Lime requirement is defined as the amount of liming material, as calcium carbonate equivalent, required to change a volume of soil to a specific state with respect to pH or soluble Al content (Soil Science Society of America, 1997). However, in economic terms, lime requirement can be defined as the quantity of liming material required to produce maximum economic yield of crops cultivated on acid soils. Quantity of lime required to
Table 8 Influence of soil pH on micronutrient concentrations in soil and plant uptake
Micronutrient Influence on concentration/uptake
Zinc Zinc solubility is highly soil pH dependent and decreases 100-fold for each increase in pH, and uptake by plant decreases as a consequence.
Iron Ferric (Fe3þ) and ferrous (Fe2þ) activities in soil solution decrease 1000-fold and 100-fold, respectively, for each unit increase in soil pH. In most oxidized soils, uptake of Fe by crop plants decreases with increasing soil pH.
Manganese The principal ionic Mn species in soil solution is Mn2þ, and concentrations decrease 100-fold for each unit increase in soil pH. In extremely acidic soils, Mn2þsolubility can be sufficiently high to induce toxicity problems in sensitive crop species.
Copper Solubility of Cu2þis very soil pH dependent and decreases 100-fold for each unit increase in pH. Plant uptake also decreases.
Boron Sorption of B to Fe and Al oxides is pH dependent and is highest at pH 6.0–9.0. Bioaavailability of B is higher between pH 5.5 and 7.5, decreasing below and above this range mainly due to pH-dependent reactions.
Molybdenum Above soil pH 4.2, MoO42is dominant. Concentration of these species increases with increasing soil pH and plant uptake also increases. Water-soluble Mo increases sixfold as pH increases from 4.7 to 7.5. Replacement of adsorbed Mo by OHis responsible for increases in water-soluble Mo as soil pH increases.
Chlorine Chloride is bound tightly by most soils in mildly acidic to neutral pH soils and becomes negligible to pH 7.0.
Appreciable amounts can be adsorbed with increasing soil acidity.
Source:Adriano (1986), Fageriaet al.(1997, 2002), Mortvedt (2000), and Tisdaleet al.(1985).
364 N. K. Fageria and V. C. Baligar
produce maximum economic yields of crops grown on acid soils is deter- mined by soil properties, liming material quality, management practices, cropping systems, crop species or genotypes within species, calcium and magnesium interaction with other nutrients, and economic considerations.
5.1. Quality of liming material
Quality of liming material is very important in correcting soil acidity.
Chemical analysis of the liming material gives its composition. A liming material containing both calcium and magnesium is desirable for correcting soil acidity and improving contents of Ca and Mg of the soil. Two impor- tant characteristics that determine lime material quality are its neutralizing
Table 9 Influence of soil pH on acquisition of Zn, Cu, Fe, and Mn by upland rice grown in an Oxisol of Brazil
Soil pH
Zn
(mg plant1) Cu
(mg plant1) Fe
(mg plant1) Mn (mg plant1)
4.6 1090 75 4540 11,160
5.7 300 105 1860 5010
6.2 242 78 1980 4310
6.4 262 64 1630 3610
6.6 163 61 1660 2760
6.8 142 51 1570 2360
R2 0.98 ** 0.89 * 0.97 ** 0.99 **
*,** Significant at 5 and 1% probability levels, respectively.
Source:Fageria (2000).
Table 10 Influence of soil pH on shoot dry weight, grain yield, and panicle number of upland rice grown on Oxisols
pH in H2O
Shoot dry weight (g plant1)
Grain yield (g plant1)
Panicle number (plant1)
4.6 15.9 11.0 5.0
5.7 14.9 11.5 4.6
6.2 12.2 11.7 4.5
6.4 10.6 9.5 3.9
6.6 7.8 6.8 3.4
6.8 6.0 5.2 3.3
R 2 0.99 ** 0.91 ** 0.94 **
** Significant at the 1% probability level.
Source:Fageria (2000).
