**2. Soil acidity and liming**

The soil, from where mankind has drawn its main sustenance since the beginning of the civ‐ ilization, requires adequate management to maintain its fertility and nutrient availability sufficient to sustain the fundamental role of crops in supporting human life.

Among soil environmental factors, acidity (pH, base saturation, potential acidity and nu‐ trient solubility) is the one that affects most crop yields, particularly in tropical regions [4]. According to [5], the low fertility found in acidic soils is strongly associated with deficient levels of exchangeable bases and excessive amounts of of aluminum and manganese. The application of fertilizers that acidify the soil aggravates this problem, unless a well-planned liming program is implemented.

Some soils are naturally acidic due to relative shortage of basic cations in the original ma‐ terial or to processes that causes the loss of elements like potassium, calcium and magne‐ sium [6]. Other soils, although not originally acid, become so due to the removal of exchangeable cations from the surface of colloids, caused by: *a*) rainwater; *b*) alteration of clay minerals; *c*) ion exchange of roots; *d*) decomposition of organic matter; and *e*) addi‐ tion of nitrogen fertilizers.

Although liming is recognized as a beneficial practice to reduce soil acidity, it is often not employed, or is conducted inadequately. Limestone raises soil pH, neutralizes toxic alumi‐ num and supplies calcium and magnesium to the crops. These factors promote the develop‐ ment of root systems and enhance the use of nutrients and water by the plant [7]. In soils of tropical regions, acid reaction and low levels of basic cations such as calcium are ever-last‐ ing problems. Under these conditions, the application of limestone is an inexpensive, fast and efficient way to tackle both problems [8].

Among Brazilian minerals, limestone occupies first place: the country has estimated reserves of some 53 billion metric tons well distributed throughout the country and generally of good quality, making it a relatively inexpensive agricultural input. However, despite the abun‐ dance of limestone and the need for liming, this soil corrective measure is not used at a suffi‐ cient extent in the majority of Brazilian farming regions [5].

The application of limestone on annual crops, with homogeneous incorporation in the soil, is a common practice, although not recognized as it should be. In perennial crops, the incor‐ poration of limestone ismore complex due to the intrinsic characteristics of these plants and the lack of scientific and technological information [9]. This is the case, for example, for the majority of fruit crops in Brazil.

liming is more complicated due to the characteristics of these plants and the lack of scientific knowledge on this subject. Fruit trees, like all other perennials, keep producing for many years in practically the same volume of soil, which is the reason why soil acidity requires special attention. Despite the high importance of lime application for most fruit trees, there is a lack of information on the effects of this soil treatment technique during the planting,

Therefore, it is important to study the effects on orchards of soil acidity correction, espe‐ cially through liming, by monitoring soil chemistry and the response of the trees. Better knowledge in this respect can improve fruit crop productivity that translates into higher

The soil, from where mankind has drawn its main sustenance since the beginning of the civ‐ ilization, requires adequate management to maintain its fertility and nutrient availability

Among soil environmental factors, acidity (pH, base saturation, potential acidity and nu‐ trient solubility) is the one that affects most crop yields, particularly in tropical regions [4]. According to [5], the low fertility found in acidic soils is strongly associated with deficient levels of exchangeable bases and excessive amounts of of aluminum and manganese. The application of fertilizers that acidify the soil aggravates this problem, unless a well-planned

Some soils are naturally acidic due to relative shortage of basic cations in the original ma‐ terial or to processes that causes the loss of elements like potassium, calcium and magne‐ sium [6]. Other soils, although not originally acid, become so due to the removal of exchangeable cations from the surface of colloids, caused by: *a*) rainwater; *b*) alteration of clay minerals; *c*) ion exchange of roots; *d*) decomposition of organic matter; and *e*) addi‐

Although liming is recognized as a beneficial practice to reduce soil acidity, it is often not employed, or is conducted inadequately. Limestone raises soil pH, neutralizes toxic alumi‐ num and supplies calcium and magnesium to the crops. These factors promote the develop‐ ment of root systems and enhance the use of nutrients and water by the plant [7]. In soils of tropical regions, acid reaction and low levels of basic cations such as calcium are ever-last‐ ing problems. Under these conditions, the application of limestone is an inexpensive, fast

Among Brazilian minerals, limestone occupies first place: the country has estimated reserves of some 53 billion metric tons well distributed throughout the country and generally of good quality, making it a relatively inexpensive agricultural input. However, despite the abun‐ dance of limestone and the need for liming, this soil corrective measure is not used at a suffi‐

sufficient to sustain the fundamental role of crops in supporting human life.

formation and production stages of orchards.

profits for farmers.

174 Soil Fertility

**2. Soil acidity and liming**

liming program is implemented.

tion of nitrogen fertilizers.

and efficient way to tackle both problems [8].

cient extent in the majority of Brazilian farming regions [5].

In acid soils with high aluminum saturation, liming promotes the neutralization of the toxic Al in the surface layers, hence enabling more intense proliferation of roots with positive ef‐ fect on plant growth. However, it is important to consider the need to incorporate the lime‐ stone thoroughly into the soil at the time of planting perennial crops because surface application alone acts slowly on the deeper soil layers and a soil insufficiently corrected at the establishment of the orchard can impair crop productivity for a long time [10]. The ho‐ mogeneous incorporation of limestone allows greater contact between the amendment and the sources of acidity, speeding up the corrective effects that support efficient use of water and nutrients by the plant in the amended layer.

