Soil fertility assessment and microbial enhanced solubility of Minjingu phosphate rock in neutral to slightly alkaline soils of Kongwa district
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Date
2023
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Sokoine University of Agriculture
Abstract
To optimize production of maize and other crops, assessment of soil fertility should be
done in agricultural areas from which cite specific fertilizer recommendations will be
established. This study was conducted in selected fields of Kongwa district in Dodoma
Region to assess the soil fertility status and maximize availability of Phosphate from
Minjingu phosphate rock (MPR) for maize production through the use of phosphate
solubilizing bacteria (PSB).
The specific objectives of the study were to determine the levels of N, P, K, S, Mg, Ca,
Na, Zn, Cu, Fe and Mn in soils and evaluate capability of phosphate solubilizing bacteria
(PSB) on enhancing P availability from Minjingu Phosphate Rock (MPR) and evaluate
field characteristics in slightly acid to neutral soils.
Assessment of soil fertility status was done by analysing soils from twenty-four maize
growing areas of Kongwa district. Composite soil samples of top soils (0 – 20 cm depth)
were collected for physical and chemical analysis. Organic carbon was determined by the
Walkley-and Black wet oxidation method (Nelson and Sommers, 1982) and total nitrogen
(TN) by the micro-Kjeldahl procedure (Loria et al., 2005). Available P was extracted
using Bray-1 and Olsen methods of Bray and Kurtz (1945) and measured by
Spectrophotometer following colour developed by molybdenum blue method (Murphy
and Riley, 1962). Exchangeable bases were extracted by ammonium acetate saturation
method (Thomas, 1986) and cation exchange capacity was determined from NH 4+
saturated soil colloids and displaced using 1 M KCl, then determined by Kjeldahl
distillation method for estimation of cation exchange capacity (CEC) of the soil
(Robertson and Philip et al., 1999). Extractable sulphur (SO4 2- S) was extracted using
calcium monophosphate (Ca(H 2 PO 4 ) 2 .H 2 O), then determined by the turbidimetric method
as described by Moberg (2001). The EC for soil samples from Kongwa was measured by
using EC meter in a 1:2.5 soil: water (or CaCl 2 for pH only) extract as described by
McLean, (1982).Extractable metallic micronutrients e.g., Cu, Fe, Zn and Mn, were
extracted by diethylenetriaminepentaacetic acid (DTPA) as described by Lindsay and
Norvell (1978).
Results indicated that 48% of the sampled fields were sandy clay loam, 26% were sandy
loam and the remaining whad a clay or loamy sand textural classes. Soil pH ranged from
extremely acid (3.52) to moderately alkaline (7.7), organic carbon ranged from very low
to medium (0.19-1.60%) and total N values were very low to low (0.01-015%). Also,
results indicated 42% of soils had P deficiency and 16.7% with inadequate S. In addition,
45.8% of the soils had inadequate exchangeable K and exchangeable Mg were very low to
high (0.29-4.06cmol (+) kg -1 ). Exchangeable Ca was low to very high (1.06 to 10.04 cmol (+)
kg -1 ) with favourable base saturation for crop production. The CEC ranged from very low
(2.62cmol (+) kg -1 ) to medium (18.9 cmol (+) kg -1 ). Extractable micronutrients such as Cu,
Fe, and Mn were adequate but Zn was inadequate in 58% of the soils. The study area had
poor soil fertility is poor with deficiencies in N, P, K, Zn, Mg, and Ca, hence calling for
proper management to improve crop production.
Phosphorus (P) is one of the major growth-limiting macronutrients required for proper
plant growth and it is contained within nucleic acids, enzymes, coenzymes, nucleotides,
and phospholipids. One of the ways to replenish P in soils is the use of inorganic
fertilizers such as Minjingu Phosphate rock. The challenge is that, phosphate rock (PR)
generally requires P-deficient acidic soils with pH of less than 6.2 for P to be released and
become available for uptake. Application of PR in neutral to slightly alkaline to soils (pH
>6.2) is therefore discouraged since PR solubility is very low in such soils and therefore
can not supply sufficient available P to plants. Phosphate solubilizing bacteria have a
capability among others, of producing organic acids which in turn lowers the local acidity
of the soil in which they operate. The lowering of the pH will/may bring about right
conditions for solubility of MPR .
Maize trials were established in two villages (Nghumbu and Mlali) of Kongwa District to
assess effectiveness of PSB in solubilizing MPR in neutral to slightly alkaline soils and
two factors were tested, which are field characteristics (in which treatments were sites),
the second factor was PSB co-applied with MPR. Maize trials were established using
randomized complete block design (RCBD), the seven sites ,PSB inoculum co-applied
with different rates of Minjingu phosphate rock (MPR) (0 – 60 kg ha -1 ). Limiting nutrients
such as N and S were corrected then Minjingu RP was applied as basal fertilizer during
planting by placing it below the seed and covering with a thin layer of soil followed by a
co-culture of PSB inoculant at 5 mL (10 9 bacteria per litre) poured around each plant four
days after emergence. Maize plants were harvested at maturity then grain and straw
samples were analysed for N, P and K content, phosphorus uptake and phosphorus use
efficiency were calculated.
Using the soil data, it was found that Nitrogen was the main limiting nutrient in all seven
fields. The limiting nutrients in Nghumbi were N and K while N, P, K and Zn were
limiting nutrients in Mlali village. In the trial fields, the main effect of fields’
characteristics indicated that the performance of maize and its analyzed parameters (N, P,
K, S, Mg, Ca, Na, Zn, Cu, Fe and Mn) were significant (P < 0.001) across the fields
reflecting heterogeneity in nutrient levels. There was no significant effect of PSB
treatments on maize grain yield and straw yield. Results also did not show significant (P
=0.427) interaction effects of field characteristics and treatment combinations on the
measured variables in maize because of drought experienced in the study area.
The findings of the present study, apart from setting site-specific fertilizer
recommendations, also suggest that the performance of maize in the study area depends
on the soil nutrient status, especially for native P, N, and K. The studied soils need
external nutrient inputs and proper management to optimize crop production and more
research is recommended on PSB for more than one season in neutral to slightly alkaline
soil before ascertaining the technology to farmers because in this season PSB did not
enhance solubility of MPR due to drought experienced in the study area.
Description
MSc. Dissertation in Soil Science and Land Management
Keywords
Maize, Soil fertility, Alkaline soil