Evaluation of Soil Chemical Properties under Paddy Production System in Central Kenya : Soil Exchangeable Cations

Lowland irrigated schemes contribute the most rice produced in Kenya. However, production is low and highly variable due to management problems. Production could be increased with appropriate soil management which requires that baseline fertility status of the soils and how they vary be known. This study examined the variability of selected soil chemical properties in the Mwea Irrigation Scheme in Central Kenya. Soil samples were collected from the top 0-15 cm depth in August 2013 and 2014 and analysed for pH, electrical conductivity (EC) and the exchangeable cations potassium (K), calcium (Ca), magnesium (Mg) and sodium (Na). Significant variability in soil EC as well as soil cation concentration was observed among units. Overall results showed soil pH ranged from 4.56 (very strongly acidic) to 8.05 (moderately alkaline). Soil EC varied from 0.17 to 1.52 dS m with higher elevation areas recording lower values (< 0.50 dS m) and lower elevation areas recording higher EC values (> 0.50 dS m). On average, exchangeable Ca was 38.17 cmolc kg, Mg 23.80 cmolc kg, Na 1.24 cmolc kg and K 0.35 cmolc kg. The soil exchange complex was mainly dominated by Ca and Mg and cation concentration in the soil was in the order Ca > Mg > Na > K. Soil K is low and severe cation imbalances exist with regard to K and other cations thus making K deficient for plant uptake. Management practices and farming systems which enhance soil K status should be encouraged to help boost and sustain rice yield.


Introduction
Soils are highly variable both spatially and temporally as a result of land use and management strategies and this variability is expressed in soil physical and chemical properties (Bailey, Wang, Jordan, & Higgins, 2001;Jin & Jiang, 2002;Corwin et al., 2003;Cerri et al., 2004).As a consequence, soils can exhibit marked spatial variability at the macro-and micro-scale (Brejda, Karlen, Smith, & Allan, 2000;Vieira & Gonzalez, 2003).Under such conditions, crop yields are often varied and less than optimum due to nutrient deficiencies as well as excessive fertilizer application that may potentially result in environmental degradation (Mzuku et al., 2005).
Agricultural production in most sub-Saharan countries has been under threat due to diminishing soil fertility (Sanchez, 2002) and feeding the bulging population has been a serious challenge.Kenya has not been spared and in the recent years has experienced food shortages arising from declining farm productivity, high input costs and unreliable weather in the face of rising population (Nyang'au, Mati, Kalamwa, Wanjogu, & Kiplagat, 2014).A major factor in soil degradation is the soil chemical fertility and in particular its decline as a result of the lack of nutrient inputs (Hartemink, 2010).Smaling (1993) described tropical soils as often having negative soil nutrient balances and in addition to lack of inputs causing soil degradation while Bationo et al. (2006) highlighted the inherently low fertility status, inappropriate land use, poor management, erosion and salinization as other factors.On the other hand, the Global Rice Science Partnership [GRiSP] (2013) noted that the major constraints to production are poor crop management, lack of disease-resistant varieties and unavailability of labour at critical times.
While rice is the third most important staple cereal in Kenya after maize and wheat, the country is only able to produce 20% of its national needs (Ministry of Agriculture [MoA], 2009).Recent years have seen rice grow in importance in Kenya as per capita consumption, particularly in urban areas, has increased far more rapidly than that for other cereal crops (MoA, 2009; European Cooperative for Rural Development [EUCORD], 2012).Approximately 95% of the rice consumed in Kenya is produced from government managed irrigation schemes and the remaining 5% under rain-fed conditions (United States Agency International Development [USAID], 2014).Data from the Ministry of Agriculture indicate that irrigated areas cover approximately 13,000 ha and include irrigation schemes in Nyanza; West Kano and Ahero (3,520 ha), Western Bunyala scheme (516 ha) and Mwea irrigation scheme (9,000 ha) (MoA, 2009).