Ameliorating Soil Acidity 365
power or reactivity and fineness. The chemical effectiveness of agricultural limestone is measured by its CaCO3 equivalence. If neutralizing value is lower than CaCO3, a higher quantity of liming material is required and vice a versa. The neutralizing power or value of a liming material is defined as the acid-neutralizing capacity of the material by weight in relation to CaCO3. A dolomitic limestone has a higher neutralizing capacity than a calcitic limestone because of the lower atomic weight of Mg. Pure dolomite has a CaCO3 equivalence of 1.08 (Barber, 1984). Table 11 shows various liming materials and their CaCO3equivalent values (neutralizing values).
The degree of fineness indicates the speed with which lime materials will neutralize soil acidity. Fineness is measured by the proportion of processed agricultural lime which passes through a sieve with an opening of a particu- lar size. A 60-mesh sieve, which is the standard for comparisons of lime finesse and efficiency rating of 100%, is assigned (Caudle, 1991).
5.2. Soil texture
The relative proportions of sand, silt, and clay in a given soil determine soil texture, a basic physical property of the soil that remains unchanged by cultural and management practices. The sand group includes all soils whose
Table 11 Liming materials, their composition, and neutralizing power Commercial
name
Chemical formula
Neutralizing
value (%) Characteristics Dolomitic
lime
CaMg(CO3)2 95–109 Contains 78–120 g kg1of Mg and 180–210 g kg1 of Ca
Calcitic lime CaCO3 100 Contains 284–320
g kg1of Ca Dolomite lime MgCO3 100–120 Contains 36–72
g kg1of Mg
Burned lime CaO 179 Fast reacting and
difficult to handle Slaked lime Ca(OH)2 136 Fast reacting and
difficult to handle
Basic slag CaSiO3 86 Byproduct of pig-iron
industry, also contains 1–7% P
Wood ash Variable 30–70 Caustic and water
soluble
Source:Fageria (1989), Bolanet al., (2003), Brady and Weil (2002), and Caudle (1991).
366 N. K. Fageria and V. C. Baligar
sand content by weight is 85% or more, the silt group has a silt content of 80% or more, and the clay group includes those whose clay content is 40%
or more (Soil Science Society of America, 1997). Soil texture determines the buffering capacity of a soil, which refers to the ability of the solid phase soil materials to resist changes in ion concentrations in the solution phase.
For liming purposes, the resistance of the soil solution to changes in pH is a main component of soil buffer power. Oxisols contain predominantly iron and aluminum oxide minerals and kaolinite and have characteristically low to moderately low cation exchange capacity, but they also have high buffering capacity. Hence, Oxisols require large amount of liming materials to raise soil pH to a desired level for maximum crop yields. Maximum legume grain yield (common bean, soybean) is achieved at a pH of about 6.5 in Brazilian Oxisols (Fageria, 2001b, 2006; Fageria and Stone, 2004). Data inFig. 2show that to raise a pH from 5.3 to 6.5 in Oxisols, a lime rate of about 7 Mg ha1is needed.
5.3. Soil fertility
Soil fertility has a significant influence on the quantity of lime required to correct soil acidity to produce maximum economic crop yields. Quantity of nutrient present in the soil is defined in term of soil fertility, and, generally, soil analysis is used as a criterion to make fertilizer recommendations for
0 5.0 5.5 6.0 Soil pH in H2O 6.5 7.0
3 6 9
Lime applied (Mg ha-1) Y= 5.2809 + 0.2181X-0.0061X2
R2= 0.9656**
12 15 18
Figure 2 Relationship between lime rate and soil pH in Brazilian Oxisols.
Ameliorating Soil Acidity 367
field crops (Fageria and Baligar, 2005b). High fertility soils in terms of exchangeable Ca2þ, Mg2þ, and Kþ require less lime than do those with lower soil fertility. When Ca2þ, Mg2þ, and Kþcontents are higher, a lower lime rate is required, because of higher levels of these basic cations in the soil, meaning relatively higher base saturation and higher pH than with lower levels of these cations (Table 12).
