Article first published in The GROWER, December 2013
We tracked water use since planting in the first two crops of peas at the MicroFarm. We want to learn as much as we can about our soil and irrigation.
HydroServices’ Melanie Smith, our specialist support for soil moisture monitoring, established three neutron probe access tubes in each of our first two crops. These were read weekly and analysed to give a Paddock soil moisture content down to 80 cm.
Both pea crops were planted on the same day with the same drill. One had some nitrogen starter-fertiliser because our discussion group wondered if it would make a difference, but that is another story. For now, we are talking irrigation management.
We saw significantly different patterns from our two crops. Significant in that considering the usual 30 cm root depth, one crop needed irrigating a week before the other. Significant in that one would get through to harvest at the start of December without needing to be irrigated. The other would need irrigation. What was going on?
Let’s look at two graphs: Paddock 3a and Paddock 4a which are the two crops in question.
The top parts of these graphs show soil water content in the top 30 cm. We see that in each case the Full Point (116 mm) and Refill Point (82 mm) is the same. So we have 34 mm of readily available water our plants can access from the first 30 cm depth of soil.
The graphs show Paddock 4 reached Refill Point a whole week before Paddock 3. In fact, Paddock 4 hit Refill Point almost three weeks before Paddock 3, and but for a chance 12 mm rainfall would have gone into critical deficit in early November.
Let’s compare these graphs a bit more closely.
We see they tracked about the same to start with, then at the beginning of November Paddock 4 suddenly used significantly more water from the 0 ‑ 30 cm root zone than did Paddock 3. This is around the time the canopies reached full ground cover.
Our observations of the crops suggest Paddock 3 had more canopy so we thought it would be using more water than Paddock 4. Looking at the lower parts of Graphs 3a and 4a, we see that Paddock 3 used more water from deeper in the profile at 40 – 50 cm.
We did some Visual Soil Assessments and found more evidence of soil compaction in Paddock 4. Being the main gate access into the area it has seen more tractors, trucks and paddock forklift activity. So we expected to see compaction limiting root development.
Now lets look at the water content in the whole soil profile, right down to 80 cm, presumably well past any pea roots (Figures Paddock 3b and Paddock 4b).
The first thing to notice is much higher water storage, because 80 cm of soil has more readily available water than 30 cm of soil. So now Full Point is 314 mm and Refill Point is 232 mm giving 82 mm of readily available water for our crop to grow before we would need to irrigate.
When we compare these two graphs we get a different picture. Now we see the two crops using similar amounts of water through until 14 November. After that, Paddock 3 (the fuller canopy and better soil condition) used slightly more water than Paddock 4, and actually hit Refill Point a day or so earlier.
Overall, it seems our Paddock 3 crop is getting more water from deeper in the profile, accessing water from 50 ‑ 80 cm deep.
For a lower price crop like peas, reducing costs makes a big difference. Can avoiding compaction save the need to irrigate?
How deep are your crops’ roots?