Paddy soils are naturally heterogeneous in their physico-chemical properties which impact on rice productivity.This means that uniform management of fields will often result in over-application of inputs in areas with high nutrient levels and under-application in areas with low nutrient levels (Ferguson et al., 2002).However, good agricultural practices can be achieved if soil and nutrient variations within a farm are established and properly managed (Chan, Amin, Lee, & Mohammud, 2008).Soil chemical properties form the basis for soil fertility evaluation and chemical concentrations in the soil must be regularly tested to develop fertilizer recommendations and site-specific management considerations for optimum crop production (Omonode & Vyn, 2006).Low rice yields from farmers' fields have been continuously reported (MoA, 2009) but the variability in soil fertility and rice growth for formulating soil fertility recommendations has not been investigated in a long time (Kondo, Ota, & Wanjogu, 2001).
Although knowledge on soil fertility status plays a vital role in enhancing production and productivity of the agricultural sector on sustainable basis, no information is available on rice growing soils of Kenya in general and the specific study area of Mwea irrigation scheme.Therefore, this study was proposed to characterize the fertility status of the rice growing soils in Kenya based on selected soil chemical (soil salinity and exchangeable cations calcium (Ca 2+ ), magnesium (Mg 2+ ), sodium (Na + ) and potassium (K + )) properties in the rice growing soils of Mwea.

Description of the Study Area
Mwea Irrigation Scheme is located on the lower slopes of Mt.Kenya, Kirinyaga County in Central Kenya (Figure 1).It is one of the oldest public irrigation schemes in Kenya (Mati, Wanjogu, Odongo, & Home, 2011).It lies within latitudes 37°13′E and 37°30′E and longitudes 0°32′S and 0°46′S with an annual average precipitation of about 950 mm.The area experiences bimodal rainfall with the long rains falling between March and May and short rains between October and December (Kihoro, Bosco, Murage, Ateka, & Makihara, 2013).According to Kenya's agro-climatic zoning by Sombroek, Braun, and van de Pouw (1982), the scheme traverses three agro-climatic zones, with maximum moisture availability ratios ranging from 0.65 for zone III towards the highland slopes to 0.50 for the vast area covered by zone IV and to 0.40 for the semi-arid zone V.The area is generally hot, with average temperatures ranging between 23 and 25 °C, with about 10 °C difference between the minimum temperatures in June/July and the maximum temperatures in October/March.The predominant soils of the rice-growing areas of Mwea are Vertisols characterized by imperfectly drained clays, very deep, dark grey to black, firm to very firm and prone to cracking (Sombroek et al., 1982).
Vertisols popularly known as 'black cotton soils' occupy approximately 2.8 million ha which constitutes about 4.9% of Kenya's total land area and are found under different climatic conditions, but about 80% are in the semi-arid to arid areas (International Board for Soil Research Management [IBSRAM], 1987).According to Muchena and Gachene (1985), these Vertisols are developed on parent materials ranging from Precambrian Basement System rocks, volcanic rocks to alluvial/colluvial deposits derived from various rocks.The Vertisols in the Mwea-Tebere area are reported to have developed on olivine basalt (IBSRAM, 1987).Kondo et al. (2001) also observed predominant presence of Vertisols in the rice growing fields and Alfisols occurred at higher elevation.Studies in Mwea irrigation scheme by Mukiama and Mwangi (1989) noted that the most appropriate season for rice cultivation is from August to December when temperatures are opportune for grain filling and with less risk of disease incidence.However, this same period is also when the river flows are at their lowest, coinciding with the dry season thus straining on availability of irrigation water.
Data from MoA (2009) indicate that the entire irrigation scheme covers an area of about 12,282 ha of which about 9,000 ha has been developed for paddy production.Mwea irrigation scheme, first developed in 1953 is the  (1979) and McLean (1982) using 8 g air-dried soil.EC was measured with the conductivity meter (HORIBA D-24) after a soil saturation paste was prepared with a soil-water ratio of 1:5 (w/v) using 5 g of air-dried soil.Exchangeable Ca 2+ , Mg 2+ , K + and Na + in the soil were first extracted by mixing 1.25 g of air dried soil with 25 ml of 1M neutral ammonium acetate (1 M NH 4 0Ac pH 7.0) and shaking for 60 minutes according to Thomas (1982).The solution was then filtered through ADVANTEC Whatman filter paper No. 6 and the cation concentration in the leachate determined by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICPE-9000 Shimadzu).