5.4. Crop rotation
Crop rotation is defined as a planned sequence of crops growing in a regularly recurring succession on the same area of land, as contrasted to continuous culture of one crop or growing a variable sequence of crops (Soil Science Society of America, 1997). This is a very old system of growing crops and was practiced during the Han dynasty of China more than 3000 years ago (Karlen et al., 1994; MacRae and Meheys, 1985). Romans recognized the benefits of alternating leguminous crops with cereals more than 2000 years ago (Karlen et al., 1994; Robsonet al., 2002). Continuous cultivation of a given crop on the same land may reduce productivity. This decline in productivity is associated with a decrease in soil fertility, infestation by weeds, insects, and diseases, loss of soil by erosion, reduction in soil biological diversity, and buildup of allelopathy (Fageria, 2002). Ability of legumes to fix atmospheric nitrogen discovered in the late 19th century was a major reason for crop rotation (cereals with legumes) that became popular in the early 20th century (Karlenet al., 1994).
Crop rotation has several benefits. Diversifying crops in rotation can improve crop yields, a response known as the rotation effect (Anderson, 2003). Phosphorus deficiency or unavailability is a major yield-limiting
Table 12 Influence of liming on base saturation and exchangeable Caþand Mg2þat 0–20 cm depth in an Oxisols
Lime rate (Mg ha1)
Base saturation (%)
Ca2þ (cmolckg)
Mg
(cmolckg1)
0 40 1.9 1.0
4 44 2.3 1.1
8 51 3.0 1.2
12 53 3.1 1.3
16 56 3.3 1.3
20 66 3.8 1.4
R2 0.80 ** 0.7 2** 0.2 3*
*,** Significant at the 5 and 1% probability level, respectively.
Source:Fageria (2001a,b).
368 N. K. Fageria and V. C. Baligar
factor in Oxisols, and in the Central Great Plains of the United States, crop rotations have been reported to improve P-use efficiency (Bowman and Halvorson, 1997). However, there are some disadvantages in adopting crop rotation. One such disadvantage is soil acidification, a serious form of land degradation associated with crop rotation (Coventry et al., 2003). In crop rotations, the N cycle and C cycle contribute most to the acid input (Coventry and Slattery, 1991; Helyar et al., 1997). Acidification of soil can also result from increasing addition of crop residues due to crop rotation (Posset al., 1995). It is reported in several studies in southern Australia that in the wheat-based crop rotation, the rates of acidification can be 0.16–3.6 Hþ ha1 year1 for pastures (Loss et al., 1993; Ridley et al., 1990) and 1.0–7.5 Hþha1year1for cereal–legume rotations (Coventry and Slattery, 1991; Dolling, 1995; Moody and Aitken, 1997). The rate of acidification in intensive cropping systems is also associated with removal of large amounts of calcium and magnesium in the grains. In addition, acidification in the tropical rainforest such as the cerrado region of Brazil is also associated with the replacement of perennial vegetation with shallow-rooted annual pastures and crops. With this change, more water has drained through the soil profile, which leads to removal of bases (Coventryet al., 2003). The rates of acidifi- cation are generally more pronounced in higher rainfall areas and soil pH in the range of 4.8–5.5 (CaCl2) is likely to be more susceptible to rapid acidification (Coventryet al., 2003; Haynes, 1983).
5.5. Use of organic manures
Organic manures are products from the processing of animal or vegetable substances that contain reasonable amount of plant nutrients to be of value as fertilizers. Brosius et al. (1998) reported that plant- and animal-based organic byproducts may substitute for commercial fertilizers and enhance chemical and biological attributes of soil quality in agricultural production systems. Use of organic manures can also affect lime requirement of a crop.
Organic matter increases the soil’s ability to hold and make available essential plant nutrients and to resist the natural tendency of soil to become acidic (Coleet al., 1987). Furthermore, addition of organic manures to acid soils has been shown to increase soil pH, decrease Al saturation, and thereby improve conditions for plant growth (Alter and Mitchell, 1992; Reis and Rodella, 2002; Wong and Swift, 2003).Miyazawaet al. (1993)reported that crop residues of wheat and corn and 20 plant species utilized as green manure increased soil pH and decreased Al content of the soil. Several mechanisms have been proposed for reducing acidity by organic manures.
