Precision soil pH mapping

Throughout South Australia there are four to five million hectares of agricultural land prone to soil acidification.

Soil acidification is a natural process but is accelerated by productive and intensive farming systems due to nitrogen fertilisers and product removal and is now emerging in previously unaffected areas throughout much of the low to medium rainfall cropping areas of SA. Once the soil pH falls below pH 5.5 (CaCl2) it can have a considerable effect on crop and pasture growth.  Soil pH can vary significantly across paddocks and down the soil profile. It is estimated that soil acidity causes a production loss of about $88 million per year in SA (Department of Environment and Water).

Lime is the most effective and economical method for the treatment and prevention of acid soils. Previously, the amount of lime required for a paddock has generally been based on a single soil test with the lime applied as a uniform (blanket) rate across the whole paddock. In recent years, the cost of lime and freight has significantly increased.

Precision soil pH mapping is a new and innovative technology in which machines are used for measuring and mapping soil pH variation across the paddock. The maps created identify soil pH zones within a paddock that allow appropriate rates of lime to be calculated for each zone. This not only results in better soil pH conditions for crop and pasture growth through targeted lime applications but in most cases can reduce the cost of lime applications.

There are now a number of soil sampling machines that are commercially available for mapping soil pH. This includes Veris® machines and quad bikes or ATV’s with soil sampling units.

Veris® machines

Veris® Technologies Pty Ltd (USA) have developed two types of machines for soil pH mapping. They are the MSP-3 machine that can be mounted on the back of a tractor or towed by a 4WD or the U3 series that can be towed by an ATV.

In 2015, PIRSA invested and imported a MSP-3 from the USA (Figure 1). This was one of the first machines in SA.

There are now four businesses owning and operating Veris® machines in SA.

Figure 1: Veris® MSP-3 soil pH machine

As these machines are towed across the paddock at about 11 kph they take a soil sample on-the-go, measure the soil pH from direct soil contact with antimony electrodes and record its geographic position.  After each sample the electrodes are washed with high pressure water to avoid contamination between one point and the next.  

The machine takes a sample at every 30 metres along the row but the total number of points sampled per hectare depends on the swath width. At a swath width of 36 metres wide the soil pH machine tests about 8 -10 samples per hectare. For a 50 hectare paddock this equates to about 500 sampling points. Approximately, 20 hectares can be sampled per hour.

Quad bikes or ATV with soil sampling units

Soil sampling units can be mounted onto quad bikes or ATV’s to take soil samples.  A number of organisations are now offering a soil service where soil samples are taken across the paddock (down to a depth of about 10cm or more) on a pre-determined geo-referenced grid basis.  All the work is done from the driver’s seat. Due to cost–effectiveness sampling is generally done on one to two hectare grid basis. The soil samples are then sent to a laboratory for a range of analyses including soil pH and the data is then used to produce maps

Soil pH maps

All the data from the Veris® MSP-3 machine are converted to pH (CaCl2). Once the data has been downloaded into a software mapping program then the ‘dot’ pH map is converted to a pH (contour) map of the paddock.  Figure 2 shows the map from the Veris® MSP-3 on a 51.25 hectare cropping paddock.  The map shows that there is a large variation in soil pH (CaCl2) from 4.4 to 7.1.  The optimum soil pH for plant growth is between 5.5 and 7.5 (CaCl2).

       Figure 2: Soil pH (CaCl2) contour map              

Figure 3: Lime Prescription map (t/ha) (Rounded)

Lime application

 From the soil pH maps, lime prescription maps can be prepared. Figure 3 shows the rate of lime for each soil pH zone to raise the soil pH to pH 5.5 (CaCl2) for a loam to clay loam. The map shows that 31 percent of the paddock requires no lime and that the remainder of the paddock requires a variable rate of lime from about 1 to 4 t/ha.  The lime prescription maps can be converted to an application map (lime rate in kg/ha) and the shape files sent to the spreaders for variable rate spreading.    

 Table 1 shows the cost savings (from the above maps) by applying a targeted rate of lime rather than a uniform (blanket) rate of lime.  Taking into account the cost of soil pH mapping there has been a cost savings of $2,204 or $43 per hectare. 

Table 1: Economics of liming

 Case studies have shown that the cost savings of applying the appropriate amount of lime for different areas of the paddock compared to applying a uniform rate to the whole paddock can be in the order of about 30 percent or more. The highest cost savings are on those paddocks with a high degree of variability, particularly those with a large proportion of soils above pH 5.5 (CaCl2). In some cases, more lime may be required in large areas with highly acidic soil, but the cost will be out-weighed by the improvement in productivity.   

 Mapping early in the year (e.g. Feb– April) provides the opportunity to apply lime well before seeding.                          

Summary

The use of soil pH precision mapping is increasing in SA. Soil pH mapping and the identification of pH zones will enable more accurate targeting of lime applications. This will not only help to save costs but will also result in improved soil pH conditions over the paddock and an overall improvement of crop and pasture productivity.  

For further information contact Andrew Harding on 0417 886 835 or Brian Hughes 0429 691 468 Department of Primary Industries and Regions.