The next step in improving nitrogen use efficiency
Cut and
carry fertilisers
Geert-Jan van der Burgt
In organic farming, nitrogen is often a limiting
factor for production. Leguminous crops can fix
this ‘fuel’, but how to use it in an efficient way?
Cut and carry fertilisers offer new prospects for
increasing the production potential of arable
farming.
L
Cut and carry fertilisers are usually clover/
grass, clover or alfalfa. The crop is used
directly as ‘green manure’ for fertilising
another field without going through the
usual cycle of being fed to an animal
to produce nitrogen-rich manure. This
allows for an increase in nitrogen use
efficiency as feeding the crops to cows
to produce manure involves a 20-30%
eguminous crops are a well-known
But what is a farmer to do if there is no
nitrogen loss. The option of redistributing
component of organic farming and
organic dairy farm close by? In such a
nitrogen around the farm offers increased
gardening. As a cash crop they reduce
situation, cut and carry fertilisers are an
freedom for nitrogen use, compared to
the overall need for fertiliser. As a green
interesting option.
mulching with a grass-clover crop. It gives
manure they provide nitrogen for the
freedom of location (it can be put on any
next crop on the same field. Clover/
field), freedom of amount (you choose
grass, clover or alfalfa can be sold to a
how much you apply on a certain field)
nearby dairy farm in exchange for manure.
and freedom of timing (if you don’t need
the nitrogen now you can conserve it as
silage and use it later).
This idea is not new, but it has been picked up again in the Netherlands among
arable farmers who don’t have organic
dairy farms nearby and who are interested
46
1-2013 | ECOLOGY & FARMING
soil
The material must
be finely cut, evenly
spread and not too dry
in farming without using animal inputs;
green fertiliser acts best on crops with a
were built. We also learned that digging it
some refer to it as “vegetarian arable far-
long growing period. We also tested it on
into the soil is necessary. In experiments
ming”.
spinach, and there it only worked when
with spring wheat, the green mass was
a very large quantity of nitrogen was
applied to the crop at the end of April /
For three years, the Lois Bolk Institute
applied. The timing aspect also became
beginning of May and only lightly dug into
hasv been comparing the nitrogen value
clear in an experiment with potatoes.
the soil, when the field was tilled for weed
of cut and carry fertilisers with that of
The cut and carry fertiliser gave a better
suppression. The effects of the nitrogen
dairy slurry and poultry manure. We found
result when applied before planting than if
were noticeable, but not sufficient. We
this green mass, freshly used or after a
applied three weeks later, when the ridges
also learned that the material must be
period of storage, resulted in comparable
cut into short enough strips to be easily
or slightly higher yields than the use of
and evenly spread and tilled into the soil.
manure. However, the release of nitrogen
Finally the material should not be too dry.
shows a different pattern. About half of
the nitrogen in dairy slurry is inorganic
and is only available to the plants shortly
after application. In the case of cut and
carry fertiliser, more time is needed for the
material to decompose and the nitrogen is
released little by little. For this reason, this
Mineral
nitrogen
green fertiliser
acts best on
crops with a long
growing period
After these three years of experiments on
small plots we upgraded the experiment
to the farm scale at two locations. We
Soil
fertility
When selling clover/grass, clover
or alfalfa one is not only selling
nitrogen. Other nutrients and organic matter are sold that
could be kept on the farm if the plants were used as cutand-carry fertiliser.
About 50% of
the nitrogen in
dairy slurry is in mineral form. This has the advantage
of being directly available to the plants. But there are
also two disadvantages: the risk of leaching, especially
on sandy soils, and the risk of increased denitrification,
especially on clay soils.
ECOLOGY & FARMING | 1-2013
47
Mulching
A cheap way of providing
nitrogen input is to grow
clover/grass, clover or alfalfa and to cut this and use it as mulch in situ three or four
times a year. This increases soil fertility in the field, but the
overall nitrogen fixation is less than when the produce is
removed. When mulched, the decay of nitrogen-rich material creates plant-available nitrogen for the next cut, which
reduces the nitrogen fixation. Leguminous plants prefer
easy N-uptake above energy-demanding fixation. So mulching in situ reduces the N-input at the farm level.
The approach is
practical and
under local
conditions
might become
economical
continued to measure the fertilising value
tory to produce feed pellets. The main
and compare it to the use of manure, but
determining factor in this new system is
we also looked at new and very practical
the price saved by not buying manure,
questions: At what growing stage should
including the costs of transporting the
the cut and carry fertilizer be cut? How
green mass to the dairy farm and the
do you know or estimate the nitrogen
manure to the arable farm. We estimate
After five years of research, we have con-
content? How long should it be dried (up
that a price of between €12 and €15 per
cluded that the fertilising potential of cut
to 30-35% dry matter?) before spreading
tonne of manure is the point at which
and carry fertilisers is comparable to that
it on the target field? How short should it
it becomes worthwhile to grow clover/
of manure and that the overall nitrogen
be cut after harvesting to create a product
grass, clover or alfalfa for cut and carry
use efficiency is better. We have shown
with good spreading properties? What
fertiliser, rather than for sale. Some far-
that this alternative approach to using
type of manure spreader is best to use
mers are close to paying that much, and
clover/grass, clover or alfalfa is techni-
with this high-volume, low-weight fertili-
the price of organic manure is expected
cally and practically possible at the farm
ser? How do you ensure that the correct
to rise.
scale and that, under local conditions, it
might become economical. So if you farm
amount is applied?
We plan to continue the experiments
under these conditions, give it a thought
And then there is the financial aspect.
at farm scale for at least another three
and even try it yourself.
Under Dutch conditions there are two
years. This will allow us to observe the
alternative pathways: selling the produce
development of soil fertility in the medi-
to a dairy farm and buying manure in
um term when exclusively using cut and
return, or selling the produce to a fac-
carry fertilisers.
48
1-2013 | ECOLOGY & FARMING
Geert-Jan van der Burgt works as researcher
soil fertility with the Louis Bolk Institute in the
Netherlands. Contact [email protected]