Tuesday, February 3, 2015

Manure Analysis - As Is versus Dry Basis, what does it mean, and which one to use

I received a question the other day about poultry litter and the variability of the manure samples they were collecting over the last few years. There are lots of things that can cause variability in our manure, for example, the diet of the birds, the management of the litter, the age of the birds, weather conditions and a host of others. We discussed this but my follow-up question was are your sample results on an “as-is/wet” basis or were they on a “dry” basis. So today’s post is about why I asked that, and just as importantly, what the difference is, and which one should I be using in my manure planning?

So let’s start with the difference, when we talk about an “as-is” or “wet” basis we are talking about the amount of some item (let’s say phosphorus) per unit of mass of the manure in its current condition, that is as it sits in our storage or loaded in our spreader. When samples results are reported on a “dry” basis, the laboratory has measured the solids content of the sample and is reporting how much of that item is present per unit of solids.
So when is one presentation advantageous over the other one. When we apply manure we are always applying it on an “as-is” basis, so when we are figuring application rates it is most useful to have the  nutrient contents on an as-is basis. For example, if we know our manures phosphorus content on an as is basis,110 lb P2O5/ton, and the amount of that nutrient we want to apply is 250 lbs of P2O5 per acre, then we can estimate that we want to apply 2.3 tons of that manure per acre. Is we were told that the manure had a phosphorus content of  225 lbs P2O5/ton on a dry basis, we still wouldn’t be able to estimate how much of our manure to apply, because our actual manure has some of its mass from water. So typically in our manure management planning we want to work on an “as-is” or wet basis.

So when is it advantageous to work on a “dry-basis?” Well, there are a few times when this may be useful. For example, the consistency of our manure samples from one year to the next or between barns. Adjusting to a dry-basis in these cases provides us a way to normalize our data for a specific variable, in this case moisture that can be influenced by the amount of water used in the barn. One way to think of this is in a pig barn it’s a way to account for differences in water wastage (or perhaps if I’ve washed my barn 3 times in one year and only twice the next), or in the case of feedlot manure, how wet it was when I scraped my lots. As an example of this I recently looked at a series of 275 manure samples from swine deep-pit barns in two ways, on an as-is basis and a dry-basis. What I found, is that by looking at the phosphorus content on a dry-basis as opposed to a wet-basis, the variability was reduced from 49% to 35%. Similarly, in the case of nitrogen, the variability was reduced from 19% to 11%. For similar reasons your soil sample results are also presented on a dry basis.

 So the take home message is usually the wet basis presentation is more useful to us in terms of our manure management planning, but if we are trying to learn something about the consistency of our manure samples it is often useful to look at the data on a dry-basis.