Pasture production and soil phosphorus fractions resulting from six previous annual applications of triple superphosphate or Sechura phosphate rock

Abstract

Abstract The effects of six annual applications of either Sechura phosphate rock (SPR) or triple superphosphate (TSP) on pasture production and soil phosphorus (P) fractions were compared at eight New Zealand sites. At most sites, pasture production resulting from residual fertiliser P was significantly ( P < 0.05) greater for SPR than it was for corresponding rates of TSP. Molybdenum (Mo) present in the SPR may have contributed to this effect at four of the sites where Mo deficiency was suspected. Sequential extraction (alkali followed by acid) was used to determine chemical forms of residual fertiliser P in soil samples from all sites. Fertiliser P which had reacted with the soil or been adsorbed on soil surfaces was alkali‐soluble whereas undissolved apatite residues were acid‐soluble. SPR‐treated soil had significantly ( P < 0.05) higher amounts of acid‐soluble P than TSP‐treated soil, lower amounts of alkali‐soluble P (except at two sites), and higher amounts of total P (except at three sites). The greater effect of previous SPR applications on pasture production at Mo‐adequate sites, compared with TSP, was attributed to the dissolution of some of the apatite residues accumulated in the soil. At fertiliser application rates equivalent to 1 or 2 times the maintenance P requirement for pasture production, Olsen P values for TSP‐treated soil were higher than for the corresponding SPR‐treated soil at most sites. Olsen P values for TSP‐treated soil were linearly correlated with alkali‐soluble P at each site. Olsen P values for the SPR‐treated soils were close to values predicted from the amounts of alkali‐soluble P present. The Olsen P test underestimated the phosphate status of the SPR‐treated soils because it did not extract any of the residual apatite.