Data Analysis
Data for all the measured parameters were subjected to statistical analyses using SPSS (IBM SPSS Statistics version 22).Descriptive statistics that included mean, maximum, minimum and standard deviation (SD) were obtained.The coefficient of variation (CV) which is a ratio of standard deviation to mean and expressed as a percentage was also calculated for each measured variable.We used the CV as a measure of variability to gauge the degree of heterogeneity among the studied parameters.Correlation analysis was performed to assess relationships among the soil parameters.The soil fertility status of the study area was evaluated basing on the concentrations of the respective parameters obtained from the laboratory analyses and were compared with established ratings and critical levels for optimum rice growth.

Soil Solution pH and Electrical Conductivity (EC)
Mean values for soil solution pH and EC of the Mwea irrigation scheme paddy fields are presented in Table 1.Note.Means followed by the same superscript letter within a column are not significantly different at p ≤ 0.05.
The soil pH water values ranged from 4.56 to 8.05 while pH KCl ranged from 3.33 to 6.63 and was consistently less than the values observed for pH water an indication of soils with a net negative charge.Variations within the units were observed in soil solution pH.In the Mwea production unit, soil solution pH water from the sampled fields ranged from 4.56 to 8.05.In Wamumu, the lowest soil solution pH water recorded was 4.91 and the highest value was 7.86.In the Karaba unit block, soil solution pH water varied from 5.53 to 7.99 while in Tebere unit it ranged from 4.90 to 7.74.In Thiba unit, soil solution pH water ranged from 4.90 to 6.96.Unit-wise, Mwea paddy soils had the lowest mean value of 5.99 while the other units had pH values above 6.Compared to Tropical Asia (pH 6.0 and 5.6) and Japan paddy fields (pH 5.4) (Kawaguchi & Kyuma, 1974;Kyuma, 2004) Mwea paddy fields have higher mean pH value (6.27) (Table 1).
Soil EC ranged from 0.08 to 1.52 dS m -1 in surface soil samples indicating that these soils have a low content of soluble salts and that there is no risk of salinity.In Mwea unit, soil solution EC ranged from 0.08 to 1.12 dS m -1 with field differences.In Thiba, soil EC values recorded ranged from 0.11 to 0.59 dS m -1 while in Wamumu fields, it varied from 0.10 to 0.89 dS m -1 .Down in Karaba, soil solution EC ranged from 0.13 to 1.22 dS m -1 whereas in Tebere, values ranging between 0.10 and 1.52 dS m -1 were recorded.On average across the sampled fields in the Mwea rice production mandate have moderately high soil pH and low EC.Coefficient of variation (CV) statistic for pH and EC was 12% and 62% respectively (Table 1).

Soil Cation Concentration
Variations were observed in the concentration of soil exchangeable cations in the sampled fields.Mean values and deficiency level as described by Dobermann and Fairhurst (2000) for soil exchangeable cations in our study site are shown in Table 2. Note.Means followed by the same superscript letter within a column are not significantly different at p ≤ 0.05.
In the larger Mwea region, soil Ca concentration ranged from 8.66 to 69.60 cmol c kg -1 , soil Mg from 5.00 to 40.30 cmol c kg -1 .Soil Na concentration on the other hand ranged from 0.28 to 5.01 cmol c kg -1 while exchangeable K ranged from 0.07 to 1.47 cmol c kg -1 with unit differences.In Mwea unit, the concentration of soil exchangeable Ca ranged from 8.66 to 51.70 cmol c kg -1 with field variations.Exchangeable Mg, K and Na were in the ranges of 5.00 to 40.30 cmol c kg -1 , 0.07 to 1.26 cmol c kg -1 and 0.28 to 5.01 cmol c kg -1 respectively.In Thiba unit, soil cation concentration ranged from 37.42 to 51.80 cmol c kg -1 for exchangeable Ca, 19.30 to 38.30 cmol c kg -1 for exchangeable Mg, 0.32 to 1.78 cmol c kg -1 