**3.1 Soil acidity and calcium and magnesium contents**

As it was expected, liming significantly increased soil pH, Ca, and Mg contents and reduced the exchangeable Al at the three assessed depths (**Table 2**). Despite the 2 t ha−1 of limestone application significantly reduced the exchangeable Al content at the three evaluated depths, it still remained high at deeper layers (1.01–2.00 cmoc kg−1) [13]. The liming effect was more pronounced in the 0–10 cm layer, mainly in the treatments with the application of organic residues. There occurred an increase of the soil pH, as well as a more pronounced reduction of the Al content in the treatments with an application of organic residues, in the limestone plots, especially in the superficial layer of the soil. The increase in soil pH with the addition of plant residues has been observed [14]. Castro [15] observed Al content marked reduction with the application of organic fertilizers on an Ultisol in Central Amazonia. The intensity of the effects is linked to the characteristics of the plant material used. In general, legumes provide higher pH and Al neutralization in the soil than grasses, and this effect is linked to the cation content in the plant material [16]. This result would be due to the complexation of free H+ and Al3+ with anionic organic compounds from the residues and the increased saturation of the cation exchange capacity by Ca, Mg, and K added with the plant residue, which would reduce the potential acidity [14]. The Oxisols of the Amazon are characterized by high acidity and the presence of toxic Al, and the application of organic matter has been suggested as an alternative for correcting the acidity and the neutralization of the exchangeable Al in the soil [17, 18].

#### **3.2 Exchangeable K, available P, and organic C contents**

The results of the **Table 3** showed a beneficial effect of liming on the decomposition of the organic material added, resulting in the increase of the mineralization of K in the soil, improving the efficiency of the organic fertilizers used. In that case, K concentration on the plant materials used on the organic fertilization may


*Organic Fertilization with Residues of Cupuassu (*Theobroma grandiflorum*) and Inga… DOI: http://dx.doi.org/10.5772/intechopen.100423*

*Means followed by different lowercase letters in the columns, and different upper case letters in the lines, differ from each other, by Tukey test at 5% probability.*

*12 t ha−1 organic fertilization of residues of cupuassu shell.*

*22 t ha−1 organic fertilization of residues of cupuassu shell +3 t ha−1 Inga pruning.*

#### **Table 2.**

*Values of pH (H2O), exchangeable aluminum (Al), calcium (Ca), and magnesium (Mg) in an Oxisol, cultivated with cupuassu tree, as a function of lime and organic fertilizer.*

have been the determinant factor for such an effect, since in the treatment with *cupuassu shells + Inga prunings*, we applied almost twice the amount of K we had done in the treatment with just cupuassu shells. Similar results have also been observed by Alfaia et al. [9] in cupuassu agroforestry systems in Western Amazonia.

In general, the average levels of K in the soil observed in this study were low (<0.05–0.10 cmolc kg−1), according to Moreira and Fageria [13] criteria. Soils in Amazonia hold low K contents [6, 13, 19, 20] and, due to the relatively high demand many native plants have for it, K becomes one of the most limiting nutrients for


*Means followed by different lowercase letters in the columns, and different upper case letters in the lines, differ from each other, by Tukey test at 5% probability.*<sup>1</sup> *2 t ha−1 organic fertilization of residues of cupuassu shell. 2 2 t ha−1 organic fertilization of residues of cupuassu shell +3 t ha−1 Inga pruning.*

#### **Table 3.**

*Values of exchangeable potassium (K), available phosphorus (P), and organic carbon (C) in an Oxisol, cultivated with cupuassu tree, as a function of lime and organic fertilizer.*

producing fruits in this region [21]. Studies conducted in the Manaus region have shown K to be the most exported nutrient through agroforestry products originating from Amazonian native species, such as cupuassu, peach palm (*Bactris gasipaes*), and assai (*Euterpe oleracea*), with higher concentration in shells, seeds, and infructescence petioles, which must be reincorporated into the planting areas to maintain sustainability [22].

