Organic amendments (composts and manures) are a key fertility source that recycle nutrients, add organic matter to the soil, and reduce/eliminate the need for synthetic fertilizers (e.g., urea).
However, organic amendments are enriched in phosphorus (P) relative to nitrogen (N) compared to crop need, hence farmers fertilizing exclusively with organic amendments can
- Target crop N needs, over-fertilize for P, and potentially favor water pollution;
- Target crop P needs, under-fertilize for N, and produce lower yields.
In addition, a large fraction of N in organic amendments is not directly available to plants, with manures having more available N than composts, and N availability varying among manures.
Also, Vancouver citizens have to compost their waste, generating large quantities of municipal compost that is less nutrient-rich than manures and might be less effective as a fertility source.
The goal of this experiment is to determine how to better balance N and P inputs and outputs when using organic amendments to:
- Optimize N supply to obtain high yields;
- Minimize P over-fertilization to prevent risks of water pollution (eutrophication);
- Supply sufficient organic matter to maintain soil quality.
We are comparing four different treatments in this experiment:
1. Composted poultry manure to meet crop N demand;
2. Municipal compost to meet crop N demand;
3. Municipal compost to meet crop P demand (6.5 times less compost than #2);
4. Hybrid system: same as #3 but with a N-rich fertilizer (blood meal) to compensate for the N deficit caused by the low compost addition.
We are measuring the effects of these treatments on several variables
- Cash crop yields – total biomass, marketable biomass, harvest index – and eventually nutrient content (C, N, P) to make nutrient budgets;
- Cover crop yields – total biomass, clover/rye/other biomass and relative abundance – and eventually nutrient content (C, N, P);
- Soil fertility – available nitrogen (ammonium and nitrate), net mineralization and nitrification, conductivity, pH and (eventually) total C and N, and Mehlich P;
- Greenhouse gas emissions – nitrous oxide, carbon dioxide and methane.
The goal with measuring all of this is to evaluate trade-offs. For example, the hybrid treatment should have the best N and P balance but it
- Adds the least organic matter to the soil, which may impact long-term fertility;
- Adds a significant amount of N at one time, potentially increasing nitrous oxide fluxes;
- Is more costly (blood meal) and complex to manage (multiple nutrient additions).
Ultimately, this research will provide tools for farmers to optimize C, N and P benefits from organic amendments to maintain yields and soil fertility while reducing risks of water pollution. It will also test the capacity of municipal compost to produce yields comparable to composted poultry manure when targeting a similar N input.
Note: I didn’t put anything in there relative to the impact of lower yields on the area of land that must be cultivated to produce the same amount of food, and whether this has a net positive or net negative impact on the environmental footprint of agriculture. But this can be added to the mix as well, especially when people question the need to maintain high yields.