Buffering capacity and levels of macronutrients found in leaves of selected plants
Njagi, Silas Edward
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Due to fast growth in population, there has been need to expand agricultural activities since land owned by nuclear families is no longer sufficient for large scale farming as a result of land subdivision. The pieces of land are under cultivation throughout which has led to reduction in soil fertility. 'The use of commercial inorganic fertilizers in these small pieces of land tends to be the ultimate solution in maintaining high yield. Too much use of these inorganic fertilizers has led to low soil pH which reduces availability of essential nutrients to plants, results in formation of toxic chemicals in the soil and leads to low rates of decomposition of organic matter which is required to increase buffering capacity of the soil and also release nutrients to plants. Soil acidity is reduced by liming which is an expensive exercise and does not add fertility to the soil. There is need therefore to look for alternatives to inorganic fertilizers and liming materials. This study was aimed at identifying potential plants that can raise soil pH while at the same time add nutrients to soil. The study aimed at investigating buffering capacity, levels of macronutrients, humification and mineralization of leaves of selected plants found on highlands east side of Mount Kenya. The pH trends of leaf water extracts (in which leaves were soaked for a period of 90 days) and the effect of this extract on acidic soil was also studied. Determination of potassium was done using flame photometry, calcium and magnesium using atomic absorption spectrometry, nitrogen and phosphorous using UV/visible spectrometry and sulphate -- sulphur using turbidimetry. The pH was determined using a pH meter. humification using UV/visible spectrophotometer and buffering capacity by titration method_ The pH of leaf extracts ranged from 3.85 to 8.08 for the 90 days it was monitored, the volume of acid required to reduce pH of leaf extracts to 5.0 ranged froin 0.1 to 20.7 m1 and most of the plant species studied were found to have leaves that are good to make humus. The percentage of nitrogen in leaves ranged from 0.037 to 1.605, phosphorous 0.152 to 0.792. potassium from 0.044 to 1.331, calcium 0.354 to 2.034, magnesium 0.073 to 0.977 and sulphur from 0.883 to 1.153. The levels of minerals released in water (m./g) ranged from 21.92 to 287.61 for potassium, 7.74 to 399.90 for calcium. 13.42 to 160.3 for magnesium, 15.49 to 52.37 for sulphate, 2.154 to 31.325 for phosphate and 8.387 to 72.054 for nitrate. The acidic soil pH (initially at 5.32) ranged from 5.26 to 6.10 upon addition of leaf extracts. It was found that species of plants such as Helianthus annul (sunflower), Markhamia lulea (muu), Cordia africana (muringa), Croton macrostachyus (mutuntu) and Tithonia diversifolia (mexican sunflower) from the species studied are the best for the purposes of this study. Others like Cassia spectabilis (cassia). Jacaranda mimosifolia (jacaranda). Manihot esculenta (cassava), Carica papaya (pawpaw), Vitex keniensis (mere oak) and Albizia gummifera (peacock flower) are also fairly good. The other species also have some little value that may also be useful to improve the soil. However, Grevillea robusta (silky oak), Eucalyptus saligna (sydney blue gum) and Mangifera indica (mango), produced leaf extracts that were quite acidic and on being added to acidic soil did not raise the soil pH significantly and so may not be appropriate for acidic soils. It is therefore recommended that the species that are good for soil be planted in farms so that farmers will be using their leaves to improve their farm output and those species that do not improve soil or produce acidic extracts be reduced in farms.
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