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 fertilizer practices. Objectives were to quantify changes in Mehlich-3 extractable 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 cm) 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 plus inorganic fertilizer P at 132 kg ha−1 yr−1 increased extractable P 1.0 kg for every 4.2 kg added P if only topsoil was considered, for 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 uptake excluded, and 2.1 kg added P with subsoil P included and corn P removal deducted. Without manure, 132 kg P ha−1 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 rate of 44 kg P ha−1 with manure P, Mehlich-3 levels increased by 1.0 kg P for every 5.2 kg added fertilizer P. Extractable P remained constant when inorganic fertilizer P was added without manure, probably because crop removals equalled fertilizer additions. Mehlich-3 extractable K increased in both top soil and subsoil at rates of 141 and 332 kg ha−1 yr−1 inorganic K except for the 141 K rate in the final four years when no manure was applied. When both soil depths were included and 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 added 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 manure. Either form or rate of added K was equivalent in increasing extractable K. Key words: Long-term fertilization, continuous corn, organic manure P, inorganic P, Mehlich-3 extractable P, Mehlich-3 extractable K