Introduction

Tea (Camellia sinensis L.O. Kuntze) is a perennial plant used in the manufacturing of beverages and traditional medicines for asthma and coronary diseases. Tea, owing to its favourable effects on human health, currently enjoys a great popularity among other beverages worldwide (Ruan and Hardter, 2001). In Kenya, tea is mainly grown in the East and West highlands of the Great Rift Valley. Tea is a major economic crop for some developing countries. In Kenya, tea is a key player within the agro-industrial crops and is the single commodity leading foreign exchange earner accounting for about 26% of the total export earnings and 4% of the gross domestic product (GDP) and is a source of livelihood to over 3 million people. Over 62% of the Kenyan tea is produced by the smallholder growers living in the rural set ups where industrialisation is low and economic activities are rare. The crop is viewed as a source of rural development in many developing countries. Teais harvested by plucking off its young tender shoots normally comprising two leaves and a bud under manual harvesting resulting in excessive depletion of soil nutrients annually (Omwoyo et al., 2014). Application of fertilizers is therefore necessary in order to maintain optimum production of tea yields and thus minimize crop loss due to nutrient deficiency for high and sustainable income to the farmer. Green leaved plants grown as agricultural crops require essential nutrient elements for sustainable growth and high yields. To remedy nutrient deficiency, fertilizer usage and evaluation techniques should be considered so as to guard against crop loss and instead target high crop yields (Kamau et al., 2005).

Most small holder tea plantations in Kenya are of average 30-50 years. Declining productivity and moribuncy has been reported in many of these fields mainly due to prolonged period of monoculture under tea plantations. Tea requires various nutrients for normal growth and productivity like all other plants (Kamau et al., 2008). Mostly plants acquire the nutrients from the soil except for small quantities of nitrogen and other elements that can be obtained from air and rain water through absorption by the leaves (Kamau, 2008). Excessive and inappropriate fertilizer use may lead to reduced yields, low tea quality, and nutrient imbalances in the soil with consequent environment degradation. Since soil cannot supply adequate quantities of the plant nutrients, growers should supplement the soil nutrients with fertilizers (Jondiko, 2010).

In tea, the harvesting involves the removal of young tender shoots which contain considerable amounts of nutrients (Omwoyo et al., 2013). The main elements that are removed during harvesting are nitrogen, phosphorus, potassium, magnesium, copper and zinc. Nitrogen removal during plucking ranges from 40 to 160kg/ha assuming made yields of 1 to 4 t/ha (Kamau, 2008). Harvested shoots contains 3-5% nitrogen on dry matter. Other nutrients are removed during harvesting of the tea shoots in small amounts and these include Mg, S, Ca, Fe, Mn, B, Cu, and Zn (Kamau, 2008; Kamau et al., 2008).

Foliar spraying of fertilizers is the application of nutrients to the plants through the leaves. Foliar spraying is meant to raise the concentration of element directly in the leaves since in the soil nutrients are victimised by a number of processes such as mineralisation, leaching, run-off which leads to the unavailability of the nutrient to the plant. Foliar application may also be expensive since it does not have a residual effect, more spraying rounds are required than soil application The effectiveness of foliar applications is influenced by factors like the rate at which the element is absorbed and transported by the plant from the leaves to other parts of the plant that includes the roots (Kibeney et al., 2010). Elements are transported via the phloem so their transportation depends on the mobility of element in the phloem. Some elements are not phloem mobile. Absorption of foliar applied nutrients depends on the permeability of the leaf cuticle. This permeability is affected by factors such as leaf surface area, stomata aperture, density of the stomata and concentration of the nutrients. Other factors that affect foliar application includes leaf area index where the larger the leaf area index the larger the area for interception. Studies have shown that for perennial crops like tea, foliar application of micronutrients has been ineffective.

