More Value from Hot Water Soil Tests | Hill Laboratories - NZ


More Value from Hot Water Soil Tests

Date: 13 May 2022
Author: Hill Laboratories

In response to our customers growing interest in soil health, the laboratory set up the Hot Water Extractable Carbon (HWEC) test a few years ago and it has been gratifying to see the uptake of this test. The HWEC test is a measure of the labile (easily broken down) carbon fraction in the soil organic matter, and includes carbon from soluble carbohydrates, amino acids, root exudates and microbes. Microbial biomass carbon (MBC) is considered a key measure of soil health, but it is a relatively difficult and expensive test to carry out in the laboratory.

Late in 2020, Hill Laboratories commissioned soil scientist Dr Anwar Ghani to write this science view, which discusses the hot water extractable carbon test in relation to soil health. The strong correlation between HWEC and MBC, as described by Dr Ghani in his article and in the references, has led Hill Laboratories to now offer an Estimated Microbial Biomass Carbon (MBCest) measure whenever a HWEC test is requested. It is important to understand the estimated MBC result that will be reported is simply a derived result, but it is thought to be of value for monitoring the status of soil health over time.

Excitingly, Hill Laboratories will soon be able to offer the HWEC test using NIR spectroscopy, rather than the slower reference wet chemistry method. The NIR validation is looking promising, except for raw peat soils which are not highly suitable for the HWEC test, given their very high organic matter content. The laboratory uses qualification statistics to monitor the prediction performance of the HWEC test by NIR, and “code-swap” to the reference method any results flagged as outliers.

Another “hot” test is the Potentially Mineralisable Nitrogen (PMN) test, developed in conjunction with a project run by Plant & Food Research towards better nitrogen management for crops. The PMN test uses the result from the Hot Water Extractable Organic Nitrogen method in its calculation, described in more depth here. 

Test information for both HWEC and PMN (HWEON) can be found in the Client Resources section of our website, under Technical Notes - Agriculture. We know these test acronyms can be maddening, so if further clarification is needed, we have a team of Client Services Managers ready to assist