Apparent nitrogen fertilizer replacement value of grass-clover leys and of farmyard manure in an arable rotation. Part II: Farmyard manure

The apparent nitrogen fertilizer replacement value of farmyard manure (NFRV^FYM) was studied in a long-term experiment on a loamy sand soil at Grabow, Poland. The experiment combined five rates of FYM with four nitrogen (N) fertilizer rates in two rotations (RotA and RotB) started in 1980 (Expt1) and, in parallel, 1981 (Expt2). Rotations consisted of potatoes, winter wheat and spring barley, followed by silage maize (RotA) or grass-clover ley (GCL, RotB). We analysed six consecutive cycles. NFRV^FYM was much larger in RotB than RotA, due to high N supply from GCL residues combined with likely deficiencies in potassium (K) and/or phosphorus (P), causing weak crop responses to N fertilizer in RotB relative to FYM effects. Devoid of non-N effects, 'true' (as opposed to apparent) NFRV^FYM was estimated at 2.0 kg fertilizer-N per tonne of FYM (at equal N offtake), or 2.7 kg/t (at equal DM yield). These values correspond with equivalencies of 0.37 and 0.50 kg fertilizer-N per kg FYM-N. RotB required 25 kg P and 350 kg K/ha/cycle more input than RotA to compensate for higher P and K offtake in GCL compared to maize. The study highlights differences between approaches to assess NFRV^FYM, giving larger values at high compared with low N fertilizer rate, and larger values at low compared with high FYM rate. These contrasts reflect drift in the ratio between the agronomic N-use efficiencies of FYM and fertilizer, respectively, and in the ratio between the apparent recoveries of N from these sources. Journal compilation