# Lime-Aluminium- Phosphorus Interactions in Acid Soils of the Kenya Highlands

E M Muindi, J P Mrema, E. Semu, P W Mtakwa, C K Gachene, M K Njogu

► To cite this version:

E M Muindi, J P Mrema, E. Semu, P W Mtakwa, C K Gachene, et al., Lime-Aluminium- Phosphorus Interactions in Acid Soils of the Kenya Highlands. American Journal of Experimental Agriculture, 2015, 9 (4), pp.1-10. 10.9734/AJEA/2015/20220. http://dx.doi.org/10.9734/AJEA/2015/20220

HAL Id: hal-04883832

https://hal.science/hal-04883832v1

Submitted on 13 Jan 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

# Lime-Aluminium- Phosphorus Interactions in Acid Soils of the Kenya Highlands

E. M. Muindi<fnref n="1" />*, J. P. Mrema<fnref n="2" />, E. Semu<fnref n="2" />, P. W. Mtakwa<fnref n="2" />, C. K. Gachene<fnref n="3" /> and M. K. Njogu<fnref n="4" />

## Authors' contributions

This work was carried out in collaboration between all authors. Author EMM designed the study, wrote the protocol and wrote the first draft of the manuscript. Authors JPM, ES, PWM and CKG guided on data collection, evaluation and interpretations while author MKN was involved in data collection, data analysis, interpretation and review. All authors read and approved the final manuscript.

## Article Information

DOI: 10.9734/AJEA/2015/20220

Editor(s):

(1) Rusu Teodor, Department of Technical and Soil Sciences, University of Agricultural Sciences and Veterinary Medicine Cluj-Napoca, Romania.

Reviewers:

(1) Ade Onanuga, Biology, Dalhousie University, Canada.

(2) Anonymous, Yaradua University, Nigeria.

Complete Peer review History: http://sciencedomain.org/review-history/10546

## Original Research Article

Received 17th July 2015

Accepted 30th July 2015

Published 14th August 2015

## ABSTRACT

Liming and phosphorus (P) applications are common practices for improving crop production in acid soils of the tropics. Although considerable work has been done to establish liming rates for acid soils in many parts of the world, information on the role of the lime-Al-P interactions on P fertility management is minimal. A green house pot experiment was conducted at Waruhiu Farmers Training Centre, Githunguri to evaluate the lime-Al-P interactions in acid soils of the Kenya highlands. Extremely acidic (pH 4.48) and strongly acidic (pH 4.59) soils were used for the study. Four lime (CaO) rates and phosphorus (Ca (H₂PO₄)₂ rates were used. The liming rates were: 0, 2.2, 5.2 and 7.4 tonnes ha⁻<fnref n="1" /> for extremely acidic soil and 0, 1.4, 3.2, and 4.5 tonnes ha⁻<fnref n="1" /> for the strongly acidic soil. Phosphorus applications rates were: 0, 0.15, 0.30 and 0.59 g P kg⁻<fnref n="1" /> soil for the extremely acidic soil and 0, 0.13, 0.26, and 0.51 g P kg⁻<fnref n="1" /> for the strongly acidic soils. The experiments were a 4² factorial laid in a randomized complete block design (RCBD) and replicated

<footnote n="1">¹Department of Crop Science, Pwani University, Kenya.</footnote>

<footnote n="2">²Department of Soil Science, Sokoine University of Agriculture, Tanzania.</footnote>

<footnote n="3">³Department of Land Resource Management, University of Nairobi, Kenya.</footnote>

<footnote n="4">⁴Department of Plant Sciences, Chuka University, Kenya.</footnote>

<footnote>*Corresponding author: E-mail: e.muindi@pu.ac.ke, muidiede@gmail.com;</footnote>
three times. Data collected included: soil chemical properties and P adsorption. The soils had high exchangeable Al (>2 cmol Al kg⁻¹), Al saturation of (>20% Al) and low P. Lime-Al-P interaction significantly (P≤0.05) increased soil pH, extractable P, reduced exchangeable Al, Al saturation, P adsorption and standard phosphorus requirements (SPR). Use of 7.4 tonnes ha⁻¹ lime in extremely acidic soils and 4.5 tonnes ha⁻¹ lime in strongly acidic soils significantly reduced exchangeable Al and SPR by >70%. Lime positively correlated with soil pH, extractable P, and Langmuir maximal adsorption constant and negatively correlated with SPR and exchangeable Al. It was therefore concluded that lime and P positively interact to reduce Al and P adsorption rates in acid soils in the Kenya highlands.

Keywords: Lime; phosphorus; aluminium; standard phosphate requirement; acid soils.

