Soil Acidity and Liming
Soil acidity is one of the environmental factors which can influence plant growth and seriously limit crop production. In order to correct soil acidity, ground agricultural limestone should be applied to the soil at rates determined by soil test or diagnostic chart. While the response of crops to lime may not be seen as quickly as their response to fertilizer, failure to use lime limits crop yields. Agricultural lime will also increase the efficiency with which crops use the fertilizer applied.
Most of the soils used for crop production in Zambia need lime for optimum yield. Soil analysis results from studies done by faculty in the Soil Science Department, University of Zambia between 1998 and 2003, revealed that although soil acidity is stronger in the upper regions of Zambia (Figure 1), it is widespread in all parts of the country and all soil will benefit from liming. The extent of the problem varies from field to field therefore the amount of lime required also varies. A proper liming strategy combined with other sound agronomic practices will increase crop production.
What is Soil Acidity?
Soil acidity is the term used to express the quantity of Hydrogen (H) and Aluminium (Al) in the soil.
Measure of Soil Acidity
pH is the indirect “indicator” of soil acidity. It is usually measured by suspending soil in water, but in Zambia calcium chloride solution is also widely used. The scale of pH values in water ranges from 1 to 14. Soils with pH less than 7 (or 6 in calcium chloride) are said to be acidic and may require lime while those above pH 7 are alkaline and do not need liming. The lower the pH value the more acidic the soil is and therefore needs more lime. As the pH scale is logarithmic, a soil pH of 4 is 10 times more acid than a soil pH of 5, and 100 times more acid than a soil of pH 6.
Desirable pH range for best crop performance
The tolerance of various crop species to soil acidity varies (Figure 2), but most crops will grow well in soils that range in pH from 5.5 to 7.0.
Causes of Soil Acidity
Soil acidity occurs as a result of:
- Acidic parent material that forms the soil.
- High volumes of rainfall and leaching of elements like Ca, Mg, and K out of the soil plow layer.
- Decay of organic matter leading to release of organic acids into the soil.
- Harvest of high yielding crop which removes plenty of Ca, Mg, and K from the soil.
- Widespread use of nitrogen fertilizers.
Why are we concerned about Soil Acidity?
- Acidic soils may contain harmful concentrations of aluminum and manganese that limit plant growth.
- The solubility and availability of elements in the soil such as N, P, K, Ca, Mg, B, and Mo is usually low in acidic soils (Figure 3)
- Activities of microorganisms that are needed to decompose organic matter, to convert ammonium to nitrate and to fix nitrogen is negatively affected in acidic soils.
- The effectiveness of fertilizer and some chemicals applied to the soil is reduced.
- Crop yields are generally lower in acidic soils.
Combating Soil Acidity
The quickest way to neutralize soil acidity is by applying agricultural limestone to the soil. A good liming program is based on a professional laboratory soil test that determines the degree of soil acidity and the correct amount of lime to use. Other options include the use of field test kits or a diagnostic chart.
What is agricultural lime?
This is material that is capable of neutralizing soil acidity. There are two basic types of agricultural lime used in Zambia: calcitic limestone, which consists of calcium carbonate and dolomitic limestone, is a mixture of both calcium and magnesium carbonates. As they neutralize soil acidity, these lime materials also add calcium and magnesium to the soil. Choice of type of lime to use depends upon the balance between calcium and magnesium levels in the soil. If the soil is low in magnesium, dolomitic lime should be used.
Amount of lime to use
The goal of liming can be to raise soil pH to a specific value, or to neutralize acidity generated by soil. The first option usually requires more lime. After the extent of soil acidity is known, the quality of lime material is another important factor to arrive at the amount of lime to use.
Quality of Lime
The primary factors that affect the quality/value of agricultural limestone are neutralizing value and fineness.
Neutralizing Value (NV) is the amount of soil acidity which the material can neutralize when it is compared to an equal amount of calcium carbonate. Calcium carbonate has been assigned a value of 100. So, if for instance, 1 kg of the lime material neutralizes the same amount of soil acidity as 1 kg of calcium carbonate, the NV of that lime is 100. If ¾ of acidity is neutralized, the NV is 75, and if ¼ more acidity is neutralized then the NV is 125. The higher the NV of a lime material the better it is.
Fineness of the lime material determines how quickly it will react with the soil to neutralize soil acidity. The finer the material, the faster it will work as it will be more soluble and contact more soil surface area. Fineness value is usually arrived at by determining the percentages of the lime material that will pass through different sieve mesh sizes. Low sieve number means that larger particles can pass through while high sieve numbers allow only smaller particles to pass through. An ideal lime material should have 90-95% material pass through 20-mesh, 50-60% through 60-mesh and 30-50% through 100-mesh. This allows for some material to react quickly while the rest provides residual reaction.
How long will it take for the lime to react with soil and how long will it last?
It normally takes lime 2 to 3 years to react completely with soil. Apart from the quality of the lime, reaction time will also depend upon amount of soil acidity, soil organic matter, type and amount of clay and the cropping practices. The effect of lime can last up to 3 or 4 years, but this does not mean that it will not be necessary to apply more lime during that period. Not all acidity is neutralized at once, so regular applications of lime may be necessary. Lime should be applied well ahead of planting acid sensitive crop so that there is enough time for the reaction to begin. It is important to note that the chemical reaction of lime in the soil requires moisture.
How should lime be applied?
Lime does not move very much in the soil, so it should be evenly mixed in with the soil plow layer, especially the rooting volume.
Will liming pay-off?
Yes. Where the need for lime has been established and the correct amount is used, studies have shown that applying lime to soil can return between $5 to $25 for each $1 spent. Do not over-lime the soil as excess lime may cause micronutrient deficiencies and reduce crop yields.
Is lime the same as fertilizer?
No. Lime works to neutralize soil acidity and make the soil environment more suitable for roots to grow in. However, several studies have shown that applying lime to acidic soils significantly enhances the performance of fertilizers that are used on such soils (Figure 4). This can result in significant crop yield increases and more income to the farmer.
Benefits of Liming
- Combats soil acidity by reducing metals toxicity, making soil P more soluble and
- microbes more active.
- Supplies Ca, Mg to plants.
- Improves soil physical structure with better water infiltration and reduced energy for roots to penetrate soil.
- Improves the performance of fertilizer.
Some of our valued and satisfied customers include:
- Agriculture procurement of Zambia
- Lisitu Farms
- Lwimba Ranch
- Magoye tobacco Company Ltd
- Mpongwe Development Company Ltd
- Sable Farms
- Syringa dairy Farms
- Tobacco Association of Zambia
- Tombwe Processing
- Wangwa Farms
- Zambezi ranching & Cropping Ltd etc.
For Further Information:
Turtle Agro-mining Ltd
Plot 12913, Mumbwa Road,
About Soil Acidity and Soil Testing:
Soil Science Department,
University of Zambia
P.O. Box 32379, Lusaka.
Zambia Agricultural Research Institute
Mt Makulu, P/Bag 7 Chilanga.