The low exchangeable K contents in the soils may also be related to its export through fruit harvesting in addition to losses by leaching, according to what was observed by Alfaia et al. [9]. The results of that study confirmed that mineralization and, the addition to the soil, of K-rich organic matter, such as cupuassu shells and Inga plant material, have the potential to restore this nutrient in the soil when associated with the correction of the soil acidity [9, 23].

With regard to P, it was observed that only at a depth of 20–30 cm the levels of this nutrient were significantly higher in the presence of liming, although the data in **Table 4** show a significant effect of liming on the absorption of P by cupuassu plants.

According to Khorramdel et al. [24], soil organic matter content is a result of the balance between the processes of addition of organic materials (plant residues, among *Organic Fertilization with Residues of Cupuassu (*Theobroma grandiflorum*) and Inga… DOI: http://dx.doi.org/10.5772/intechopen.100423*


*Means followed by different lowercase letters in the columns, and different upper-case letters in the lines, differ from each other, by Tukey test at 5% probability.*

*12 t ha−1 organic fertilization of residues of cupuassu shell.*

*22 t ha−1 organic fertilization of residues of cupuassu shell + 3 t ha−1 Inga pruning.*

#### **Table 4.**

*Concentrations of nitrogen (N), phosphorus (P), and potassium (K), in cupuassu tree, leaves planted in an Oxisol of Central Amazonia as a function of lime and organic fertilizer.*

others) and their loss (mineralization and decomposition by the decomposing organisms present in the soil). Under the conditions this work was performed, the dolomitic liming might have stimulated the mineralization of the organic matter added to the soil in the form of plant residues, which, combined with the high temperatures and humidity moisture (rainy season), favored and accelerated the decomposing process.

Inputs of exogenous organic matter may accelerate or retard the mineralization of native soil organic carbon (SOC) through a priming effect, and thus have a potential to change SOC dynamics [25]. In general, the priming effect is induced by the exogenous organic C but its intensity is controlled by soil nutrient availability [26]. Under the conditions of this work, liming increased the mineralization of N, increasing the availability of this nutrient in the soil (**Figures 1** and **2**), which probably accelerated the decomposition of the native SOC, [26, 27]. However, more research is needed to clarify the influences of organic amendments on SOC build-up [28].

#### **3.3 Mineral nitrogen contents (N-NH4 + and NO3 − )**

**Figure 1** shows the results of the mineral N, in the soil, as nitrate (N-NO3 − ) and ammonium (N-NH4 + ). The predominant mineral N was the form of N-NH4 + , especially in the treatment with the addition of Inga pruning, with presented this nutrient's highest releasing rate (0–5 cm layer) 60 days after its application. Mineral N predominant in the form of ammonium (N-NH4 + ) confirms the findings by other authors who claimed this ion to be a major N source for plants on Amazonian Oxisol [29]. The lower content of N in the form of nitrate (N-NO3 − ) might result from its greater losses through leaching and denitrification, such as documented by other authors [30, 31] and by its use by plants. On the other hand, the presence of the two forms of nitrogen in the soil can be highly positive for plant nutrition to maintain the internal ionic balance, since their uptake is in the form of N-NH4 + (positive charge) or in the form of N-NO3 − (negative charge) keep the electrical equilibrium.

**Figure 2** shows the liming positive effect on the organic fertilizer mineralization 60 days following their application. The three depths showed to have occurred an increase in the total mineral N (N-NH4 + + N-NO3 − ) contents, confirming the data shown in **Figure 1** and adding the information of it that it was leached down to the depth of 20 cm, at least. At 60 days after the application of organic fertilizers, the mineral N total (NH4 + + NO3 − ) contents along the soil profile were higher in the treatments containing liming and cupuassu shells either with or without Inga residues, because adding

#### **Figure 1.**

*N mineral content in the form of NH4 + and NO3 − in an Oxisol, cultivated with cupuassu tree, as a function of lime and organic fertilization.*

that organic material and raising the pH as well as the addition of Ca and Mg through dolomitic limestone increased the nitrogen contents through soil layer (**Figure 2**). It must take into account that part of this nutrient was uptake by the plants, or its contents would have shown to be higher in the soil profile due to leaching. The results obtained in this work confirm that the applications of green manure (leguminous plant prunings) are able to increase the mineralizable N in cupuassu crops in Oxisols.