# 1. INTRODUCTION

Inorganic phosphorus (P) in soil occurs mainly as H₂PO₄⁻ and HPO₄²⁻ adsorbed onto the surfaces of iron (Fe) and aluminium (Al) oxide, and hydroxides, organic matter layer and amorphous alumino- silicate clay minerals or bound to cations like calcium [1]. Soil acidity is associated with aluminium (Al), hydrogen (H), iron (Fe) and manganese (Mn) on the soil colloids and in the soil solution [2-6] and corresponding deficiencies of the available P, molybdenum (Mo), calcium (Ca), magnesium (Mg) and potassium (K) (3, 4, 5, 6). Although high concentrations of H⁺ are toxic to plant roots [7], the main constraint to crop production in acid soils is not high H⁺ but increased concentration of Fe³⁺ and Al³⁺ at pH< 5.5 [8]. The occurrence of high levels of Al and Fe in soils gives rise to low pH and low P availability. Thus, wherever Fe and Al are high, a corresponding P stress occurs [9].

Soil acidity and P deficiency are not independent and one of the primary reasons commonly proposed for liming acid soils is to increase the availability of P to plants [10]. The effects of liming on the P supplying power of highly weathered acid soils has, however, been argued differently by different authors. Liming has been reported to increase, decrease, or have no effect on the amounts of soil extractable P [11-15]. Many of these apparent inconsistencies have been attributed to the differences in initial soil pH, nature of soil acidity, soil mineralogy, and the soil P extraction methods used [16].

According to [17], liming can increase P sorption in highly weathered acid soils. The increased P sorption with liming of acid soils was attributed to formation of active x-ray amorphous Al hydroxyl polymers which actively sorb more P than Al³⁺ [18]. The active Al hydroxyl polymers formed can also coat the surface of minerals, thereby

modifying their surface charge characteristics [19,20] hence influencing the P- sorption mechanisms by such soils.

Kenyan soils, similar to other agricultural soils of the tropics, are generally low in available P attributable to low soil pH and oxides or/ and hydroxides of Al and / or Fe. Several authors have reported that the available P in the Kenya highland soils is low, ranging from 2 to 5 mg P kg⁻¹ soil [21-24]. Various P studies have further indicated that these soils have high P-fixation capacities that transform about 80% of the added inorganic P to forms unavailable for plant uptake [21,25,26]. Several scientists have tested and documented the effect of liming on nutrient availability in acid soils of the western highlands of Kenya [27,28,14]. However, information on the interaction of lime and P on availability of P, as assessed by soil-testing procedures and plant uptake in the Kenya highland, is scanty. The objectives of this study were therefore: (i) to evaluate the lime-P interaction effect on soil acidity and available P, and (ii) To determine the lime-Al-P interaction effect on the P adsorption by two acid soils from Kenya Highlands.

# 2. MATERIALS AND METHODS

## 2.1 Experimental Layout and Design

A greenhouse pot experiment was carried out at Waruhiu Farmers' Training Centre, Githunguri, Kiambu County Kenya to determine the lime-Al-P interactions in acid soils. Two composite soil samples representing extremely acid (pH 4.0-4.5) and strongly acidic (pH 5.0-5.5) soils, as described by [29] were used in the study. The experiments were a 4² factorial laid down in a Randomized Complete Block Design (RCBD) and replicated three times. Liming rates were chosen to attain 0, 30, 70 or 100% reduction in the amounts of M KCl-extractable Al originally
<table><caption>Table 1. Actual amounts of phosphorus and lime added to the two acid soils</caption><thead><tr><th rowspan="2">Soil</th><th colspan="4">Lime added (tones ha<sup>-1</sup>)</th><th colspan="4">P added (g P kg<sup>-1</sup> soil)</th></tr><tr><th>0%</th><th>30%</th><th>70%</th><th>100%</th><th>0 SPR</th><th>0.5 SPR</th><th>1 SPR</th><th>2 SPR</th></tr></thead><tbody><tr><td>Extremely acidic</td><td>0</td><td>2.2</td><td>5.2</td><td>7.4</td><td>0</td><td>0.15</td><td>0.30</td><td>0.59</td></tr><tr><td>Strongly acidic</td><td>0</td><td>1.4</td><td>3.2</td><td>4.5</td><td>0</td><td>0.13</td><td>0.26</td><td>0.51</td></tr></tbody></table>

