Green Manures booklet - Institute of Organic Training and Advice

An alternative is to save seed from current crop though in practice it is the exception rather than the rule. Much
will depend on having the correct equipment on the farm and this will include cleaning and drying equipment as
well as harvesting machinery. The other problem is that the crop will often need to be left in place for longer
than normal to allow seed to set and mature.
3.5. Mowing
Most short-term green manures are grown to generate bulk and this is turned in at the end of the growing period,
ideally before the crop has set seed. Longer-term green manures need active management if they are to generate
the required benefits. In stockless systems this is achieved by periodic mowing in the so-called ‘cut and mulch’
approach. The frequency of mowing will depend on the performance of the crop but as a general rule the crop is
mown when it reaches about 45cm; any higher than this and the volume of cut material is likely to cause
problems with smothering.
Unless there is a severe weed problem the cut should take the crop height down to no lower than ankle height
(10-15cm). This is to ensure that re-growth is rapid and is particularly important for non-grass crops where new
growth often arises from aerial buds. To avoid swathes of plant material that can smother new growth a rotary or
flail topper should be used to mince up the cut material and distribute it evenly across the sward.
Some farmers may wish to remove the cut material to use as forage or graze their fertility building leys. This will
obviously provide some cash return. There is evidence (Goodlass et al, 2006) that removal of the herbage will
stimulate nitrogen fixation (because less nitrogen is available though fixation) and it may be possible to apply
the mowings to a cash crop elsewhere, or to compost them for use later. However, with this approach there is a
danger of depleting nutrients such as potassium to such an extent that regrowth may be inhibited.
3.6. Incorporation techniques
Effective incorporation of the green manure crop is as important a job as the growing of the crop. Breakdown of
the green material should happen quickly and this will depend on good mixing and adequate aeration of the soil.
It is also important that the green manure has not become too mature and woody.
The top growth should ideally be wilted before incorporation. If there is a lot of bulk a topper should be used to
chop the growth, which is then wilted for up to 7 days before incorporation. An alternative approach uses disc
harrows or a shallow rotavator to chop the green material into the soil surface. This material is then wilted in situ
for 4-5 days before a further pass is made to finish the job.
The relative merits of different incorporation machinery are listed in Table 3. The final choice may depend on
the following crop eg the material must be well chopped up if there is to be a subsequent destoning operation
before a root crop like potatoes are grown. Ploughing can be very effective – it should bury all the material and
leave clean furrows for working down to a seedbed. Care should be taken with heavy soils not to bury the
material too deeply as further decomposition could be slowed down or prevented. The use of powered
implements can be more effective in mixing but they are more expensive in terms of their power requirements
The use of green manures
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