1 Materials and Methods

1.1 Site description and experimental layout

The study was carried out in James Finlays Kericho-Tiluet Estate, Kericho County which is at an altitude of 7 m above sea level, latitude of 0º22′ south and a longitude of 35º21′ east. The annual rainfall range is between 1200-2700 mm and a mean monthly air temperatures between 15 to 20℃.The area was (0.257 Ha or 3456 plants) while the spacing 1.22 m by 0.61 m (13448 plants per Ha). This will be split-plot with clones as the main treatment and the micronutrients split on clones replicated three times. The four main treatments consist of the clones: (1) 31/8, (2) 303/577, (3) JFK S15/10, and (4) JFK 12/28. The sub-treatments are made of eight micronutrient combinations of (a) Control (Nil), (b) Zn, (c) Mo, (d) B, (e) Zn+Mo (f) Zn+B, (g) Mo+B, (h) Zn+Mo+B.

1.2 Soil sampling

Surface plant and Littre materials were removed from a random selected auger site within the area of 96 plots of the four clones. At each plot 0-10 cm, 10-20 cm, 20-30 cm and 40-60 depths was augured and the soils of each depth placed in polythene bags of 1 kg per sample as documented in the method adopted from Kamau et al. (2012). The sampled soils were delivered to the lab for pH testing thereafter dried under a shade, sieved using a micro meter sieve, weighed into 5 kg using a weighing scale balance, extracted, diluted to the required amounts and set for analysis using an ICPE machine.

1.3 pH determination

This was done by placing 25 g of soil samples in 50 mL beaker with 25 mL distilled water stirred using a stirring rod and left to settle for 30 minutes then tested using a pH meter, according to documented procedures (Kamau et al., 2012).

1.4 Elemental analysis in the soils

This was done according to procedures documented by Mokgalaka et al. (2004). This involved preparing standards, blank determination and preparation of the specific soil samples; 96 samples from each plot each containing four depths. 5 g of soil sample was weighed in a 100 mL plastic bottle and extracted using 50 mL of extractant containing 20 mL of EDTA, Acetic acid, 30 mL HCl, HNO₃ acid then subjected to an orbital shaker for 10 mins and filtered into 50 mL test tubes using Whatman filter paper Number 2. The samples were diluted by pipetting 1 mL of the filtrate into 50 mL volumetric flask. 5 mL of 0.5% strontium chloride solution was added to minimise interferences (Matsuura et al., 2001) and analysis performed by an ICPE machine.

1.5 Statistical analysis

The data was subjected to the analysis of variance ANOVA using MSTAT-C software package. In case of significant treatment effects, a comparison of means was performed by means of least significant difference (LSD) at (P≤0.05) significant level.

2 Results and Discussions

2.1 Effect of micronutrient soil application on nutritional status of different clones

From the results below, different clones have different ways of absorbing and taking up nutrients when the soils are subjected to different micronutrient applications. The results demonstrate how Aluminium (Al) is influenced by different clones and micronutrient application. In the uptake of Al there were no significant interactions between micronutrient soil application (B, Mo, Zn, Zn+Mo+B, Zn+B, Mo+Zn, Mo+B and the control). The mean levels of clones had no significant interactions towards the absorption and uptake of Al. This implies that the subjection of different clones to different micronutrient applications did not bring an impact in the uptake and absorption of Al. Boron had significant (P≤0.05) interactions between clones and micronutrient application. There were no significant (P≤0.05) interactions of B with the micronutrients andthis implies that none of the micronutrients which were split on the clones brought an impact in the uptake of Boron. In Calcium there were significant (P≤0.05) interactions between clones and micronutrient application. Mo and Zn interacted significantly (P≤0.05) where Zinc had a significantly higher uptake and absorption of Ca. Clone 31/8 and 12/28 interacted significantly (P≤0.05) together with clone 31/8 and 303/577. Clone 31/8 recorded the highest the highest uptake of Ca. In Molybdenum, the clones and micronutrient applications interacted significantly. The clones had significant interactions amongst upon subjection of micronutrient combinations with clone 303/577 recording the highest uptake of Mo.

