Or. Admin. R. 603-095-5055
(1) A landowner subject to this rule has met or followed an annual nitrogen plan if, for each field, application of fertilizer at an agronomic application rate has resulted in achieving a crop’s estimated yield, post-harvest soil nitrate levels are low or decreasing and nitrate leaching is minimized or prevented.
(a) A landowner may estimate post-harvest soil nitrate levels using any one or more of the methodologies provided in sections (2) – (5) of this rule.
(b) A landowner has minimized or prevented nitrate leaching only as consistent with section (6).
(2) A/R Ratio. Especially when evaluated over multiple years, the A/R ratio may provide a reliable measurement of the nitrogen left in the field post-harvest. In each consecutive year, the nitrogen left in the field from the prior year as approximated by the A/R ratio will either be utilized by the next crop or move further down the soil with the potential of leaching into the groundwater. A landowner has met or followed their annual nitrogen plan if for each specific crop type, the A/R ratio is low or dropping.
(a) The A/R ratio is calculated as the ratio of total nitrogen applied divided by the total nitrogen removed. (A/R = All nitrogen added to a field from any source/ All nitrogen removed from a field in harvested or other materials).
(b) Total nitrogen applied shall be the sum total of all nitrogen from any source applied to a field (lbs/unit).
(c) Total nitrogen removed shall be calculated by multiplying crop yield (lbs/acre) by the specific nitrogen coefficient (CN) (lbs/unit) for that crop. (Nitrogen removed (lbs/acre) = Crop Yield (units/acre) x CN (lbs/unit)). Crop yield includes the harvested crop removed from the field and other materials. Other materials include wheat straw, crop residues, and orchard prunings that are removed from a field.
(3) A-R Difference. A landowner may estimate post-harvest soil nitrate levels as the A-R difference (A – R = storage of N in soil). An annual nitrogen plan is met or followed if a landowner is approaching or meeting a steady-state condition, i.e., soil nitrogen storage is within an acceptably small value for the crop type. The A-R difference may also be paired with the A/R ratio, especially where use of only the A/R ratio may mask significant quantities of nitrogen left in the field.
(a) The A – R difference is calculated as the sum total of nitrogen (lbs/acre) added to a field from any source minus the sum total of all nitrogen removed (lbs/acre) from a field. (Nitrogen Applied minus Total Nitrogen Removed = Change in Soil N storage).
(b) Total nitrogen applied (lbs/acre) is calculated as the sum total of nitrogen added to a field from any source.
(c) Total nitrogen removed (lbs/acre) is calculated as actual crop yield (units/acre) multiplied by the specific nitrogen coefficient (CN ) (lbs/unit) of the harvested crop. (Nitrogen removed (lbs/acre) = Crop Yield (units/acre) x CN (lbs/unit)). Crop yield includes the harvested crop removed from the field and other materials. Other materials include wheat straw, crop residues, and orchard prunings that are removed from a field. (Nitrogen Removed (lbs/acre) = Crop Yield (units/acre) x CN (lbs/unit)).
(4) Post-harvest soil nitrate samples. A landowner may determine post-harvest nitrate levels using soil nitrate levels derived from post-harvest soil samples taken consistent with the soil sampling protocol in OAR 603-095-5065(2).
(a) Low or decreasing post-harvest soil nitrate concentrations may indicate that a landowner has applied fertilizer at an agronomic application rate and the risk of nitrate leaching is lowered.
(b) Increasing post-harvest soil nitrate concentrations may indicate an increased risk of nitrate leaching.
(5) Estimated Crop Yield. A landowner may estimate post-harvest soil nitrate levels by determining whether they met or exceeded their estimated crop yield.
(a) If a landowner has met or exceeded their estimated crop yield this is an indication that a landowner has applied fertilizer at an agronomic application rate and post-harvest soil nitrate concentrations are low.
(b) If a landowner has not met or exceeded their estimated crop yield, this may be an indication that a landowner has not applied fertilizer at an agronomic application rate and excess plant available nitrogen remains in the soil at the crop root depth.
(6) Nitrate leaching is prevented or minimized if a landowner has followed their irrigation water management plan and has achieved the goal in OAR 603-095-5025(1) for each irrigation event.
(7) For each field, a landowner shall determine and record in their post-harvest summary report whether they met or followed an annual nitrogen plan or not.
(8) Adaptive Management Measures. For each field where an annual nitrogen plan was not met or followed, a landowner shall record in the following calendar year’s annual nitrogen plan, the adaptive management measures they will employ according to Table 1 Adaptive Management Measures.
[ED. NOTE: To view attachments referenced in rule text, click here for PDF copy.]
ORS 561.191 & ORS 568.900-933
ORS 568.900-933 & 468B.025(1)