Phosphorus management in continuous wheat and wheat-legume rotations

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 crop ping systems: continuous wheat (W-W) and chickpea-wheat (CP-W). Experiments were conducted in 1994-96 at two locations in the arid and semiarid region s of Morocco. Phosphorus was applied the first year at rates of 0, 9, 18, a nd 27 kg P ha(-1) on both wheat and chickpea. The second year, plots were s plit 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 rang e where a third successive crop could be grown without fertilization at loc ations 1 and 2, respectively. Also, soils with the same initial NaHCO3-P so il test levels required different amounts of fertilizer P to produce maximu m yields. Inclusion of chickpea in the rotation resulted in a greater respo nse to residual P by wheat at location 2. Differences in wheat grain yield between rotations were not significant. The maximum increase in yields abov e 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 maximu m yields. Yields (GY, DM) could be predicted by the inclusion of both P app lied in previous year (PR) and P applied in the current year (CP) by the fo llowing model: GY or DM = a + b*Ln(RP+1) + c*Ln(CP+1). Based on the range o f 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 firs t year plus 18 kg P ha(-1) the second year is recommended for chickpea-whea t rotations. We suggest that either using single large applications of P or performing repeated applications should take into account the range of tar geted application rates.