| dc.description.abstract | Soil fertility depletion, unreliable rainfall patterns and poor access to nutrient supplements
continue to impede crop production in much of Sub-Saharan Africa (SSA). There is increasing
need to develop holistic approaches to deal with soil fertility depletion and food security in the
region. Farming in Zimbabwe is characterised by blanket fertilizer recommendations that fail to
address farm-specific niches which potentially pose as hotspots for crop production. Farmers have
developed adaptive mechanisms that are specifically tailored to increase production at the farm
level. In light of this argument, the current study sought to gain an improved understanding on the
effects of basin-based conservation agriculture (BCA), furrow-based conservation agriculture
(FCA), and conventional tillage (CT) on maize grain yields under rain fed conditions and soils
with different soil organic carbon (SOC) contents in Murewa district, Zimbabwe. An assessment
of farmers’ local knowledge of soil fertility was carried out using a survey during the 2013/2014
cropping season. Yield estimates on 152 fields were quantified using yield cuts on fields that each
of the farmers identified as either rich or poor during the 2013/2014 cropping season as well.
Another set of experiments was set up as nutrient omission trials on both the rich and poor fields
with contrasting SOC in 3 cropping seasons (2013/2014 to 2015/2016) using nitrogen (N),
phosphorus (P), potassium (K), cattle manure (M) and their combinations on twenty farms in Study
1, and ten farms in Study 2 under rain-fed conditions in Murewa. Phosphorus fertilizers also
contain sulphur (S) but adding an NKS fertilizer combination was not possible due to lack of
fertilizers containing S only in Zimbabwe. Grain yield data was analysed in Genstat 14 to test for
differences between grain yields from poor and rich fields. Questionnaire data was subjected to
analysis of simple proportions in SPSS version 20. Indicators of soil fertility that were consistent
with farmers’ perceptions of soil fertility were also identified. Farmers in Murewa district, were
grouped into four recommendation domains (RDs), that is the very rich, rich, poor and very poor
using draught power, product orientation, mineral fertilizer use, labour use and annual remittance.
Host farmer depiction of both rich and poor soils agreed with the laboratory indices (p < 0.001).
Poor fields had SOC < 0.4 % and had significantly lower maize grain yields than rich soils (p=
0.001). The poor soils had significantly lower (p < 0.001) bases, % clay, SOC, total N and were
more acidic than the rich fields. Most farmers are constrained in inputs and harvested average
maize yields of less than 1 Mg ha-1
. Farmers have a deep knowledge and understanding of their
farm niches that is passed down generations. Farms had SOC ranging from 0.18% to 0.89%, and
clay content from 6 to 15%. Maize grain yields were significantly higher under B-CA compared
to both F-CA and CT (P <0.01). For Study 1, nutrient management showed positive effects on
yields across the three years (P<0.001) but there were no significant tillage effects observed. Grain
yields increased from 0.3 Mg ha-1 for unfertilized control to 4.1 Mg ha-1
for the nitrogen +
phosphorus + potassium + manure (NPKSM) treatment. Yield response to N was consistently
larger than for P or K, irrespective of soil fertility status. Response to N significantly increased
with increase in soil fertility, suggesting higher N use efficiency for soils with higher SOC. Except
for NPKSM, no significant yield differences were observed under the residual and additive plots
for treatments, when N was added each year. At productivity levels of <4 Mg ha-1
, there was no
yield gain in applying both P and K for consecutive years, suggesting that nutrient investments by
resource constrained farmers for Year 2 could target only N application. For Study 2, maize grain
yields were significantly higher under B-CA compared with both F-CA and CT (P <0.01), iv
suggesting a positive effect due to concentration of nutrients in the second year. The consistently
larger NPKSM yields highlight the importance of integrated nutrient management, combining
mineral and organic sources of nutrients to ensure maize productivity on poor soils in agro ecologies receiving unreliable rainfall. | en_ZW |
