The art of soil test­ing _______________________________

Tak­ing soil tests to as­cer­tain nu­tri­ent lev­els for sub­se­quent crop pro­duc­tion is a very small but es­sen­tial cost which can pay back large div­i­dends.

NZ Grower - - News - Robin Boom

Not only will a good soil test iden­tify which el­e­ments the fer­tiliser bud­get should fo­cus on ap­ply­ing to the crop and what quan­ti­ties are re­quired, but it will also iden­tify which el­e­ments are not re­quired. Ap­ply­ing un­nec­es­sary or ex­ces­sive nu­tri­ents to the soil is not only a waste of money, but they can also be bad for the en­vi­ron­ment when these are leached into ground wa­ter or lost as run-off into streams. Too much of one el­e­ment can also neg­a­tively af­fect over­all crop yield, mak­ing crops more sus­cep­ti­ble to at­tack from pathogens, and af­fect the qual­ity of crop grown it­self, as well as the shelf life of cer­tain fruits and veg­eta­bles.

When tak­ing soil tests off a par­tic­u­lar field, you need to iden­tify if there are any soil type, aspect or con­tour dif­fer­ences within the area you are sam­pling. If the field is flat and uni­form with a known sim­i­lar fer­tiliser his­tory, and pre­vi­ous yields across the field have been sim­i­lar, then a ran­dom walk across the field tak­ing 10-20 plugs should suf­fice. If how­ever there are pond­ing ar­eas, or if there are dis­tinct soil type or con­tour dif­fer­ences, then these should be sam­pled sep­a­rately, or not be in­cluded in the sam­ple should they only be a small frac­tion of the area the sam­ple is to rep­re­sent, as in­clud­ing these ano­ma­lous ar­eas in the bulk sam­ple will skew the re­sults and lead to in­cor­rect di­ag­no­sis of what fer­tiliser nu­tri­ents are needed.

If the pad­dock has pre­vi­ously been in pas­ture, ar­eas around wa­ter troughs, gate­ways, shel­ter belts, hedges or trees should be avoided as these are ar­eas where livestock will have con­gre­gated and will be higher in fer­til­ity than other parts of the pad­dock due to more dung and urine be­ing de­posited there. If the pad­dock has been re­cently grazed with cat­tle in par­tic­u­lar, you should wait for at least 2-3 weeks af­ter graz­ing to iden­tify urine patches and avoid in­clud­ing these in the sam­ple as this will lift the ni­tro­gen and potas­sium lev­els in par­tic­u­lar. Sheep and deer urine patches are not as big and con­cen­trated so will not skew the sam­ple as much as cat­tle urine patches.

When to take sam­ples is an­other im­por­tant con­sid­er­a­tion. Soil sam­ples should not be taken if fer­tiliser has been ap­plied within the pre­vi­ous three months as this could con­tam­i­nate sam­ple. The mois­ture con­tent of the soil is also im­por­tant and tests should not be taken in drought con­di­tions or if the soils have been

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