Variable-Rate Phosphorus on a Leased Block
A grower leases 500 hectares of cropping land on a five-year lease and has soil test data showing Colwell phosphorus ranging from 8 to 34 mg/kg across the block. The low-testing areas are on rises with a history of erosion; the high-testing areas are near the old sheep yards. A variable-rate spreader and prescription mapping service is available at an extra $12/ha over uniform spreading, and the total phosphorus budget is $95,000 for the year. The grower's agronomist recommends building phosphorus on the low areas and maintaining or drawing down the high areas. The landowner is reluctant to co-fund soil improvement on land the grower may not farm after year five.
1. Decide whether to invest in variable-rate spreading and how to allocate the phosphorus budget, given the lease.
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Variable-rate spreading is technically justified because the soil test range (8 to 34 mg/kg Colwell P) is wide and the pattern is systematic - low on eroded rises, high near old sheep yards - so a single rate would over-fertilise some areas and under-fertilise others. The extra cost is $12/ha × 500 ha = $6,000, which is about 6% of the $95,000 budget, and the savings from not spreading high-P areas can offset it.
The lease changes the economics. Building phosphorus on the low-testing areas is a long-term investment that raises the land's value, and the grower may not capture that after year five. The grower should therefore:
- Allocate most of the budget to maintaining the productive areas and correcting the worst low-P rises only to a level that pays back within the remaining lease term.
- Draw down the high-P areas (near the sheep yards) with reduced rates, since that phosphorus is already available and drawing it down costs nothing and saves budget.
- Negotiate with the landowner: a written agreement that compensates the grower for unused phosphorus if the lease is not renewed, or a shared cost on the low areas, is the standard way to resolve the reluctance. The landowner benefits from the improvement, so co-funding is reasonable.
- Keep soil test records to document the improvement, which supports the negotiation.
A defensible answer uses variable rate, avoids building capital phosphorus it cannot recover, and resolves the lease conflict by agreement rather than by either party absorbing the risk.
2. The grower proposes ignoring the soil tests and applying a uniform high rate to 'be safe'. Evaluate this approach.
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A uniform high rate is the least defensible option on this block.
Agronomically, the high-testing areas near the sheep yards already have 34 mg/kg Colwell P; adding more there gives little or no yield response and increases the risk of phosphorus loss to waterways. The low-testing eroded rises do need phosphorus, but a uniform rate under-supplies them relative to their need while over-supplying the rest.
Economically, the $95,000 budget is fixed. A uniform high rate spreads it thinly across the whole block, so the areas that would respond get less than they need, and money is spent where there is no response. The extra $12/ha for variable rate is small compared with the waste from misallocation.
Environmentally, phosphorus above crop requirement accumulates and can move with erosion from the rises - the same erosion that created the low-P pattern - into waterways.
There is a legitimate kernel in the proposal: a 'safe' strategy of maintaining adequate phosphorus everywhere avoids the risk of under-fertilising a responsive area because of a single soil test. But that is achieved by variable rate with a sensible floor, not by a uniform high rate. The answer should reject the uniform approach, recommend variable rate with a minimum rate on the eroded rises, and note that the lease still limits how much capital phosphorus is worth building.