Defatted oilseed meals from sunflower (Helianthus annuus L.), and some Brassica species have a well
known economic value as feed for animals, whilst their value as organic nitrogen fertilizers has been not
fully explored so far. Compared to sunflower, the seed meals of Brassica species are reported to have a
potentially higher capacity of nutrient supply, due to their lower content of indigestible fiber. The high
content of glucosinolates may conversely reduce the availability of nitrogen, because of their inhibitory
effect on nitrification processes.
In this lysimetric study we compared the effect of the application of three seed meals: partially defatted
meals of sunflower and of Ethiopian mustard (Brassica carinata A. Braun), and completed defatted meal of
sunflower on the yield and N utilization of three vegetable crops (lettuce, chard and spinach), compared
to an unfertilized control and one fertilized with ammonium nitrate. Furthermore, we also investigated
the residual effect of fertility of oilseed meal application on winter barley and fallow. Both sunflower
meals revealed a fertilization effect comparable to that of the mineral fertilizer, whilst B. carinata meal
reduced the yields of chard and spinach, due to low N availability as shown by the lower N use efficiency
index. We argue that this lower efficacy of B. carinata meal was due to the technique of the application of
the meal, that might have produced a high concentration of glucosinolates in the soil layer explored by
roots, concurrently determining a significant impact on soil biota and direct phytotoxicity phenomena.
A relevant part of the N applied with fertilizers was not absorbed by the three vegetables and hence
accumulated in the soil. Despite the high values of unabsorbed soil N in the plots fertilized with oilseed
meals, nitrate leaching was lower than with ammonium nitrate. The impact on global warming potential
of fertilization with defatted oilseed meals was assessed and shown to be lower than or comparable to
the impact of the synthetic fertilizer. In particular, defatted sunflower meals caused a lower impact per
hectare and total biomass, and a similar impact per fresh marketable product.
Overall, our findings confirmed the high value of oilseed meals as a sustainable alternative to mineral
fertilizers. However, their efficacy strongly depends on the technique of application to the soil.