Switchgrass and Giant Miscanthus Agronomy - Engineering and Science of Biomass Feedstock Production and Provision - page 43

3.2.3 Fertilization

Switchgrass can grow without fertilization in natural or conservation settings.
However, the grass responds to fertilization, which can increase biomass yields
[ 42 - 44 ]. During the establishment year, nitrogen (N) fertilization is not recom-
mended, because switchgrass seedling growth is slow and annual weeds are better
able to take advantage of the applied N [ 33 ]. Nitrogen fertilization during the seed-
ing year may be recommended, however, if the fi eld is relatively free of weeds and
the switchgrass seedlings are not competing with weeds for N.
Post-establishment year N-management recommendations for switchgrass bio-
mass production are determined by agronomic factors including yield goals, the
production potential of the cultivars, soil conditions and fertility levels, the harvest
timing and frequency, and the weather. In general, N-fertilizer application rates can
be calculated based on N removal by switchgrass biomass. Nitrogen concentration
in biomass harvested at fl owering is approximately 1-2 %, and this concentration
can be decreased by up to 0.5 % if harvest takes place after a killing frost [ 42 , 44 ,
45 ] since a signifi cant portion of the N is translocated into belowground biomass
and recycled during the following season [ 13 , 46 ]. Therefore, N applications can be
made based on harvested biomass; when switchgrass is harvested for biomass after
a killing frost, approximately 0.5 kg N ha −1 needs to be applied for each ovendry Mg
ha −1 of harvested biomass. For example, a switchgrass biomass yield of 10 Mg ha −1
harvested after a killing frost removes 50 kg N ha −1 with a nitrogen concentration of
0.5 % in biomass. This fertilizer calculation should be adjusted based on soil test N
and soil N-mineralization rates. Nitrogen fertilizers should be top-dressed in late
spring when switchgrass is initiating growth.
There is limited information about switchgrass responses to phosphorus (P) and
potassium (K) fertilization. Brejda [ 47 ] found that switchgrass may respond to
P and K fertilization if available soil P and K are low. Continuous forage and hay
production depletes soil P and K making P and K application necessary for long-
term biomass production [ 47 - 51 ]. Additional research is needed to determine
switchgrass responses to P and K application for long-term biomass production and
annual removal of P and K in biomass. In general, when soil testing determines that
P and K levels are low, these minerals should be applied before planting and incor-
porated into the soil. Following establishment, soil P and K should be continuously
monitored to maintain recommended levels.

3.2.4 Harvest

Switchgrass harvest management for biofuel feedstock production is very different
from that for hay and forage production where nutrient value is an important quality.
Harvest timing and frequency of dedicated energy crops should be optimized for
maximum sustainable biomass production and for year-round feedstock supply to
conversion facilities. Switchgrass reaches a peak standing crop at fl owering stage,