Seed should be planted immediately after inoculation, and protected from direct
sunlight.
Although you can buy seed that has already been inoculated, this has a very short life
and it is normally farmers and gardeners themselves who apply one of the four types of
inoculum:
- Powdered—the most common form, in which finely ground peat is normally used
as the inert substance; - Granulated—micro granules are produced from clay flakes and powdered
inoculum. It is applied into the seed furrow and is used when the seed is treated
with a fungicide or insecticide, which can be harmful to Rhizobia. Seed dressings
of Thiram are generally safe to use with inoculum; - Liquid—either applied into the seed furrow or added to the seed before planting;
- Pellet support—placed together with the seed, either by machine or by hand.
Failures with Inoculation
Inoculation can fail to stimulate active nodules for several reasons:
- incorrect strain of Rhizobium for either the crop or the conditions;
- old inoculum, or stored or applied at high temperatures;
- seed kept too long between inoculation and planting;
- if treated seed is also subsequently inoculated, unless precautions are taken;
- soil too dry;
- soil too acid (especially when below about pH 5.5);
- excessive nitrogen in the soil;
- deficiency of phosphorus, boron or molybdenum.
Deciding Whether to Inoculate
There are no firm rules to follow when inoculation is advisable, but if a leguminous crop
has been grown often and successfully in a field then it is usually not necessary.
However if a new leguminous crop is grown for the first time in that field, especially
one which is specific in its Rhizobium requirement, then inoculation is very often
worthwhile and cost effective.
The crop species which are most specific in their requirements are: chickpeas, lima
beans, soybeans, lupin, lucerne and subterranean and Kenya White clovers. Pigeon peas
are less specific, while cowpeas are fairly nonspecific and can normally use the strains
of Rhizobia which are commonly found naturally in tropical soils.
The whole process of inoculating seed can be quite daunting, expensive and
logistically challenging so if possible conduct some field trials in the first growing
season which comes along. In this way, if you can wait for a few months (one growing
It may be a good idea to plant out a number of trial size areas, half with inoculated
seed and half with uninoculated seed. During the growing season make observations of
the relative nodulation, plant growth/health and finally yields of the different trial areas,
and then decide if inoculation is a worthwhile operation for future years.
56 TONY WINCH
season) to find out the answer to the question “Is inoculation a cost-effective exercise?”.