Evaluation of Plant Growth on Expanded Black Cotton Soil

In aggregate hydroponics, the suitability of a particular medium is often guided by its physical properties. However these may not provide adequate evaluation. By growing a crop, the growth characteristics can also be used to guide medium preparation and selection. Tomatoes (Anna F1 variety) were grown in containers under a greenhouse and provided with equal amount of nutrient solution. The expanded black cotton soil was prepared by mixing with rice husk at a ratio of 90:10, fired at 750C for 30 minutes and size reduced to various textures. The crops planted in the expanded clay aggregates performed better than the black cotton soil both in terms of stem elongation and enlargement due to improved drainage, nutrient flow and aeration conditions. The root length density was 25654 m m in black cotton soil, being the highest and lowest in the coarse aggregates which had 9433 m m.


Introduction
Expanded clay has been applied in various forms to aggregate hydroponic systems. These systems use inert solid material which has most of the desirable physical properties of soil such as, being friable, good water holding properties, easily aerated and well drained. The growing medium cannot grow anything on its own since it does not supply plant nutrients. The nutrients are supplied to the plants by a nutrient solution. One of the most commonly adopted methods of preparing expanded clay aggregates involves forming round pellets out of clay in rotary kilns at about 1200 o C. This causes the clay to expand and become porous resulting in pellets which are inert, lighter in weight and does not compact over time (Singh, 2009).
In this study the aggregate medium was prepared using black cotton soil mixed with rice husk at a ratio of 90:10 by mass and then fired at 750 o C for 30 minutes. The resulting material was the size reduced and tested for various physical properties.  concluded that black cotton soils can be converted into aggregate hydroponic medium in this way thus improving on its physical characteristics to make it light weight with low bulk density, enabling it to remain loose and friable over wide ranges of moisture contents, drain easily and have moderate permeability to permit water and nutrient movement through it. These parameters may however not adequately describe the performance of the expanded black cotton soil as a plant growth medium hence the need for further evaluation in terms of effect on stem growth and root development. Root and stem growth are affected by the composition and texture of the medium (Maloof, 2004). Root length density (RLD) is a critical factor in determining the crops potential to uptake water and nutrients. RLD can be affected by irrigation method, growing medium texture and bulk density (Rangjian et al., 2017). Water and nutrients taken up by the roots are transferred to the leaves through the stem for use in photosynthesis. The roots also provide anchorage to the plant. The root system has also been found to have more surface area than the shoot system (Wahome et al., 2011).

Materials and the Methods
The experiment was carried out at Egerton University's Tatton Farm. The area is located 00 o 22' South of the equator, 35 o 55' East and 2286 m above sea level. The experimental site receives an average annual rainfall of 1200 mm, and the averages of temperatures in the field ranges between 10.2 and 22.0 °C (Ng'etich et al., 2014). The experimental trial was carried out using tomato crop (Solanum lycopersicum) grown in a greenhouse.
The expanded clay was categorized as fine aggregates, coarse aggregates and a mix of fine and coarse aggregates. The fines had particle sizes corresponding to fine sand ranging from 0.075 -0.85 mm, while the sizes for the coarse aggregates ranged between 0.85 -2.00 mm. The mixed size aggregates comprised of materials greater than 2 mm and those less than 0.075mm at a ratio of 10% each. The remaining 80% was materials within the 0.75 mm -2.00 mm size range. The black cotton soil (clay) was also used as sampled from Kenya Agricultural and Livestock Research Organization (KALRO) field in Kibos, Kisumu. The area lies within the Kano plains in Kenya. The soil sample had the following particle size distribution as given in table 1. The crops were planted in one-liter containers, each supplied with an average 130 ml of the nutrient solution daily throughout the growing period. The containers were perforated on the sides and at the bottom to enhance aeration and free drainage. They were placed in a single row, buried into the soil to provide better support. Figures 1a and 1b show the setup at various stages of the experiment. The hydroponic nutrients were obtained from Hydroponics Africa with the following nutrient composition; The nutrients are packaged as Hydro A and Hydro B and each was mixed at a rate of 2 g per liter of water. The nutrient solution was supplied from a raised tank through drip lines to each plant.
The plant stem diameter and height were measured weekly. The stem height was measured using a meter rule with the measurements taken from the border of the container to the top of the main plant stem while the diameters were measured using a vernier caliper. The first readings were taken 12 days after transplanting. This was to allow proper establishment of the seedlings. RLD was determined at the end of the trial period when the crops started to flower after six weeks. The total length of the roots was obtained by carefully cleaning off the medium and measuring the length of individual roots.

Results and Discussion
The results shown in table 3 were obtained from the stem diameter (D) and height (H) measurements. Subscripts 0 -5 indicate the number of weeks elapsed since commencement of data collection. From these results, the percent change in stem height (elongation) and diameter (enlargement) was determined and averaged over the test period as given in tables 4 and 5 respectively. The plants grown in fine aggregates of the expanded clay had the highest mean weekly stem elongation rate of 51.4%. This could be attributed to better water holding capacity, nutrient flow and root contact. The black cotton soil had the least stem elongation at an average weekly rate of 45.2% attributed to poor drainage and aeration conditions. The plants grown in the coarse and mixed size aggregates had higher weekly growth in stem diameter of 28%. This could be attributed to improved water and nutrient flow, better drainage and aeration. The black cotton soil had the least change of 25.8%. This is because clay although has high total available water, the bulk of this water is not readily available for uptake by plant. From week two there is a decline in stem enlargement rate in all the samples due to development of more leaves hence higher evapotranspiration rates. The decline is more pronounced in black cotton soil between weeks three and four after which the crop seems to recover as the roots spread deeper into the soil.
The RLD was computed from equation 1 and the mean values presented in table 6 below;  Since the crops were planted in containers, the RLD was generally high for all the samples. This is because RLD tends to be higher at depths between 0 to 20 cm and decreases down the profile. Under drip irrigation, 70-75% of the total RLD of tomatoes is concentrated in 0 -15 cm soil layers because of the high irrigation frequencies.
Smaller soil volumes also tend to give higher RLD values (Rangjian et al., 2017;de Azevedo et al., 2011). The containers concentrated the roots within the limited space resulting in increase in RLD. In this study coarse aggregates had the lowest RLD value. The finer aggregates had numerous small pore spaces which encouraged the development of more root hairs. The plants in black cotton soil had the highest RLD hence providing more surface area to be able to extract the nutrient feed. This is response to water stress due lower readily available water in clays. For the expanded clay, the mixed aggregates had high RLD. Mixing fine and coarse aggregates makes the medium to have proportionate ratio of micro and micro pores hence moderate drainage and water holding capacities. Considering both root and stem development, black cotton soil performed well under the test