SPR-Standard phosphate requirement

present in the soil while the phosphorus levels added were: 0, 0.5, 1 or 2 times the standard phosphorus requirement (SPR) (Table 1) of the soils used in the study. Burnt lime (CaO) containing about 21% calcium oxide was used in this study while triple superphosphate (TSP) fertilizer (Ca(H₂PO₄)₂ was used as P-source. Lime requirement of the soils was calculated using the equation of [30]. The equation aims at reducing the % Al saturation to a level that is commensurate with crop Al tolerance, given as: Lime required CaCO₃ equiv. tons/ha = 1.8[Al - RAS (Al + Ca + Mg)/100] where Al = cmol kg⁻¹ soil in the original exchange complex, RAS = Required percentage Al saturation, Ca = cmol kg⁻¹ soil in the original exchange complex, Mg = cmol kg⁻¹ soil in the original exchange complex. A RAS value of 20% was used. Soil characterization data in Table 2 below was used for lime requirement determinations.

Forty eight, 5 kg composite soil sample portions for both extremely and strongly acid soils were weighed and transferred to nine litre plastic pots. The different lime levels were weighed and thoroughly mixed with the soils and incubated at friable state for a period of 21 days. Water was added every 2 days to compensate for evaporative losses and the soils were remixed thoroughly. After incubation, soils from each liming level were air-dried, sieved and returned to the plastic pots. The various P levels were then added and the soils re-incubated at the same conditions for 14 days after which they were air dried. One kilogramme subsamples of the soils were then used for chemical analyses in the laboratory.

## 2.2 Laboratory Analysis

Soil physiochemical analysis and P adsorption were determined before and after application of treatments. The analysis conducted include: soil pH, exchangeable aluminium, CEC, exchangeable bases and particle size were analyzed as described by [31]. Extractable P was determined Using Mehlich double acid method as described by [32] while phosphorus sorption capacities of the soils were evaluated as

described by [33] and the P adsorbed data for the two soils fitted into the linearized form of the Langmuir equation.

## 2.3 Data Analysis

Data collected was subjected to Analysis of Variance (ANOVA) using the Genstat statistical software [34] and means were ranked using Duncan's New Multiple Range Test. Dependency tests and correlations were also conducted to find out if there was a relationship between the various variables.

<table><caption>Table 2. Physiochemical properties of the two acidic soils before the experiment</caption><thead><tr><th></th><th>Extremely acidic soils</th><th>Strongly acidic soils</th></tr></thead><tbody><tr><td>pH (CaCl<sub>2</sub>)</td><td>4.48</td><td>4.95</td></tr><tr><td>N (%)</td><td>0.19</td><td>0.16</td></tr><tr><td>OC (%)</td><td>1.75</td><td>1.83</td></tr><tr><td>K (cmol kg<sup>-1</sup>)</td><td>0.15</td><td>0.20</td></tr><tr><td>Na (cmol kg<sup>-1</sup>)</td><td>0.58</td><td>0.95</td></tr><tr><td>Ca (cmol kg<sup>-1</sup>)</td><td>1.80</td><td>1.23</td></tr><tr><td>Mg (cmol kg<sup>-1</sup>)</td><td>0.75</td><td>3.50</td></tr><tr><td>P (mg kg<sup>-1</sup>)</td><td>10.50</td><td>13.50</td></tr><tr><td>CEC</td><td>10.82</td><td>11.68</td></tr><tr><td>Exch. Al (cmol kg<sup>-1</sup>)</td><td>3.85</td><td>3.90</td></tr><tr><td>Exch. Acidity (cmol kg<sup>-1</sup>)</td><td>5.79</td><td>5.67</td></tr><tr><td>AL saturation (%)</td><td>55.82</td><td>49.66</td></tr><tr><td>% Clay</td><td>56.32</td><td>50.00</td></tr><tr><td>% Silt</td><td>21.00</td><td>17.00</td></tr><tr><td>% Sand</td><td>22.68</td><td>33.00</td></tr><tr><td>Textural class</td><td>Clay</td><td>Clay</td></tr></tbody></table>

# 3. RESULTS

## 3.1 The Initial Physical and Chemical Characteristic of the Soils

The tested soils were acidic with pH<sub>CaCl2</sub> <5.5 (Table 2). Exchangeable Aluminium levels for both soils were > 2 cmol kg⁻¹ and % Aluminium saturation > 20%. Both extractable P and Ca were low while CEC was < 15 cmol kg⁻¹ soil. The tested soils had clay texture.
## 3.2 The Effect of Lime - Phosphorus Interaction on Soil Chemical Properties

Lime-Al-P interaction significantly (P<.01) increased soil pH levels (Table 3). Lime rates resulting to 100% reduction in Al³⁺ increased soil pH levels by 2.20 and 2.31 in extremely and strongly acidic soils respectively, compared to the control. Phosphorus did not significantly influence soil pH.

