Characterization of vermiculites from the Mozambique b elt of Tanzania for agricultural applications
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Date
2009
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University of Aberdeen
Abstract
Crop productivity in Tanzania is generally low and this is partly due to inadequate and poor
retention of plant nutrients and moisture by some soils. The country has several vermiculite
deposits, but none of them are exploited to improve the soil properties because of lack of
information on their potential suitability. The aim of this research was to establish the
suitability of vermiculites from Tanzania as soil improvers for crop production through
characterization. The study involved five samples from Tanzania and one from South Africa,
which was included for comparison purposes. Mineralogy of the samples was studied by a
combination of X-ray diffraction (XRD) and scanning electron microscopy (SEM) fitted with
energy dispersive system (EDS), whereas electron probe microanalysis (EPMA) and
inductively coupled plasma-mass spectrometry (ICP-MS) were used to establish elemental
compositions. Extractability of some heavy metals in vermiculites was assessed by diethylene
triamine pentaacetic acid extraction (DTPA), whilst extractable P was determined by acetic
acid extraction. The pH, water release characteristic, and cation exchange capacity (CEC)
were among the physical and chemical properties assessed. Pot and field studies were then
carried out in Tanzania to assess maize response and retention of macronutrients in a sandy
soil amended with vermiculites. Maize was used as a test crop. The results found indicate that
not all samples are vermiculites, some are hydrobiotites. Analysis shows that none of them
contain hazardous accessory minerals. However, some have elevated concentrations of Cr and
Ni, but these heavy metals are insignificantly plant available and do not inhibit the uptake of
essential plant nutrients. Hence, the studied vermiculites are safe to exploit for crop
production. In addition, all are slightly alkaline with high CEC and, thus, they are suitable as a
growing medium with ability to retain plant nutrients from leaching. However, heating above
600 °C should be avoided as it reduces the CEC of vermiculites by more than 90 % and makes some of the exfoliated products strongly alkaline and, thus, unfavourable for crop production.
The Tanzanian vermiculites can retain plant-available water but their ability is less than
vermiculite from South Africa. The P in these vermiculites is extractable and extractability
increases on heating to 400 °C. Further heating makes P insoluble and less extractable. Maize
vegetative growth, dry matter yield, and nutrient uptake were significantly enhanced by adding
vermiculite to the soil over the control. In addition, it retained and fertilized the soil with P.
Pre-heated vermiculite at 600 °C performed better than unheated vermiculite and it inhibited
the fixation of the applied K and N. It was concluded that the Tanzanian vermiculites have a
recommendable potential of improving soil properties for crop production when heated at a
temperature of not more than 600 °C. However, more field trials are recommended on other
types of soils and crops other than sand and maize used in order to widen the scope of their
utilization in Tanzania.
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Keywords
Agriculture, Mozambique belt, Restoration in Tanzania, Characterizaof vermiculites, Tanzania