Strategies for high nitrogen production and fertilizer value of plant-based fertilizers

BackgroundOrganic vegetable production has a demand for alternative fertilizers to replace fertilizers from sources that are not organic, that is, typically animal-based ones from conventional farming. AimsThe aim of this study was to develop production strategies of plant-based fertilizers to maximize cumulative nitrogen (N) production (equal to N yield by green manure crops), while maintaining a low carbon-to-nitrogen (C:N) ratio, and to test the fertilizer value in organic vegetable production. MethodsThe plant-based fertilizers consisted of the perennial green manure crops-alfalfa, white clover, red clover, and a mixture of red clover and ryegrass-and the annual green-manure crops-broad bean, lupine, and pea. The crops were cut several times at different developmental stages. The harvested crops were used fresh or pelleted as fertilizers for field-grown white cabbage and leek. The fertilizer value was tested with respect to biomass, N offtake, N recovery, and soil mineral N (N-min). Poultry manure and an unfertilized treatment were used as controls. ResultsThe cumulative N production of the perennial green manure crops ranged from 300 to 640 kg N ha(-1) y(-1) when cut two to five times. The highest productions occurred at early and intermediate developmental stages, when cut three to four times. Annual green manure crops produced 110-320 kg N ha(-1) y(-1), since repeated cutting was restricted. The C:N ratio of the green manure crops was 8.5-20.5, and increased with developmental stage. The fertilizer value of green manure, as measured in white cabbage and leek, was comparable to animal-based manure on the condition that the C:N ratio was low (<18). N recovery was 20%-49% for green manure and 29%-42% for poultry manure. A positive correlation was detected between soil N-min and vegetable N offtake shortly after incorporating the green manure crops, indicating synchrony between N release and crop demand. ConclusionsPlant-based fertilizers represent highly productive and efficient fertilizers that can substitute conventional animal-based fertilizers in organic vegetable production.