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Julie 2017
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Conservation agriculture and
soil fertility management
F
ollowing an introduction to some theoretical principles and
practices of conservation agriculture (CA) and integrated
soil fertility management in Part 1 (page 20), this article
presents a case study of a one-season soil rehabilitation
process of degraded soil on the farm Humanskraal of Mr 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 mm to 600 mm 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 Steyn,
decided to initiate a biological soil rehabilitation process on these
degraded fields through the establishment of a ten species cover
crop 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 cover crop mixture, enhances and speeds up the biologi-
cal (ecosystem) processes in the soil. No fertilisers were used be-
cause the previous crop was not harvested due to a poor stand and
performance.
The summer annual cover crop 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 mix had mainly annual grasses that are not
easily decomposed (such as babala and fodder sorghum), while
the winter cover crop mix had temperate crops that decomposed
fairly quickly.
The impact of the different cover crop mixtures on maize grain
yield will be determined after harvest this year.
The dry matter determined from the summer cover crop mix dur-
ing the growing season was between 11 t/ha and 14 t/ha with an
average of 12 t/ha. This gives us a good indication of the poten-
tial 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 amounts to an estimated total amount of nutrients of 168 kg
nitrogen (N), 24 kg of phosphorus (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 cover crop 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 last year during the fully de-
veloped growth stage of the summer cover crop stand, whereafter
a Haney soil health analysis was done. Most nutrients that were
available in the soil were effectively taken up by the cover crops.
The Haney soil health analysis (
Table 1
) shows the available N, P,
and K in kg/ha in the field established by the warm season cover
crops. The nutrient levels can be regarded as below average due to
the degraded state of the soil.
Table 3
shows that phosphorus saturation is below 5%, which
indicate that additional inorganic phosphorus fertiliser is needed.
FOCUS
Fertiliser
Special
GERRIE TRYTSMAN
, ARC-DPI and
DR HENDRIK SMITH,
conservation agriculture facilitator, Grain SA
Part 2: Case study on degraded soil in the North West Province
COST
R/KG
NUTRIENT
NUTRIENT
VALUE (KG/HA)
TOTAL
ORGANIC
INORGANIC
17
Nitrogen
23,3
R396
R133
R263
40
Phosphorus
42,5
R1 700
R72
R1 628
15
Potassium
209,7
R3 146
R0
R3 146
R5 242
R205
R5 037
Saving
TABLE 1: AVAILABLE ORGANIC AND INORGANIC NUTRIENTS (KG/HA) AND VALUE (R/HA) IN SOIL UNDER WARM
SEASON COVER CROPS.
PRICE
R/KG
NUTRIENT
NUTRIENT
VALUE (KG/HA)
TOTAL
ORGANIC
INORGANIC
17
Nitrogen
19,9
R339
R156
R183
40
Phosphorus
54,6
R2 185
R84
R2 101
15
Potassium
285,6
R4 284
R0
R4 284
R6 808
R240
R6 568
Saving
TABLE 2: AVAILABLE ORGANIC AND INORGANIC NUTRIENTS (KG/HA) AND VALUE (R/HA) IN SOILS UNDER COOL
SEASON COVER CROPS.