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.

Irrigation Water Testing

Send your irrigation water to FAS for testing

Testing your water quality can improve soil health

Poor irrigation water quality, particularly salt-affected sources, can cause soil degradation and induce crop stress, resulting in a decline in crop yield. Most commonly, water quality is classed on its potential to lead to saline and sodic conditions in the soil. Excessive amounts of soluble salts (salinity) in water can cause an increase in soil salt content and increase crop water stress, while unfavourable Na levels can also lead to sodic soil conditions that lead to poor soil physical conditions. Remediation of salt affected soils can be costly and should be prevented from occurring. An essential requirement to prevent the buildup of salts in soil is good drainage and periodic flushing with good quality water.

To better manage potential negative impacts of irrigation water, it is good practice to regularly monitor irrigation water quality. Ideally, water sampling should occur in each season to gain an understanding of seasonal shifts in water quality and guide practice to improve scheduling and use of the irrigation water throughout the year. Where this is not possible, attempt to get at least a wet and dry season water sample analysed. Essential parameters, as assessed by FAS Agricultural Laboratory are:

·         Water pH: This is an indicator of the acidity or alkalinity of the water. Ideally good irrigation water will have a pH of 6.5 to 7.

·         Soluble base cations: The concentration of Ca, Mg and Na in the water is used to determine the sodium adsorption ratio (SAR).

·         Alkalinity (HCO3): Excessive alkalinity is used to adjust the SAR value for the impact of the excess bicarbonate in solution.

·         Electrical conductivity (EC): An indicator of the amount of total dissolved salt of water.

·         Effective EC (EEC): This is an adjusted EC to account for the diluting effect of rainfall received in conjunction with the irrigation amounts.

·         Sodium adsorption ratio (SAR): High SAR values indicate a sodicity impact risk of using this water.

·         Adjusted SAR (ASAR): This accounts for the alkaline ions present that can precipitate Ca and Mg, effectively increasing the SAR.

The water quality is classed according to the relationship given in the figure below. It is important to note that the SASRI classification is based on the EEC and ASAR that account for effects of rainfall dilution and residual alkalinity, respectively, on the effective water quality. Depending on your soil type and water quality class, the water may be used with certain limitations.

 

 

 

The figure above shows the relationship between the Adjusted Sodium Adsorption Ratio (ASAR) and Effective Electrical Conductivity (EEC) on suitability of water for irrigation. The quality class determines the suitability for different soil types and conditions. The white lines in the graph define the assigned water quality class for reporting purposes, while the green-yellow-red gradient transitions highlight the increasing risk gradient across these categories (see Important considerations when interpreting the water class).

Further information and guidance on managing irrigation water quality and salt affected soils is available in Information Sheet 5.12 (Water quality for Soil Health) and 5.11 (Soil Salinity and Sodicity) (available from www.sasri.org.za and navigate to Knowledge Hub) or visit our website for sampling guidelines and submission forms www.fasagrilab.co.za.

Important considerations when interpreting the water class:

When a water sample is classified it is assigned to one of the water quality classes based on the discrete categories shown in the above figure. However, this can lead to confusion as to the suitability of the water for irrigation under different conditions, especially where samples lie near category class borders. To better guide the suitability and risk associated with using the water it is useful to evaluate where the sample lies relative to the colour gradients shown in the figure. Samples in the darker green regions indicate low risk except in very dispersive clay soils. Samples in the light green to yellow indicate increasing risk to all dispersive and poorly drained soils (and drainage is advised). The orange to red transition indicates very high risk to soil quality and the water should not be used for irrigation purposes without treatment.