| dc.description.abstract | Phosphorus deficiency limits crop production in western Kenya due to low native soil
P, continuous cultivation without adequate P fertilization and often high soil P fixation.
The use of commercial P fertilizers by small scale farmers is limited by their cost. The
use of finely ground phosphate rock (PR) is viewed as an attractive option to alleviate
phosphorus deficiency.
Two 16-week laboratory soil incubations were carried out to evaluate the effect of
organic and inorganic amendments on the dissolution of two PR materials (Minjingu
and Busumbu) in an Oxisol from western Kenya. Treatment with triple superphosphate
(TSP) served as a reference. The effect of Tithonia diversifolia leaves (tithonia) added
at 2 or 10 tonnes ha' on dissolution of moderately reactive Minjingu PR (Tanzania) and
unreactive Busumbu PR (Uganda) was investigated in the first incubation experiment.
The effect of inorganic amendments (CaCOj or CaCl2) on dissolution of the moderately
reactive Minjingu PR was investigated in the second incubation study. Dissolution was
determined from the increase in anion resin (AER), NaHCOj and NaOH-extractable P
in soil amended with PRs, compared with the TSP treated soil. In the third study maize
(Zea mays L.) and cowpea [Vigna unguiculata (L.) Walp] were grown as test crops in
pots on soils treated with 30 mg P kg" soil as Busumbu PR and TSP applied either
singularly or in combination with 5 or 10 tonnes ha' of tithonia to determine P uptake
and dry-matter yield.
In soils without added tithonia, AER-P followed the order TSP > Minjingu PR »
Busumbu PR. By week 16, AER-P from Minjingu PR and TSP were similar. Busumbu
PR solubility was low, and did not increase signifcantly in 16 weeks. ACER-extractable
P was generally greater than AER-P. The difference was greater for PRs than for TSP.
The ACER extraction may be a better estimate of plant P availability, particularly when
poorly soluble P sources are used. This may be due either to the ability of the Ca sink
(CER) to remove accessory CaCOj and Ca from the decomposition of tithonia, or to act
directly to dissolve solid PR in the soil.
Application of tithonia at 10 tonnes ha' in combination with Minjingu PR caused a
small but significant depression in AER-P relative to Minjingu PR alone at all sampling
dates except week 16 while ACER-P was not significantly affected by tithonia addition.
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Soil samples treated with TSP had the highest NaHC03-Pi and NaOH-Pi (labile and
moderately labile inorganic P) in the sequential extraction, followed by Minjingu PRo
The tithonia + Minjingu treatment showed decreases in NaOH-Pi and NaHC03 Pi
fractions, and increases in HCl-P (undissoved Ca-P) relative to Minjingu PR alone.
Solubility of Busumbu PR was uniformly low, and was unaffected by the addition of
either tithonia or TSP.
The addition of inorganic amendments (CaC03 and CaCI2) showed a depression in
AER-P as compared to application of Minjingu PR alone. This was presumably due to
the Ca common-ion effect. The addition of CaC03 gave a significant increase in ACERP
relative to all other treatments. The higher extractability of P on inclusion of both
AER+CER may be due to direct interaction of CER with the Ca of undissolved PR,
which presumably was the dominant form of PR at the higher pH of the CaC03
treatment. The lowest extracted P (both AER and AER+CER) was in CaCl2 treatments.
It is possible that the decreased P extraction was due to Ca common ion effect and
strong preference for CI- over orthophosphate by the anion exchange resin.
In the pot study, addition ofTSP up to 100 kg P ha' gave a linear increase in dry-matter
yield and P uptake for both maize and cowpea. At any given P rate, highest yields were
obtained with tithonia alone. Organic-inorganic combinations resulted in yield
decreases in most cases. Combination of Busumbu PR with TSP or tithonia did not
enhance the effectiveness of the PR. MPR was very reactive in this soil whilst BPR was
the least reactive. Addition of tithonia showed no effect on the solubility of the less
reactive BPR. The solubility of the more reactive MPR was depressed. | en |
