Maintain soil pH rates to optimise production

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Soil pH needs to be maintained at or above 5.5 in the topsoil (0 to 10 centimetres) and 4.8 in the subsurface (10 to 30cm). Levels below these pH targets will limit root growth and affect nutrient availability.

PHOTO: Evan Collis

Pure lime (calcium carbonate) has a 100-per-cent neutralising value and applied at 1t/ha will increase topsoil pH by about 0.7 on sands, 0.5 on loams and 0.3 on clays.

Lime profit calculator

The Lime Profit Calculator developed by the Liebe Group can be used to determine:

  • the amount of lime required to change pH;
  • cashflow and profit of different liming programs over 10 years; and
  • payback period of liming programs.

To avoid constraints to crop growth and yield, it is best to maintain pH at or above 5.5 in the topsoil and 4.8 in the subsurface.

If the topsoil pH falls below 5.5 there will be insufficient alkalinity to move down and treat subsurface acidification.

If the top and subsurface soil pH values are at or above target values, only maintenance levels of liming will be required to counter ongoing acidification due to export of alkalinity in crop and animal products (Table 1). These alkaline exports need to be replaced in the form of lime to prevent soil acidification.

The amount of lime required to lift pH will depend on the current pH profile, soil type, rainfall, farming system and lime quality. Pure lime (calcium carbonate) has a 100-per-cent neutralising value and applied at one tonne per hectare will increase topsoil pH by about 0.7 on sands, 0.5 on loams and 0.3 on clays.

Greater quantities of agricultural lime (which has a neutralising value less than 100 per cent) will be required to achieve these pH changes, with the actual amount dependent on the neutralising value of the lime.

A typical dryland cropping system in a winter crop-pasture rotation has an acidification rate equivalent to 25 to 345 kilograms per hectare per year of pure calcium carbonate.

This is the amount of calcium carbonate (in lime) that must be applied to keep up with the ongoing seasonal acidification caused by grain and animal exports (Table 1) and the inefficient use of acidifying nitrogen fertilisers (some of which is leached below the root zone).

Keeping up with the farming system’s natural acidification rate is called maintenance liming. However, the liming rates needed to keep up with the acidification rate can only be estimated so monitoring of the soil pH profile every three to four years is recommended to enable the liming program to be refined.

 Product removed
CaCO3 removed (kg) with each tonne of product
Cereal grain (wheat, barley, triticale)
8
Cereal – whole tops
20
Canola grain
2
Lupin grain
20
Lupin – whole tops
60
Lucerne hay
60
Sheep dung
25
Sheep – lambs
3
Sheep wool (6kg/sheep)
0.4

Recovery liming

If the topsoil pH is below 5.5, recovery liming is recommended to prevent the development of subsurface acidity, even if the subsurface pH is at 4.8 (Table 2).

If the subsurface pH is below 4.8, liming to maintain (or recover) topsoil pH at or above 5.5 is essential and subsurface pH should be monitored in three to four years so that the liming rates can be adjusted if insufficient alkalinity has moved down to treat the subsurface acidity.

Soil depth (cm)
pHCa Lime amount over five years (t/ha)
0–10 <5.0 2
0–10 <5.5 1
                                              Add to
 10–20  <4.5  2
 10–20  <4.8  1
                                              Add to
 20–30  <4.5  1
 20–30  <4.8  Measure pH in 3 years

* Table 2 indicates of the amount of lime that may be required on sandy soils over five years to achieve a pH above 5.5 in the topsoil and 4.8 in the subsurface after 10 years. Increases in pH will depend on soil type, rainfall, lime quality and quantity applied and other farming practices, as well as the soil pH profile. Expert advice should be sought for individual recommendations as ongoing acidification resulting from agricultural production will require additional lime.     

SOURCE: DAFWA

More information:

Liam Ryan, DAFWA,
08 9622 1902,
liam.ryan@agric.wa.gov.au

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GRDC Project Code DAW00236, DAW00222

Region National