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The Agriculture Victoria Pasture Soil Moisture Monitoring project provides real time soil water content and temperature data, using soil moisture probes. Monitoring sites have been installed by Agriculture Victoria on a range of pasture and soil types in the medium to high rainfall zones. The sites record soil water content at one source point from 10 cm down to 80 cm as a reference point for a paddock. As a pilot project, how the data can be used is still in development, but is expected to have application in the dairy, beef and sheep grazing systems in the future.
This newsletter is the first of a series of seasonal updates on what is happening at these sites and we hope it will provide some insights into how to understand and use the information collected.
Dale Boyd and Jane Court
In this Issue
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| Locations of Agriculture Victoria Pasture soil moisture monitoring sites |
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To access information provided in the graphs, log onto the website:
www.INTELLiWEB.mait.com.au Login and password are both: agvic
Dairy sites are at Longwarry, Yarram and Jancourt.
Beef and sheep sites are at Baynton, Greta, Pigeon Ponds, Dartmoor, Lawloit and Bairnsdale.
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| Soil moisture across the state |
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The BoM map gives a statewide picture of soil moisture in mid October, as estimated by satellite imagery. The darker the blue, the wetter the soil. Imposed on this is some data from a number of the Agriculture Victoria soil probes and also from some probes owned by the Perennial Pasture Systems group around Ararat. Sites that did not have enough winter rainfall to fill the soil profile, and hence could not provide a measure of full water capacity are not included. The colour of the marker at each site indicates whether the soil is wetter, drier or the same as this time a month ago. The percentage figure indicates how wet the soil is (proportion of full capacity) and the depth to which plant roots are accessing moisture is in centimetres. A range indicates more than one pasture at this site. For example, at Harrow, the soil is significantly drier than a month ago, soil
moisture is 69% of full capacity and plants are accessing water to 50 cm.
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The late spring rainfall that fell over much of the east of the state in October, generally managed to halt (briefly) the fast declining soil moisture reserves at all locations as pasture plants drew on these reserves.
Areas that did not fill the moisture profile over winter are experiencing poor spring growth. Examples of these areas are the sites at Bairnsdale in Gippsland and Greta in the North East Victoria, further illustrated in the individual site graphs below.
The second lowest September rainfall on record in Victoria has seen all pasture soil moisture monitoring sites deplete soil moisture reserves.
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| Central Victoria - Baynton |
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The Baynton soil moisture monitoring site on basalt soil (phalaris pasture) shows that field capacity was reached in early August where moisture infiltrated down to 80 cm. Since early September the moisture profiles declined with limited rainfall. Moisture was being drawn from 40 cm before 10 mm rain on October 10, which started to fill the moisture deficit but did not boost soil moisture levels.
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The soil moisture monitoring site at Greta, growing phalaris, peaked with water infiltration in early August down to 50 cm. As infiltration did not improve past this depth, full water capacity was not reached and so neither full field capacity or the plant available water zone could be set. Since early September the moisture profiles have declined with little rainfall. Moisture was being drawn from 50 cm before rain on October 10 and so without further rain, the phalaris will run out of moisture soon.
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The Greta site sown to Arrowleaf clover, shows that near field capacity was reached in early August where moisture infiltrated down to 80 cm. Moisture infiltration was better compared to the phalaris paddock with the same rainfall, probably because the clover was still establishing roots. The phalaris has a well developed and deeper root system and so was able to use the moisture down the profile. Since early September the moisture profiles have declined with limited rainfall. Moisture was being drawn from 60 cm before rain on October 10.
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The soil moisture monitoring site at Bairnsdale, growing perennial pasture, has had limited moisture infiltration this season due to low rainfall. Moisture moved down to 40 cm which occured in June. As water infiltration did not improve below this depth, full water capacity was not reached and and so neither the the plant available water zone or field capacity could be determined. With little soil moisture at depth, there has been limited soil moisture to draw from in September.
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The soil moisture monitoring site at Dartmoor growing lucerne shows that field capacity was reached in mid-June where moisture infiltrated down to 80 cm. Since early September the moisture profiles have declined slightly but rainfall has largely kept up with plant water demands. Moisture was being drawn from 10 cm and the soil appears to have a big reserve of moisture to draw from this spring which may last until summer. Lucerne will be able to respond to summer rain if it falls and the moisture probe data will be a valuable tool to determine water infiltation and potential pasture growth for grass fed finishing cattle.
