Phosphorus management in continuous wheat and wheat-legume rotations

Abstract

The sustainability of cropping systems is closely related to the judicious use of fertilizers. Little research has been conducted on the management of P in rotations in Morocco. The purpose of this study was to determine the effects of direct, cumulative, and residual P on wheat (Triticum aestivum) and chickpea (Cicer arietinum L.) yields under field conditions in two cropping systems: continuous wheat (W-W) and chickpea-wheat (CP-W). Experiments were conducted in 1994–96 at two locations in the arid and semiarid regions of Morocco. Phosphorus was applied the first year at rates of 0, 9, 18, and 27 kg P ha⁻¹ on both wheat and chickpea. The second year, plots were split into treatments with P and without P fertilizer. The changes in NaHCO₃-P in soil showed that after two years of cropping, P rates of 9 and 18 kg P ha⁻¹ were needed to increase and maintain soil test P level in the range where a third successive crop could be grown without fertilization at locations 1 and 2, respectively. Also, soils with the same initial NaHCO₃-P soil test levels required different amounts of fertilizer P to produce maximum yields. Inclusion of chickpea in the rotation resulted in a greater response to residual P by wheat at location 2. Differences in wheat grain yield between rotations were not significant. The maximum increase in yields above the nil-P treatment due to the highest amount applied in the the previous year was 1.3 t ha⁻¹, obtained for continuous wheat at location 2. Though the residual P effect was evident in this study, it did not produce maximum yields. Yields (GY, DM) could be predicted by the inclusion of both P applied in previous year (PR) and P applied in the current year (CP) by the following model: GY or DM = a + b*Ln(RP+1) + c*Ln(CP+1). Based on the range of P rates used in this study, a single P application for a 2-year rotation is not a suitable practice in these soils. The application of 18 kg P ha⁻¹ each year is recommended for continuous wheat, and 9 kg P ha⁻¹ the first year plus 18 kg P ha⁻¹ the second year is recommended for chickpea-wheat rotations. We suggest that either using single large applications of P or performing repeated applications should take into account the range of targeted application rates.