(4) RESUMO A presença de Al em níveis tóxicos no solo inibe o crescimento das raízes com reflexos negativos na absorção de água e de alguns nutrientes, especialmente do P. Sendo assim, a adição de grandes quantidades de fertilizantes fosfatados deverá tornar a absorção de P menos dependente da existência de um amplo sistema radicular e com isto diminuir a resposta das culturas à calagem. O presente trabalho objetivou avaliar o rendimento de milho de acordo com a aplicação de quantidades crescentes de P e de calcário. O experimento foi desenvolvido em Lages (SC), de 1994 a 1998, num Latossolo Bruno argiloso com pH 4,7, Al trocável = 39 mmol c kg -1 , 1 mg kg -1 de P, que requeriam 9,0 t ha -1 de calcário para elevar o pH a 6,0. Combinaram-se três doses de calcário (0, 4,5 e 9,0 t ha -1 ) com quatro de P, no delineamento de parcelas subdivididas. As quantidades de P 2 O 5 foram, respectivamente, de 60, 120, 180 e 240 kg ha -1 , na primeira safra; de 40, 80, 120 e 160 kg ha -1 , na segunda e na quarta, e de 30, 60, 90, e 120 kg ha -1 , na terceira safra, perfazendo uma média de 42, 85, 127 e 170 kg ha -1 por cultivo, respectivamente. O efeito benéfico da calagem aumentou com o passar dos anos, de inexistente, no primeiro cultivo, para incrementos de até 39% no rendimento de milho. Na maioria das safras, a produtividade aumentou somente até pH 5,4, quando foi aplicada metade da dose de calcário recomendada pelo método SMP para elevar o pH até 6,0. Nos tratamentos que receberam as duas maiores doses de P 2 O 5 (127 e 170 kg -1 ha -1 por cultivo), (média de 85 kg ha -1 por cultivo), a calagem não influiu no rendimento de milho. A adubação fosfatada aumentou a produtividade de milho em todas as safras, porém em magnitudes que decresceram com a elevação do pH. Os maiores rendimentos foram obtidos com a dose recomendada em duas delas, porém, em outras duas, a quantidade necessária foi 50% maior.Termos de indexação: calagem, adubação fosfatada, solos ácidos.
Poor accessibility and cost of soil testing reduce effectiveness of fertilizer use on small-scale subsistence farms, and inadequate funding promotes adoption of soil tests in developing countries with minimal validation. For example, Mehlich I extraction of phosphorus (P) currently used extensively in Guatemala may not be suitable for Guatemala's broad range of soils. At least four alternatives are available but relatively untested [Bray 1, Mehlich III, Olsen, and pressurized hot water (PHW)]. Pressurized hot water is relatively simple and inexpensive but is not yet tested against other extraction methods under variable P or potassium (K) fertilization levels. To determine whether PHW-extracted nutrients could be used to predict maize yield and nutrient concentration and uptake, soil, plant tissue and grain samples were obtained from a multiple-site field study, and calibration studies were conducted using five rates of P and three rates of K on soils incubated without plants or cropped with maize in greenhouse and field conditions. In the multiple-site field study, maize yield related significantly to PHW-extractable P (r 2 ¼ 0.36) and to leaf P concentration 1815 (r 2 ¼ 0.23), but Mehlich I -extractable P did not. In the two soils used in the greenhouse study, maize yield, vegetative P concentration, and total P uptake by maize were predicted by PHW-extractable P (R 2 ¼ 0.72, 0.75, and 0.90, respectively). In the field experiment, grain yield was not improved by P or K application, but P concentration of maize leaf tissue did relate significantly with PHW-extracted P (R 2 ¼ 0.40). Mehlich I did not. There were no yield responses to K application in any experiment, but relationships defined between extractable K for all five K-extraction procedures and soil-applied K were similarly significant. In comparison, PHW was as good as or better than Olsen whereas Bray 1 and Mehlich III were less consistent. Mehlich I was overall the poorest P extractant. Mehlich I extraction of P should be replaced by one of the four alternatives tested. PHW is the least expensive and, therefore, most viable for use in Guatemala soils.
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