Maize response to nitrogen and phosphorus starter fertilisation in mineral-fertilised or manured systems

Phosphorus (P) is an essential nutrient for maize production, but in temperate areas the P uptake during early growing stages can be limited due to low soil temperature, even though the soil was tested high in P. The objective of this study was to assess the effects of nitrogen and phosphorous (NP) starter fertilisation during early growth stages and its carryover until maize harvest, in mineral-fertilised or manured systems. A field experiment was carried out in north-west Italy during the 2019 and 2020 growing seasons. The trial compared sub-surface placement of NP (diammonium phosphate) or N alone (ammonium nitrate) in bands close to the maize seed furrows, in differing long-term (LT) fertilisation managements: two doses of urea (Min-L and Min-H), two doses of bovine slurry (Slu-L and Slu-H) or two doses of farmyard manure (Fym-L and Fym-H). The two rates, low (L) and high (H), corresponded to 170 and 250 kg N ha⁻¹ year⁻¹ respectively. Compared to N fertilisation, NP starter fertilisation improved early maize growth assessed by leaf area index (LAI) and shoot dry weight (SDW) in all systems. The effects differed between the two years (2019: LAI + 63%, SDW + 67%; 2020: LAI + 36%, SDW + 38%), as 2019 was cool during the first growth. Higher LAI and SDW values were confirmed at crop flowering in the mineral-fertilised systems only. As shoot growth was enhanced by NP starter fertilisation, anthesis occurred 1 day earlier in all systems. However, a response to NP starter fertilisation at harvest was recorded in mineral-fertilised systems only (+1.3 and +3.2 t ha⁻¹ in Min-L and Min-H, respectively). The uptake of P, used as a true indicator of soil nutrient availability, increased with increasing soil Olsen P until 39 mg kg⁻¹. These results suggest that soil test thresholds should be revised for points above which P fertilisation should be suspended.