Manure Minute: Back to basics: Understanding your manure nutrient analysis report
January 25, 2021 by Chryseis Modderman
You did the work of taking a good, representative manure sample, you sent it off to a nutrient testing lab, and the lab sent you a results report. Great! But how do you interpret the results and use them for accurate manure application? Let’s first walk through the common tests, and then look at how to use them for application.
Moisture and dry matter are pretty self-explanatory. It’s just the measure of how wet or dry the manure is. It’s reported as percent and the two values (percent moisture and percent dry matter) should add up to 100 percent.
Nitrogen (N) usually takes up a hefty portion of the report because there are many forms that are relevant for plant growth. Total nitrogen is all of the nitrogen forms combined. Ammonium nitrogen is the fraction of nitrogen that is immediately plant-available; and organic nitrogen, which is sometimes called “slow release N,” is the fraction that will need to be mineralized before a plant can use it.
Phosphorus might be reported as P2O5 or simply P. Even though P2O5 is not a P form used by plants, it is the preferred form for calculating application rates since it is what’s used in fertilizer recommendations. If your lab reports phosphorus as just P, you’ll need to multiply by 2.29 to convert P to P2O5.
Potassium may be reported as K2O or K. Similar to phosphorus, though plants don’t use K2O, it is the preferred form for reporting since it lines up with fertilizer recommendations. If potassium is reported as just K, convert to K2O by multiplying by 1.2.
While N, P, and K usually get all the attention, other nutrients may be important to test for if you are concerned that your soil has a nutrient deficiency for the planned crop.
Sodium, while not an essential nutrient for plant growth, may be a good test to include as very high sodium levels can damage plants. If you live in an area with sodic (high sodium) soils, adding manure with high sodium can make the problem worse.
Soluble salts are reported as EC, which stands for electrical conductivity. Much like sodium, manure with high salts can damage crops and make saline soils worse.
pH is the measure of how acidic or alkaline the manure is. The scale ranges from 0 to 14, with 7 as neutral. Higher than 7 is alkaline, and lower than 7 is acidic.
Using your manure test
The most obvious use of your manure test is to calculate application rates. Of course, to do this, you also need to have a recent soil test to know what nutrients already exist in your soil. If your manure test shows a high P content and your soil P level is already high, consider applying at a rate that meets the P needs of the crop. This will likely under-apply N, but it will avoid P buildup, which can lead to runoff and pollution.
Besides application rates, there are other nuances that manure test results can reveal. For example, potatoes tend to be fond of slightly acidic soil. If you’re applying a very alkaline manure to alkaline soil, it might not be the best growth environment for your potatoes. Similarly, sodic and saline conditions are not conducive for the growth of many plants. So, if you have saline soil and your manure also has a high EC, you might want to choose to plant a crop with high salt tolerance such as barley.
There are many tests out there for manure, and what’s available may vary based on your location. Which tests you choose to order and how you use them are up to you; in the end, it comes down to what is important for your situation. If you’re trying to save some money, you certainly don’t have to order all the tests described here. It’s generally recommended to test manure, at a minimum, for moisture, total nitrogen, phosphorus and potassium.