ABSTRACT
Potassium (K) plays an outstanding role in plants and
animals. Cassava, a widely grown staple in Ghana is noted for extracting large
amounts of K from the soil. Cassava’s ability to absorb K from the soil is
worth studying as it is vital for sustainable production and biofortification.
The study was conducted to assess cassava’s ability to absorb K under
fertilized and unfertilized systems. Two cassava genotypes, Cape Vars and Botan
were grown under different fertilizer treatments; a control, NPK and NPK + KCl
in a randomized complete block design. A survey was also undertaken to assess
the K status of some cassava farms and cassava tuber produced on such soils in
the Central region of Ghana. Composite soil samples were taken at a depth of 0
– 30 cm and analysed for exchangeable and non-exchangeable K. Aside these, the
study also compared the extractability of exchangeable K by Nitric Acid,
Ammonium Acetate and Calcium Chloride in soils cultivated to cassava. The
outcomes of the study revealed that most cassava farms have low levels of
exchangeable K (0.76 – 0.06 cmolc kg-1) likewise the K
content of cassava tuber produced from them (0.84% - 0.61%). A significant
increase in K content of tubers was however observed in cassava that was
treated with NPK + KCl fertilizer (1.13% - 0.86%). Cassava tuber yield
also saw a significant increase with the highest yield (20.5 t ha-1)
produced from NPK + KCl plots. The study also revealed that Cape Vars variety
has a greater ability to absorb K in both fertilized and unfertilized soils. NH4OAc,
HNO3 and CaCl2 extracted 0.21, 0.64 and 0.23 cmolc
kg-1 exchangeable K respectively. However, CaCl2 – K was found to be more predictive of the K status of
cassava soils. The study recommends the sensitization of cassava farmers on fertilizer
application as well as a further research into cassava’s ability to absorb K in
marginal soils.
CHAPTER ONE
INTRODUCTION
Background of the Study
Potassium is one of the key
elements for life and its abundance in the Earth’s crust lends credence to its
importance. Among the major soil nutrients, potassium is the most abundant in
soils (Elbaalawy, Benbi, & Benipal, 2016) and the second most abundant
mineral found in plants (Towett et al., 2015). Potassium is very important for
both biophysical and biochemical processes in plants. Some of these processes
include the preservation of cell turgor, maintenance of charge balance as well
as vacuole and cell expansion (Benito, Haro, Amtmann, Cuin, & Dreyer, 2014).
Potassium is of a unique significance to the carbohydrate producers as it plays
a significant role towards the translocation of assimilates in the phloem
(Patrick, Zhang, Tyerman, Offler, & Walker, 2001). Potassium also plays a
vital role in the translocation of photosynthates in root growth (Römheld &
Kirkby, 2010) which results in an increase and swelling of tuberous crops such
as cassava. Given the role of potassium in plant growth and development, it is
obvious that it is needed in massive quantities by plants. Soils have a huge
potassium reserves, especially in soils developed from igneous rocks (Mouhamad,
Alsaede, & Iqbal, 2016). However not all is available to support plant
growth at a time.
In humans, K is vital for the
maintenance of important physiochemical processes which are crucial for the
wellbeing and survival of the human body (Soetan, Olaiya, & Oyewole, 2010).
Potassium serves as an all-important electrolyte that is needed to counteract
the effect of sodium consumption contributing to the maintenance of a healthy
blood pressure (Whelton et al.,1997). The importance of
potassium to human health has been well recognised and studies continue to
emphasise its positive effects. Increased dietary intake of K improves the
functioning of the cardiovascular system and reduces the risk of cardiovascular
diseases and increased mortality from heart diseases (Soetan et al., 2010).
Diets are the main sources of
potassium in humans. Potassium dietary sources include vegetables, fruits and
nuts (Soetan, Olaiya, & Oyewole, 2010). The bulk of most Ghanaian diets is
starchy foods such as cassava, yam and rice. Amongst these, cassava has the
highest per capita consumption (Sayre, 2011). Yawson et al. (2016) ranked
cassava as the first among yams, plantain, roots (other than cassava) and rice
as the top five food items consumed in enormous quantities in Ghana. Cassava
also serve as an important staple for half a billion people in Africa and Asia
(Cuvaca et al., 2015).
Potassium is very abundant in
soil-plant systems in various forms, some of which is considered plant
available and others unavailable (Lalitha & Dhakshinamoorthy, 2014).
Intensified crop production systems require an efficient soil management
towards a sustainable crop production. Potassium content of crops is influenced
to a substantial extent by the K content of soils on which such crops are
grown. This common knowledge has been tapped to increase bioavailable concentrations
of essential elements in edible portions of crops, otherwise known as
biofortification (Garcia-Casal, Peña-Rosas, Giyose, 2016). Globally,
biofortification of cassava has received due attention but for other equally
important nutrients such as protein, minerals, starch, and vitamins (Montagnac,
Davis, & Tanumihardjo, 2009). Cassava has a wide geographical distribution
(Montagnac et al., 2009). In Ghana, it is grown in all the six agroecological zones
(Adowa, 2009). This makes cassava an important crop for reaching parts of the
world with mineral deficiencies through biofortification. As cassava provides
food for over 500 million people in Africa on daily basis (Montagnac et al.,
2009) serving as a dominant staple in areas where mineral deficiency is
widespread, especially in Africa, it remains a perfect crop for targeting
mineral deficiency in Africa and other parts of the world where mineral
deficiency and associated non-communicable diseases constitutes a major health
challenge.
Statement of the Problem
Root tubers such as cassava are
noted for removing substantial amounts of nutrients from the soil, especially K
during their growth period (John & Imas, 2013). However, its ability to
thrive and produce reasonable yields on infertile and marginal soils remains a
mystery. These foods constitute the bulk and form a major part of the diet of
many Ghanaians. Regardless of cassava’s big K budget on the soils, it is grown
mostly on marginal soils and fertilization is not a customary practice among
cassava farmers.
Cassava, an important staple for
Ghanaians and Africans at large is mostly produced by small-holder subsistent
farmers who use little or no fertilizer at all (Fermont, Tittonell, &
Giller, 2010). Fertilizer application in cassava production is not common in
Ghana as farmers attribute several reasons such as high prices of chemical
fertilizers and reduction in quality of cassava tuber’s cooking quality and
storage. (Yawson, Armah, Afrifa, & Dadzie, 2010)
As the nutrient concentrations in
the plant are influenced to a large extent by soil fertility and the
fertilisation practices that are undertaken by farmers (CIAT, 2011), the
observed high concentrations of K in cassava roots grown on even marginal soils
might be due to a special ability of the crop to mobilize K from soils. It is
not yet clear why this happens but there is also little information on the
relationships between soil K content before and after harvest of cassava and K
content of harvested cassava roots from farmers’ fields. Hence, this study aims
to analyse the K content of soils of selected cassava farms in relation to K
content of harvested cassava roots in the Central Region of Ghana.
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