Fertigation and Irrigation Scheduling
Current Irrigation practices
- Most protected cropping vegetable growers on the NAP have generally been over irrigating for many years.
- Due to there being an abundance of relatively cheap reclaimed water available, a lack of irrigation scheduling knowledge and the vegetable crops looking water stressed in hot weather.
- The Northern Adelaide Plains has very shallow topsoil with underlying heavy clay layers.
- Years of applying saline reclaimed and bore water has filled the clay layer with salt, making it sodic.
- Over irrigation has also lead to very shallow, highly saline water tables throughout a lot of the area.
- The problem most commonly encountered with these saline/sodic soils is that in summer when the temperature increases the vegetable crops look stressed and therefore most growers irrigate more often and for longer amounts of time to try and alleviate the problem.
- Stressed looking crops are not due to the lack of water (in most cases the soil is actually saturated) but due to the effects of the salt accumulation in the plant root-zone.
- High salt concentrations reduce water uptake in plants by lowering the osmotic potential of the soil.
- The effects of irrigating more to combat what looks like water stress, also leads to many other problems, like: root and fungal diseases, shallow saline water tables and excessive drainage to the ground water.
Traditional Methods of Leaching Saline and Sodic Soils
- Drainage carries the salts down through the soil profile and out of the plant root-zone.
- Without drainage, salts will accumulate in the soil profile regardless of any applied soil amendments. Creating a drainage event is commonly known as “leaching” and this practice is very effective in free draining sandy/sandy loam soil types. Leaching is much more complex on the Northern Adelaide Plains due to the underlying heavy clay layers that are characteristic of most of the district.
- Implementing scheduled irrigations will help to leach salts sideways and away from the root-zone.
- A few large irrigations before, midway and at the end of each crop cycle will induce some leaching of salt down through the clay and to drainage.
Pulse Irrigations with Saline Water
- The concept behind pulsing irrigation applications is that water is applied to the crop in short bursts of time, but more often.
- After a few water applications, this technique allows the water to spread more widely across the planting beds which then starts to push salts sideways to the edge of the planting beds and away from the plant root-zone.
- Applying small amounts of water more often, (even though it still contains salt) not only moves excess salts out of the plant root-zone, but gives the plants a fresher flush of water each time water is applied
- Irrigating for longer periods of time, less often, only results in leaving a larger salt concentration in the plant root-zone.
- The addition of compost to the soil also assists this process by allowing the topsoil to hold more water, thereby reducing the overall amount of water that has to be applied to match the plant water requirements.
Short bursts of irrigation water allows salts to be pushed away from the plant rot mass. Notice the lack of salt rings around the wet soil.
- Soil testing provides only part of the nutrient picture.
- Plants extract from the soil only certain nutrients that are readily available to the plant roots.
- Measuring leaf nutrient levels allows nutrient deficiencies to be corrected through the addition of liquid fertilisers delivered by irrigation water.
- Target deficient nutrients on a prescriptive basis.
- Need to leaf test every three weeks to get optimal plant nutrient balancing so that plant growth is smooth and not stilted.
- Plant nutrient requirements change during the growing season and fruit production draws heavily on nutrient reserves in both the soil and plant.
- Applying nutrients with irrigation water allows the plants to be fed regularly with smaller doses of fertiliser. This means better nutrient use efficiency by the plant and no tie up in the soil.
- The regular addition of Calcium during elevated temperatures (>35oC) has meant that blossom rot levels have been dramatically reduced and the quality of fruit has been maintained to a very high level.