The uptake of magnesium recorded no significant interactions between the micronutrient applications and the clones. Clones significantly varied in the uptake of Mg as well as micronutrient applications. The application of different micronutrients did not bring an impact in the absorption of Mg since the original Mg concentration present had a significant higher concentration of 46.708 as observed from Table 1. The application of Zn+Mo, Mo, Zn+Mo+B, Zn, Mo+B, B, Zn+B had no significant interactions in the uptake of Mg. Iron did not show any influence on the application of the micronutrients; there were no significant interactions. The clones showed interactions where clone 12/28 and 31/8 had significantly higher interactions. Clone 12/28 recorded the highest uptake of Fe whereas clone 15/10 and 303/577 were observed with significant interactions which implied that different clones absorbed Fe at different concentrations. In zinc, different clones showed varied abilities to absorb nutrients when subjected to different micronutrient combinations. The clones had varied abilities of uptaking Zn. This indicates that the clones have varied abilities to absorb any of the micronutrients from the tea soils and this corroborates with previous studies (Omwoyo et al., 2014). Clone 12/28 and S15/10 did not interact significantly whereas clones 12/28 and 31/8 significantly interacted. There were no significant interactions with the micronutrient combinations and the clones. This implies that different clones showed varied abilities to absorb nutrients when subjected to different micronutrient combinations. The uptake of Zinc was significantly higher in clone 12/28.

2.2 Effect of depth and micronutrient application on the levels of various elements in tea soils

The results presented herein demonstrate that the micronutrient levels in the soils vary significantly (P≤0.05) depending with the depth. The soils were strongly acidic with a pH range of 3.4-4.1 which categorizes the soils under acidic soils (Yemane et al., 2008). The pH was more acidic in depth 0-10 cm and less acidic in depth 40-60 cm. Clone 12/28 was strongly acidic and had significantly (P≤0.05) higher mean levels of Zn as a sub-treatment than other clones while clone 31/8 had the lowest mean levels, meaning it was less acidic. When the profile depth was increased, there was a significant increase in the soil pH. This was reported that one reason for the decreased soil acidification was less accumulation of fertilizer particularly nitrogen (Yemane et al., 2008). Excess nitrogen input acidifies soil and the acidification rate usually increases with increasing nitrogen application (Wallace, 1994). Yemane et al. (2008) reported that nitrogenous fertilizers are known to produce H⁺ by the following reaction, which is induced by soil bacteria.

Thus, during the application of these fertilizers to the soil, the rate of nitrification is reported to be higher and inorganic nitrogen may be rapidly converted to nitrate producing H⁺ which acidifies the soil. Another possible reason for the general low pH near the surface could be attributed to decomposition of fallen tea leaves, which induced an increase of Al³⁺ in the soil, thereby leading to soil acidification (Wang and Chen, 1992). In high and well distributed rainfall areas, where the soil is deep and well drained, a requirement for economic growth of tea (Carr, 1972) natural soil acidification is enhanced through leaching of bases. Continuous use of nitrogen fertilizer to increase yields similarly accelerates the same process, whether the fertilizer is used for tea or for other crops. In addition, crop harvesting (tea has a high content of bases) involves removal of bushes and hence soil acidification (Kebeney et al., 2010). Where soil conservation measures are not adequate, as reported by (Wanyoko et al., 1990), the erosion of the fertile top soil under the high rainfall can contribute towards removal of bases and this effect also contributes to soil acidification. The erosion of the fertile top soil also removes with it a high content of the organic matter, which is important in increasing the soil cation exchange capacity (Brady, 1990). The remaining soil has a lower capacity to retain the cations and thus becomes acidic faster. Regardless of its main case, acidification ultimately causes chemical degradation of the soil, thereby making it less productive (Goedert, 1993).

2.2.1 Variation of Boron with the clones and depths

There were no significant interaction effects between depths and clones significantly interacted with clone 303/577<12/28<15/10<31/8 having the highest levels of B respectively. The mean levels of B varied significantly (P≤0.05) from clone to clone meaning that clones have varied abilities to absorb Boron from the soil. There were no significant interaction effects between 12/28 and 15/10, 15/10 and 31/8. There were significant interactions between 303/577 and 31/8, and 15/10 and 303/577. This means that different clones absorbed Boron differently.

