Phosphorus retention and the interactive effects of phosphorus, lime and tillage on maize in acid soils of the Kenya Highlands
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Date
2016
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Publisher
Sokoine University of Agriculture
Abstract
Maize (Zea mays) is the most important food crop in Kenya. However, its
productivity is greately constrained across the Kenya highlands. Soil acidity and low
available phosphorus (P) are among the major hindrances to maize production in acid
soils of the Kenya highlands. A survey, greenhouse experiment and field trials were
carried out to assess the extent of soil acidity, soil nutrient levels and common soil
management practices by farmers that contribute to the poor growth performance of
maize in the Kenya highlands. The study also sought to evaluate the interactive
effects of tillage, P and lime on maize yields in the acid soils of the Kenyan
Highlands. The survey was carried out in nine counties across the Kenya highlands
namely: Meru, Embu, Kirinyaga, Nyeri, Kiambu, Nyandarua, Siaya, Busia and
Eldoret. Soil acidity and nutrient levels were determined on soil samples taken from
the study sites, while farmers’ practices were established by administration of a
structured questionnaire. Phosphorus adsorption rates and lime requirements of the
soils were also evaluated. The adsorption data were fitted into the Langmuir and
Freundlich adsorption equations and the relationships between sorbed P and soil
properties determined by correlations. Evaluation of the interactive effects of lime,
phosphorus and aluminium was carried out by establishment of a greenhouse pot
experiment at Waruhiu Farmers Training Centre, Githunguri - Kiambu. Extremely
acidic (pH 4.48) and strongly acidic (pH 4.59) soils were used for the study. Four
lime (CaO) rates and phosphorus (Ca (H2PO4)2 rates were used. The liming rates
were: 0, 2.2, 5.2 and 7.4 tonnes ha-1 for the extremely acidic soil and 0, 1.4, 3.2, and
4.5 tonnes ha-1 for the strongly acidic soil. Phosphorus application rates were: 0, 0.15,
0.30 and 0.59 g P kg-1 soil for the extremely acidic soil and 0, 0.13, 0.26, and 0.51 g
P kg-1 for strongly acidic soil. The experiments were a 42 factorial laid down in a
Randomized Complete Block Design (RCBD) and replicated three times. After the
greenhouse experiment, field experiments were carried out during the 2014 long
rains and 2014/15 short rains in Waruhiu Farmers Training Centre, Githunguri,
Kiambu County. The objective of the study was to assess the interactive effects of
tillage, P and lime in the management of soil acidity, P-uptake and maize growth
performance. Tillage practices evaluated included: ploughing, strip tillage and use of
hand hoe while fertilizer lime combinations included: DAP + lime (DAPL), TSP +
lime (LP), TSP alone (P) and lime alone. The experiment was laid down in the
Randomized Complete Block Design (RCBD) with split plot arrangement. Lime and
P rates applied were 3.2 tonnes ha-1 and 52kg P ha-1, respectively. Data collected
included: maize plant height, number of leaves, internode interval, P-uptake at 30
days after emergence and at tasseling stage, grain yield, and soil chemical
characteristics. Information gathered through personal interview via questionnaires
revealed that most farmers (>96%) were not aware of the soil acidity problems and
management strategies. The farmers also lacked sources of information about new
technologies or innovations and < 37% were members of farmers’ training groups.