for exchangeable Na and from 0.08 to 1.26 cmol c kg -1 for soil exchangeable K.In Karaba unit, soil cation concentration ranged from 32.46 to 69.60 cmol c kg -1 , 16.74 to 37.80 cmol c kg -1 , 0.63 to 1.70 cmol c kg -1 and 0.09 to 1.16 cmol c kg -1 for exchangeable Ca, Mg, Na and K respectively.In Wamumu unit, soil exchangeable cation concentrations were from 17.17 to 66.40 cmol c kg -1 for exchangeable Ca, from 8.25 to 32.93 cmol c kg -1 for exchangeable Mg, from 0.52 to 2.56 cmol c kg -1 for exchangeable Na and from 0.12 to 0.99 cmol c kg -1 for exchangeable K.In Tebere unit, Ca and Mg dominated the exchangeable sites and ranged from 17.70 to 65.60 cmol c kg -1 and from 9.76 to 39.20 cmol c kg -1 of soil respectively.In the same unit, the concentration of soil exchangeable Na ranged from 0.62 to 3.31 cmol c kg -1 and exchangeable K from 0.20 to 1.47 cmol c kg -1 of soil.Exchangeable Ca and Mg were the dominant cations and on average, all the exchangeable cations exceeded the deficiency criteria as outlined by Dobermann and Fairhurst (2000).The high level of exchangeable Ca, Mg and low exchangeable K is consistent with the findings of earlier studies by Kondo et al. (2001).Ca + Mg:K ratios exceeding 100 were observed in all units which may indicate low K availability for rice growth.The imbalance was extremely high in Thiba and Mwea units at 455.03 and 325.86 respectively (Table 2).The coefficient of variation statistics indicated that Mwea region is highly heterogeneous in terms of soil exchangeable K and Na with CV of 83% and 65% respectively and least heterogeneous in exchangeable Ca and Mg with CV of 27% and 29% respectively (Table 2).

Correlation between Soil Analytical Parameters
Correlation analysis was performed to assess relationships among the assessed soil parameters; soil pH, soil electrical conductivity and exchangeable cations of paddy field soils from the Mwea irrigation scheme (Table 3).Note.* and ** : significant at 0.05 and 0.01 levels respectively.
There was a positive correlation between soil solution EC and other measured soil parameters except with exchangeable Mg which showed a negative but significant correlation (r = -0.197,p ˂ 0.05).The correlation was significant but weak with pH KCl , exchangeable Na and exchangeable K (r = 0.237, r ˂ 0.233 and r = 0.333, p ˂ 0.01).Soil solution pH water showed a very good correlation with pH KCl (r = 0.973, p ˂ 0.01), exchangeable Ca (r = 0.578, p ˂ 0.01) and exchangeable Na (r = 0.370, p ˂ 0.01).Among the exchangeable cations, Mg and Ca showed moderate positive correlation with each other (r = 0.323, p ˂ 0.01) and positively correlated with exchangeable K and Na (r = 0.285, p ˂ 0.01 and r = 0.154, p ˂ 0.05).Exchangeable K showed a significant and positive correlation with exchangeable Ca (r = 0.285, p ˂ 0.01) and exchangeable Mg (r = 0.215, p ˂ 0.01).

Discussion
The degree of soil acidity or alkalinity is expressed as soil pH and is a master variable affecting a wide range of soil chemical and biological properties.It also influences greatly the availability and uptake of many elements both nutrients and toxins by plant roots (Brady & Weil, 2014).It plays an important role in soil microbial activity and decomposition of mineral substances and organic matter thus influences the release, fixation and migration of soil nutrients (Fageria & Baligar, 1999).Across the units in the scheme, soil pH was moderately high tending towards neutral (pH 7).Soil pH of the paddy fields ranged from very strongly acid to mildly alkaline.The maximum pH observed was 8.05 and minimum 4.56 with a mean of 6.27 (Table 1).On average, the lowest pH value was recorded for Mwea unit soils (5.99) and the highest for Karaba unit (6.65).It was noticeable that soil pH gradually increased from the high elevation areas of Mwea towards the low topographical fields in Karaba.