Lime-Al-P interaction significantly (P<.01) increased extractable P in both extremely and strongly acidic soils (Table 4). Lime rates resulting in to 70% reduction in Al³⁺, promoted the highest amount of extractable P in both soils.

Lime-Al-P interaction significantly (P<.01) reduced exchangeable Al, and % Aluminium saturation in both extremely and strongly acidic soils (Tables 5 & 6). Liming to achieve 100% Al³⁺ reduction, resulted to >50% reduction of the exchangeable Al in both soils compared to the control. Phosphorus use however, did not significantly influence either exchangeable Al or % Aluminium saturation in both soils.

Significant positive relationships were observed between soil pH, lime and extractable P (Table 7). On the contrary, pH, lime and extractable P negatively correlated with exchangeable Al, and aluminium saturation.

<table><caption>Table 3. Soil pH (water) levels determined at the end of the experiment</caption><thead><tr><th rowspan="2"></th><th colspan="8">Phosphorus levels applied (g P kg<sup>-1</sup>)</th></tr><tr><th colspan="4">Extremely acidic soils</th><th colspan="4">Strongly acidic soils</th></tr></thead><tbody><tr><td>Lime to give</td><td>0</td><td>0.15</td><td>0.30</td><td>0.59</td><td>0</td><td>0.13</td><td>0.26</td><td>0.51</td></tr><tr><td>0% reduction in Al<sup>3+</sup></td><td>3.96a</td><td>4.31a</td><td>4.17a</td><td>4.19a</td><td>3.91a</td><td>4.41a</td><td>4.07a</td><td>3.96a</td></tr><tr><td>30% reduction in Al<sup>3+</sup></td><td>5.30b</td><td>5.37b</td><td>5.22b</td><td>5.15b</td><td>5.29b</td><td>5.24b</td><td>5.29b</td><td>5.30b</td></tr><tr><td>70% reduction in Al<sup>3+</sup></td><td>6.24c</td><td>6.33c</td><td>6.27c</td><td>6.37c</td><td>5.62c</td><td>5.69c</td><td>5.53c</td><td>5.45b</td></tr><tr><td>100% reduction in Al<sup>3+</sup></td><td>6.63d</td><td>6.40c</td><td>6.42c</td><td>6.39c</td><td>6.54c</td><td>6.51c</td><td>6.38d</td><td>6.27c</td></tr><tr><td>CV%</td><td>0.60</td><td>0.60</td><td>0.60</td><td>0.60</td><td>0.90</td><td>0.90</td><td>0.90</td><td>0.90</td></tr></tbody></table>

Values followed by the same letter(s) on the same column are not significantly different at p<0.05

<table><caption>Table 4. Mean extractable phosphorus (mg kg⁻¹) determined from soils at the end of the experiment</caption><thead><tr><th rowspan="2"></th><th colspan="8">Phosphorus levels applied (g kg<sup>-1</sup>)</th></tr><tr><th colspan="4">Extremely acidic soils</th><th colspan="4">Strongly acidic soils</th></tr></thead><tbody><tr><td>Lime to give</td><td>0</td><td>0.15</td><td>0.30</td><td>0.59</td><td>0</td><td>0.13</td><td>0.26</td><td>0.51</td></tr><tr><td>0% reduction in Al<sup>3+</sup></td><td>6.80a</td><td>12.30a</td><td>18.00a</td><td>25.00a</td><td>6.60a</td><td>11.30a</td><td>11.30a</td><td>11.00a</td></tr><tr><td>30% reduction in Al<sup>3+</sup></td><td>13.00a</td><td>16.20a</td><td>22.80b</td><td>26.50a</td><td>12.40b</td><td>16.30a</td><td>16.80a</td><td>23.40b</td></tr><tr><td>70% reduction in Al<sup>3+</sup></td><td>28.20c</td><td>32.20c</td><td>36.30c</td><td>43.20b</td><td>41.00c</td><td>34.20b</td><td>30.60b</td><td>35.00c</td></tr><tr><td>100% reduction in Al<sup>3+</sup></td><td>18.40ab</td><td>21.20a</td><td>35.50c</td><td>43.90b</td><td>10.90c</td><td>30.00b</td><td>27.60b</td><td>39.70c</td></tr><tr><td>CV%</td><td>3.10</td><td>3.10</td><td>3.10</td><td>3.10</td><td>7.60</td><td>7.60</td><td>7.60</td><td>7.60</td></tr></tbody></table>

Values followed by the same letter(s) on the same column are not significantly different at p<0.05

