Results!

The interpretation of soil organic carbon is fraught because the initial measurements were made after sowing (hence soil throw is a factor in No Till paddocks but less so in Till paddocks). Apart from Patchewollock, the No Till paddocks have tended to have lower organic carbon, and these patterns have been reinforced by measurements at harvest in 2007. For the paddocks with decreases in organic carbon, there is a negative relationship with change in topsoil mineral nitrogen over a similar period (sowing 2006 to sowing 2008), perhaps indicating that soil organic matter in No Till paddocks is being mineralised to provide nitrogen.

Table 13. Soil organic carbon 0-10cm at emergence 2006 and harvest 2007.

Soil mineral nitrogen differences in the systems will relate to differences in mineralisation, leaching, nitrogen application (fertiliser/legumes) and removal. Surface soil nitrogen was generally maintained at a higher level in the No Till paddocks across all measurements (Table 8). This may reflect different dynamics of mineralisation (eg. more before-sowing with cultivation, more in-season with No-Till).

In root zone measurements none of the paddocks were especially high in nitrogen (Table 9), indicating a lack of chemical or physical limitation, but also potentially a limit to yield in higher rainfall years. The stability of the sowing measurements was interesting – there was very little change from year to year despite harvest measurements often showing evidence of use during the crop phase. This may reflect mineralisation of soil organic matter buffering any depletion by crop offtake, rather than fertiliser nitrogen being sufficient.

The subsoil was often very low in nitrogen (Table 10). The Till paddocks were often (not always) an exception to this, containing high nitrogen at sowing at Sea Lake, Yaapeet and Culgoa.

Table 8. Soil mineral nitrogen 0-10cm at sowing in 2006-8.

Table 9. Soil mineral nitrogen 10-70cm at sowing and harvest in 2006-8.

Table 10. Soil mineral nitrogen 70-100cm at sowing and harvest in 2006-8.

Soil EC1:5 is a measure of salt content. To put it into perspective, the salt in one year’s rainfall for the Mallee (11 kg/ha) is enough to raise the EC1:5 of 10cm of soil by 0.002 dS/m. Apart from sampling error, most of the change in EC1:5 is likely to be caused by water movement taking salt with it from depth to depth.

Soil EC1:5 was not measured at every sampling, but did change much more from measurement to measurement than was initially expected. Topsoil EC1:5 at harvest in 2007, following a relatively wet autumn/winter, was much lower than measured at sowing in 2006 or 2008 (Table 5). Sowing 2008 measurements were generally higher than 2006, particularly at Minyip. It was difficult to relate changes in topsoil EC1:5 to particular systems or crops; rather the pattern seemed to be consistent for a particular pair of paddocks (probably reflecting the EC1:5 of water stored deeper in the profile).

There were some striking changes in rootzone EC1:5 measurements, particularly increases between Sowing 2006 and harvest 2007, and harvest 2007 and sowing 2008 (Table 6). It is tempting to relate the more dramatic changes in EC1:5 to water use patterns (eg. Table 3) but they are not completely consistent – the large increase in Donald No Till is paired with high in-crop water use (after high in-fallow water storage), but a similar increase in the Culgoa Till paddock is not related to similar water use.

Unfortunately many of the subsoil measurements are missing. There is a large fall in the Sea Lake No Till subsoil that helps to explain the rise in the rootzone measurement (Table 7). The Culgoa and Donald Till paddocks and the Minyip paddocks all show concerning increases in EC1:5 that may also be related to the draw-up of water from deeper down.

Table 5. Soil EC1:5 0-10cm at measurements in 2006-8.

Table 6. Soil EC1:5 10-70cm at measurements in 2006-8.

Table 7. Soil EC1:5 70-100cm at measurements in 2006-8.

There was no obvious pattern in surface soil water between treatments, although it did change in consistent ways between measurements (Table 2). Soil water in the root zone (10-70cm) tended to be higher in the ‘Till’ paddocks following cereal crops in northern paddocks after the drought year of 2006 (Table 3). This probably reflects generally higher clay content in these paddocks. A similar pattern in all paddocks at harvest in 2007 reflected fallow at Sea Lake and Donald, and Chickpea crops at Yaapeet and Minyip No-Till paddocks (late-season water use). It is difficult to interpret changes in water content during summer fallow without also looking at rainfall patterns (and differences between widely spaced paddocks) but, apart from textural differences, some farmers seem to be able to make better use of fallow rainfall in 2006 (eg. Yaapeet Till and Minyip No-Till). Differences in 2007 need to be interpreted with consideration of fallows in 2007.

The “subsoil” depth (70-100cm) was less regularly sampled at some sites because of dry soil – this indicates the state of the subsoil where a sample is missing! The same pattern of more water (clay) is present in the northern paddocks as in the southern ones (Table 4). Notable features at this depth are the relative lack of water use at Culgoa and substantial water use in the Donald ‘Till’ paddock during the 2006 crop. Most paddocks gain water at this depth in 2006/7 fallow except at Culgoa, but there is little water use in 2007 crop except at Minyip. There is substantial water loss during the 2007/8 fallow at Yaapeet and the Patchewollock Till paddock (no record for the No-Till paddock), probably reflecting ‘draw up’ from the layer above.   

Table 2. Soil water 0-10cm and change in soil water during the crop (Harvest – Sowing) and fallow (Sowing – previous year Harvest) periods.

Table 3. Soil water 10-70cm and change in soil water during the crop (Harvest – Sowing) and fallow (Sowing – previous year Harvest) periods.

Table 4. Soil water 70-100cm and change in soil water during the crop (Harvest – Sowing) and fallow (Sowing – previous year Harvest) periods.