I just read an article from the latest edition of California Agriculture: “Nitrogen fertilizer use in California: Assessing the data, trends and a way forward”. The article compiles data about inorganic N fertilizer use by major crops in California and compares data from the 1970s to the 2000s. Rice is one of the crops included, accounting for 10% of the total N fertilizer used by the 33 crops considered. Two points jumped at me. First, the article found that “experts” believe growers apply more N fertilizer than growers report applying. The authors used the UC Davis Cost and Return Studies as the experts' opinion, and grower surveys conducted by the USDA for growers input on N fertilizer rates.

The second point is that the average N fertilizer rate for all 33 commodities in the study increased by 25% from 1973 to 2005. In the case of rice, for the same time period, the N fertilizer rate increased from 86 to 130 lbs N/acre (52% increase). The authors caution that these numbers shouldn't be taken in a vacuum, that yield increases should be considered. I calculated just that: if you consider the 5-year moving average rice yield, you find that the lbs of N fertilizer used per lb of rice produced hasn't changed from 1973 to 2005, it's 0.02 lbs N fertilizer/lb rice.