoid unnecessary delay into shattering | Representative maturity evidence, weather and harvest date |
| Handle and store | Harvest cobs; dry and shell under the approved process; clean/sort and verify storage conditions | Harvest mature plants/pods; dry and thresh under the approved process; clean/sort and verify storage conditions | Lot identity, moisture basis, quantity, damage flags and store history |
| Use | Sound suitable grain may enter human-food or properly assessed livestock-feed chains | Sound suitable dry beans may be used for food; any planting-seed use also needs seed-quality checks | Destination, acceptance criteria and quantity; maturity alone is not food/feed certification |
The broad sequence is shared: plan → establish → manage → assess harvest → handle/store → allocate to an appropriate use. The crop-specific work cannot be replaced by changing the label C to L. Shelling removes maize kernels from cobs; threshing releases beans from pods. A bush-bean task does not automatically need the support system used for climbing beans. Numerical seed rates, chemical use and real calendars are deliberately left to a local crop plan. The factual crop context is supported by KALRO’s dry-bean manual and KALRO’s maize manual.
Worked scheduling conflict
Two fictional ready-to-harvest plots share six worker-hours on Day H. Bean harvest needs four hours and its assessor notes a new shattering risk if delayed. Maize harvest needs four hours and its assessed crop can wait to Day H+1 under this case’s supplied conditions. Partial plot jobs are not allowed. Plan: beans on H, maize on H+1, with suitable separate post-harvest capacity confirmed for each. Eight required hours cannot fit into six. The choice is justified by the supplied urgency, not a universal rule that beans always come first.
Practice with an answer
A learner schedules bean sowing after maize harvest, skips the bean fertility assessment “because legumes make nitrogen”, then shells the beans from cobs. Identify three errors. Answer: a sequence needs suitable locally verified planting conditions, not merely an empty plot; fixation does not remove other nutrient or soil constraints; common beans are threshed from pods, whereas this maize crop is shelled from cobs. Record crop-specific operations before comparing the seasons.
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