Conservation Agriculture and soil
fertility management
Part 2: Case study on a degraded
soil in the North West Province
F
Hendrik Smith, Conservation Agriculture Facilitator,
Gerrie Trytsman, Independent Scientist,
Grain SA
ollowing an introduction to some
theoretical principles and practices of CA
and integrated soil fertility management
(ISFM) in Part 1 (November 2018
edition), this article presents a case study of
a one-season soil rehabilitation process of
a degraded soil on the farm Humanskraal
of George Steyn in the Ottosdal area. The
soil was degraded due to continuous tillage
and excess water run-off, leading to severe
sheet, rill and gully erosion. The soil type is
an Oakleaf soil form, a fairly common soil
for crop production in the region with a
depth of about 500-600mm and underlain by
weathered rock material. To prepare the soil
for crop production, the gullies were closed
with a disk during winter.
The Grain SA CA research project team, which
included George Steyn, decided to initiate a
biological soil rehabilitation process on these
degraded fields through the establishment
of a ten species cover crop (CC) mix, planted
with an Amazon spreader (for the small
seeds) and a John Deere no-till planter (for
the big seeds). As described in Part 1, the
use of crop diversity, in this case a summer
CC mixture, enhances and speed-up the
biological (ecosystem) processes in the soil.
No fertilisers were used because the previous
crop was not harvested due to a poor stand
and performance.
The summer annual CC mixture included
functional groups such as legumes, cash
crops, grasses, as well as a brassica in the
form of radish. The winter mixture included
the same functional groups. The summer
AgriKultuur |AgriCulture
mix had mainly annual grasses that are not
easily decomposed (such as Babala and
fodder sorghum), while the winter CC mix
had temperate crops that decomposed
fairly quickly. The impact of the different
CC mixtures on maize grain yield will be
determined after harvest in June 2017.
The DM determined from the summer CC mix
during the growing season was between 11
and 14 t/ha with an average of 12 t/ha. This
gives us a good indication of the potential
amount of nutrients locked-up in an organic
form within the biomass. This plant biomass
contains on average 1.4%N, 0,3% P and 2%
K considering previous sample analyses.
This amount to an estimated total amount
of nutrients of 168 kg nitrogen (N), 24 kg of
phosphorous (P) and 249 kg of potassium (K),
of which around 40% of N could potentially
be made available for the next crop through
decomposition by microbes.
The CC was left to be killed by frost; a decision
was made not to roll it flat in order to enhance
the mulch durability and to escape possible
decomposition by microbes before planting.
Assessment of soil health
Soil samples were taken on 19 April 2016
during the fully developed growth stage of the
summer CC stand, where after a Haney soil
health analyses were done. Most nutrients
that were available in the soil were effectively
taken up by the cover crops.
The Haney soil health analyses (Table 1)
show the available N, P, and K in kg/ha in the
field established by the warm season CCs;
the nutrient levels can be regarded as below
average due to the degraded state of the soil.
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