LONG-TERM CHANGES IN MEHLICH-3 EXTRACTABLE P AND K IN A SANDY CLAY LOAM SOIL UNDER CONTINUOUS CORN (ZEA-MAYS L)

Rates of change of soil nutrient levels during crop production can be used to evaluate the long-term economic value and sustainability of fe rtilizer practices. Objectives were to quantify changes in Mehlich-3 e xtractable P and K due to additions of inorganic and manure P and K in corn (Zea mays L.) production. Surface (0-20 cm) and subsoil (20-40 c m) samples were collected after harvest from 1984 to 1993 on a Chicot sandy clay loam soil (Grey Brown Luvisol) fertilized with two rates of inorganic P and K and with dairy manure. Manure P at 60 kg P ha(-1) p lus inorganic fertilizer P at 132 kg ha(-1) yr(-1) increased extractab le P 1.0 kg for every 4.2 kg added P if only topsoil was considered, f or every 3.2 kg added P if corn grain P removal was deducted, and for every 2.8 kg added P if subsoil increases were included but corn P upt ake excluded, and 2.1 kg added P with subsoil P included and corn P re moval deducted. Without manure, 132 kg P hal yr(-1) increased Mehlich- 3 extractable P by 1.0 kg for every 8.1 kg P added, or 5.3 kg P added if corn P removal was deducted from added fertilizer P. At the low rat e of 44 kg P ha(-1) with manure P, Mehlich-3 levels increased by 1.0 k g P for every 5.2 kg added fertilizer P. Extractable P remained consta nt when inorganic fertilizer P was added without manure, probably beca use crop removals equalled fertilizer additions. Mehlich-3 extractable K increased in both top soil and subsoil at rates of 141 and 332 kg h a(-1) yr(-1) inorganic K except for the 141 K rate in the final four y ears when no manure was applied. When both soil depths were included a nd summed over all years, values of added K to increase extractable K by 1.0 kg ranged from 4.2 to 5.5 kg, regardless of source or rate of a dded K. To increase soil test P, manure combined with high P rates was more effective per unit added P than low P rates or added P without m anure. Either form or rate of added K was equivalent in increasing ext ractable K.