The Kenya Soil Survey (KSS) 1987 classification of soil reaction (Table 4) was used to categorize the soils from our study on the basis of pH.As per the KSS 1987 rating (Table 4), the soils of Mwea region have a medium acidic to slightly acidic reaction with a medium level soil pH.In all sampled fields, pH values measured in water were higher relative to pH values in salt solutions; an indication that our study site has soils with a net negative charge.This moderately high soil pH condition had also been reported by Kondo et al. (2001) who attributed it to high carbonate concentration in the soils of this semi-arid region.Kawaguchi and Kyuma (1974) in their studies associated the occurrence of high soil pH to climate and/or parent material noting that drier conditions coupled with calcareous parent material favor higher pH.Additionally Murthy, Bhattacharjee, Landey, and Pofali (1982) related soil pH to the nature of the parent material, climate and topographic situations adding that elements of climate and topography remaining the same, Vertisols develop from parent materials rich in alkaline earths thus higher pH values.Descriptive statistics showed a low variance in soil pH with a CV of 12%.Similar low variance in soil solution pH has been reported elsewhere for instance Aimrun, Amin, Ahmad, Hanafi, and Chan, (2007); Abu and Malgwi (2011) and Addis, Klik, and Strohmeier (2015).This is because pH values are indicated on log scale of proton concentration in soil solution; otherwise there would be a much higher variability if soil acidity is expressed in terms of proton concentration directly (Sun, Zhou, & Zhao, 2003).Given the results generated and from the low CV statistics, we can construe that the soils in the study site are relatively homogenous in view of their pH status and this could be attributed to the homogeneous calcareous parent material and the drier climate in the area.
Soil EC is one of the major parameters in assessing salinity status of a soil because it indicates the presence of soluble salts in a soil.Saline soils refer to soils with an EC above 4 dS m -1 , exchangeable sodium percentage (ESP) values less than 15% and pH water values less than 8 and usually contain sufficient soluble salts that adversely affect growth and yield of crops (Yoshida, 1981;Dobermann & Fairhurst, 2000;Kyuma, 2004;Fairhurst, Witt, Buresh, & Dobermann, 2007;Allotey, Asiamah, Dedzoe, & Nyamekye, 2008).The soluble salts are mainly chlorides and sulphates of sodium, calcium and magnesium (Allotey et al., 2008).According to Corwin, Rhoades, and Simunek (2007), salinity and sodicity affect a significant fraction of irrigated soils in semi-arid climates.Nayanaka, Vitharana, and Mapa (2010) further added that salinity is a problem situation in soils where there is excess accumulation of soluble salts as a result of high evapotranspiration, poor drainage and poor quality irrigation water.The application of soil EC to agriculture has its origin in the measurement of soil salinity, which is an arid-zone problem associated with irrigated agricultural land and with areas having shallow water tables.The condition is however influenced by a combination of physico-chemical properties including soluble salts, clay content and mineralogy, soil water content, bulk density, organic matter, and soil temperature.As a result, measurements of EC have been used to map out variation of several edaphic properties such as soil salinity and soil nutrients (Tao 1998;Corwin et al., 2003;Godwin & Miller, 2003;Corwin & Lesch, 2005).
Rice has been regarded as moderately salt tolerant crop but an EC of 6-10 dS m -1 decreased yield by up to 50% (Yoshida, 1981;van Mensvoort, Lantin, Brinkman, & van Breemen, 1985).Salt tolerance however varies with rice growth stage and variety (Kyuma, 2004) and sensitivity to salt is reported to be high during the seedling and heading stages.Rice yields in the Mwea region do not exceed 5 ton ha -1 and varies greatly from farmer to farmer.This low yield is far below the optimum of about 10 ton ha -1 according to MoA (2009).This low yield has been attributed to several factors such as lack of newly improved and adapted varieties, certified seeds, high input costs, climate change, soil chemical and physical degradation due to continuous mono-cropping and an over-reliance by farmers on production techniques that are inefficient (Kimani, Tongoona, Derera, & Nyende, 2011;Nyamai et al., 2012).
The EC values ranged from 0.08 to 1.52 dS m -1 in the study area.On average, the lowest value was found in Thiba unit soil (0.20 dS m -1 ) and highest in Karaba (0.54 dS m -1 ).Lower values of EC were recorded for upstream and topographically higher areas of Mwea and Thiba.According to the KSS 1987 key to salinity classes, the soils of Mwea region are categorised as non-saline (Table 5) with EC lower than 4 dS m -1 and therefore no salinity related risks.The low EC in our study site shows that the presence of salts in these soils is negligible.In our results, the soil EC coefficient of variation statistic of 62% observed indicates that the Mwea region soils are heterogeneous in terms of salt concentration which could be attributed to the differences in topographical elevation where high elevation areas of Mwea and Thiba soils showed EC values of < 0.5 dS m -1 while in the lower position areas it was > 0.5 dS m -1 .Similar diffences in salt accumulation with respect to elevation has been reported in Vietnam by Nguyen, Watanabe & Funakawa 2014 where they observed differences in soil salinity over very small elevation differences.