#### **3.4 Cupuassu plant nutrient contents**

Liming significantly increased the plant absorption of P, K (**Table 4**). On the other hand, in the plot without liming occurred a significant increase in the absorption of N in the treatment with *Inga* + *cupuassu shell* compared to the control. Probably, the incorporation of organic waste has contributed to increasing the mineralization of organic N in the soil, as has been reported in other studies [29, 32]. Studies on the Oxisol and Ultisol of Central Amazonia have mentioned the absence of N response in cupuassu fruit production, due to the high mineralization rate and, consequently, high availability of mineral N in these soils, especially in soils with leguminous cover [33, 34].

The average concentrations of N and P obtained in this work are below the levels found by other authors in cupuassu tree plantations in the Oxisols of Central Amazonia [33], while the average concentration of K is found well above the values obtained in other works in Amazonia [1, 33], which may be related to the effect of organic fertilizers on the supply of K to the cupuassu plants in the present work. On Cambisols of Central Amazonia, Ayres and Alfaia [1] observed that liming promoted a small increase in K uptake by cupuassu plants.

*Organic Fertilization with Residues of Cupuassu (*Theobroma grandiflorum*) and Inga… DOI: http://dx.doi.org/10.5772/intechopen.100423*

**Figure 2.**

*N mineral total (NH4 + + NO3 − ) content in a Oxisol, cultivated with cupuassu tree, as a function of lime and organic fertilization.*

#### **3.5 Fruit production**

The data in **Figure 3** showed that organic fertilization induced a slight, non-significant increase in fruit production. Alfaia et al. [9], in experiments using cupuassu bark residues with and without liming, also did not observe significant effects on fruit production during the first fruit harvest; however, the increase in production was most pronounced during the harvest of the following year. It is probable that in this work, the time after the application of the treatments was not enough for the decomposition of organic matter to occur.

The largest increases of nitrogen in the soil occurred at 60 days after the application of fertilizers, shown a slow decomposition of organic material (**Figure 3**), even in the treatment with the addition of Inga, which has a lower C/N ratio and could help to accelerate the decomposition process [35]. The Inga, although being a legume, has low rates of decomposition and release of nutrients, when compared to other legume species [36]. In an Oxisol in the Central Amazon, Schwendener et al. [37] observed that the mixture of cupuassu litter (of low nutritional quality) and leaves of Inga (of slow decomposition) did not contribute to the increase of mineral N in the soil in the short term, in contrast to a legume such as gliricidia (*Gliricidia sepium*), of rapid decomposition. The results of this work show the potential of organic fertilization with cupuassu shell + Inga pruning as a supplier of nutrients for the cupuassu plants. However, more studies are needed, both on the effect of the application of residues in the long term, and tests of doses and mixing with other legumes and its use in the production of biochar and composting, which would prevent the immobilization of nutrients.

#### **Figure 3.**

*Fruit production of cupuassu tree as a function of lime and organic fertilizer. Columns represent the mean of four replications and lines the standard error of the mean.*

#### **4. Conclusions**

The organic fertilization with *cupuassu shell + Inga pruning* improved the chemical characteristics of the soil, mainly for the replacement of K and Ca, since applied with liming.

Without liming, there was a significant increase in the absorption of N by the cupuassu plants, showing that applications of green manure (legume pruning) can increase the N mineralization in Oxisols.

Liming also stimulated the mineralization of the organic fertilizers added to the soil, bringing about significant increases in P and K uptake by the cupuassu trees.

The assessed organic residues sources can result in a great nutrient-bearing organic fertilizer and become a low-cost alternative for recycling cupuassu processing residues.

#### **Acknowledgements**

We thank the National Technological Development Council (CNPq) and the Amazonas State Research Support Foundation (FAPEAM) for their financial support.

#### **Conflict of interest**

The author declares no conflict of interest.

*Organic Fertilization with Residues of Cupuassu (*Theobroma grandiflorum*) and Inga… DOI: http://dx.doi.org/10.5772/intechopen.100423*
