Tony White, Miling: lime incorporation by spading
Average growing season rainfall: 230mm
Enterprise: Cropping and livestock
Dominant soil: Yellow sand plain
After extensive subsoil testing for pH, Tony White recognised a significant acidity constraint in the 10-30cm layer. Tony decided to trial incorporation by spading after surface application of limesand to one of his paddocks to speed up the amelioration of the subsurface layers.
Trial treatments and management
In 2012, limesand was spread on the surface of the paddock at 3t/ha. The paddock was then spaded to 35-40cm depth, leaving an area approximately 40m x 150m unspaded. The paddock was managed as usual and sown to wheat (Figure 1).
Effectiveness of lime incorporation
Visual inspection of small soil pits (stained with universal pH indicator) showed that limed topsoil was reasonably well incorporated by the spading treatment (Figure 2). The distribution of lime was uneven, but there was enough limed soil throughout the acidic 10-40cm layer to provide pathways for roots to grow into the nonacidic soil below. This is reflected in the large spread of pH values at each depth from the replicate samples (Figure 2).
The soil pH profile of the undisturbed limed area was less variable and showed that there was a significant barrier to root growth from 10cm down to 40-50cm with average soil pHCa of 4-4.3. Beyond this, soil pHCa increased to around 5 where aluminium toxicity would not constrain root growth.
Root growth and water usage
In early November 2012 when the crop was ready for harvest, roots in the unspaded profile were present at a depth to 20-25cm. In the spaded profile, many more roots were observed to a depth of 40-50cm (Figures 3). Detailed soil sampling showed that the spaded profile was dryer to a depth of 1m (Figure 4). The crop in the spaded area had been able to access and use subsoil moisture to considerable depth.
Deeper rooting depth, and therefore access to more subsurface moisture, resulted in significantly greater grain yield (Figure 5). The value of the extra 0.7t/ha of wheat is enough to have paid for the lime and spading and the benefits of the improved profile will be ongoing.
Tony White's work demonstrates how incorporation of lime by spading can improve crop growth on an acidified soil profile.
Lime applied to the surface at 3t/ha was sufficient to bring the 0-10cm soil pHCa to almost 5.5, which would have improved microbial activity and nutrient availability in this layer. In the first year after liming, where the soil was not spaded, the subsurface soil was still very acidic.
Over time, some alkalinity will move down the profile and improve the subsurface soil pH. However, this will take many years and require approximately 5t/ha more lime to be applied over the next 10 years to recover the subsurface pHCa to 4.8.
Incorporation of the limed surface soil by spading resulted in a highly variable pH in the subsurface soil but provided pathways of higher pH down to 40cm. Roots were able to grow down through these pathways to access subsoil moisture. Tony will need to apply lime to the surface in the future to counter the ongoing acidification that is an inevitable part of agriculture.
Tony did not set out to do a scientific study and there were no areas in the paddock that had no lime and no spading or no lime with spading for comparison. Spading is known to have effects such as removing compaction and distributing nutrients and organic matter.
Therefore, we can't attribute all the improved growth to an improved pH profile. However, there was no evidence of compaction and the root growth and soil moisture observations strongly indicate that pathways of improved soil pH were primarily responsible for the improved crop performance on the spaded areas of Tony's paddock.
Incorporation of surface-applied lime by spading is a good option to recover very acidic profiles on deep sands.