Magnesium (Mg), Calcium (Ca) and Potassium (K) constitute macronutrients required by plants for growth and metabolism.Sodium (Na) is termed a beneficial element because it stimulates growth but is not essential or is only essential for certain plant species or under specific conditions (Marschner, 2012).Mg is a component of chlorophyll and is involved in photosynthesis and protein synthesis.It activates several enzymes and regulates cellular pH and cation-anion balance.Mg is very mobile and is retranslocated readily from old to young leaves.In rice plants, Mg deficiency that can be exacerbated by Fe toxicities can result in reduced spikelet number, reduced grain weight and reduced grain quality (Dobermann & Fairhurst, 2000;Fairhurst et al., 2007;Marschner, 2012).A critical deficiency level of 1 cmol c kg -1 soil is recommended for rice (Dobermann & Fairhurst, 2000;Fairhurst et al., 2007).In our study area, the soil solution concentration of Mg was above the critical deficiency level and hence sufficient for rice production.According to Murthy et al. (1982) and IBSRAM (1987), parent material that is primarily basic in nature and containing a high proportion of alkaline earths, favour the development of Vertisols and soils of vertic subgroups.
Ca is a constituent of Ca pectates which are important cell wall constituents involved in bio membrane maintenance (Dobermann & Fairhurst, 2000).It is important for cell wall and membrane stabilization, osmoregulation and as second messenger allowing plants to regulate developmental processes in response to environmental stimuli.It is less mobile in rice plants and is required in the maintenance of cation-anion balance in cells (Dobermann & Fairhurst, 2000;Fairhurst et al., 2007;Marschner, 2012).An adequate supply of Ca to plants increases its resistance to diseases such as bacterial leaf blight or brown spot (Dobermann & Fairhurst, 2000;Fairhurst et al., 2007;Marschner, 2012).A soil concentration of below 1 cmol c kg -1 results in deficiency and can result in impaired root function and predispose rice plants to Fe toxicity.Under extreme deficiency cases, plant stunting and death of growing points may occur.However according to Dobermann and Fairhurst (2000) and Fairhurst et al. (2007), Ca deficiency is uncommon in lowland soils because it is usually sufficient enough in the soil, is supplied from mineral fertilizers and also from irrigation water.From our study, Ca and Mg concentration in the soil solution is sufficiently enough and therefore it is unlikely that their deficiency can occur in this region.On average, soil Ca and Mg content in our study site was very high (Table 2) according to KSS 1987 criteria (Table 6).In our results, it was observed that soil Ca and Mg concentration was very high which could be attributed to the parent material composition.The high content of exchangeable Ca and Mg of these soils corroborate the findings of Kondo et al. (2001) who observed Ca and Mg as the dominant cations in the region.Furthermore, Muriithi, Karoki, and Gachanja (2012) while studying the mineral composition of Mwea clays also observed that the clays contained substantial amounts of Mg and Ca oxides.Karaba unit in the lower elevation had the highest mean soil Ca concentration that are likely to have been leached from the upslope and subsequently deposited there.Additional basic cations are also supplied through irrigation water movement and thus a higher concentration especially in the depression areas.Fageria, Caryalho, Santos, Ferraira, and Knupp (2011) further reiterated that calcium and magnesium deficiencies are rare in lowland rice and changes in their concentrations are infinitesimal in flooded soils.In our study site, exchangeable Ca and Mg were very high compared to Tropical Asian and Japanese paddy soils; which could be attributed to differences in parent material composition and climate.With such high calcium levels, the pH of soils is expected to be high.In our results, the soils in the Mwea production unit had lower exchangeable Ca that could be attributed to its higher topographical elevation and low pH that promote leaching thus less cation retention.Funakawa, Tanaka, Kaewkhongkha, Hattori, and Yonebayashi (1997) showed that in soils where pH is less than 6, there is a limited negative charge thus limited cation retention.On the contrary, where soil pH is higher than 6, there is an increase in the amount of variable negative charge thus an increase in the retention of exchangeable cations.
