TROPSOILS TECHNICAL REPORTn.1986-1989, Fevereiro. 19861111111111111111111111 11111 1111111111111111111 111111 111111111111111 11111111CPAA-237!58-1

Oxisols ofManaus

Phosphorus FertilizerPlacement and Profitability

T.fOI Smyth, N. C. State University- .Manoel S. Cravo, EMBRAPA

PlacementThe effects of P fertilizer placement methods

were investigated during eight consecutive crops of acorn-cowpea annual rotation. Treatments inc1udedcomparisons between rates of broadcast P (O, 22, 44,88 and 176 kg/ha) applied before the initial comcrop, in factorial combination with banded P rates (O,11,22 and 44 kg/ha), applied to each crop. Cowpearequíred greater amounts of extemal soil P than didcom, with Bray l-extractable critical soil P levels of19and 9 mg'/kg, respectively. Total yields duringfour years of cultivation, for com and cowpea or bothspecies combined, increased as a function of total Papplied, regard1ess whether P was applied in bands orbroadcast. Although these results suggest thatplacement method is not an important parameter in Pfertilization management for these Oxisols, economicconsiderations indicate advantages to banded Papplications in small increments to each crop.

ProfitabilityProfitability for three P fertilization strategies

during eight consecutive crops is shown in Figure 1.Without P fertilization, yields did not exceed 0.2 t/ha,obtained during the initial com crop. Increasingeconomic losses resulted from expenses incurred byinputs other than P fertilizer. Cumulative yields for Ptreatments of both 176 kg/ha, broadcast-applied oncebefore the initial crop, and 22 kg/ha, banded to eachof the eight crops, were all similar, 17 t/ha, Similarprofits were obtained after a total of eight crops, butbanded P applications were advantageous during theinitial four crops because P fertilizer costs were

2.0....u.. ~ 1.5 o 176 kg P BROADCAST ONCEo c A 22 kg P BANDED I CROPQ:La.. ...•• 1.0

o o WITHOUT PILIO 0.5>0S* 0.0...J VJ -0.5::J ::J::E~ -1.0::Ju

-1.52 3 4 5 6 7 8

CROP NUMBER

Figure 1. Cumulatlve profitablllty of P fertlllzerlevels and placement over a four-year growlngperlod. Manaus, 1981-1985.

even1y distributed over each crop. Frequent bandedapplications of moderate amounts of P fertilizer arealso compatible with the limited capital available tofarmers in the region.

ImplicationsThesé data confirm that continuous cultivation of

annual crop rotations involving com and cowpea ispossible in c1ayey Oxisols, the other extreme in therange of well-drained acid soils of the humid tropicsfrom the sandy or loamy Ultisols of Yurimaguas.

Since P deficiency is the first constraint encoun-tered, the profitability of continuous cultivationdepended on how P was managed. Without P fertiliz-ers, no annual crops were profitable regard1ess ofother inputs applied. Removing P as a constraint witha broadcast application of 176 kg P/ha cost so muchthat the e..conomic break-even point was not reacheduntil the fourth crop. Banding small rates of Ppermitted continuous profits, beginning with thesecond crop. This is obviously the recommendedoption.

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