Additionally, among the 4% who were aware of soil acidity, only less than 8% had
ever tested their soils for soil acidity. Farmers who had applied lime on their farms at
least once were also < 3% in all sites. On the basis of overall soil fertility
manangement, >80% of the farmers in the research area applied both fertilizer and
manure in their farms, with the majority using diammonium phosphate (DAP),
calcium ammonium nitrate (CAN) and farmyard manure. However, a negative balance was identified between livestock keeping and fertility manangement, with
<8% of the farmers returning crop residues back to the farm. Correlation analysis
showed that there were significant relationships (P ≤ 0.05) between farmer education
level, farmer training groups, fertilizer usage and crop yields. It is, therefore,
concluded that more farmers’ awareness is critical for effective manangement of acid
soils in the Kenya Highlands. Soils in all the study sites were strongly to extremely
acidic (pH 4.83 - 3.76), had high exchangeable Al (>2 cmol Al kg-1), and high Al
saturation (> 20% Al), which most maize germplasms grown in Kenya are sensitive
to. The cation exchange capacity was low (CEC < 15 cmol kg-1) in Siaya, Busia and
Eldoret while available P was low (<10 mg P kg-1 Bray 1) for all sites. The P
adsorption data showed that average maximal adsorbed P of the soils were high (1
383- 2 238 mg kg -1 soil) while SPR varied from 189.43 mg P kg-1 in Eldoret to
529.76 mg P kg-1 in Nyeri. Freundlich P adsorption isotherms were found to have the
best fit for all the tested soils (r2 = 0.882 - 0.990). Additionally, analysis of the
interaction between P adsorption maxima and soil properties revealed that there was
a positive relationship between P adsorption maxima and clay content, extractable P,
exchangeable acidity and aluminium saturation. Conversely, organic matter and
electrical conductivity negatively correlated with P adsorption maxima. Greenhouse
pot experiments revealed that there existed good lime-Al-P interactions in acid soils
of the Kenya Highlands. The interactions significantly (P ≤ 0.05) increased soil pH,
extractable P, Ca, plant P uptake and maize yields. The interactions were also found
to negatively affect exchangeable Al, oxalate Al, dithionate Al and P adsorption
levels in the soils. Use of 7.4 tonnes ha-1 lime for the extremely acidic soils and 4.5
tonnes ha-1 or the strongly acidic soils significantly reduced exchangeable Al and standard phosphate requirement (SPR) by > 70% compared to lower lime rates.
Similary, use of 0.59 mg P kg-1 soil reduced SPR of the soil by 40% while 0.51 g P
kg-1 reduced SPR by > 73% in strongly acidic soils. It was, however, observed that
there was no significant difference between combination of 7.4 tonnes ha-1 with
either 0.59 g P kg-1 or 0.3 g P kg-1 in extremely acidic soils or combination of 4.5
tonnes ha-1 with either 0.51 g P kg-1 or 0.26 g P kg-1 in strongly acidic soils. This
shows that the lower lime and P rates can be efficient in the management of soil P in
these soils. Analysis of the relationship between lime and P availability showed that
liming significantly (P ≤ 0.05) and positively correlated with soil pH, available P,
exchangeable Ca, Langmuir maximum adsorption constant and Langmuir constant
related to binding energy. Lime also negatively correlated with SPR and
exchangeable Al. It was, therefore, concluded that lime and P positively interact to
reduce Al and P adsorption in acid soils in the Kenya highlands. Further research is,
however, required to evaluate the short and long term effects of the lime-P
interaction on crop yields, availability of other plant nutrients and the soil
environment under field conditions. Results from field experiment revealed that
tillage-P- lime interactions significantly (P ≤ 0.05) increased soil pH, available P,
CEC, Ca, maize growth, maize plant P-uptake and yields, and reduced soil
exchangeable Al. Plough-LP interactions significantly (P ≤ 0.05) led to > 90%
increase in soil pH, extractable P, CEC, Ca, and > 90% reduction of exchangeable Al
as compared to the control. Plough-DAPL interactions on the other hand also led to >
96% increase in soil CEC and extractable P. Similary; plough-DAPL and plough-LP
interactions significantly (P ≤ 0.05) increased maize plant height, P- uptake,
drymatter yields and grain yields. Plough-DAPL or LP interactions significantly
increased grain yields by > 70% as compared to the control treatment. Integrating
plough with either DAPL or LP was, therefore, found to be effective in the
manangement of soil acidity for increased maize production in Kiambu in the Kenya
highlands. Further research is, however, required to ascertain the longterm effects of
the integration on maize production, soil acidity and overall soil fertility status of the
soils in the Kenya highlands.
Description
PhD Thesis
Keywords
Phosphorus retention, Phosphorus interactive effects, Lime effects, Acid soils, Kenya Highlands, Maize production, Soil fertility