Na is one of the most mobile elements in the soil and according to Kawaguchi and Kyuma (1974), where climate is humid; it is leached out of all soils regardless of their physiographic position.In our study area, soil exchangeable Na is comparable to Tropical Asian soils with almost similar soil concentration probably due to the dry climate or alternating wet and dry conditions.The case is however different for Japanese soils, whose mean Ex-Na content was low at 0.4 cmol c kg -1 as discovered by Kawaguchi andKyuma (1974), andKyuma, (2004).
Exchangeable K represents the fraction of potassium which is adsorbed on external and accessible internal soil surfaces.K has a main role in osmoregulation and affects loading of sucrose and the rate of mass flow-driven solute movement within the plant.It provides strength to plant cell walls and is involved in lignification of sclerenchyma tissues.It increases leaf area and leaf chlorophyll content, delays senescence and thus contributes to greater canopy photosynthesis and crop growth.In rice plants, K increases the number of spikelets per panicle, percentage of filled grains and grain weight.It also improves tolerance to adverse climatic conditions, lodging, insect pest and diseases (Dobermann & Fairhurst, 2000;Fairhurst et al., 2007).Depending on soil texture, clay mineralogy and K input from natural sources, a critical concentration for deficiency to occur is set in the range of 0.1 to 0.4 cmol c kg -1 but a soil concentration of 0.2 cmol c kg -1 is commonly used (Dobermann & Fairhurst, 2000;Fairhurst et al., 2007).In the Mwea region paddy fields, soil K content ranged from 0.07 (very low) to 1.47 cmol c kg -1 (moderate) according to KSS 1987 (Table 6).On average in the units, soil K content was very low in Mwea unit and low in Thiba, Wamumu, Tebere, and Karaba (Table 2) and therefore not sufficient enough for plant growth.
Furthermore, Dobermann and Fairhurst (2000) noted that a Ca+Mg:K ratio exceeding 100 may indicate low K availability for rice growth because of stronger K adsorption to cation exchange sites thus reduced K concentration in the soil solution for plant uptake.In our study site, the Ca+Mg:K ratios were high over 100 in all units (Table 2) thus an indication that K uptake by rice is limited in Mwea paddy soils.Dobermann, Sta Cruz, andCassman (1996a), andDobermann, Cassman, Sta Cruz, Adviento, andPampolino (1996b) also indicated that excessive Ca and Mg compared to K reduces K uptake by rice.In addition, management practices like removal of straw and excessive use of N or N and P fertilizers with insufficient K application further exacerbates K deficiency problems.During our survey in the area, we noted that while a few farmers were using K fertilizers, rice straw was removed from the fields after harvest; a common practice that has been going on as was reported by Kondo et al. (2001).With the increasing mining of native soil K without additional reserves, there is bound to be a continuous increase in K deficiency and cation imbalance ratios especially with Ca and Mg in relation to K in the Mwea irrigation scheme.A Study by Dobermann, Cassman, Mamaril, and Sheehy (1998) observed that most rice-based production systems have been known to have a negative K balance that was attributed to the lack of K fertilizer use in correspondence with the increased use of nitrogen fertilizers.Moreover, the practice of removing all straw from farms by many resource poor farmers in most rice growing countries for fuel and cattle feed or burning for easy tillage operation further exacerbates the removal of nutrients.Similar negative K balances were observed by Ghiri and Abtahi (2011) in their study on K dynamics in calcareous Vertisols in Southern Iran.

Conclusion
Soil chemical property variability in the Mwea Irrigation Scheme is evident.The soils of Mwea irrigation scheme have slightly high pH, have low salt accumulation and the soil exchange complex is rich in exchangeable bases predominantly the divalent cations (exchangeable Ca 2+ followed by Mg 2+ ).In addition, high cation imbalance exists.In view of the variability and cation imbalance, the soils are deficient in K availability for rice uptake.Soil management practices and farming systems which enhance long term soil K availability should be encouraged to help boost and sustain rice yield.Understanding soil nutrient patchiness is essential in applying location-specific management strategies because the more variable the properties, the more variable the crop growth and yield.

Table 1 .
Mean soil solution pH and EC across the units

Table 2 .
Soil exchangeable cation concentration across the units

Table 3 .
Pearson correlation between soil analytical parameters assessed