Steps to calculate nitrogen applied in irrigation
Step 1
Analyse a sample of your bore water to determine its nitrogen content. Collect at least 100mL of water in a clean bottle with a tight lid. Keep the sample cool and deliver it to the laboratory (Chemistry Centre WA or private laboratory) within a few hours. Frozen samples will last up to four weeks. (Fill the bottle only two-thirds full, to allow for expansion during freezing.)
Step 2
Calculate the volume of irrigation water — cubic metres — applied per hour over 1 hectare:
Output of one sprinkler (L/h) multiplied by 10 divided by distance (m) between sprinklers along each lateral multiplied by distance (m) between laterals.
Step 3
Calculate how many kilograms of nitrogen are applied per hectare per hour in the irrigation water:
Nitrogen analysis results (Step 1) multiplied by volume of water/ha/h (Step 2) divided by 1000.
Step 4
Calculate how many kilograms of nitrogen are applied per hectare over the crop's life. Hours of watering will vary with time of year, crop stage and rainfall:
Kg of nitrogen/ha/h (Step 3) multiplied by total number of hours of watering over the crop's life.
The figure obtained in Step 4 is the extra nitrogen that is applied to the crop per hectare from the nitrogen in the groundwater.
Example
If the nitrogen concentration of bore water used on a vegetable farm is 15mg/L, how much nitrogen is being applied through the irrigation system over one crop's lifetime?
Assume the following:
- output of an impact sprinkler (size 12 nozzle) at a pressure of 300kPa = 1452L/h (24.2L/min)
- sprinklers 12m apart along the laterals
- laterals 13m apart
- crop watered 1.5 hours per day (on average)
- a cabbage crop that takes 72 days to mature.
Step 1
The nitrogen content of water is 15mg/L.
Step 2
Calculate the volume of irrigation water (cubic metres) applied per hour over 1 hectare:
Output of one sprinkler (L/h) multiplied by 10 divided by distance (m) between sprinklers along each lateral multiplied by distance (m) between laterals).
That is: 1452 L/h multiplied by 10 divided by 12m multiplied by 13m = 93 cubic metres/ha/h
Step 3
Calculate how many kilograms of nitrogen are applied per hectare per hour in the irrigation water:
Nitrogen analysis results (Step 1) multiplied by volume of water/ha/hour (Step 2) divided by 1000.
That is: 15mg/L multiplied by 93 divided by 1000 = 1.40kg N/ha/h.
Step 4
Calculate how many kilograms of nitrogen (N) are applied per hectare over the crop's life:
Kg N/ha/hour (Step 3) multiplied by total number of hours of watering over the crop's life
That is: 1.40 multiplied by 108 (1½ hours/day multiplied by 72 days) hours = 151kg N/ha/crop.
A cabbage crop generally requires about 400kg of nitrogen per hectare for optimum growth. This grower only needs to apply 249kg of nitrogen per hectare (400kg minus 151kg) because of the contribution from the irrigation water.
The next step is working out at what stage of the crop’s life to leave off the nitrogen. In this example, it could be as much as the first half of the crop’s life while nitrogen demand is relatively low. Leaving a fixed amount of nitrogen out every time fertiliser is applied is still potentially wasteful because in this case, early in the crop’s life, nitrogen may not be needed at all.