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 and navigate to Knowledge Hub) or visit our website for sampling guidelines and submission forms

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.


The Fertiliser Advisory Service analyses soil & leaf macro- and micronutrients for a range of agricultural crops. FAS realises the value in creating a ‘one-stop’ analytical service as many growers are not just in the business of sugarcane. This diversification will make it easier for growers to test the nutrient value of most of their crops on their farms in one place, saving the logistical nightmare of having to submit samples to several labs. In addition, a grower will be able to receive all their results at the same time for all farms and in a uniform easy-to-understand format, easing the record keeping process.If you’re in the business of growing macadamias, avocados, bananas, maize, or garden vegetables, then fill out the appropriate form for the crop to be analysed and we will take care of the rest! 

FAS: your one-stop-shop for every agricultural crop!

Sample submission 101

When sending leaf, water or soil samples to FAS, always remember to include the appropriate submission form labelled with the correct crop and analysis required. Label all samples (soil boxes, leaf labels, fertilisers and/or water vials) accordingto what is written on the submission form. This ensures that the correct analyses are done on the appropriately labelled sample in the quickest time possible. Here are some guidelines:

·       Select the appropriate submission form according to analysis requested (e.g. leaf, soil fertility, soil salinity, fertilisers).

·       All samples must have an identification (e.g. field number) and correspond to the submission form.

·       For soil and leaf samples, the information supplied on the submission form has a major bearing on the recommendations appearing in the analytical reports.  For example, in the case of soil fertility samples, the ‘Attainable Yield’ is crucial for the calculation of the N and K requirements of the crop.

·       To submit samples, courier them to our physical address or drop them off at the designated drop-off points at the SASRI Extension Offices.

FAS on the ground..

FAS exhibition at SASTA (Durban ICC): The FAS research team networked with Africa and beyond at this year’s SASTA symposium! With a few conference presentations and an exhibition stand, the team showcased some of the lab’s latest research developments and new products.

A trip to the Midlands: The FAS team had another fantastic chance to spread their wings and interact with our local growers and existing loyal clients at the Nedbank Eston Show in the Midlands.

If you visited us at any of these events, we thank you for your support and hope to hear from you soon. To find out where we will be next, watch this space in our next newsletter for upcoming FAS events.


See for your ’sulph’

Sulphur (S) is a macronutrient that is commonly overlooked during soil testing. Sulphur deficiencies can be found in all soil types but are most common in sandy soils with low organic matter. Insufficient sulphur can lead to reductions in yield and crop quality. In addition, there is a very strong interaction between S and nitrogen (N) in the growth of the crop. Adequate supplies of S are therefore of great importance in ensuring efficient response to applied N fertiliser.

Most laboratories offer S as an extra-cost supplementary analysis. This is due to the need for more complex analytical methods to test for this nutrient.  At FAS, we understand the importance of this macronutrient and, in response, our scientists have developed a new, reliable and inexpensive method to test for S. This analysis has now been included as part of the routine fertility analysis package at no additional cost to the customer.

The resin-extractable S method is a cost-effective test that mimics root uptake of sulphur by the plant. This means that there is far greater accuracy of results as the method is reliably estimating the amount of plant-available S in the soil. From now on, all FAS customers will receive their soil test reports with this nutrient included!

The addition of this nutrient to FAS’s growing routine package is part of our ongoing commitment to provide more value to our clients without burdening them with additional costs and add-ons. Now, for as little as R 210.00 per sample, clients will receive:

pH (CaCl2) copper
sulphur zinc
phosphorus manganese
potassium silicon
calcium volume weight
magnesium estimates of clay and organic matter
sodium potential nitrogen volatilisation
exchangeable acidity (Al+H) nitrogen mineralisation
total cations, acid saturation reserve potassium
exchangeable sodium  

Don’t guess, soil test with FAS!

Smart tips:

  • With the approach of winter, rainfed growers are advised to not apply nitrogen.
  • Where lime is being applied, remember to periodically check for subsoil acidity down to 80 cm.
  • Periodically open soil pits and check for compaction.

New FAS webpage

We have just launched our new FAS Agricultural Laboratory webpage.

Check out the latest newsletters and prices ● Download submission forms ● Get information on how to do soil and leaf sampling for a variety of crops ● View drop off points for soil and leaf samples ● Get information on how to submit samples to the lab ● Other helpful publications are also available on the site.

Leaf Sampling 2

As the African proverb goes, “Roots do not know what a leaf has in mind”. In many cases, this is true as a healthy soil does not necessarily reflect what is taken up by the crop. A South African North Coast grower recently experienced severely stunted growth on patches of his field, with no clear leaf symptoms. FAS soil analysis revealed acidity was not the culprit and most soil nutrient levels were above threshold, with zinc being marginal. The leaf sample, however, confirmed a definite zinc deficiency on the areas of reduced growth.

