The Loam Ranger – The benefits of subsoil testing

SESL Australia

Loam Ranger logoDear Loam Ranger,

I’ve always tested my topsoil, but now I hear that the DPI wants to encourage farmers to test the subsoil too. What’s the point of this?

Most routine soil testing is done on the top 150 mm of the soil. Probably 95% of our clients sample just that layer. Yet chemical analysis of subsoils can provide vital information in several situations:

  1. To determine whether the subsoil can contribute to plant nutrition. The most obvious example is magnesium. Most subsoils contain more magnesium than the topsoil. Thus, even if the topsoil is deficient, plants may get all the magnesium they need from the subsoil. This can also be the case with iron, manganese and sometimes potassium. In addition, in drier climates nitrate can accumulate in subsoils, providing nitrogen to the crop.
    When you are devising a fertiliser program, it is important to keep in mind that most crops can put down roots to as deep as 1 metre (and some, such as lucerne and grapevines, to much deeper). Without deeper sampling, you won’t know whether you’re benefiting your crop by applying fertiliser or just wasting your money. For example, a CSIRO study in south-eastern Queensland showed that 35% of phosphorus removed by grain crops came from the 100–300 mm layer. A single topsoil sample would not have shown this.
  2. To determine whether deep inversion (deep ploughing to mix topsoil with subsoil) could be of benefit. For example, subsoil may be less acidic than topsoil, and a simple deep ploughing could reduce the need for liming.
  3. Where topsoils are being stripped or exposed, as in land developments and during earthmoving for orchard development. These situations provide an opportunity to apply lime or gypsum in acidic or calcium-deficient soils. It is not usually feasible to apply calcium to subsoils unless they are exposed.

Subsoil sampling can also reveal potential drainage problems, salinity or sodicity, and pH imbalances leading to nutrient deficiencies or toxicities. All of these problems can interfere with a crop’s ability to draw up water and nutrients.

Case study

The results of triple-depth soil testing we conducted this year in 80 paddocks in the Central West of NSW clearly show the pitfalls of basing nutrient management programs on soil testing at 1 depth (i.e. 0 to 150 mm).

In general, surface testing underestimates the reserves of calcium, magnesium (Figure 1) and potassium available to plants, as these elements generally increase with depth (along with pH). Conversely, the plant availability of nitrogen (Figure 2), phosphorus, zinc, iron and manganese decreases with increasing depth, so an adequate value in the topsoil may hide an overall deficiency.

Figure 1. Magnesium increases with increasing depth.

Graph of Mg in soil

Figure 2. Nitrogen decreases with increasing depth.

Graph of nitrogen in soil

These results emphasise that a scientifically sound approach to plant nutrition should seek to quantify and understand the contribution from the entire rooting depth, not just the topsoil. In this case study from the Central West, an assumed magnesium deficiency based on surface soil testing may not happen, because plant roots can draw on subsoil reserves. Conversely, an assumed nitrogen adequacy based only on the surface soil result would imply more N than is actually available to plants. Thus, if we looked only at the nutrients present at the surface and ignored the nutrient reservoir below, we would come up with a very different nutrient management program.

Of course, this approach is applicable only to plants that put down roots deeper than 150 mm. Urban soils and, in particular, sports turf do not generally have deep roots. Many of the sports fields we assess show root growth ceasing at about 100 mm. This highlights the drawbacks of frequent irrigation, the use of shallow-rooting turf species, or ill-conceived soil profiles (e.g. clay capping on landfill with 150 mm of alluvial sandy loam on top).

What to sample

We recommend sampling at 3 depths: ideally 0–150, 150–300 and 300–600 mm. Remember to combine subsamples at each depth.

When collecting subsoil samples, take care not to get any topsoil mixed with the subsoil, as this can skew results.

In summary

Subsoils provide essential functions for plants: root anchorage, water supply and nutrition. It pays to know if anything needs to be done to improve how they do this. Remember the following rules of thumb about subsoils compared with topsoils:

  • They are usually higher in clay and hence cation exchange capacity.
  • Magnesium and sodium contents are usually higher.
  • Organic matter content is much lower.
  • Salinity increases with increasing depth.
  • pH increases (subsoils are not as acidic).

Further reading

Nash D, Brown A. 1998. Sampling soils used for growing pastures, field and fodder crops. Agnote AG0375. Department of Primary Industries Victoria.

Keys M. 1996. Management of profitable and sustainable pastures – a field guide. NSW Agriculture (NSW DPI).

Wang X, Lester DW, Guppy CN, Lockwood PV, Tang C. 2007. Changes in phosphorus fractions at various soil depths following long-term P fertiliser application on a Black Vertosol from south-eastern Queensland. Australian Journal of Soil Research 45(7): 524–532.