Manure chemistry: What’s happening to my phosphorus

What happens to manure from the time it is excreted until it is land applied? Well, that’s simple right, we put it in some sort of storage (a pit, a lagoon, a Slurrystore, a concrete or earthen basin, or store it dry on a stacking pad) and it just stays there, waiting for us to land apply it and take advantage its fertility. Of course, it is not really that simple; manure waits for no man (or woman), while it is sitting there waiting it is changing (it is alive after all – it’s filled with microbes). Because of this, almost no matter how we store the manure, these microbes will be doing something – breaking down some of the organic material in the manure, converting elements from one form to another, or just causing things to be different.

This process can be complex and still isn’t completely understood, but we can give some generalities about what is happening and understand at least some of the jargon that gets thrown around. There are many ways to cover this topic, but what I am going to try to do is break it down by element, carbon, nitrogen, phosphorus, ect. In this post, I’m going to turn my attention to phosphorus.

Phosphorus

When we think of phosphorus we are probably most familiar with it in some of the fertilizer compounds we can purchase, such as mono-ammonium phosphate, di-ammonium phosphate, or triple super-phosphate. Otherwise we might think of it as the plant-available ion orthophosphate. Regardless of the actual chemical form of phosphorus we purchase, the analyses of phosphorus fertilizer is usually given as phosphate (P2O5).

       

Alright, so back to phosphorus in manure. As we’ve discussed previously, about 70% of the phosphorus consumed by the animal will be excreted, so it ends up in the manure. The actual amount is dependent on lots of things, animal age, species, diet composition, and numerous others, but the majority of the P we feed, will be in the manure. At the time of excretion, virtually all the phosphorus is associated with the fecal, or solid, material ( > 90%), but as the manure ages and the organic material in it breaks down from microbial action, more and more of the phosphorus becomes dissolved in the liquid fraction. For example in swine manure from a deep-pit, a good guess is that around 75% of the phosphorus will be dissolved reactive phosphorus (ortho-phosphate).

Although numerous fractionations schemes can be used to fractionate the manure phosphorus into pools of different characteristics and availabilities, the use of this information remains academic. From a practical standpoint, there are really four basic types of phosphorus in manure: dissolved inorganic phosphorus, precipitated inorganic phosphorus, and dissolved organic phosphorus, and particulate organic phosphorus.

Essentially all the dissolved, inorganic P in manure exists in the form of orthophosphate (or other derivatives of phosphoric acid (H₃PO₄, H₂PO₄^-, HPO₄^2-, and PO₄^3-). The exact form of this phosphate is dependent on the pH of the manure, but most of it will be either H₂PO₄^- or HPO₄^2- under normal pH conditions. This phosphorus is essentially equivalent to what would be purchased if you were to buy a mineral phosphorus fertilizer. As we store manure, two things can happen to this phosphorus, it can undergo biological immobilization and be incorporated into an organic molecule or it can react with something in the manure (often iron or calcium) and become a solid material (it is precipitated).

When we focus on the organic phosphorus, there are again two types, particulate phosphorus and dissolved organic phosphorus. In general, the flow is from organic phosphorus towards dissolved reactive phosphorus, this happens through a process called mineralization. Mineralization is just a way of saying broken down from an organic form (associated with carbon) to an inorganic from (usually H₂PO₄^- or HPO₄^2-). This process is mediated by micro-organisms living in the manure, as these “bugs” eat carbon to provide themselves with energy they break off the phosphorus and either use it to grow their cells, or release it into as dissolved phosphorus into the manure.

The final type of phosphorus is precipitated phosphorus. This is inorganic phosphorus that has reacted with something else in the manure and formed a particle. In general, I tend to think of this of being of less importance in manure, but in some cases we might add iron to our manure (as a way to acidify it – think for instance alum or ferric sulfate as amendments in poultry litter) and this makes it so our manure can hold less phosphorus in dissolved forms.

So now that we have talked about forms, where does this leave us? What we have seen is that phosphorus can change forms, but all these forms stayed in the liquid. This means that if the phosphorus was there to start with, it will be there when we land apply it (unless we spill some or if rainwater leaches through it and then flows away). However, just because it is there, doesn’t mean we’ll be able to capture it an land apply it. Since the particulate phosphorus is attached to solids, it is distributed like the solids. This means that the sludge rich manure often will have higher phosphorus contents and watery manure from the surface will have less. To get uniform phosphorus concentration we want to either get those solids evenly mixed in the manure, or to get as much of the phosphorus as possible into a dissolved form.