Soil moisture monitoring and water use
Water quality and availability is a topical issue and likely to become more controversial as demand increases and our fresh water quality deteriorates.
In this article, we discuss soil moisture monitoring and management. We consider some of the factors influencing the quality of information and compare the irrigation practices of two different horticultural sectors. Our technical discussion becomes slightly political as we consider aspects of proposed changes to the cost of water.
Fruition provides soil moisture monitoring services to horticulture. During the winter of 2017 we took time to try and answer some nagging questions. We were curious to compare the performance of the two types of monitoring equipment we use – neutron probe and diviner (electrical capacitance). We wanted to test the manufacturer’s recommendation that 3 tubes be used for each soil moisture monitoring site. How much soil, plant and irrigation system performance variation is there and how much does that affect the accuracy of readings. Why monitor 3 tubes if one would do? To do this we separated the 3 individual tube data readings at 50cm soil depth taken from 134 neutron probe and diviner sites. The amount of variation between the 3 tubes for each site was calculated using Minitab statistical software (Figure 1). Results showed that the neutron probe readings were a lot less variable than the diviner. This is probably explained by the neutron probe’s ability to ‘read’ a much larger (16x) soil volume than the electrical capacitance signal from the diviner.
In addition, the amount of variation between the 3 tubes at each site was considerable and confirmed the need for 3 tubes to account for natural site variation. In a third of the monitoring sites, the coefficient of variation was tolerable at 10% or less, but a significant proportion of sites had variation of 30, 60 even 80%. Clearly a single reading in these situations would give very misleading information. As a result of this in-house study, we are sticking with 3 tube sites and will progressively convert our sites in variable soils to neutron probe technology.
CAUSES OF VARIATION
Can we explain the data variation within sites? It turns out that soil type variation is a big part of the explanation. On closer examination we found that sites in areas of frequent changes in soil types had greater variation (Figure 2) while those in blocks of single soil types had more uniformity with their tube readings.
The Heretaunga and Ruataniwha Plains were formed from alluvial and volcanic deposition over millennia and are a veritable patchwork quilt ever changing soil types, water holding capacities and drainage classes. Correct placement of monitoring tubes is important and something we pay particular attention to.
COMPARING IRRIGATION PRACTICE
We also reviewed seasonal irrigation applied by our apple and grape grower clients. Over the past 5 years, grape growers have applied an average of 52 mm of irrigation over the whole season while apple growers have applied 191 mm (3.7 times more) (Figure 4).
Most apple growers apply frequent irrigation and maintain soil moisture at relatively high levels compared to grape growers who restrict irrigation and manage their soil moisture levels down to precise, pre-determined levels (Figure 5).
Yes, grape and apple growers face different commercial drivers. Grape growers are incentivized by fruit quality (brix and freedom from disease) while apple growers are rewarded more by fruit volume and appearance (size and colour). At the risk of over simplifying, grape growers irrigate sparingly because they know that excess irrigation may reduce fruit value and increase costs of canopy management. Apple growers, on the other hand, err on the side of ‘putting more on’ because it might increase yield.
There is a subjective aspect to irrigation. Growers want to be ‘kind’ to their trees and it feels good to put water on when its hot, dry and windy. Nevertheless, Fruition has gathered evidence from two season’s of replicated trials that show that ‘regulated’ irrigation pays. When soil moisture levels were managed down to ‘refill’ levels (at the onset of plant stress) fruit colour was improved while fruit size
and yield was unaffected.The economic case for increased fruit revenue with lower electricity pumping costsis a ‘no brainer’.
PAYING FOR WATER
Water was a hot issue in the 2017 election as New Zealanders have become more concerned about the degrading state of our freshwater. (this article is being written while post election negotiations to form a government are taking place). The Labour Party’s proposed policy to charge for water polarized urban and rural sectors during the election. HorticultureNZ responded the day after Labour’s water tax announcement saying that the 1-2 cents/cube blanket tax would make “healthy food more expensive” and “would not deliver long term benefits to New Zealanders”.
This rapid response is normal for the ‘tooth and nail’ lobbying game but is there room for a broader and longer view? Using HB 5 year average data, we estimate the proposed water tax would cost grape growers $5-10/ha pa and apple growers $19-38 (Table 1.)
At these levels, the extra cost is negligible. If growers had to pay a nominal amount for water, they might use it more efficiently and even improve their economic performance by improving fruit quality and value. Grape growers are leading the way here, but imagine if all horticulture could develop an ‘IFP’ of water use (Integrated Fruit Production, synonymous with careful and justified use of inputs).
Imagine if this example could positively influence those who seriously over-use water and pollute our rivers and streams with nutrients and sediment. Could that lead the way to a long term benefit of environmental restoration that our grandchildren would appreciate?
“Water was a hot issue in the 2017 election as New Zealanders have become more concerned about the degrading state of our freshwater.”
Figure 5. Typical Soil Moisture & Irrigation for HB Grapes & Apples
Figure 4. Total average seasonal irrigation by HB apple and grape growers over the past five years
Figure 2. High variation Makoha site showing changes in soil type.
Figure 3. Low variation Scotland Farms site with a single soil type.