An applied bioassay with tropical soil analyses for clayey oxisol fertility improvement

Abstract

There is a need for improved methodology providing reliable yield response data for tropical soils that correlate with soil testing results. However, both root development and desirable soil microbial activities are usually repressed with the heavy clayey oxisols in small pot greenhouse experiments. Fertility of the ultisols and oxisols is governed by soil organic matter content and soil microbial transformations. These are not determined with soil test extraction procedures and must be established with actual measurement of the complex biological permutations involved. The objective of these studies was to develop bioassay procedures suitable for boil fertility investigations with a heavy, clayey oxisol (56% clay) of Jaiba, a tropical region of central Brazil.

Soil dilution to 5.6% clay content with sterile, coarse quartz sand resulted in an optimum rhizosphere for plant nutrient studies using 500 g bioassay cultures. Phosphorus was the first limiting plant nutrient. Both N and K produced yield increases when applied singly although the largest increases in corn dry matter resulted with NPK combinations. Available N, P and S were related to organic matter transformations with response to S attained after exhaustive cropping. Significant responses to Zn, B and Mo were not obtained in these studies. Practical value of this procedure results from attaining reliable plant growth response to levels and combinations of fertilizer treatments with small quantities of soil. These data are fundamental for interpretation of soil test results that are utilized for highly diverse ultisols and oxisols. Sound recommendations for improved soil management practices are essential for the small, provincial subsistence type farms having very limited resources within the extensive, underutilized neotropics of the world.

Key words:

• Subsistence farms
• fertilization
• soil humus
• soil management
• soil testing

Notes

Chefe do Departamento de Fitotecnia da EPAMIG, Caixa Postal 515, 30.000, Belo Horizonte, MG, Brasil and Professor, Agronomy Department, Oklahoma State University, Stillwater, OK, 74078, USA.