The importance of the root system is obvious because there is a close dependency between root development and the aboveground portion of the plant. The greater or lesser success of applying limestone and fertilizers, in turn, depends on the nature of the root system and on the volume of the soil effectively exploited by the particular plant species. Correction of acidity is the most efficient way to eliminate chemical barriers to the full development of the roots, and consequently, of the plant.

Unlike other agricultural inputs such as fertilizers, herbicides and insecticides, limestone can be considered an investment, because its benefits last over more than one harvest. This is due to the low solubility of the common limestones and the variability of particle sizes in crushed limestone, giving them different capacities to neutralize acidity over time. There‐ fore, two factors should be considered: the rate at which the acidity is corrected and the du‐ ration of the effects of liming. Fine particles promote rapid acidity correction, but this effect declines more quickly due to their faster solubilization. Therefore, the most efficient liming involves application of material with varied grain sizes to promote fast initial acidity correc‐ tion with sufficient residual effect as well. The Brazilian law (2006) states that the reactivity of liming materials after a period of three months following soil application is zero for large particles more than 2 mm in diameter, 20% for particles in the range of 0.84 and 2 mm in diameter, 60% for particles from 0.30 to 0.84 mm in diameter, and 100% for fine particle less than 0.30 mm.

Because of the residual effect of limestone, liming materials applied to the soil at the time of planting orchard seedlings can keep the soil within acceptable acidity range for a certain pe‐ riod of time. However, determining the duration and intensity of the residual effect of lim‐ ing at the moment of planting fruit orchards has not been widely studied, both due to experimental constraints and the time necessary to obtain satisfactory results [11-13].

Based on the above aspects, the best approach for liming is to apply limestone with larger grain size at the time of planting fruit orchards, with homogeneous incorporation in the soil, to prolong the residual effect, followed by use of fine material on adult trees, limited to the surface, because the incorporation of corrective materials into the soil may induce phytosa‐ nitary problems to the plants. Materials with finer particles can move more easily through the soil profile, correcting the acidity only in the surface layers.

results do not accurately reflect the true soil fertility. Soil sampling is a common practice among farmers for annual crops, but it has not been widely studied for perennial crops such as fruit trees, raising doubts on its reliability. The present recommendations are to sample the area that receives soil treatment. However, some works have shown a higher correlation between leaf nutrient levels of fruit trees and soil nutrient levels in the paths between rows than in the rows [12, 20]. So, which soil samples should be analyzed, those from the treated area or between the rows? How should correlations be interpreted? And at what depth should the soil be sampled? These are difficult questions to answer, and according to [8], it

Soil Acidity and Liming in Tropical Fruit Orchards

http://dx.doi.org/10.5772/53345

177

At the time of planting fruit orchards, the soil sampling procedure is the same as for annual crops, namely across the entire representative area. In producing orchards, it is important to sample the region under the projection of the tree crowns, which is the area that usually re‐ ceives fertilizers. Samples should be collected at the end of harvest from 20 points in each homogeneous plot (same cultivar, age, productivity, soil type, management and fertiliza‐ tion). At the same time, samples should be taken between the rows to measure lime require‐ ments if necessary. Studies have shown that acidification occurs more intensely under the projection of tree crowns due to nitrogen fertilization, application of organic wastes and ac‐ cumulation of plant material from pruning. As a rule, limestone is more often applied in

The most common method to calculate lime requirement in Brazil is base saturation [10].

TRNP is the total relative neutralization power, which considers the quantity of carbonates

The soil layer typically sampled is the surface 0 to 20 cm. However, fruit trees exploit a much larger soil volume compared to annual plants, so it is important to analyze the proper‐ ties of the deeper layers, especially regarding calcium and aluminum concentrations. This may lead to gypsum application, which neutralizes toxic Al and allows increasing Ca con‐ centration in deeper layers, an important factor for the proliferation of the root system and

Gypsum is indicated, for crops in general, when analysis of the soil from the 20-40 cm layer reveals calcium concentrations lower than 4 mmolc dm-3 and/or aluminum saturation above

40%. The need for gypsum is estimated by the following equation [10]:

also is not easy to design studies for this purpose.

strips under crown projection than onto areas between rows.

The formula is as follows:

)=

V1 is base saturation of the soil;

(*V*<sup>2</sup> −*V*1)× *CEC TRNP* × 10

LN is the need for limestone, in ton ha-1;

V2 is the target base saturation for the crop;

CEC is soil's cation exchange capacity; and

its exploitation of a larger soil volume.

present in the limestone and lime's granulometry.

*LN* (*ton h a* <sup>−</sup><sup>1</sup>

where:

Considering the perennial nature and cultivation conditions of fruit trees, the path of limestone particles in the soil can vary along with various factors, including physical ones, through the channels left by the decomposition of roots [14]. According to [15] and [16], another explanation for particle flow through the soil profile is the formation of ion pairs (Ca2+ and Mg2+) and organic acids (RO and RCOO- ) of high solubility and low molecular weight that can be leached to deeper layers. Besides these mechanisms, according to [17] other compounds may form such as Ca(HCO3)2 and Mg(HCO3)2. Nitrogen fertilization, in turn, can promote the formation of soluble salts, such as calcium nitrate, which percolate down through the soil in forms dissolved in water [18]. According to [11], it is probable that the sum of the contributions of all these processes is more important than each one individually. Finally, the movement of these particles depends on the dose of the correc‐ tive measure employed, the time after application, soil type and the type of fertilization applied to the orchard.
