Calibration of how much N to apply was potentially different for every crop and the method had a tendency to predict shortages after the fact, thus risking loss of yield potential. Scientists at Plant & Food Research, Lincoln, created a potato growth model in order to help growers better schedule nitrogen and water applications for Canterbury process potato crops. 

The model, developed using well tested science from international publications, tracked the daily water and N interactions in the soil profile, and grew a crop by partitioning biomass accumulation between the canopy and tubers.

The potato model (the Potato Calculator, PC) was simultaneously developed and tested in 15 Canterbury commercial crops via a Sustainable Farming Fund (SFF) project (2002-2005). The PC recommended from between 40 to 200 kg/ha less N be applied to the crops, saving from $60 to $280/ha (2005 dollars), enabling a reduction of leachable N ending up in the soil at harvest. However using the PC recommendations gave an associated yield loss which ranged between 0.3 and 2.6 t/ha, although modelling showed that water deficit was probably a far greater cause of yield limitation (5 -15 t/ha).

A second SFF project (2005 -2008) repeated the same approach for a total of 18 crops in Manawatu, Pukekohe, Hawke’s Bay and Waikato regions. In the 13 of the crops there was no yield response to side dressed N (grower amounts ranged from 0 to 280 kg N/ha) as enough N was supplied at planting as fertiliser and by the soil (stored and mineralised). 

Once again, by far the greatest yield limitation (2 to 48 t/ha estimated loss) was caused by water deficit (some crops had no irrigation available). A leaching trial carried out each year of the project (Waikato) showed that no leaching occurred during crop growth but between 20 to 120 kg/ha was lost from the grower N treatment during the winter of the following the potato crop.

Source: FAR