Soil test for best re­sults

Shepparton News - Country News - - GRASSLAND CONFERENCE -

Ap­ply­ing phos­pho­rus to best im­prove pas­ture was the topic of CSIRO Agri­cul­ture and Food re­search sci­en­tist Dr Richard Simp­son’s talk.

Dr Simp­son said there were dif­fer­ing views on what im­prove­ments, if any, were made to pas­ture once phos­pho­rus was ap­plied, with some found to be in­cor­rect through re­cent re­search.

‘‘Typ­i­cally we think in a very lin­ear fashion about what hap­pens when phos­pho­rus fer­tiliser is ap­plied to im­prove or main­tain high pas­ture pro­duc­tion,’’ he said.

‘‘Some of us as­sume that each ap­pli­ca­tion of su­per­phos­phate feeds the pas­ture di­rectly to boost pas­ture growth.

‘‘Oth­ers have heard about ‘P-fix­a­tion’ in soils and have a pes­simistic view of how fer­tiliser works.

‘‘Re­cent re­search has demon­strated that nei­ther view is quite right and that by us­ing soil test­ing to in­form P-fer­tiliser use, we can ob­jec­tively en­hance the cycling of phos­pho­rus in a graz­ing sys­tem to sup­port high pas­ture growth rates and an­i­mal pro­duc­tion.’’

Dr Simp­son said there were three im­por­tant phases of phos­pho­rus cy­cle man­age­ment.

‘‘The first is the sub-op­ti­mal soil phos­pho­rus fer­til­ity phase, which is char­ac­terised by un­re­alised po­ten­tial for pas­ture growth and stock car­ry­ing ca­pac­ity.

‘‘Op­ti­mum soil phos­pho­rus fer­til­ity co­in­cides with the at­tain­ment of the crit­i­cal STP (soil test phos­pho­rus lev­els) con­cen­tra­tion in soil and should be ac­com­pa­nied by a recog­nis­able im­prove­ment in the clover con­tent of the pas­ture.

‘‘And over-fer­tilised pas­tures oc­cur if crit­i­cal STP con­cen­tra­tion of the soil is grossly ex­ceeded,’’ Dr Simp­son said.

To iden­tify the ideal STP level for your soil, Dr Simp­son said if us­ing the Olsen test it would usu­ally be 15 mg P (phos­pho­rus)/kg, while if us­ing the Col­well P test, he sug­gested us­ing the Phos­pho­rus Buf­fer­ing In­dex soil test to de­ter­mine the crit­i­cal Col­well P value that is ap­pro­pri­ate for your soil.

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