For as little as R162, this grower had effectively diagnosed his problem and saved thousands of rands on misdirected application of NPK fertilisers or lime – none of which would have improved yields.

Don’t leaf me behind

FAS offers a comprehensive leaf sampling package for all crops including macadamias, avocadoes, maize and vegetables. To ensure the most reliable interpretations, the best time to take leaf samples is:

• when the crop is growing vigorously (dependent on crop age and area),

• there is no water stress, and

• at least four weeks since the last fertiliser application.

The crop must also be sampled at the correct age, and in the correct month of the year. Ask your local Extension Specialist to assist you with this.

Why should I leaf sample?

  1. It provides a snapshot of the crop nutrient balance.
  2. It’s particularly valuable to detect deficiencies of nutrients for which soil tests are not wholly reliable, including nitrogen, sulphur and micronutrients.
  3. Sampling early in the growth cycle allows for corrective treatment for the current crop.

HOW TO turn over a new leaf…

When submitting leaf samples to FAS remember to fill in the leaf submission form correctly. If possible, use the appropriate FAS leaf labels on the samples.

• Ensure adequate leaf material is supplied to the lab for analysis (typically 200-250 g fresh leaves per sample).

• Leaves with discoloration or from areas of poor/patchy growth must be submitted separately. Do not mix these samples with healthy, green leaves.

• Do not store/submit samples in a sealed plastic bag as they might go mouldy. Also, do not submit your samples in an old fertiliser bag. Fertiliser nutrient contamination could ruin your results. Leaf sampling is easy to do. Conducted routinely, it can help to obtain maximum possible yields through optimal nutrition. If done as a means of trouble-shooting, leaf sampling can save large sums of money by helping to identify particular nutrient deficiencies.

Leaf Sampling


Top International Ratings for FAS Leaf Analysis! FAS participates in two quality assurance schemes, the local Agrilasa scheme and the international WEPAL leaf testing scheme. These proficiency schemes aim to ensure that quality (accuracy) of analyses is maintained in the lab.

How does FAS participate?

Samples (soil, leaf or fertiliser) are received from both schemes for routine analyses. Once the results have been generated, they are sent back to the scheme to compare against their compare against other laboratories. This ensures that all FAS analyses are being monitored for accuracy and reliability, and promotes continuous improvement in the lab.

Agrilasa is a South African laboratory quality assurance scheme which promotes analytical accuracy and provides networking opportunities between experts.

WEPAL is a world-leading organiser of proficiency testing programmes in the fields of plant, soil, sediment and organic waste. They are a part of Wageningen University, Netherlands.

A new test for FAS!

Getting Your Reserve Potassium off the bench

FAS has just introduced a brand new test to the Routine Soils Package. For as little as R190.00 incl VAT, customers submitting soil samples for routine analyses will now have the added benefit of understanding how much Reserve Potassium (K) is in their soils without any additional cost!

Introducing this new parameter to the package is a first for South African soil analytical labs as this was previously not measured due to the difficulty associated with its determination.

Understanding the potential of your ‘benchwarmer’

Soils containing significant amounts of mica-type clays are known to store substantial reserves of potassium. These reserves contribute significantly to plant growth as a portion becomes available during the growing season. Wide variations in K reserves occur in southern African soils, with the rainfed coastal and midlands areas of South Africa generally having low reserves, while areas and countries to the north have high to extremely high reserves.

Understanding the amount of K available as a reserve can result in huge savings in K fertiliser for the farmer. Where K reserves are high, fertiliser K requirements are lower, (or may even be negligible). There are growers on high K reserve soils who have not applied this nutrient for 15 to 20 years and are still achieving top yields! The reserve K test thus represents a significant advancement in the quest for optimising efficiencies in fertiliser use.

No mess, no fuss

Testing for Reserve K has, in the past, been difficult and dangerous to carry out due to the harsh chemicals and laborious methods involved (the extraction involved boiling nitric acid!).

Recently, FAS has revolutionised this process with the purchase of a state-of-the-art mid-infrared spectrometer. This new technology has allowed the team of research scientists to calibrate the instrument to test Reserve K levels on air-dried soil samples – without the use of nitric acid! This is a major breakthrough not just for soil science but for the FAS customer as well. Following this development, K recommendations are now modified to accommodate the reserves of this nutrient measured in soil samples submitted to FAS.

With this new addition to FAS’s Routine recommendations package, K requirements are adjusted on a sliding scale according to the levels of reserve K determined in the soil samples submitted by customers. This will assist the customer with purchasing correct amounts of K for their fields, resulting in huge fertiliser savings at no additional cost.