Non-uniformly broadcast as well as row applied fertilizers under a no-tillage system increase the variability of soil chemical attributes. Therefore, to acomplish an appropriate fertility description of an area under no-tillage it is necessary to establish better sampling procedures, including number of subsamples. So, the main objective of this work was to measure the horizontal variability of soil fertility attributes, allowing, as a consequence, a definition of the number of subsamples needed to compose a whole and representative fertility sample. In November 1997, in eight chosen areas of commercial agriculture production in the state of Rio Grande do Sul, Brazil, 36 single samples were collected in 0-10 cm layer depths with a cutting shovel, with the slices being 5 cm-thick, and 10 cm-wide in broadcast areas and 5 cm-thick/interrow-wide in row fertilized areas. The minimum number of subsamples to acceptably estimate (α=0,05 and 10% mean error) pH, SMP value and organic matter was low (less than 8) and high (more than 40) for phosphorus and potassium (Mehlich-1). The number of subsamples defined for conventional tillage (20) may also be used for the no-tillage system, provided a 20% mean error is allowed.
chemical characteristics; number of subsamples; fertilization methods