When it comes to managing soil health it’s easy to focus on the big three nutrients in the soil - nitrogen, phosphorus, and potassium and overlook a fourth key aspect, soil pH. Soil pH refers to how acidic (sour) or alkaline (sweet) soil is on a scale between 0 and 14, with 𝟳.𝟬 being neutral.
Most plants and crops prefer soil pH levels in the 𝟲.𝟬–𝟳.𝟬 range.
As you move towards the ends of the pH scale, different nutrients that are important for good crop growth will either become more or less available.
For example, phosphorus is readily available when soil pH is 6.5. Decreasing the pH to 5.0 reduces its availability by half, resulting in poor crop and pasture growth, crop yield reduction and smaller plant size occur as a result of inadequate water and nutrition.
By liming soil to increase the soil pH is effective in reducing the availability of aluminium to non-toxic levels.
Effects of soil acidity
Soil acidity can have negative direct and indirect effects on crop growth and yield. Acidic soils usually contain soluble forms of aluminium (Al) and manganese (Mn). As soils become more acidic, the soil pH decreases, and this increases the concentration of hydrogen in soil. As soils become more acidic, this causes aluminium and manganese to become more soluble in soil; they will gradually increase to levels toxic to plants.
Aluminium toxicity will restrict root growth and tie up phosphorus (P), reducing crop uptake of P. The indirect effect of restricted root growth is a reduced uptake of water and nutrients which further restricts plant growth.
Manganese toxicity will result in visual symptoms, including black necrotic spots or streaks on leaves of cereal crops. Manganese toxicity can cause chlorosis on leaf margins and cupping of leaves of canola and legume crops. Toxicity of aluminium and manganese can reduce yields of most crops when grown on strongly acid soils (pH < 5.5). Recent research has shown that higher concentrations of H+ ions can be directly toxic to plants.
The other major negative effect of soil acidity is on the survival and growth soil microorganisms. This affects nutrient cycling, such as the mineralization of soil organic matter. This can reduce the mineralization and release of nitrogen, phosphorus, sulphur, and other nutrients from organic matter.
Diagnosing soil acidity
Poor yields of more sensitive crops may indicate acidic soil problems. Soil sampling and analysis are the first steps to correctly diagnose and confirm a soil acidity problem. Visual crop symptoms alone are not enough to diagnose a problem. Fields of concern must be carefully soil sampled.0 Often, soil pH will vary with topography, so on land with more rolling topography, the lower, mid, and upper slope areas should be sampled separately. Often different areas of a field may be more acidic and require higher rates of lime than other areas, and some areas may not require any lime at all.
Soil samples that are moderately or strongly acidic should then have a lime requirement test to determine the amount of lime required to raise the soil pH to 6.0 or 6.5. Lime rates depend on the amount of pH change that is needed and must consider the soil buffering capacity. Buffering capacity is the amount of lime required to change pH a given amount. Sandy soils have low buffering capacity and require less lime to modify soil pH versus soils with higher clay content, which have a high buffering capacity. Once the rates of lime are determined, then the cost to purchase, transport and apply the lime can be estimated to assess the economics of liming.
How does lime change soil pH?
The most common product used to modify acidic soils is lime, which is calcium carbonate (CaCO3).
When calcium carbonate is added to an acidic soil it produces a gas (carbon dioxide) and leaves Ca+2 in the soil. The Ca+2 will exchange with exchangeable acidity on the soil exchange complex. The reaction continues with calcium carbonate until all the acidity is neutralised or all the calcium carbonate is used up. The reaction process occurs over many months to several years.
Other calcium-based products such as calcium chloride or calcium sulphate (gypsum) are neutral salts and cannot be used as liming materials and are ineffective in modifying acid soils.
Lime application
From the lime requirement test, the lab provides the rates required as pure lime. This is important as some sources of agricultural lime are not pure, they may only be 70% or 80% calcium carbonate, our ag lime has a CaCO3 (Calcium Carbonate) of 95%.
The lime must also be very finely ground. The finer the liming material, the greater its surface area, resulting in faster reactivity with the soil. Fineness of the liming material must also be considered in calculating the actual application rate of the liming product.
Ideally, apply ag lime immediately after harvest to allow time for the lime to react for greatest benefit on soil pH before the next growing season. Lime should be spread very evenly over the soil surface and thoroughly incorporated into the soil. Water is required for the reaction process between the lime and soil. Lime will react more rapidly in a very moist soil versus a drier soil. It often takes a year or more before a response can be measured even under very good soil moisture conditions.
The reaction time will depend on the type of ag lime used, the fineness or coarseness of the lime material, and moisture conditions. Remember that liming materials differ widely in their neutralising power due to variations in the percentage of calcium and magnesium content. Liming materials with a higher CaCo3 will neutralize soil acidity faster than those with a lower CaCO3.
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