Adubação orgânica em condições de irrigação suplementar e seu efeito na produtividade da quinua (Chenopodium quinoa Willd) no planalto da Bolívia

Abstract

Quinua (Chenopodium quinoa Willd.) is an Andean grain with high protein content (14%), and it is one of the main crops that generate incomes for Bolivian plateau farmers. The quinua crop grows in conditions of low temperature, rainfall, and fertility, and although a good development is observed under limited environmental and soil conditions, the yields in Bolivia are usually very low. This study aimed to evaluate the quinoa grain yield and the nitrogen content in the soil and the grains using different amounts of organic fertilization and supplemental irrigation. Four experiments were conducted: the first one was conducted in the South Highland, in a completely randomized block design, bi-factorial: quantities of sheep manure applied in the soil (0, 4, 8 and 12 Mg ha-1) and irrigation management (with and without supplemental irrigation), the second experiment was conducted in the Central Highlands, using a completely randomized block design, with three factors: sheep manure applied in three different times (May, July and September), irrigation (with and without supplemental irrigation) and application of three amounts of sheep manure (0, 15 and 30 Mg ha-1), evaluating in both experiments, the effects of these treatments on grain yield, quinua dry matter, grain nitrogen content and nitrogen use efficiency. The third experiment was conducted in the Central Highlands with the objective of evaluating two soil textures classes and two crop residues on soil surface: sandy soil with fescue and quinua crop residues on the soil surface and sandy clay loam soil with fescue and potato crop residues. On this experiment it was analyzed the effect of these treatments on the mineral nitrogen content of the soil; the fourth experiment was conducted in the Laboratory of the Agronomy Faculty of La Paz, in a completely randomized design with three replications. Soil was incubated with deferent quantities of sheep manure (0, 4, 8, 12, 16, 20, 24 and 30 Mg ha-1) to determine nitrogen and carbon mineralization in different times during a period of time of three months. Experimental results indicated that in the Bolivian highlands the quinoa yield is influenced by climate and soil conditions (rainfall of 200 mm per year and 0.03% of total nitrogen in the soil). However, the distribution of 15 Mg ha-1 of sheep manure applied five to seven months before sowing the crop and complimented with supplemental irrigation at flowering and grain milk crop development stage, increase quinua grain yield, the nitrogen accumulation and the nitrogen use efficiency. The total grain nitrogen was not affected by the quantities of sheep manure used. Results also indicated that the soil management (vegetation) and the type of the soil texture has influenced on the soil mineral nitrogen, showing value of 2 mg N kg-1 of soil. The incorporation of sheep manure to the soil resulted in nitrogen immobilization during the first 45 days, the rate of nitrogen mineralization ranged from 0.62 to 0.64 mg kg day-1 for treatments with doses of 16 and 20 Mg ha-1 of sheep manure.