Considerable soil structure degradation has occurred in many cotton fields during the past three seasons. This has been caused by picking and tillage operations during the wet winters of 1988, 1989 and 1990. Soil compaction as a result of these operations will have a significant impact on the irrigation schedule in the following year.
The following example demonstrates how a wet pick in the previous season influences the irrigation schedule. Growers need to be aware of this in the current irrigation season.
The deficit is how many mm of water the profile is below the full point. A deficit of 80-90 mm is typical for heavy clay soils around Moree which have good soil structure. It is most important to realise that the deficit will be different for every field and change from year to year depending on the history of wet picking and tillage operations in each individual field.
Frequent neutron probe readings (2 – 3 times/week) enables the daily water use of the crop to be determined. The daily water use will decline, assuming constant weather conditions, when the refill point has been reached.
The full and refill points shown in Figure 1 have been transferred from the previous seasons data in this field. Last year this field had a 79mm deficit which is typical of good fields in the area.
The rest of the neutron probe data in Figure 1 is for the following irrigation season. Following the 2nd crop irrigation on the 2/1 a low daily water use (1.3mm and 1.8mm) is evident on the 5/1 and 9/1 due to water logging and cool temperatures. On the 11/1 the water use was 7.5mm/day The daily water use then dropped to 5.0mm/day and it was decided to irrigate this crop again, based on crop symptoms and the reduced daily water use at a deficit of 49mm.
A similar pattern of water use was evident between the 3rd and 4th crop irrigations. Between the 14/1 and 16/1 a daily water use of 0.0mm/day was recorded due to water logging. This increased to 4.4mm/day as the crop recovered from the water logging to a peak of 6.3mm/day on the 22/1. By the 25/1 the daily water use had dropped to 2.2mm/day at a deficit of 47mm.
This is a similar deficit to the previous irrigation, indicating that the refill point had changed from a deficit of 79mm which it was in the previous season to a deficit of 47mm. The values (VSW%) for 25/1 should be used to replace the previous years refill point.
Wet picking reduced the deficit by 32mm in one season due to soil compaction (Figure 2). Irrigators should be aware of an expected reduced deficit following wet harvesting and tillage operations in the previous season. Careful monitoring of the crops daily water use and root extraction patterns as well as crop observations will enable refill points to be set correctly.