Get the most out of your soil tests

At this time of the year it's not easy to think about your fertiliser requirements – after all, calving and lambing are fast approaching and are bound to be top of mind. Yet that imminent busy time is exactly why you should be getting soil testing out of the way now, while there is less pressure.

Establishing a pattern of early-winter soil testing enables you to plan your annual fertiliser applications without the distractions of the busy early spring season.

Is winter soil testing right for you?

Winter soil testing doesn't suit every farm, but many parts of the South Island do offer good conditions for soil testing at this time of year. Farms that are on free-draining soils are ideal candidates for winter soil testing, as the ground conditions are usually good through the cooler months. Areas of the island that receive low rainfall can also be soil tested in winter, because they tend not to suffer from waterlogging.

However, if you have applied fertiliser late in autumn, then winter soil testing is not advisable. There should be a gap of at least three months between fertiliser application and soil testing, otherwise you are likely to get an inaccurate indication of soil fertility.

Additionally, if you have a history of soil testing at other times of the year, then you are better to persevere with this, because you will benefit from having a long-term picture of the nutrient trends on your farm.

While one-off soil test results do provide valuable information, trend lines of individual nutrients give you a better idea of your land's fertility status, and how well your fertiliser strategy is suited to your farming practice. It takes a minimum of three years' successive soil tests to start to see trends develop, and five years to see a definite trend. If the trend lines show that soil fertility is increasing and the levels are above that required for maximum production, then fertiliser inputs can be confidently decreased.

Similarly, a declining trend would mean that historical inputs are insufficient to meet current needs, and the fertiliser strategy should be reviewed.

In order to see valid trend lines, soil testing needs to be carried out in a consistent manner; soil tests should be taken along the same transect, at the same time of year and to the same depth each time. This will reduce variability and give you a much more reliable picture of your soil's nutrient trends.

Traditional soil tests or whole-farm?

Traditionally, soil testing is structured so that each unique block of the farm is tested separately, and fertiliser recommendations are extrapolated from these results. That means dividing the farm into areas with the same soil type and testing representative paddocks from each of these areas. This is a reasonably easy and cost-effective way to get an overview of farm nutrient levels and to develop an acceptable fertiliser strategy for the farm.

However, as farming has become more intensive, there has been increasing interest in whole-farm soil testing, where every individual paddock is soil tested. This approach has also been adopted in other countries, including the United Kingdom. Clearly testing every paddock is more time consuming and more expensive than simply collecting representative samples from blocks, so where is the benefit? Our experience is that taking this approach can reduce fertiliser expenditure, and that this can far offset any additional costs incurred by extra testing.

A research exercise conducted by Hill Laboratories reached the same conclusion. In this example, the traditional block by block soil tests and maintenance fertiliser approach was compared to paddock-by-paddock soil tests and tailored fertiliser applications. The outcome was a near 50% reduction in the fertiliser recommendation for the farm as a result of whole-farm testing – which would amount to a considerable cost saving.

You can see how this might come about if you imagine a block with 20 paddocks, where the range of Olsen P values might be from 15 to 50, with an average of 25. If the goal is to get the average Olsen P up to 30, and fertiliser is applied equally to all paddocks, then the actual range ends up being 20-55 – some paddocks fall short of your target, some overshoot the target.

Whole-farm soil testing allows differential application of fertiliser to each paddock, for a much more tailored solution.

The outcome is better for the farm and for the environment, as less fertile paddocks can be brought up to optimum levels, which improves production, and paddocks with high Olsen P levels can be managed down, reducing environmental impact without affecting production.

Whole-farm soil testing is worth thinking about if you are running a high-production system. Many dairy farms in the south fall into this category. A lot of high-producing dairy farms in the South Island are relatively young, and have had a lot of money invested in them during the conversion process. They've often come out of a base of low-fertility dryland farms, where soils have had low Olsen P levels. Bringing them up to an optimum level of Olsen P 25-30 has taken quite an investment in capital fertiliser, so it makes sense to take the time to keep paddocks in this range, and not let them stray too far either side.

Whole-farm soil testing is one approach that can help with this – for more information, talk to your Farmlands Technical Field Officer today.

Article supplied by Ballance Agri-Nutrients