Liquid fertilizers – compatibility and safety

FERTILIZER APPLICATION

Liquid fertilizers – compatibility and safety

Liquid fertilizers are prized because of their flexibility – especially the freedom to blend nutrients together to create customised products for growers. Dr Karl Wyant, Nutrien’s Director of Agronomy, offers guidance on compatibility and safety, two key concerns when blending and handling liquid fertilizers.

Nutrien’s production plant at Gesimar, Louisiana, manufactures large volumes of liquid fertilizers (UAN, 32-0-0).

PHOTO: NUTRIEN

The fertilizer industry has responded to demands from growers for innovation in crop nutrition by continuing to develop new fertilizer products and improved nutrient formulations. In recent years, one notable and landmark shift in product development has been the introduction of liquid fertilizers.

Introduction

Liquid fertilizers, also known as fluid fertilizers, have become increasing popular in the agriculture, horticulture and turf markets (Fertilizer International 508, p18). This is largely due to their flexibility. They are well suited to both fertigation and foliar application, for example, and can be applied as starter fertilizers alongside seeds or as a side dressing. Formulations also offer myriad possibilities for combining N, P and K with other crop inputs.

The global liquid fertilizer market is valued at around $2 billion currently and is growing at more than three percent per annum, according to market research. Liquid fertilizers represent the frontier of nutrient management for many ag retailers and their customers, having been pioneered by growers across a diverse range of row and field crops, oilseeds and legumes, and fruit and vegetables.

Nutrient suspensions versus nutrient solutions

Liquid fertilizers divide into two broad categories: firstly, suspensions of very fine solids and, secondly, solutions of dissolved nutrients.

Suspension fertilizers are commonly used in direct soil application. They consist of finely-ground solids suspended within a small volume of water or thixotropic gel. Suspending agents are used to help keep the fertilizer in a dispersed state and prevent settling. Advantageously, suspension formulations can offer a higher nutrient analysis relative to solutions.

Solution fertilizers, on the other hand, are obtained by completely dissolving solid nutrients in water. Common formulations include urea ammonium nitrate (UAN, 32-0-0), ammonium polyphosphate (10-340), and potash-derived fertilizers (0-0-17). Fluid fertilizers can also supply secondary nutrients and micronutrients, depending on the fertilizer source and the formulation.

Due to their flexibility, these nutrient solutions have a wide range of uses including starter applications, fertigation and foliar sprays. The introduction of liquid fertilizers, and their growing popularity in certain markets, has heralded a marked shift away from standard, granular commodity fertilizers like urea (46-0-0), mono-ammonium phosphate (MAP, 11-52-0), and muriate of potash (MOP, 0-0-60), etc. This specific category of liquid fertilizers – nutrient solutions – is where we will turn our focus next.

Benefits and limitations

Crops cannot tell the difference between nutrients delivered by a solid, granular fertilizer or those delivered by a liquid product. Instead, these two distinct forms of fertilizer offer different nutrient delivery mechanisms and, as a nutrient management choice, have both advantages and disadvantages. Therefore, to ensure crop performance is not compromised, the relative merits and demerits of liquid fertilizers – as set out below – must be considered when incorporating these products into an existing nutrient programme.

Liquid fertilizers are prized because of their blending behaviour – especially the freedom to combine nutrients together to create new products with customised analyses. Unique blends can be formulated, for example, by partnering nitrogen, phosphorus and potassium with micronutrients, adjuvants, biostimulants, and crop protection products. Furthermore, these novel nutrient solutions can be offered to growers at different scales, depending on logistics and convenience, ranging from large volumes delivered direct by suppliers to smaller batches provided by the local ag retail store. Either way, liquid fertilizers can be provided as custom blends tailored to match specific crop and geographical needs.

Another advantage is flexibility in both application timing and placement. Liquid fertilizers are typically administered in-season when crop nutrient demand is highest. They can be applied through an irrigation system (fertigation), as a starter or side dress blend, or to the crop itself as a foliar treatment.

Granular fertilizer applications, in contrast, come with limitations. Their bulky handling equipment and attendant height limitations can prevent spreaders from getting into the field. This often limits applications to when the field is fallow or during early crop growth stages.

The ability to make nutrients rapidly available to plants is another key advantage. Crops can quickly take up nutrients from applied liquid products, since the nutrients are already dissolved in water, a characteristic which also drives up nutrient use efficiency. Moreover, nutrients can be applied more safely as the risk of plant burn is less with liquid fertilizers – relative to granular fertilizers – as they tend to have a lower salt content.

Liquid fertilizers do, however, have several shortcomings. As a result, there are undoubtedly circumstances where the use of granular fertilizers or manures/compost might be a better choice instead.

