Improved Reserve-K analyses

As part of the FAS Agricultural Laboratory’s ongoing commitment to provide our customers with accurate, evidence-based recommendations, we are constantly refining our methods and interpretations. One of our most recently introduced analyses is the Reserve-K estimates which are part of your routine soil fertility analysis report, along with adjustments to K fertiliser recommendations.

What is Reserve-K?

Reserve-K is an estimate of slowly exchangeable K held between layers of clay in minerals such as illite (think of it as the cheese between two slices of bread). These types of soils are more common in the irrigated regions where there are soils that have not been extensively weathered, or in floodplains and valley-bottoms, where these K-rich minerals sometimes accumulate. In soils with high amounts Reserve-K, K for crop uptake can be resupplied from these reserves (the slices of bread can be opened and the cheese removed). The figure below outlines the process of K being slowly released from these layers to become ‘readily available’ to the plant.

The Reserve-K test

The FAS Agricultural Laboratory developed a system that applied reductions to the recommended K-fertiliser rate based on the amount of Reserve-K and Exchangeable K in a sample. The approach is category-based, applying either no reduction (when there is low exchangeable K or Reserve-K amounts) or reducing K fertiliser recommendations by either 30, 60 or 100% as Reserve-K amounts increased from one category to the next. While a convenient approach, some concern over large K-fertiliser reductions being applied for small changes in Reserve-K values (particular at the category boundaries) were noted.

Improvements to the adjustment scale

A recent review of this approach thus led to conversion of the category-based adjustment to a sliding scale of K-fertiliser rate reductions. This approach better aligns the amount by which the K-fertiliser recommendation is reduced to the amount of Reserve-K in a sample. This means that for smaller amounts of Reserve-K, reductions to the K recommendation are lesser, and as Reserve K increases, the amount to reduce will proportionally increase. Where Reserve K is low (or if exchangeable K is very low), no reductions to K-fertiliser are recommended. In instances where Reserve-K is very high, a maximum reduction of 90% has been introduced to ensure that your crop still has a small supply of immediately available K for early crop growth. The difference in approaches is graphically presented in the figure below.

For more information on K management and Reserve-K see Information Sheet 7.5, available  from our Publications page.

Sample submission form

Selecting the correct sample submission form

The FAS Agricultural Laboratory provides a wide variety of agricultural analyses that target better management of your soil and crop. Each suite of analysis uses specific methods, extractions and interpretations to ensure accurate and reliable recommendations are made. However, when samples are submitted with the incorrect forms or have incorrect information filled in, the resulting FAS analysis may not be what was expected and the interpretation of the results will be incorrect. It is thus CRITICAL that the appropriate form be used for your intended analysis. The available forms are:

  • Soil fertility analysis: Used for routine fertility determinations (N, P, K, other nutrients and fertiliser recommendations) and acidity (lime and gypsum for top and subsoil acidity recommendations).
  • Salinity and sodicity analysis: Used to asses if the soil profile is saline or sodic and provide gypsum recommendations – normally used in irrigated regions and some valley bottom sites. No fertiliser recommendations can be made from salinity/sodicity analysis.
  • Leaf analysis: Used to measure the status of plant nutrients in the leaf material of the crop. This can be used to guide adjustments to future fertiliser recommendations based on how the crop is responding.
  • Irrigation water quality analysis: Used to assess the amount of salts in irrigation water and the likely risk of causing saline or sodic conditions in the soil.
  • Fertiliser analysis: Used to determine the amount of total nutrients in different types of fertiliser, organic amendments or liming materials. This can be used to help with calculating fertiliser rates, but does not indicate how much of the material to use.

All forms have headings to indicate their intended use, so check that this matches your intended purpose for the sample. If in doubt contact FAS or your SASRI Extension or Research Specialist. These forms can be downloaded from here or obtained from FAS or your regional Extension Specialist.

Reminder to sample

For growers that have harvested recently or intend to replant this coming spring, it is a good time to look at getting your soil sampling completed and submitted for analysis. This will allow time for the sample analysis results to be returned to you and the fertiliser requirements can be determined. If you are replanting it is also ideal to evaluate your soil acidity status (or salinity/sodicity status in the irrigated regions) and apply corrective actions before planting.

Previous studies have shown that the major source of error in sample analysis is due to inappropriate sampling procedures in the field. For guidance on proper soil sampling procedures see Information Sheet 7.16: Soil Sampling (available from the SASRI website on the Knowledge Hub page). If you are uncertain and require further assistance contact your regional SASRI Extension Specialist.