These mechanisms include specific adsorption of organic anions on hydrous Fe and Al surfaces and the corresponding release of hydroxyl ions which increase soil pH (Hue, 1992; Wong and Swift, 2003). Adsorption of Al by organic matter sites and the subsequent isolation of the inorganic phase to
Ameliorating Soil Acidity 369
maintain the equilibrium Al activity in soil solution have been proposed to increase soil pH (Wong and Swift, 2003; Wong et al., 1998). Chelating agents released by decomposing organic matter may detoxify Al ions.
Plant roots decay in the soil and form into soil organic matter. Active roots also release organic acids such as citrate, malate, and tartrate. These organic acids react strongly with Al and convert it into less toxic organically bound forms (Yanget al., 2000). The organic Al affinities or stability constants are in the order of citrate >tartrate>malate (Hueet al., 1986). The decrease in Al-activity by addition of organic matter has been reported by Kochain (1995). The functional groups involved in metal complexation by organic matter are COOH and OH (Wong and Swift, 2003). Surface application or surface incorporation of organic matter also decreased phytotoxic subsoil Al3þ activities because dissolved organic matter (DOM) that leached into the subsoil formed nontoxic Al–DOM complexes (Hue, 1992; Hue and Licudine, 1999; Liu and Hue, 1996; Willert and Stehouwer, 2003). The combined application of CaCO3and organic matter in lime-stabilized bio- solids decreased subsoil acidity and increased subsoil Ca saturation, compared with CaCO3alone (Brownet al., 1997; Tanet al., 1985; Tester, 1990; Willert and Stehouwer, 2003). This effect was attributed to increases in Ca mobility caused by Ca–DOM complexes (Willert and Stehouwer, 2003).
Additional benefits of organic matter addition to acid soils are improving nutrient cycling and availability to plants through direct additions as well as through modification in soils’ physical and biological properties. A comple- mentary use of organic manures and chemical fertilizers has proven to be the best soil fertility management strategy in the tropics (Fageria and Baligar, 2005a; Makinde and Agboola, 2002). Enhanced soil organic matter increases soil aggregation and water-holding capacity, provides source of nutrients, and reduces P fixation, toxicities of Al and Mn, and leaching of nutrients (Baligar and Fageria, 1999). Build-up of organic matter through additions of crop and animal residues increases the population and species diversity of microorgan- isms and their associated enzyme activities and respiration rates (Kirchner et al., 1993; Weilet al., 1993). The use of organic compost may result in a soil that has greater capacity to resist the spread of plant pathogenic organisms.
The improvement in overall soil quality may produce more vigorous growing and high yielding crops (Brosiuset al., 1998).
5.6. Conservation tillage
The mechanical manipulation of the soil profile for crop production is known as tillage. Conservation tillage is minimum manipulation of the soil profile for crop production and 30% or more soil surface is covered with crop residues. In conservation tillage, weeds are controlled by herbi- cides and this practice helps in conservation of water and nutrients, and reduces soil erosion and labor cost. Conservation tillage reduces oxidation
370 N. K. Fageria and V. C. Baligar
of organic matter or conserves soil organic matter content and consequently decreases the adverse effects of soil acidity. Additional research is needed to assess the long-term conservation tillage effects on soil acidity.
5.7. Crop species and genotypes within species
Crop species and genotypes within species differ significantly in relation to their tolerance to soil acidity (Baligar and Fageria, 1997; Devine, 1976;
Fageria and Baligar, 2003b; Fageria et al., 1989, 2004; Foy, 1984; Garvin and Carver, 2003; Reid, 1976; Sanchez and Salinas, 1981; Yang et al., 2000). Hence, lime requirements also vary from species to species and among cultivars within species. Many of the plant species tolerant to acidity have their center of origin in acid soil regions, suggesting that adaptation to soil constraints is part of the evolution processes (Foy, 1984; Sanchez and Salinas, 1981). A typical example of this evolution is the acid soil tolerance of Brazilian upland rice cultivars. In Brazilian Oxisols, upland rice grows very well without liming, when other essential nutrients are supplied in adequate amount and water is not a limiting factor (Fageria, 2000, 2001a).
