NEWS
Conservation Agriculture and soil
fertility management: Part 2
Case study on a degraded soil in the North West Province
Hendrik Smith 1 and Gerrie Trytsman 2
Current Address: 1 Conservation Agriculture Facilitator and 2 Independent Scientist, Grain SA
Reprinted From: http://bit.ly/2C0BQiO
F
ollowing an introduction to some
theoretical principles and practices
of CA and integrated soil fertility
management (ISFM) in Part 1 (Novem-
ber 2018 edition), this article presents a
case study of a one-season soil rehabili-
tation process of a degraded soil on the
farm Humanskraal of George Steyn in
the Ottosdal area.
The soil was degraded due to continu-
ous 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 rehabilita-
tion 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 per-
formance.
The summer annual CC mixture includ-
ed 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 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 de-
termined from the summer CC mix dur-
ing the growing season was between 11
and 14 t/ha with an average of 12 t/ha.
17
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 aver-
age 1.4% N, 0,3% P and 2% K consid-
ering 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 po-
tassium (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. Assess-
ment of soil health Soil samples were
taken on 19 April 2016 during the fully
developed growth stage of the sum-
mer CC stand, where after a Haney soil
health analyses were done. Most nutri-
ents 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.
Table 3 shows that phosphorous satu-
ration is below 5% which indicate that
additional inorganic phosphorous ferti-
liser is needed. With less than 1% SOM
the soil can be seen as highly degraded
due to a long period of continuous till-
age practices.
The establishment of a multi-specie CC
system are seen and applied as the start
of a process to build-up the degraded
soil, which could take up to 7 years or
more, depending on the situation and
the quality of CA application, the soil
type and the climate. However, it is an-
ticipated that this type of CC system, ro-
tated every second year by a cash crop
producing high amounts of residues
(such as maize), could quickly restore
Table 1: Available organic and inorganic nutrients (kg/ha) and value (R/ha) in a
soil under warm season cover crops.
Cost
R / Kg Nutrient Nutrient value
(kg/ha) Total Organic Inorganic
17 Nitrogen 23.3 R 396 R 133 R 263
40 Phosphorus 42.5 R 1 700 R 72 R 1 628
15 Potassium 209.7 R 3 146 R 0 R 3 146
R 5 242 R 205 R 5 037
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Table 2: Available organic and inorganic nutrients (kg/ha) and value (R/ha) in soils
under cool season crops.
Price
R / Kg Nutrient Nutrient value
(kg/ha) Total Organic Inorganic
17 Nitrogen 19.9 R 339 R 156 R 183
40 Phosphorus 54.6 R 2 185 R 84 R 2 101
15 Potassium 285.6 R 4 284 R 0 R 4 284
R 6 808 R 240 R 6 568
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Grassroots
Vol 19
No 1
March 2019