2.2.2 Variation of calcium with the clones and depths

There were no significant interaction effects between depths of the soils and the clones. The mean levels of calcium varied significantly (P≤0.05) from clone to clone meaning that clones had varied abilities to absorb Ca from the soils. Clone 12/28 recorded higher mean levels of Ca uptake at different depths. 12/28<31/8<15/10<303/577. Clone 12/28 and 303/577, 12/28 and 15/10, 12/28 and 31/8, 31/8 and 303/577. Clones S15/10 and 303/577 did not significantly interact.

2.2.3 Variation of iron with the clones and depths

No significant interactions between clones and depths. Clones interacted significantly as the clones. Depth 0-10 recorded the highest uptake of Fe, 10-20, 20-30 and 40-60 increased the order. The uptake of Fe was observed to decrease down with increase in depth.

Clones and depths did not interact significantly. Depths had no significant interactions with the clones. Clone 12/28 recording the highest uptake of Zn from 15/10 <303/577 <31/8 <12/28 and 31/8 respectively.

2.3 Effects of micronutrient combinations on the depths of various elements

In the uptake of Aluminium and Boron, no significant interaction effects were observed between the micronutrient combinations and the depths. In Calcium, there were no significant interactions between the micronutrient combinations and the depths, depths did not interact significantly while as the micronutrients significantly interacted (P≤0.05) at 35.4963 from Table 2. Zn+Mo and Zn+B significant interaction effects. The mean levels of Zn+Mo, Mo, Mo+Zn+B, Mo+B, B affected the original concentration of calcium on there application since control recorded a higher mean level as observed from Table 2. Mo&Zn+B and Zn+B&Mo+B interacted with significant effects.In Fe, there were no significant interaction effects between micronutrient combinations and depths. The micronutrients did not interact while the depths interacted significantly. The depth from 0-10cm recorded highest mean levels of Fe of 98.844 at (P≤0.05) from Table 2. The mean levels of Fe were recorded in the order 0-10>10-20>20-30>40-60. The depth increased with decrease in mean levels. This implies that the uptake of Fe increases with increase in depth. No significant interactions were observed with potassium.

In magnesium, no significant interactions between the micronutrient combinations and depths while the micronutrient combinations interacted with each other. The control recorded the highest mean levels of 46.708. This illustrates that none of the micronutrient combinations applied improved the levels of Mg. The micronutrient combinations affected the original Mg concentration in the soils and this could be attributed to the antagonism effect. Molybdenum showed no interaction effects in the depths. There was a significant interaction effect between the micronutrients where Zn+B recorded the highest mean level of 3.167. Zn+Mo&Zn+B, Zn+B&Mo, Zn+B&Zn, Zn+B&Zn+B+Mo, and Zn+B&Mo+B significantly interacted in the uptake of Mo. There were no significant interactions with Zn. Copper recorded no significant interactions between the depths and micronutrient combinations. The depths interacted significantly with LSD of 1.5716 at (P≤0.05) a depth of 0-10 cm gave a highest mean level of 18.615 in the figure above. The mean levels in the uptake of Cu decreased with increasing depth 0-10>10-20>20-30>40-60.

3 Conclusions

It is important to determine the level of micronutrients in both leaves and soil before application of fertilizers to be able to tell which type of nutrient or fertilizers to be applied. Clone TRFK 303/577 produced the highest yield as compared to the others and had the highest uptake of nutrients. Clone S15/10 was the second since it had a higher yield as compared to clone 31/8 and 12/28 respectively. A micronutrient combination of Zn+Mo reflected a higher uptake of nutrients, more so in clone 303/577. Mo as a sub treatment also gave a better result for the uptake of elements. Antagonism of the elements was observed in this case since the major elements were interacting with one another.

Author’s contributions

The reported study is a product of the intellectual environment of the whole team and that all the authors of this publication have contributed equally in various degrees to the concept and design of the study, acquisition of data, analysis and interpretation of data, preparation of the manuscript and revising critically the concerns of the reviewers for important intellectual content.

Acknowledgments

The authors wish to acknowledge Maasai Mara University and Tea Research Institute of Kenya for facilitating this research work through field sampling and laboratory works.

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