Gut instinct or past experience is no match for a soil test when farmers want to manage budgets and maintain pasture productivity, says an industry expert.
Ballance science extension manager Ian Tarbotton said keeping soils fertile was good insurance with pasture an essential feed source, "but gut instinct or pastexperience won't lead to good decisions on what to spend or save".
"Soil tests will show you what you have to work with and they are the best guide to decisions around a fertiliser budget. The last thing farmers want to do is to compromise future productivity, so understanding what nutrients are available now is the best basis for decisions on fertiliser budgets."
There were three main nutrients to focus on when it came to soil fertility, phosphorus, sulphur and potassium, and how they behaved in soil provided a clue for application decisions, he said.
"Sulphur is very mobile in the soil, so usually requires annual applications. Potassium can leach too, notably on coarse textured soils. On the other hand, phosphorus gets stored in the soil so, depending on reserves, it may be possible to defer phosphate fertiliser application. Some nutrients, such as magnesium and calcium, could be supplied directly to stock as a short-term measure."
Phosphorus was a key driver of production so, if possible, levels should be kept in the optimum range and testing would confirm whether levels need attention, he said.
If sedimentary soil was above the optimum Olsen P range of more than 40, then cutting out phosphate maintenance for a year would not affect production. If the range was more than 30, then half maintenance of phosphate would not lead to noticeable impacts.
If a farmer made the call to omit or reduce phosphate, maintaining sulphur levels was important, Mr Tarbotton said.
He cautioned another consideration when planning to apply nutrients was the drop in the New Zealand dollar over the last 12 months. It was only a matter of time before the price of imported fertilisers would increase, he said.
