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Soil acidity : a guide for WA farmers and consultants

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The Chief Executive of the Department of Agriculture and Food and the State of Western Australia accepts no liability, whether negligent or otherwise, arising out of the use or release of this information or any part of it. This guidance was produced as part of the Wheatbelt NRM soil acidity project 'Optimizing soil pH for sustainable agricultural practices', delivered jointly by the Department of Agriculture and Food, Western Australia and Precision SoilTech. Estimates of wheat belt production losses due to acidity range from $300-400 million, or about 10 percent of the annual crop.

In the Avon River basin (Figure 1), approximately 80 percent of the surface soil and almost half of the subsoil are below the pH targets recommended by the Department of Agriculture and Food, Western Australia (DAFWA) (Figure 2). The only way to diagnose soil acidity is to take soil samples and test the pH. While liming to combat soil acidity should be part of normal agricultural practice throughout much of the wheat belt, accurate knowledge of soil pH allows for more accurate management decisions.

Figure 1 Location of the Avon River Basin and wheatbelt towns referred to in this guide
Figure 1 Location of the Avon River Basin and wheatbelt towns referred to in this guide

Management of soil acidity

Diagnosing soil acidity

Prepaid commercial soil sampling kits are available and may be the most convenient method for the do-it-yourself approach as they include complete instructions, sample bags, postage and laboratory testing of the samples. These kits are primarily aimed at topsoil sampling and testing for fertilizer recommendations, but the subsurface 10–20 and 20–30 cm layers can be sampled for pH at the same time (a suitable sample tube, or commercial soil sampling contractor who can accurately sample the subsoil, will need to be used ). Field testing with a hand-held pH probe can indicate areas that require accurate soil sampling and testing.

When field testing soil pH, it is usually more convenient to use deionized or distilled water instead of 0.01 M calcium chloride and so the results will need to be converted by subtracting 0.7. It is important to store the probe in good condition and calibrate it with standard pH buffer solutions each day it is used. Soil pH test kits that use indicator solutions and dye to estimate pH are inexpensive and easy to use.

Monitoring soil pH

Liming acidic soils

Target pH

Soil acidification is an inevitable and ongoing consequence of productive agriculture. Whether soil becomes acidic depends on how well ongoing soil

The amount of lime required will depend on the current pH profile, soil type, rainfall, the amount of lime required will depend on the current pH profile, soil type, rainfall, farming system and lime quality.

Maintenance liming

Recovery liming

Improving liming recommendations

The correct amount of lime will be added at three locations, but at locations 2, 4 and 6 the subsoil will continue to acidify. Option 3—considering pH values ​​from and 20–30 cm, lime applied to target areas at recommended rates The total amount of lime applied would be 1.8 times more than Option 1. The method used would be to immediately spread 2 t/ha on all areas that require liming.

Then, over two to four years, 1 or 2 t/ha will be distributed on the areas that require 3 or 4 t/ha over the 10-year period. Re-sampling of the soil in three to four years will allow adjustment of liming rates as needed. Where only the topsoil pH values ​​are considered, the recommended lime application rate can only be given as a range because the pH of the subsoil can be anywhere from ideal to extremely acidic.

Table 4 Soil pH at six sites in one paddock (Figure 9) and the recommended liming rates when only  pH of topsoil or pH of all three depths is taken into account
Table 4 Soil pH at six sites in one paddock (Figure 9) and the recommended liming rates when only pH of topsoil or pH of all three depths is taken into account

How to apply lime

Research has shown that successful direct injection is possible (Figure 12, right) and subsurface acidity can be quickly removed as a production constraint. When the distribution of lime is correct, responses of 20-30 percent in wheat are common. Poor distribution can result in the lime being placed under an untreated acidic layer (Figure 12, left) and root growth will still be limited.

Poor distribution of the lime (left) is ineffective as it leaves an untreated acidic layer above that continues to form a barrier to root growth. A good substrate distribution of the lime (right) is difficult to achieve; the presence of additional surface lime can also be seen in the topsoil.

Figure 11 Extensive modification of machinery  is required for good distribution of deep-placed  lime.
Figure 11 Extensive modification of machinery is required for good distribution of deep-placed lime.

Timing of liming

Lime and general nutrition

Complementary strategies

Acidification rate

Tolerant crop and pasture species

Benefits of liming

Sustainable production and resource protection

Off-site impacts

Profitability

Production benefits

Biomass was 70 percent greater in the 2 t/ha lime treatment plot than in the no-limit plot. Soil After being above the target soil pH for many years, the soil pH for the lime plots (2 t/ha lime in 1996) has now returned to pre-lime 1996 pH levels, indicating that the applied lime has been used completely. Where no lime was applied, the soil profile continued to acidify (Table 8) and aluminum levels are now toxic (Table 9).

Aluminum rule of thumb: < 2 ppm Al non-toxic, 2-5 ppm toxic to sensitive species, > 5 ppm toxic to tolerant species. Lime was surface applied to plots at 1, 2.5, and 5 t/ha, control plots were unrestricted, and all plots had three replications. The differences are considered more meaningful as a productivity loss of between 7 and 17 percent in insufficient or unrestricted plots.

Treatment differences of at least this magnitude were commonly observed by the farmer during the lifetime of the trial. Applying 5 t/ha of lime in one year is unlikely to be the most efficient application strategy due to the cost of incurring the expense up front. A more effective strategy would be to apply enough lime, in several applications, to treat the soil acidity over a longer period of time.

