The Nitrogen Response Curve
Diminishing returns, environmental cliffs, and the rotational advantage
First 48 kg N/ha yields 58 kg grain per kg N — the last 48 kg yields only 6 kg grain per kg N
Why This Matters
Broadbalk features a graded N series (0-288 kg N/ha) under identical basal PKMg. Plots 5-16 receive different N rates, allowing us to perfectly isolate the impact of nitrogen fertilizer on crop performance and the environment.
Goulding et al. (2000) measured tile drain nitrate directly per plot. The data reveals a critical threshold: Below N₃-N₄ (144-192 kg N/ha), the crop absorbs nearly all applied N. Above this optimum, yield plateaus but leaching surges exponentially.
Furthermore, the introduction of crop rotations to Broadbalk demonstrated that the rotational break delivers a +2.0 t/ha bonus compared to continuous wheat, largely by reducing soil-borne take-all disease.
Interactive Response Model
The Environmental Cliff
Yield plateaus while nitrate leaching grows exponentially
Marginal Returns
Grain gained per additional kg N (Rotational Wheat)
The Rotation Premium
Continuous vs 1st Wheat in Rotation
References & Methodology
- Goulding, K.W.T., et al. (2000). Nitrate leaching from arable and horticultural land. Soil Use and Management, 16, 145-151.
- Macdonald, A.J., et al. (2018). The Broadbalk Wheat Experiment: yield trends and long-term effects. Journal of Agricultural Science, 156(6), 747-758.
- Data represents smoothed long-term means for modern cultivars (post-1985).