What's in a Liquid Phosphorus Fertilizer?
Both ortho-P and poly-P are found in nearly all P fertilizers. Poly-P converts to ortho-P before plants can use it or soils can bind it.
A friend asked me the other day about phosphorus fertilizer (starter fertilizers). He was concerned about what source to use. His questions were specifically about the liquid sources 10-34-0 or 9-18-9 or 9-24-3...... the list goes on. After visiting with him a bit, his real questions were: "What is the difference in the actual makeup of the various materials?" and "Does that difference in chemical makeup change the availability of the key ingredients (nitrogen (N), phosphorus (P) and potassium (K)) to my corn and wheat?" A third (and probably more important) implication of these questions is: Does the chemical makeup of one liquid fertilizer give it an advantage over another? Is there enhanced nutrient availability, because of inherent chemical makeup? His reasons for asking the question were because the costs of the various products (per actual pound of nutrient) were very different.
If you visit with a fertilizer dealer, he might indicate "this product is 70% orthophosphate (ortho-P) and 30% polyphosphate (poly-P)" (some call it pyrophosphate). Another dealer may indicate "this product is 35% ortho-P and 65% poly-P". Well what is ortho-P? What is poly-P? And what difference does it make to plants growing in soil that is P deficient versus plants growing in soil that is not P deficient?
It is easy to answer the last part of this question. If the soil is not P deficient, then spending your dollars on P fertilizer is not going to provide you with a return on your investment. Those dollars could have been spent on a family vacation or on putting child number three through college. Differences in liquid-P fertilizers takes a little more explanation.
Ortho-P in its purist form, is phosphoric acid (H3PO4). Strong acids are corrosive, nasty stuff to work with, but strong acids are neutralized by strong bases. And so, fertilizer companies use ammonia, which in water forms the strong base: ammonium hydroxide (NH4OH which is also highly corrosive) to neutralize this acidity. Other chemicals are used to make dry fertilizers, but that is another story. When the acid and the base are mixed together, they neutralize each other and the final result is a safe, salty liquid. Don't try this at home. The actual mixing of the acid and the base usually causes a violent reaction and produces a lot of heat. When ammonium hydroxide is mixed with phosphoric acid (ortho-P), ammonium phosphate is formed, and water and heat are given off. It is the ammonium (NH4) from the injected ammonia, that gives us the 10 (10% N) in 10-34-0 and the 9 (9% N) in 9-18-9 or 9-24-3.
We measured the salt content of a batch of 10-34-0 and 9-24-3 in 200:1 dilutions using an electrical conductivity meter. The electrical conductivities of the two products were similar. We measured an EC of 37 in the 9-24-3 and an EC of 41 in the 10-34-0. You might expect a lower EC in the 9-24-3 because it has 10% less N and 30% lower P than the 10-34-0. Electrical conductivity values decrease as you increase the water content of a liquid fertilizer. One could dilute 10-34-0 with water to make a 7-24-0, add KCl and a little ammonia to get the N concentration back up to 9%, and you would have a 9-24-3 product. The pH of the 10-34-0 and the 9-24-3 were both about 6.2 indicating their corrosive characteristics due to pH (a pH of 6.2 is just slightly acidic) should be similar.
And so, after mixing with ammonia, and at the concentrations we work with, liquid P fertilizer materials are essentially harmless, but very salty, liquids. Salty liquids are naturally very corrosive materials to metal, and both of the products we evaluated would be expected to corrode steel toolbars and metal fertilizer applicators in about the same way.
Poly-P is made by heating phosphoric acid (ortho-P) to remove water. The result of this reaction is H5P3O10 (tri-poly P). Why do we need poly-P? It turns out that poly-P's are more soluble in water and are easier to maintain at a near neutral pH in solution than other ortho-P types of fertilizer. Therefore, poly-P fertilizers stay dissolved in solution and don't "salt out" as easily as other forms of P. In storage, some of the poly-P will split to form ortho-P (phosphoric acid, which drops the pH of the solution and is more corrosive). And so ammonia (which forms NH4OH in water) is added to neutralize that pH drop and to improve the nutrient status of the fertilizer. The result is ammonium-poly-P (10-34-0, 9-24-3, 9-18-9, 7-21-7 are mixtures of poly-P and ortho-P and ammonia). The amounts of poly-P versus ortho-P in each liquid fertilizer might be different, but the chemistry is essentially the same. Of course, the 9-24-3, 9-18-9 and 7-21-7 have K added, probably as potassium chloride. But remember that K is deficient in less than 1% of the soils in our four state region, and you don't need fertilizer K if you are farming soils that are already high in available K. In general, sandy and low organic matter soils are more often low in K, but most of our silt loams are very high in K.
Do
corn and wheat plants really care? Corn, wheat, millet, sunflowers,
barley, grain sorghum.....and most other crops that have been researched
all take P up through their roots in the HPO42-
or H2PO4- forms
(ortho-P ion forms). And so, poly-P fertilizers first have to break down
(hydrolyze) into ortho-P, and the ortho-P has to dissociate into an ion
form for uptake by plant roots. The ortho-P in a liquid starter, is essentially
100% available for plant uptake. The poly-P is initially not available
for uptake, but it hydrolyzes fairly quickly (in about 7-14 days depending
on temperature and moisture) and is essentially 100% available in the
time span that the plant is going to need it, even if it is applied at
planting. Remember, germination takes 3-10 days (depending on moisture
and temperature) and the seedling doesn't really do a whole lot of nutrient
accumulation for the first few days after emergence.
The flip side of rapid availability with ortho-P is rapid fixation of P into less available forms. In our neutral to high pH, calcareous soils, P fertilizers will form dicalcium and octacalcium phosphate minerals which are much less soluble in the soil and therefore less available for plant uptake than the original fertilizer P. The poly-P form maintains fertilizer solubility for a few days longer than pure ortho-P because it has to hydrolyze into ortho-P before it can be either taken up by plants or fixed into a less soluble mineral. And so poly-P might be thought of as a somewhat "slower release" fertilizer than pure ortho-P fertilizer. Typical batches of 10-34-0 are about 65% poly-P and 35% ortho-P, or 35% immediately available, and 65% "slowly available but available fairly soon after application."
The bottom line
Crop plants need P in ortho-P ion forms before plant uptake will occur.
Both ortho-P and poly-P fertilizers will provide the needed nutrition
if applied at recommended rates to P deficient soils. Both ortho-P and
poly-P are found in nearly all typical liquid P fertilizers. Poly-P provides
some delay in fixation of P into less plant available forms. Buy your
fertilizer based on price per pound of nutrient and based on how well
the company/dealer handles the product. A good clean batch of 10-34-0,
or 9-24-3 coming from a clean storage tank is as good of a fertilizer
product as a good clean batch of 9-18-9 or 7-21-7. If the dealer has dirty
tanks then you will get a dirty batch of any of these products. The nutrient
availability for the crop is essentially the same for all of these products.
Differences in how well the dealer keeps the products clean of dirt and
other impurities is an issue. Dirty batches plug applicator nozzles and
are a real pain to deal with.
