**Abstract**

The cupuassu (*Theobroma grandiflorum* (Willd. Ex Spreng.) K. Schum.) is a native fruit tree which has, in the past years, acquired great social and economic importance for the regional farmers. The nutrient-rich and often wasted cupuassu tree fruit shell residues can contribute to the improvement of the low fertility soil of Amazonia. A trial was carried out on a small holder's cupuassu plantation in Central Amazonia to ascertain the effect of organic fertilization on the recovery of soil fertility and plant nutrition by using material from cupuassu shell residues and *Inga edulis* pruning (branches and leaves). The fertilization with cupuassu rinds + Inga prunings improved soil fertility, mainly by the increase of K and Ca in the soil, but only with liming, which appears to favor the mineralization of these nutrients. At the 0–10 cm depth, the Ca level increased about 50% compared to the control and the K level increased 75% compared to the cupuassu shell treatments. The significant increase of about 30% in N absorption by trees in the plots without liming shows that the application of green manure can increase the mineralization of N in Oxisols. These results show that the organic residue sources used can result in a nutrient-bearing organic fertilizer and become a low-cost alternative for recycling cupuassu processing residues.

**Keywords:** cupuassu shell, Inga pruning, decomposition, nutrients, liming

#### **1. Introduction**

The cupuassu (*Theobroma grandiflorum* (Willd. Ex Spreng.) K. Schum.) is a native fruit tree which has, in the past years, acquired great social and economic importance to the regional farmers. Most soils, where this species is cultivated, are of low natural fertility. Successive crops with no nutrient replenishment can bring about soil exhaustion and become detrimental to the system's sustainability [1, 2]. Problems, such as population unevenness, nutritional deficiencies, the occurrence

Of witch's broom disease caused by a fungi (*Moniliophtora perniciosa*), and cultivation of plants susceptible to it, have imposed a crop yield decrease, causing losses to the rural producers [2–4].

The municipality of Presidente Figueiredo in the Amazonas state has great potential for cupuassu agribusiness, despite the producers have been faced with problems related to the decrease in cupuassu yield due to the soil's natural fertility loss throughout years of cultivation, plus the occurrence of pests and diseases. Since most producers in this municipality cannot afford agricultural inputs, an alternative technology for conserving and recovering soils must be economically viable to be adopted by the farmers in the region. Hence, low-cost soil management alternatives, such as the use of organic residues, must be prioritized.

The often wasted cupuassu tree fruit shells could be used in contributing to fertilize the soils. The recycling of these wastes, as well as a better use of natural resources, has become very important when it comes to sustainable agriculture since it reduces production costs and minimizes environmental impact [5, 6]. Furthermore, organic residues can bear phytopathogenic suppressing microorganisms present in the soil, in addition to stimulating the growth of a more diversified and competitive/suppressing microbiota, reducing the occurrence of plant diseases [7, 8].

As most fruits, especially cupussu, are rich in potassium (K) and their productivity depends on the contents of this nutrient in the soil, their replenishment becomes necessary to maintain the cupuassu productivity [1, 9, 10]. Large amounts of organic residues from cupuassu pulping processing are currently found available piled up near the dwellings and/or agroindustries in the municipality of Presidente Figueiredo. These residues are a low-cost fertilizer alternative source for the replenishment of nutrients exported by harvesting or lost by leaching. On the other hand, the mixing of the cupuassu shells with N-rich materials could help the cupuassu shells' decomposition and mineralization without the soil's N immobilization in the soil. Thus, the addition of pruning of some N2-fixing leguminous species, such as the Inga tree (*Inga edulis* Mart.), also known as ice cream bean, which is very frequent in the properties of the farmers of the Amazonas, might be used for this purpose. Hence, an experimental assay was implemented in the site of a smallholder farmer in the municipality of Presidente Figueiredo, in Central Amazonia, aiming to evaluate the effect of an organic fertilizer, by using plant materials from cupuassu shell residues and Inga tree pruning, on recovering the fertility of an Oxisol.
