Productive Pastures on Light Soils
Much of the light soils on the Southern Tablelands is considered “sheep country” and is well known for its production of fine wool. Many of the pastures are based on native grasses with sub clover and, while adequate for wethers growing wool, do not adequately support production systems like steer fattening or cattle breeding that are more demanding.
Pasture production on the Tablelands has a definite winter “trough”. Cattle production is usually centred on calving in spring and selling offspring in autumn after weaning. At Cadfor our young cattle make little growth between April and August even when supplemented with hay or silage. Cows and calves require better pastures than native grasses if they are to approach their genetic potential.
Producing pastures of higher quality during the warmer months and reducing hand feeding during winter are challenges which we are undertaking.
Acid soils
The light soil country in the Southern Tablelands, of which Binda is typical, now has very low pH and toxic aluminium levels. Only about 5% of the area is considered arable and much of it is very prone to erosion.
Establishing clover based pastures and perennial grasses such as phalaris has become very difficult. Acidic conditions have a deleterious effect on the rhizobia which form the nodules on legume roots and utilise atmospheric nitrogen to create protein. Phalaris, our most productive and persistent perennial grass, is not tolerant of high aluminium levels or acidic soils.
The usual recommended solution is to incorporate lime to increase the pH in the root zone but this risks erosion. A suggested alternative is to select plants that are more tolerant of acid conditions.
We seek the best long term practical solution which provides a pathway to regenerating the health of our soils or at least to stop further degradation.
What were the soils like before conventional agriculture changed the landscape? The trees in the area were predominately Yellow Box which prefers a pH in the range 5.4 to 6.5 with no aluminium. Soil tests conducted at Binda cemetery are pH 6. Clearly we started with a pH around 6. [All pH readings are in 0.01 molar Calcium Chloride.]
Much of the soil in this area now has pH of 4.2 with aluminium of 30%. The graph[i] below indicates that the decline in pH has slowed which might suggest things are stabilising. This is incorrect because pH is a logarithmic scale so that a decrease in pH from 6 to 5 is a 10 fold increase in hydrogen ions and decrease from 5 to 4 is another 10 fold increase in hydrogen ions. Since settlement of the area the pH has dropped from 6 to 4 which is a 100x increase in acidity. The second schematic graph more correctly displays what has happened to acidity.
If we deal with the acidity by selecting acid tolerant species we are doing nothing to arrest the continuing increase in acidity and eventually this approach will fail[ii]. To retain ground cover and to address increasing acidity, the pasture base must comprise deep rooted perennial grasses.
Advice is commonly given on the Southern Tablelands that liming has no effect on pasture production. This is based on a trial[iii] where a comparison between limed and unlimed pastures produced similar amounts of pasture. However, in the unlimed plots the deep-rooted phalaris was replaced over time by Yorkshire Fog, Vulpia and sorrel which are annuals or shallow rooted. The limed plots retained the perennial species. The advice not to lime ignores the potential loss of production in summer and winter if annual species dominate the pasture.
Work by Hackney et al[iv] has shown that establishment of clovers in soils with pH less than 4.6 is ineffectual because of the failure of nodulation.
While incorporation is the best and quickest method of increasing pH at depth, topdressing with lime has been shown to be effective in light soils in high rainfall areas within 3 years[v]. Other research suggests that topdressing with gypsum accelerates the movement of lime and increases pH at 10 cm. We intend to trial these options to see what will work best for us.
We plan to stop the ongoing acidification of our soils and to return the pH of our topsoils to above 5.
Phosphorous
The soils in the district are inherently low in phosphorous and supplementation with superphosphate gave excellent results initially. Over time this increased production has been lost due to a number of factors:
- Loss of superphosphate subsidy (the super bounty)
- Increase in price of fertilisers
- Export of product such as hay and wool leading to deficiencies of potash limiting production.
- Increased acidity reducing the effectiveness of clover to provide nitrogen.
Many soils have very low Colwell P levels, below 10 ppm is quite common. The graph below shows the response in clover with increasing P levels.
We have a number of concerns with this graph and the way these data are interpreted.
- The “Critical Level” at which 95% of spring growth is obtained has become the “Target Level” when it should be considered the “Minimum Level”.
- On light soils, where phosphorous is not highly bound to clay particles, the P level has been shown to drop by up to 10 ppm in the course of a year which would greatly reduce production. Prudent management would suggest that a buffer of 10 ppm above the Critical Level would be advisable.
- This graph is only relevant for sub clover pastures with native grasses. Research in other production systems such as irrigated phalaris / clover has shown the Critical P level to be 70 ppm or above.
In production systems other than running Merino wethers for wool production, such as cattle breeding, the sub clover dominant pastures are far from ideal:
- Wethers might do well in summer on dead subclover plants and seed but lactating cows do not.
- Dominant subclover increases the bloat risk in cattle.
- The clover dominant pastures and the nitrates they produce tend to increase soil acidity
- In summer the bare ground created by the subclover dominance can lead to soil erosion and weed incursion.
Contrary to opinions given above, accompanying this graph was a statement that there is no point in exceeding the Critical P level.
We believe that these points are not supported by research.
i. In the basic production system on which the trial was run, this statement may be true. However, this mantra has been repeated for all pasture systems in the Southern Tablelands when practical and research experience suggests that a higher P level may be beneficial. For a simple example, in irrigated nitrogen fertilised pastures the Critical Level may be closer to 70 ppm[vi].
ii. Research evidence is that the response of P level to P applied is a straight line relationship. The more P applied, the higher the Colwell level and it does not level off. The statement is wrong.
iii. Research shows that as long as ground cover is maintained there is no lateral movement of P into the wider environment. In fact, by increasing P level and improving pastures and ground cover there will be less movement of P compared to pastures where there is bare ground.
To ensure that our pastures are not being limited by the phosphorous supply, we aim to maintain a Colwell level around 50 ppm.
[i] Clements B, Keys M, Schumann W (2005) Landscan Manual, session 4, page 5. ISBN 0 7347 1627 3.
[ii] Scott BJ, Ridley AM, Conyers MK (1997) Soil acidity in non-arable permanent pastures, in the Amelioration of acidity in non-arable soils of the tablelands and slopes. Ed Scott BJ, NSW Agriculture.
[iii] Keys, M. & Darby, M. (1997) Lime Responses from Prime Pasture Sites, in the Amelioration of acidity in non-arable soils of the tablelands and slopes. Ed Scott BJ, NSW Agriculture.
[iv] Hackney B, Jenkins J, Powells J, Edwards C, Orgill S, DeMeyer S, Edwards T, Howieson J and Yates R (2017), Legumes and Nitrogen – it’s time to stop assuming. Proceedings of the 30th Conference of the Grassland Society of NSW Inc 44-50
[v] Kelso HG, Clements B, Vimpany I (1997) Lime topdressing demonstrations – Bathurst and Oberon in the Amelioration of acidity in non-arable soils of the tablelands and slopes. Ed Scott BJ, NSW Agriculture.
[vi]Euie Havilah, Helena Warren, Roy Lawrie, Ashley Senn and Paul Milham (2005) Fertilisers for Pastures, NSW Department of Primary Industries, ISBN 0 7347 1646 X
