Introduction
Nitrates from nitrogen fertiliser are readily leached from all soils. Leaching is particularly rapid on sandy soils because of their limited capacity to hold nutrients and moisture.
High nitrate concentrations in the groundwater below horticultural properties are common on the Swan Coastal Plain. This is of concern for several reasons:
- health concerns from drinking water with high nitrate levels
- the growth of algae in surface water
- the effects of nitrate directly and indirectly on aquatic life in lakes and rivers supplied by groundwater
- the effects of nitrate on native vegetation downstream from the nitrate source dependant on groundwater
- the amount of uncontrolled nitrogen applied to crops in irrigation water.
Health concerns
Drinking groundwater containing high levels of nitrates may be dangerous to your health and that of your children.
In 1991, just under half of 40 market gardeners' bores sampled contained concentrations in excess of the World Health Organisation Guideline of 10mg/L nitrate-nitrogen.
Drinking water with nitrate levels exceeding this limit is especially serious in infants. It is important that people who drink groundwater from private bores have the water analysed for its nitrate content. The Chemistry Centre (WA) or a private laboratory can conduct the analysis.
If your domestic bore is located near a septic tank or poultry manure heap, have a sample analysed for bacteria that can cause gastroenteritis. Samples taken by local health surveyors can be analysed by State Health Laboratory Services.
Growth of algae in surface water
In estuaries and shallow coastal waters, nitrates can cause growth of algae and phytoplankton. Algal blooms choke waterways, give off foul odours and may kill seagrass, fish and birds.
Groundwater flowing from agricultural and urban areas can carry nitrates, which may ultimately reach estuaries or the ocean and contribute to algal blooms.
Nitrogen applied in irrigation
Irrigation with nitrogen-rich groundwater can supply a considerable proportion of a crop's nitrogen requirement. Calculate the amount of nitrogen applied in irrigation water and adjust your fertiliser program accordingly — see below for how to do this.
Nitrogen in groundwater can be so high that the crop suffers nitrogen toxicity. In this case, use no further nitrogen fertiliser, or mix water high in nitrogen with a different, uncontaminated source.
Nitrogen levels in groundwater are likely to be higher in areas where horticulture has been practised for a long time and where groundwater is shallow. Check the nitrogen concentration in bores regularly and adjust fertiliser programs if nitrogen concentrations change.
Other nutrients
Potassium concentrations in groundwater beneath horticultural properties may also build up to the point that irrigation with this water supplies a significant part of the crop's requirement. A potassium analysis can be done at the same time as a nitrogen analysis. You may be able to reduce the amount of potassium fertiliser needed.
Phosphorus concentrations in groundwater under horticultural properties are unlikely to increase since this nutrient is held tightly by most soils and is not readily leached. However, levels may increase in the shallow groundwater below horticultural properties located on white sands.
Nitrogen analyses
The result should be expressed as milligrams per litre (mg/L) or parts per million (ppm) of total nitrogen. Note that 1mg/L = 1ppm.
If the nitrogen concentration is expressed as nitrate-nitrogen, this figure can be used but may underestimate the total amount of nitrogen being applied. Additional small amounts of nitrogen are usually present in the groundwater as ammonia and organic nitrogen. If the water analysis is expressed as nitrate, divide by 4.5 to get the nitrogen concentration. (4.5mg/L nitrate = 1mg/L nitrate-N)
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.
Ways to reduce nitrate leaching
- Apply no more nitrogen fertiliser than the crop needs for good growth.
- Do not over-water. Excessive applications of water infiltrate through the soil and leach nutrients away. Small frequent waterings are best on sandy soils, keeping the root zone moist without excessive water drainage.
- Ensure that your irrigation system applies water evenly. Uneven application leads to over-watering in some areas to supply enough water to the drier spots. Excess water drains below the root zone, taking nutrients with it.
- On sandy soils, apply nitrogen fertiliser in small, regular doses throughout the life of the crop. This will limit leaching caused by heavy rain or over-watering to the most recent application.
- Slow-release nitrogen fertilisers can reduce leaching because they supply nitrogen at a steady rate over an extended period. This can result in efficient nitrogen use by crops, with fewer nutrients available for leaching. These forms of nitrogen are generally expensive.
- Match nitrogen application rates with crop growth stage. Young crops require lower rates of nutrients than in mid-growth. Apply nutrients in more frequent, smaller doses when crops are young, because their root systems are smaller. Reduce nutrient applications as the crop approaches maturity.
- When plants are young, place nitrogen fertiliser with droppers immediately adjacent to plants. When crops develop more extensive roots they are better able to extract nutrients spread over the whole growing area.
- Use tissue testing to determine whether the crop has sufficient nitrogen and adjust nitrogen applications. Sap testing kits provide a quick method to determine the nitrogen status of a crop.
Acknowledgement
Neil Lantzke authored the original version of this material.