First, liquid fertilizers tend to have a lower nutrient density relative to their dry counterparts. The available nitrogen in one tonne of urea (46-0-0) granules versus one tonne of liquid UAN-32 (32-0-0), for example, is heavily weighted in urea’s favour. Consequently, liquid fertilizers are not as well suited to supplying nutrients in larger doses, versus dry fertilizers, as their unit nutrient cost makes them much less affordable. Granular fertilizers are therefore the most economical choice if large nutrient doses are needed, say at the end of harvest or at the pre-plant stage.

Second, liquid fertilizers generally require more specialised storage and application equipment, relative to dry fertilizers. Growers need to store liquid fertilizers in large tanks (e.g., tank farm), for example, or in small batch totes, whereas dry fertilizers only need simple pile-and-bin storage. Liquid fertilizers also require special equipment for field application including:

• Fertigation systems (centre pivot, drip and mini sprinkler, flood, linear, etc.)
• Liquid starter kits on planters
• Side dress rigs
• Spray bars for foliar application.

Upfront equipment costs can therefore prohibit the use of liquid fertilizers unless the grower already has the necessary items.

Finally, liquid fertilizers also have long-term storage challenges with the settling out of ingredients from solution (‘salt out’) when cooler weather arrives. Getting ingredients back into solution, once a blend ‘salts out’, can cause headaches, on farm and for ag retailers, due to the considerable remixing effort involved.

Overall, careful consideration of the strengths and limitations of liquid fertilizers is required when incorporating these formulations into a crop nutrient programme. As liquid fertilizers continue to gain in popularity, honest accounting of nutrient needs and logistics requirements, plus realistic crop performance expectations, should set up the supply chain for success.

Storage guidelines and tank maintenance

As mentioned previously, liquid fertilizers have distinct storage requirements versus dry fertilizers. Guidance on storage tank maintenance and advice on how to manage ‘salt out’ is set out below. (Further information on regulatory policy for large-scale storage of liquid fertilizers and site preparation, if needed, is provided in the bibliography.)

Growers increasingly prize the on-farm storage of liquid fertilizers. This enables them to take advantage of pre-pay programmes and accept delivery of liquid fertilizers during the off-season when prices are typically lower. Ag input suppliers are also storing liquid fertilizers in bulk to ensure availability to growers. As a result, seasonal tank maintenance now regularly features on annual ‘to do’ lists. The following tank maintenance tips should help keep liquid fertilizers inside tanks where they belong – and out of the environment where they can behave as a pollutant and/or become a safety risk:

• Inspect the interior and exterior of liquid fertilizer tanks annually – look for issues with corrosion, cracking, pitting, and blistering of the tank floor and walls.
• Keep annual records and photos available for comparison.
• Interior walls and floors should be gently cleaned annually.
• Repairs should be left to specialists as welding repairs on metal tanks is not a standard procedure. Epoxy coated tanks also require paint that can be lethal if managed incorrectly.
• Valves and plugs should be inspected annually and replaced with corrosion resistant materials such as stainless steel, if necessary.
• Growers should contact their fertilizer dealer as and when tank issues arise or if they are unsure of how to proceed with repairs.

The following storage guidance is specific to weather-related challenges: l In cold climates, avoid storing liquid fertilizers during the winter, if possible.

• Alternatively, move liquid fertilizer tanks into warm storage, if available.
• Do not store liquid fertilizers for more than one winter season.
• If liquid fertilizers do salt out, allow time to recirculate and resuspend any settled-out materials during spring warm up.
• Circulate product in the tank for at least 48 hours post-storage and use agitation or introduce turbulent flow, if possible.
• The time required for recirculating salted out materials depends on the tank volume. The larger the tank volume, and the higher degree of salting out, the longer the re-circulation. A 25,000 gallon (94,635 litre) tanks may need two days of circulation, while a 47,000 gallon (177,914 litre) tank may need at least five days.
• Contact the dealer for fresh product if salt out cannot be reversed after recirculation of the liquid fertilizer.

Compatibility and safety

Individual liquid fertilizers can be combined to produce final blends with unique analyses. This high degree of customisation is valuable as it allows for innovation, product differentiation, and unique formulations. However, two important concepts – compatibility and safety – require close examination before starting to combine liquid fertilizers.

Two liquid fertilizers are generally considered incompatible if they interact to produce solid particles (precipitation), separate out, or produce a hazardous by-product. The blending compatibilities of common liquid fertilizers are shown in Figure 1. This scheme uses a ‘traffic light’ classification system. Green confirms that the two liquid fertilizers are compatible, yellow shows limited compatibility, while red indicates incompatibility. An additional colour (purple) is included for those combinations with very limited compatibility.

While a yellow or purple classification does not rule out the formulation of stable blends, special attention should be paid to the solubility of the mixture with efforts made to keep the ingredients together. For combinations marked ‘H’, care must also be taken to avoid injury due to the heat generated by exothermic reactions during mixing.