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| Early learnings at Dartmoor |
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Andrew Speirs (site co-ordinator)
The soil moisture probes, in conjunction with Agriculture Victoria's Dale Grey’s Seasonal Outlook modelling, are proving very powerful information tools especially given that they also supply us with soil temperature. Combined they have given us a better look into the potential pasture growth and hence livestock performance such as slaughter turn off dates, condition of cows for re-joining or meeting heifer joining weight targets for example. Over winter we could see the soil profile wet up and knew when moisture was non-limiting. So depending on the timing of having that available moisture and the potential to grow pasture e.g. whether soil temperature is limiting, we can then choose to do nothing if feed is sufficient or apply gibberellic acid or urea depending on the need to fill a feed gap.
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Monitoring the plants drawing moisture from all depths has shown us that soil conditions are favourable (i.e. pH okay, salinity not an issue, soil structure not an impediment). The spring monitoring is showing us that soil moisture at 0-10 cm and 10–20 cm is being drawn quickly and hence the local rule of thumb of needing roughly 20 mm rain per week in spring to maintain strong pasture growth looks to be about right in this environment. The moisture storage bucket isn’t huge in these soils, especially when pastures are doing 80 kg per day growth rates.
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The soil moisture monitoring site at Lawloit, (growing barley undersown with lucerne) saw water infiltration peak in early August with moisture down to 80 cm. The plant available water zone has not been set with the installation occuring in winter in wet conditions and the site is yet to dry down to wilting point but is expected to reach this in late spring. This site is a non wetting sand and has been clay lined to assist the water holding capacity. On this soil type, with a recent 30 mm of rain, water infiltrated down to 50 cm.
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| South West - Harrow (Pigeon Ponds) |
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The site at Pigeon Ponds (Harrow), with perennial ryegrass, shows that field capacity was reached in mid-July where moisture infiltrated down to 80 cm. Since mid-September the moisture profiles have declined slightly with moisture drawn from 10 cm. This site appears to have a good reserve to draw from this spring but it is unknown how quickly this soil will give up the freely available water in the absence of rain. The wilting point was established in the autumn after the dry summer. Ten millimetres of rain on 10 October infiltrated down to 10 cm.
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Soil temperature is also measured at each 10 centimetres along the probes to 80 cm. While temperature is of less value to croppers in the warmer north of the state, it is and has been a major limitation to plant growth in many sites of Victoria that have had adequate or good soil moisture, over winter. Soil temperature at all sites dropped over May – July to ranges between about 5 and 8 degrees. The coldest sites were in North East Victoria (Greta) and Central (Baynton) where temperatures in the top 10 cm fell below 5 degrees on some days. Soil temperature, like moisture and fertility, plays a major role in controlling plant growth. Plant species have different temperature requirements and so the soil temperature at sites can be monitored to note when this will start to limit plant growth and legume nitrification. White clovers (and presumably sub clover) will stop fixing
much nitrogen below about 9 degrees which can provide some opportunities for responses to urea. Research in Tasmania indicated that perennial ryegrass will grow down to about 5 degrees and annual ryegrass as low as 2 degrees. Australian phalaris has shown to also grow down to about 5 degrees and a ‘guesstimate’ for more winter active cultivars is 1.5-2 degrees lower.
The effect of frost?
The graph below shows soil temperature in the top 10 cm of the granite soil site at Baynton, with minimum and maximum daily air temperatures. The dotted line at 2 degrees represents the frost line so several days in July experienced frosts at this site. It appears that soil temperature has been more affected by daily minimum temperatures than maximum as soil temperatures dropped below 5 degrees only on days with quite severe frosts and did not drop as much on the very cold days on the 7 and 11 July when maximum temperatures didn’t get above 9 or 6 degrees respectively. While soil temperature started to rise with warmer days in early September, a significant frost and snow event on 16 September sent this back down to below 10 degrees. As a result, paddocks with adequate soil moisture had the opportunity to respond to giberellic acid in mid winter and to urea.
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