Soil In the long term, where sufficient lime has been applied (the treatment of 5 t/ha), the acidic subsoil has improved (Figure 22). However, to restore the soil pH profile for the other treatments, a much higher amount of lime (7–10 t/ha) and time (10–20 years) is required. The Tammin tennis courts provide a peculiar field demonstration site (Figures 23 and 24) showing the impact of continuous fertilization and product removal on acidification and the response to the lime used to mark the lines on the old grass tennis courts (Table 11 ).

Table 7 Basic economic analysis of liming on  wheat rotations at Bindi Bindi
Table 7 Basic economic analysis of liming on wheat rotations at Bindi Bindi

Chemistry and biology of

Chemistry and biology

Soil pH

Soil pH buffering

Measurement of pH

Effects of soil acidity

In WA, the major problem when soils acidify is aluminium toxicity in the subsurface soil. Low pH in topsoils primarily affects nutrient availability and decreases

Aluminium toxicity

Liming of soil to increase soil pH is effective in reducing the availability of aluminum to non-toxic levels. The seedlings on the left were grown in soil that was limed to raise the pH to 5.1; aluminum concentration < 2 ppm. The seedlings on the right were grown in the same soil without lime at a pH of 4; aluminum concentration 15 ppm.

Measuring aluminum in soil solution is complicated and affected by many factors. A rough guide to aluminum levels can be obtained by measuring the aluminum concentration in the same 0.01 M CaCl2. The measurement of aluminum in topsoil is further complicated by the presence of higher levels of organic matter because aluminum can bound to organic matter (and therefore in a non-toxic form), but released when extracted with 0.01 M CaCl2 solution.

In most cases, the pH of the subsurface soil will be a good indicator of toxic aluminum levels.

Figure 30 Healthy root tip (left) compared to a root tip affected by aluminium toxicity (right)
Figure 30 Healthy root tip (left) compared to a root tip affected by aluminium toxicity (right)

Nutrient availability

Soil microbial activity

Causes of soil acidity

Soil acidification occurs naturally very slowly as soil is weathered but is accelerated by productive agriculture. Soil acidifies because the concentration of hydrogen ions

Nitrate leaching

Export of produce

Agricultural lime guide

Agricultural lime guide

Lime sources

Limesand

Limestone

Dolomitic lime

Other liming products

Lime quality

The key indicators of agricultural lime quality are neutralising value and particle size, regardless of the source. While the quality of agricultural limes can vary

How lime works

H 2 OCO2

Neutralising value

Particle size

Cost-effectiveness of limes

Comparing limes

Lime comparison calculator

Worked example

WA lime suppliers

XYZ LIME SUPPLIERS

Common questions

Common questions

They are natural, mined products composed mainly of calcium carbonate with some magnesium carbonate (Figure 50). Quicklime or quicklime is produced by heating (burning) calcium carbonate materials, such as limestone, to over 825 °C, causing chemical changes and forming calcium oxide. Hydrated or slaked lime is made by adding water to calcium oxide, forming calcium hydroxide.

Calcium oxide and calcium hydroxide are not recommended for use as agricultural lime because they are difficult to handle and store. They are corrosive and can burn the skin if contacted and cause respiratory problems if inhaled. Quicklime is unstable and will revert to calcium carbonate when exposed to air.

Lime with a higher neutralizing value will treat more soil acidity and lime with a higher proportion of fine particles will treat soil acidity faster.

Figure 50 Mining limesand—limesand, limestone and dolomitic lime are natural mined products.
Figure 50 Mining limesand—limesand, limestone and dolomitic lime are natural mined products.

Liming

This is the most desirable result because no production has been lost due to soil acidity. Where liming has removed the acidity of the soil as a limitation to production, the crop and subsequent stubble will have had better access to water and nutrients and be more palatable to sheep.

If the paddock is not uniform, management areas can be based on soil type and pH profile. Soil sampling should be representative of these areas and lime can be applied where needed according to the soil pH profile. For example, if some parts of a paddock require 2 tons/ha and others 3 tons/ha, a strategy could be to apply 2 tons/ha to the entire paddock, recheck the pH after three years and add 1 ton/ha ha to the paddock. areas requiring 3 t/ha.

It is best to keep the pH at or above 5.5 in the topsoil and 4.8 on the surface. If topsoil pH falls below 5.5, there will be insufficient alkalinity to move down and treat subsurface acidification. Best practice is to sample and test the soil every three to four years and apply lime if the pH falls below targets.

Remember that it may take several years after liming to recover the acidic subsoil, and all the while acidification will continue due to agricultural practice and production will be lost. Is it possible to apply lime to the subsoil where needed if the surface is fine. Adequate lime applied to the surface will allow alkalinity to decrease to treat subsurface acidity, although it may take several years to recover acidic soil.

If compaction is also a production limitation, it will be beneficial to apply lime to the surface prior to deep ripping (Figure 53). To help with calculations, pure calcium carbonate (100% . NV) applied at 1 t/ha will increase soil pH by approx. 0.7 on sand, 0.5 on clay and 0.3 on clay. On low pH soils, especially if the subsoil is also acidic, it may be several years after liming before production benefits are achieved.

Tramline farming makes it easy to lime when it fits into the farming schedule.

Figure 54 Tramline farming at Buntine. Tramline farming makes it easy to apply lime whenever it fits  into the farming schedule
Figure 54 Tramline farming at Buntine. Tramline farming makes it easy to apply lime whenever it fits into the farming schedule

Figure

Figure 4 Soil sampling at Kellerberrin. Professional soil-sampling contractors should be able to  accurately sample the soil profile
Figure 5 An exhaust tube marked in 10 cm  increments is useful for do-it-yourself sampling.
Figure 6 Discussing the implications of a target pH profile at Casuarina, north of MingenewBack
Table 1 The lime equivalent (as pure calcium  carbonate) of various farm products (Moore  1998)
+7

References

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