Fig 1: Compatibility chart for liquid fertilizer mixtures

Liquid fertilizers are easy enough to blend. This characteristic is generally an advantage. But it also means one can get into trouble if the safety of the blend is not considered beforehand. Examples of unsafe blends and practices (with potential hazards) are shown in Table 1.

Table 1: Examples of unsafe blends and practices with potential hazards

Safety is a particular priority for mixtures with very limited compatibility and incompatible mixtures (shown as purple and red, respectively, in Figure 1). With incompatible mixtures, there is a high certainty of salt-out issues and the likelihood that precipitates or potentially hazardous by-products will form.

The general rules when blending liquid fertilizers are:

• When in doubt, pick the green boxes!
• Always follow label instructions for tank mixing to avoid blend sequence issues.

Compatibility testing and assessment

It is always advisable to perform a bench-top jar test on a new blend before making a large batch. Jar tests help predict how blends will behave (in the tank farm and in grower equipment) and if a formulation will clog application equipment with unwanted precipitates (Figure 2).

Jat test mixtures, even when they appear to be stable after initial blending, should be observed for several days – and even weeks or months, if destined for storage – to see if solids form or components separate in the solution (Figure 3). If storage in colder conditions is anticipated, the blended mix should be exposed to the lowest expected temperature to determine if salt out will occur.

The adoption of an innovative, new liquid fertilizer blend requires pre-planning to answer two crucial questions:

Fig. 2. When mixed, incompatible liquid fertilizers can form solid precipitates (left photo) that can damage and clog equipment. In another example, a jar test revealed that liquid calcium fertilizer (calcium ammonium nitrate) is severely incompatible with a phosphate-based fertilizer (ammonium polyphosphate) as they form an immobile green sludge when combined (centre and right photo). This blend would have caused considerable headaches if formulated on a large scale.

PHOTO: ALLEN HAYNES AND EDGARD JAUREGUI.

Fig. 3. Jar testing is crucial for revealing how blends will behave over the long-term in the field. In this example, the blended ingredients were compatible at initial mixing (left) but showed strong separation after sitting for 48 hours (right). This shows a clear failure that should be avoided.

PHOTO: KARL WYANT

• First, can the ingredients be blended without compatibility issues?
• Second, can the ingredients be blended safely without risk of injury?

When assessing a new blend, the use of a logical workflow (Figure 4), with its ‘pass/fail’ and ‘yes/no’ steps, can help answer both questions by anticipating hazards and revealing compatibility challenges. This workflow uses a series of check points – when scaling-up from the initial bench top (jar test), to a small batch, and then to full scale production – to help achieve a compatible and safe blend.

Concluding thoughts

Liquid fertilizer blends are a fast-growing market category, as more agricultural suppliers and growers incorporate them into their crop nutrient management portfolios and plans.

Liquid fertilizers have a wide and unequalled range of uses – these including fertigation, starter and side dress applications, and foliar sprays. Valuably, different liquid products can be combined to create unique, customised nutrient formulations.

Compatibility and safety are the two key concerns when blending and handling liquid fertilizers. For new formulations, preplanning and jar testing are especially useful, as they can help predict compatibility and mixing issues.

It is always advisable to consult the Material Safety Data Sheets (MSDS), product labels and mixing directions to reduce the risk of injury. Potential hazards can also be assessed (and therefore avoided) prior to blending by checking compatibility charts (Figure 1).

Fig 4: A viable liquid fertilizer blend needs to be both compatible and safe to produce. A logical workflow helps achieve both goals.

If all of the above steps are clear, then move onto a jar test before proceeding to a final batch at scale.

Bibliography

Nutrien eKnomics, 2020. Liquid Fertilizer Mixing Compatibility – What You Should Know. [online] eKonomics. Available at: https://nutrienekonomics.com/news/liquid-fertilizer-mixingcompatibility-what-you-should-know/ [Accessed 20/09/23].

Isleib, J., 2016. Pros and cons of granular and liquid fertilizers. [online] MSU Extension. Available at: https://www.canr.msu.edu/news/ pros_and_cons_of_granular_and_liquid_fertilizers [Accessed 20/09/23].

Technavio, 2022. Liquid Fertilizer Market size to Grow by USD 1.92 bn. [online] Available at: https://www.prnewswire.com/news-releases/liquid-fertilizer-market-size-to-grow-by-usd1-92-bn–technavio-301501900.html [Accessed 20/09/23].

WPH, 2022. Western Fertilizer Handbook. Waveland Press.

Fairweather, T. & Leikam, D., 2012. Compatibility can be a problem for many blends. [online] Available at: https://fluidfertilizer.org/wp-content/uploads/2016/05/LS12-A3.pdf [Accessed 20/09/23].