Orthophosphate vs. Polyphosphate: Understanding the Differences
Every season producers face a multitude of questions regarding best practices. Is this hybrid or is that hybrid better for seed choice? Should I plant more corn or soybean acres? Or, more pertinent to this article is the question. What are the benefits of orthophosphate vs. polyphosphate? Today's fertilizer industry has an overabundance of products in constant competition; the difference between orthophosphate and polyphosphate might initially seem like a marketing ploy. However, science-based differences should be considered when using either an orthophosphate or a polyphosphate.
1. Availability
To start, we first need to understand the manufacturing method between orthophosphate vs. polyphosphate. Polyphosphates are created when green phosphoric acid reacts with anhydrous ammonia. The heat generated by this reaction removes water and the orthophosphate molecules combine and form polyphosphate chains. However, not all orthophosphate molecules in a polyphosphate mixture are combined in this reaction. This results in a polyphosphate mixture also containing orthophosphate molecules.
Orthophosphate is created when white phosphoric acid reacts with anhydrous ammonia. While this reaction also produces heat, the temperature resulting from this process is lower. The orthophosphate molecules remain separate and in their most available state.
This leads us to our first difference. Plants can immediately uptake soil-applied orthophosphate molecules, while polyphosphate chains need to degrade back to orthophosphates to be utilized by plants.
The integrated pest and crop management news and resources for Wisconsin agriculture from the University of Wisconsin-Madison, states a combination of water and soil enzymes carries out the process of reverting polyphosphate chains to orthophosphates. It should be noted that this process is affected by soil temperature; the lower the soil temperature, the greater the time to availability. According to the University of Minnesota Extension, converting polyphosphates to a plant-available form takes days at average planting temperatures. It stands to reason that this process is drawn out over several days to weeks when soil temperatures are cold, as they often are during spring planting.
In a 2021 corn trial, at the University of Nebraska, researchers found that yield was 4 bu/ac higher with an orthophosphate fertilizer, FREE pHOS 24, versus the polyphosphate fertilizer, 10-34-0 starter fertilizer. This fertilizer is composed of about 70% polyphosphate and 30% orthophosphate.
2. Polyphosphates are not as efficient when foliar applied
Another difference between orthophosphate and polyphosphate is their efficacy when used in foliar sprays. The choice between orthophosphate vs. polyphosphate can significantly impact foliar fertilization performance. Polyphosphates rely on soil enzymes to be converted to an available phosphate form for plant use. Little, if any, research exists examining the time required for this process to occur on plant foliage. However, it is safe to assume that this process isn't instantaneous.
As previously stated, the process of converting polyphosphates back to orthophosphates can, based on soil conditions, be a long and drawn-out process. In the soil, this process can take days to weeks. Logic suggests that this process would be even longer on the leaf surface. Polyphosphate mixes regularly are known to contain orthophosphates at a ratio of 60% poly and 40% ortho. This means that over half of the mix must be converted to a usable form over time. Orthophosphates can be readily absorbed through plant foliage. While only a portion of a polyphosphate mixture is readily available for plant use on the foliage, all applied orthophosphate fertilizer is immediately available.
3. Salt index
Understanding the salt index of fertilizers is crucial when making informed crop nutrition decisions. As stated in the "Selecting Synthetic Fertilizer in New Mexico" guide by New Mexico State University, the salt index indicates the probability of fertilizer burn resulting from fertilizer application.
“Fertilizers with a low salt index are generally considered to pose a lower risk of injury when applied properly.”
When contemplating the choice between orthophosphate and polyphosphate as pop-up fertilizers, the salt index plays a significant role in determining the most suitable product. Polyphosphates, often exhibiting a higher salt index, carry inherent dangers due to this characteristic. Placing an excessive amount of polyphosphate in direct contact with seeds can be perilous, leading to an increased risk of fertilizer burn. This unfortunate consequence can compromise germination and stand establishment, compromising overall crop performance.
In contrast, orthophosphates boast a lower salt index, making them safer for larger-scale pop-up applications. Utilizing orthophosphates in greater amounts does not carry the same level of concern about impacting germination and stand establishment. This advantage empowers producers to apply orthophosphate fertilizers with confidence. This ensures more fertilizer can be placed in direct contact with the seed without the fear of impacting stand establishment or germination overall.
4. Compatibility with other fertilizers
When it comes to orthophosphates vs. polyphosphates, the comparison isn't one-sided. Producers can easily mix polyphosphates with 32% nitrogen or 28% nitrogen in 2x2 starter mixes without complications. In contrast, orthophosphate mixes require equal parts water when adding 28% or 32% nitrogen, leading to additional time and effort during planting. Adding extra water to a 2X2 starter mix increases the number of trips required when planting. More trips across the field mean more fuel is needed to finish planting. In this aspect, polyphosphates present an attractive option for those seeking easy mixing solutions.
When comparing orthophosphates vs. polyphosphates, fertilizers' choice extends beyond mere marketing tactics. Orthophosphates, with their immediate availability to plants when applied to soil or foliage, provide a clear advantage over polyphosphates, which require time-consuming degradation before becoming usable. Orthophosphates can also be applied in more significant amounts than polyphosphates due to the lower risk of seed injury. However, polyphosphates shine in their compatibility with other fertilizers, making them an attractive option for those seeking easy mixing solutions. As producers navigate the vast sea of fertilizer options, weighing these factors and their implications for crop performance is essential. Ultimately, informed decision-making based on crops' specific needs and soil conditions will pave the way for a successful and bountiful season.
CultivAce provides 24% liquid phosphate fertilizer, FREE pHOS 24. This fertilizer can be blended as a polyphosphate or an orthophosphate, depending on your needs. Reach out to a representative, or call us for more detailed specifications and information.