Experimental results obtained on Brazilian Oxisols with upland rice are good examples of crop acidity tolerance evaluation. Fageria et al. (2004) reported that grain yield and yield components of 20 upland rice genotypes were significantly decreased at low soil acidity (limed to pH 6.4) as compared with high soil acidity (without lime, pH 4.5), demonstrating the tolerance of upland rice genotypes to soil acidity. InTable 13, data are presented showing grain yield and panicle number of six upland rice genotypes at two acidity levels. These authors also reported that grain yield gave significant negative correlations with soil pH, Ca saturation, and base saturation. Further, grain
Table 13 Grain yield and panicle number of six upland rice genotypes at two soil acidity levels in Brazilian Oxisols
Genotype
Grain yield (g pot1) Panicle number (pot1) High acidity
(pH 4.5)
Low acidity (pH 6.4)
High acidity (pH 4.5)
Low acidity (pH 6.4)
CRO97505 74.3 52.0 38.0 28.3
CNAs8983 55.2 42.9 29.0 25.7
Primavera 53.0 47.2 25.0 21.7
Canastra 51.6 38.9 32.0 26.3
Bonancáa 48.8 36.5 26.3 20.7
Carisma 50.8 17.5 43.3 17.7
Average 66.7 47.0 38.7 28.1
Source:Compiled fromFageriaet al.(2004).
Ameliorating Soil Acidity 371
yield had significant positive correlations with soil Al and HþAl, confirming that upland rice genotypes were tolerant to soil acidity. Fageria (1989) reported stimulation of growth of Brazilian rice cultivars at 10 mg Al3þL1 in nutrient solution compared with a control (no Al) treatment. Okada and Fischer (2001) suggested that the mechanism for the genotype difference of upland rice for tolerance to soil acidity is due to the relationship between regulation of cell elongation and legend-bound Ca at the root apoplast.
A substantial number of plant species of economic importance are generally regarded as tolerant to acid soil conditions of the tropics (Sanchez and Salinas, 1981). In addition, there are cultivars within crop species that are tolerant to soil acidity (Fageria et al., 2004; Garvin and Carver, 2003; Yang et al., 2004). Yang et al. (2005) reported significant differences among genotypes of rye, triticale, wheat, and buckwheat to Al toxicity. These crop species or cultivars within these species can be planted on tropical acid soils in combination with reduced rates of lime input.
Combination of legume–grass pasture and agroforestry system of man- agement are the other important soil acidity management components useful in tropical ecosystems. For example, Pueraria phaseoloides is used as understory for rubber, Gmelina arborea or Dalbergia nigra, plantations in Brazil, presumably supplying nitrogen to the tree crops (Sanchez and Salinas, 1981) A detailed discussion of combination of legume–grass pasture and agroforestry in tropical America is given bySanchez and Salinas (1981).
These authors reported that when an acid-tolerant legume or legume–grass pasture is grown under young tree crops, the soil is better protected, soil erosion is significantly reduced, and nutrient cycling is enhanced. Some important annual food crops, cover or green manure crops pasture species, and plantation crops tolerant to tropical acid soils are listed inTable 14. Acid soil tolerant crops are useful to establish low input management systems.
5.8. Interaction of lime with other nutrients
Recognition of the importance of nutrient balance in crop production is an indirect reflection of the contribution of interactions to yield. The highest yields are obtained where nutrients and other growth factors are in a favorable state of balance. As one moves away from this state of balance, nutrient antagonisms are reflected in reduced yields (Fageria et al., 1997).
Nutrient interactions can occur at the root surface or within the plant and can be classified into two major categories. In the first category are interac- tions which occur between ions because the ions are able to form a chemical bond. Interactions in this case are due to formation of precipitates or complexes either in soil or in the plant. For example, this type of interaction occurs where the liming of acid soils decreases the concentration of almost all micronutrients in soil solution except molybdenum. Such reduction in ion concentration in soil solution decreases the uptake. Increasing soil pH
372 N. K. Fageria and V. C. Baligar