Collecting good information will help make cost-effective decisions. Monitoring soil fertility levels when withholding fertiliser is a good example of this. Set up sampling lines that are on different soils, contours, and if applicable, grazing pattern areas. Take 15 or more core soil samples along each sample line. Do the soil tests on the same month each year and before fertiliser is applied to the paddock. Use these soil tests to build up a trend over time. Keep using the same sampling lines, as the trend over time is more useful than a one-off test in a new area.
Before you can consider increasing the Olsen P level, you need to establish the maintenance fertiliser requirements for your farm. Use soil test results, soil type, amounts of imported feed and load this into Overseer to determine the maintenance requirement for the farm. Your fertiliser representative will help you with this. Keep an eye on the potassium and sulphur levels as well. These could have been depleted during the downturn when changes to the fertiliser type and amount were made.
Some farmers in Waikato like to lift their Olsen P levels to the upper end of optimum. This could be seen as a risk management tool so sub-maintenance fertiliser could be applied for a short
time as an option without limiting production.
If your Olsen P tests are below optimum, field trials have given us an indication of the amount of phosphate required above maintenance to lift your Olsen P by 1 unit.
Replenishing soil phosphate after a downturn
During the recent downturn in milk price a number of farmers have stopped or reduced their application of phosphate fertiliser to improve cashflow. When milk price starts to improve it is a good time to consider how and when to replenish the soil phosphate back to levels which maximise pasture production.
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