Nitrogen Budget for a Mixed Vegetable Rotation
A 20-hectare vegetable farm runs a rotation of broccoli, lettuce, and a legume green manure. The grower applies 180 kg N/ha to broccoli and 150 kg N/ha to lettuce, and incorporates a vetch green manure before lettuce. Soil tests show 45 kg/ha mineral N at planting, and the vetch is estimated to contribute 80 kg N/ha. Broccoli removes about 200 kg N/ha in the harvested crop, lettuce about 120 kg N/ha. The farm is in a catchment with a nitrogen reduction target, and the grower wants to maintain yields while cutting losses.
1. Construct a simple nitrogen budget for the lettuce crop following vetch and identify where losses are likely.
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Inputs: soil mineral N 45 kg/ha + vetch contribution 80 kg/ha + fertiliser 150 kg/ha = 275 kg N/ha available. Crop removal for lettuce is about 120 kg N/ha, leaving a surplus of roughly 155 kg N/ha. That surplus is the risk: it cannot be stored in the soil over winter and will be vulnerable to leaching after rain and to denitrification in waterlogged patches. The vetch contribution is also poorly synchronised - it mineralises as the residue breaks down, which may not match lettuce demand, so some is lost before the crop can use it. The budget shows the fertiliser rate is not justified by the vetch credit; the grower is effectively double-counting nitrogen supply.
2. Recommend changes to the fertiliser program that maintain yield while reducing losses, and explain the mechanism for each.
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First, subtract the vetch credit from the fertiliser rate: reduce lettuce N from 150 to roughly 70–80 kg/ha, since the vetch supplies 80 kg/ha. Second, split applications so nitrogen is applied when the crop is actively growing rather than all at planting, reducing the window in which surplus N sits in the soil. Third, time vetch incorporation so mineralisation aligns with lettuce demand - incorporate two to three weeks before planting and monitor mineral N. Fourth, use tissue or sap testing to confirm the crop is not deficient before adding more. Each change targets the surplus identified in the budget: reducing the rate cuts the surplus at source, splitting reduces the time it is exposed, and timing improves synchrony.
3. The grower worries that cutting nitrogen will reduce lettuce yield. Evaluate this concern.
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The concern is understandable but not supported by the budget. The crop requires about 120 kg N/ha and the soil plus vetch already supply about 125 kg/ha before any fertiliser, so a reduced rate still leaves adequate supply. Yield loss from nitrogen deficiency occurs when supply falls below crop demand, not when surplus is trimmed. The greater risk to yield is actually excess nitrogen, which can produce soft, sappy heads prone to disease and poor shelf life. The sound approach is to cut the rate, monitor crop colour and tissue N, and retain the ability to side-dress if deficiency appears. This preserves yield while meeting the catchment target.