Scott Shaw, Plant and Food Research

Maintaining soil pH within acceptable limits is important for crop performance. For most crops this is somewhere between 5.5 and 7.0, which ensures nutrients are freely available to plants and that mineral toxicities don’t occur. Equally, we know that soil conditions in paddocks can be variable and this includes soil pH. Recently there has been a groundswell of interest in variable rate technology (VRT) to manage variable soil conditions within individual paddocks. Soil pH variability can be minimised effectively using zone mapping and VRT to tailor lime applications. This approach is termed site-specific or zone management.

Two key questions should be asked before investing in zone management – will it pay dividends? and over what time frame? Clearly the costs associated with zone maps and VRT need to be offset by increased revenue from higher yields, better quality crops, or savings from a reduction in total fertiliser volumes.

There have been numerous studies done over the years looking at the effects of soil pH on a large number of crops and soil types. These have included looking at things like root growth, yield, soil biology and disease incidence. The good news is that in good agronomic conditions most crops are capable of producing high yields of quality produce over a relatively wide range of soil pH’s.

If you are considering using zone management to reduce variation in soil pH, a good approach is to identify some basic zones within your paddocks (either using soil maps or knowledge of the land) and then collect a soil sample for pH analysis from within each of these zones. If the difference between zones is less than 0.5 pH units (e.g. 6.0 vs. 6.5) the likelihood of zone management paying dividends is unlikely. If the zones vary by up to 1 pH units (e.g. 5.5 vs. 6.5) and the size of the most extreme zone is significant (e.g. >20% of the paddock area) there may be an economic benefit investing in zone management over the medium to longer term for some crops. When significant sized extreme zones vary by >1.5 pH units (e.g. 5.0 vs. 6.5) using zone management to mitigate variability in soil pH should pay dividends over the short to medium term.

In summary, zone management can be useful and economic in certain situations. There are many factors to consider including the cost of implementing the technology. Do your homework and think about the pros and cons. Get into your paddocks, dig some holes and take some soil samples from different zones. Understand the size of the issue. There is no doubt that soil pH is important, but is variability in soil pH really an economic concern? You be the judge!