At the MicroFarm, we just harvested our second lot of peas.  We tracked their water use since planting to build on learning from our first crop (see the December 2013 issue of “Grower”, reproduced here>).

Once again, HydroServices’ Melanie Smith established three neutron probe access tubes in each of two crops. These were read weekly and analysed to tell us paddock soil moisture content down to 80 cm.

Both pea crops were planted on the same day with the same drill. Paddock 1 is dryland and Paddock 2 has drip irrigation installed 200mm deep.

Figures Paddock 1 and Paddock 2 show soil water content for each crop.

We see the crops tracked about the same at the start. In mid-December, Paddock 2 received two 9mm irrigations from our buried dripline.

Melanie estimated that the irrigation was 80% efficient, so only added about 7.5mm to the budget each time. Paddock 1 is not irrigated, and continued to drop towards stress point.

Paddock 1 reached Refill Point on Boxing Day two days before rain fortunately lifted it back out of stress. Paddock 1 again hit stress point on about 6^th January. With no more significant rain, it stayed stressed. With irrigation applied as required, Paddock 2 remained stress-free throughout.

Overall, the two crops used similar amounts of water through until early January. After that the 0 – 30 cm soil reached stress point, and water use from the unirrigated Paddock 1 began to taper off.  The steeper lines in the bottom part of the graphs show it began to get more water from deeper in the profile.

By harvest, Paddock 1 was using only about half as much water as the drip irrigated Paddock 2 and drawing it from much deeper in the profile.

The difference in what a crop did use and what it could have used if the water was available is described by Potential Soil Moisture Deficit.

We estimate that by harvest, Paddock 1 suffered about 100mm of PSMD. I am not sure what the pea response is to stress. I am told it is a “very elastic” crop. For many crops this would indicate a growth reduction of about 20%.

So did irrigation pay?

We sampled each crop pre-harvest and found Paddock 2 had about 30% more fresh weight canopy than Paddock 1. The difference was easily seen, being significantly taller and generally more “lush”. The peas in Paddock 1 reached harvest maturity at least three or four days before the irrigated peas in Paddock 2.

We get paid for peas not canopy. We also sampled yields and quality as measured by TR (pea tenderness) and found differences.

There was a lower tonnage in Paddock 2, but the quality (and pay-out value) was much higher.

At harvest the Paddock 1 tonnages were reasonable at 6.85 t/ha paid yield. But TR was 137; a bit high and the lowest pay-out grade.

We delayed harvesting Paddock 2 for two days. The paid yield was similar at 6.55 t/ha but the TR was 102, a 30% higher pay-out grade.

Paddock 1 returned $2,059/ha and Paddock 2 returned $2,625/ha gross, so a benefit of $566/ha from irrigation.

We applied 81 mm so our return from irrigation was $6.99/ha/mm applied. Many people quote an irrigation cost of about $2/ha/mm so let’s claim a benefit of $5/ha/mm applied.

Looking at it another way. If we had a 20ha paddock, irrigation would have made us about $8,000 better off. If we also sold pea hay, the benefit would be even greater.

Answer: Irrigation pays!

Thanks to: Centre for Land and Water, ThinkWater, Netafim, HydroServices, McCain Foods, Ballance AgriNutrients, BASF Crop Protection, FruitFed Supplies, Agronica NZ, Nicolle Contracting, Drumpeel Farms, Greville Ground Spraying, True Earth Organics, Tasman Harvesting, Plant & Food Research, Peracto NZ

The MicroFarm has two crops of peas almost ready for harvest. We have been tracking their water use since planting. We want to learn as much as we can about our soil and irrigation.

HydroServices’ Melanie Smith is our specialist support for soil moisture monitoring. She established three neutron probe access tubes in each of our first two crops. These are 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 are seeing 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’s 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?

Many thanks to HydroServices for the soil moisture monitoring at the MicroFarm