The incorporation of compost into your production spaces is a great way to recycle nutrients and carbon back into the soil, turning waste into something that will feed future crops and soil microbes. The variety of materials incorporated into compost, however, is highly variable. Plant residue, municipal solid waste, food processing waste, pulp or paper mill solids, manure and seafood wastes are all contenders for being included in a compost. The range of materials, each with their own broad ranges of nutrient concentrations, makes it a highly inconsistent product between batches, even where the supplier remains consistent. This variation in composition is going to have implications for nutrient management planning year to year - we can't necessarily apply the same volume from year to year and expect the same impact on our soils.
In order to approach our nutrient management and compost applications with increased accuracy, it is recommended to submit compost samples for analysis. Nova Scotia's provincial lab services offer this as an option, and the report will break down the nutritional composition of the compost for next season's planning. Once we can better quantify the amount of nitrogen (and other key nutrients) in the compost, application volumes and a plan for additional supplementation throughout the growing season can be put together to meet the crop needs, and reduce accidental over-fertilization.
For greenhouse soils, where all overhead precipitation has been excluded, compost analysis is especially important. Some composts tend to run high in salts (ex. calcium, magnesium etc), and while these are crucial nutrients for crop production, it is important that they are not being over-supplied to the soils. With a lack of a seasonal reset via rain and snow in protected spaces, these salts can accumulate quickly over time, impacting crop health and productivity.
Now that we have the data...what does it mean?!
The Department of Agriculture has an interpretation document outlining the line items included in a compost analysis report and the impact that each nutrient has on crop production. Once you have a good understanding of what is included in the report, Perennia has created a fact sheet that provides further information on some of the metrics included, and the implications that these metrics have on our nutrient management plan. Below is a brief summary of some of the information presented in the factsheet, but I highly recommend you check it out for yourselves!
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Finished composts typically contain low levels of a wide variety of nutrients (though this is highly dependent on the materials included in the mix). This lower level of nutrients can translate to a tendency to apply large volumes of compost, however this can lead to excessive levels of certain nutrients in the soil in some cases. It is typically recommended that compost is not the sole fertility source for crops, but one of a handful of additives that come together to provide the target nutrient ranges for the crop in question.
C:N Ratios
The amount of carbon present in a compost, in relation to nitrogen, impacts nutrient cycling, and how quickly nutrients will become plant available. Composts typically range from 8:1 - 30:1. If your compost has a ratio:
- >40:1, there is a good chance that any nitrogen that is present in the soil/compost will be 'tied up' through the process of breaking down carbon. This can lead to nitrogen deficiencies in your crop.
- 12-22:1, there is more nitrogen available than the microbes need, meaning it will become plant available more readily
- 25-32:1, it will slowly become plant available over the course of a few years.
Nitrogen
The provincial lab analysis of compost reports on total nitrogen in the sample, however nitrogen has many forms in compost. 'Organic nitrogen', which is the nitrogen included in microbial, animal and plant tissues, is not immediately plant available. It must be mineralized in the N cycle before a plant is able to take it up and use it for growth. The rate of mineralization is difficult to forecast, as it is dependent on a handful of variables (temperature, moisture, C:N ratio of the compost). When compost is the only source of nitrogen in a production system, it is challenging to match the times of peak demand for the crop, with the availability of high amounts of useable (i.e. mineral) N.
'Mineral nitrogen', specifically ammonium-nitrogen (NH4-N) and nitrate-nitrogen (NO3-N), are the plant-available forms of nitrogen. While these are key forms for plant production, they are easily lost to the environment if they aren't immediately taken up.
How do we know the amount of available nitrogen that is present in our compost analysis? Low N composts typically have low mineral (readily available) nitrogen. High N composts usually carry a fair amount of NO3-N, which is immediately plant available. Take a look at the table below, which estimates the breakdown of organic and mineral N based on your C:N ratio, and the total N (% dry weight). This chart was taken from Perennia's fact sheet linked above, and sheds some light on the percentage of organic, or plant -unavailable, nitrogen in a compost.
From here, we can calculate how much compost we need to add in addition to our other amendments, in order to meet the target N needs of the crop in question.
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Utilizing and understanding the tools available to us is a key step in maximizing on-farm productivity, and the inputs invested on the farm. Compost analysis, along with soil tests and water tests, are great ways to quantify and characterize our key production components, and figure out how they can work together to hit our crop-specific nutrient targets without overshooting them.
For those who are interested in better understanding their compost analysis results, check out Perennia's fact sheet here, or contact your commodity specialist for further discussion!
-Talia