<table><caption>Table 5. Mean exchangeable Aluminium (cmol kg⁻¹) of the soils at the end of the experiment</caption><thead><tr><th rowspan="2"></th><th colspan="8">Phosphorus levels applied (g kg<sup>-1</sup>)</th></tr><tr><th colspan="4">Extremely acidic soils</th><th colspan="4">Strongly acidic soils</th></tr></thead><tbody><tr><td>Lime to give</td><td>0</td><td>0.15</td><td>0.30</td><td>0.59</td><td>0</td><td>0.13</td><td>0.26</td><td>0.51</td></tr><tr><td>0% reduction in Al<sup>3+</sup></td><td>3.75a</td><td>3.53a</td><td>3.57a</td><td>3.48a</td><td>3.83a</td><td>3.69a</td><td>3.76a</td><td>3.97a</td></tr><tr><td>30% reduction in Al<sup>3+</sup></td><td>2.91b</td><td>2.92b</td><td>2.85b</td><td>3.03b</td><td>2.80b</td><td>2.86b</td><td>2.74b</td><td>2.84b</td></tr><tr><td>70% reduction in Al<sup>3+</sup></td><td>2.41c</td><td>2.22c</td><td>2.34c</td><td>2.29c</td><td>2.34c</td><td>2.05c</td><td>2.22c</td><td>2.27c</td></tr><tr><td>100% reduction in Al<sup>3+</sup></td><td>1.82d</td><td>1.70d</td><td>1.88d</td><td>1.62d</td><td>1.81d</td><td>1.63d</td><td>1.72d</td><td>1.68d</td></tr><tr><td>CV%</td><td>1.60</td><td>1.60</td><td>1.60</td><td>1.60</td><td>0.80</td><td>0.80</td><td>0.80</td><td>0.80</td></tr></tbody></table>

Values followed by the same letter(s) on the same column are not significantly different at p<0.05
<table><caption>Table 6. Mean Aluminium saturation (%) of the soils determined at the end of the experiment</caption><thead><tr><th rowspan="2"></th><th colspan="10">Phosphate levels applied (g kg<sup>-1</sup>)</th></tr><tr><th colspan="5">Extremely acidic soils</th><th colspan="5">Strongly acidic soils</th></tr></thead><tbody><tr><td>Lime to give</td><td>0</td><td>0.15</td><td>0.30</td><td>0.59</td><td>0</td><td>0.13</td><td>0.26</td><td>0.51</td></tr><tr><td>0% reduction in Al<sup>3+</sup></td><td>69.10a</td><td>62.00a</td><td>64.90a</td><td>60.80a</td><td>53.83a</td><td>59.35a</td><td>51.35a</td><td>59.09a</td></tr><tr><td>30% reduction in Al<sup>3+</sup></td><td>45.90b</td><td>47.00b</td><td>48.30b</td><td>55.70a</td><td>45.33b</td><td>44.70b</td><td>46.37b</td><td>45.76b</td></tr><tr><td>70% reduction in Al<sup>3+</sup></td><td>45.50b</td><td>39.10c</td><td>45.20b</td><td>44.70c</td><td>41.24c</td><td>37.45c</td><td>35.47c</td><td>39.17c</td></tr><tr><td>100% reduction in Al<sup>3+</sup></td><td>31.70c</td><td>29.30d</td><td>40.50bc</td><td>37.80d</td><td>28.97d</td><td>26.95d</td><td>26.66d</td><td>26.73d</td></tr><tr><td>CV%</td><td>7.90</td><td>7.90</td><td>7.90</td><td>7.90</td><td>3.20</td><td>3.20</td><td>3.20</td><td>3.20</td></tr></tbody></table>

Values followed by the same letter(s) on the same column are not significantly different at p< 0.05

<table><caption>Table 7. Relationship between soil pH, extractable P, exchangeable Acidity, exchangeable aluminium and liming in acid soils</caption><thead><tr><th></th><th>pH</th><th>Lime</th><th>Exch. P</th><th>Exch. Al</th><th>Al. Sat.</th></tr></thead><tbody><tr><td>pH</td><td>-</td><td>0.94</td><td>0.38</td><td>-0.93</td><td>-0.84</td></tr><tr><td>Lime</td><td>0.94</td><td>-</td><td>0.29</td><td>-0.95</td><td>-0.86</td></tr><tr><td>P</td><td>0.38</td><td>-</td><td>-</td><td>-0.33</td><td>-0.28</td></tr></tbody></table>

All correlations had high significant relationship of p<.01

## 3.3 The Effect of Lime – Al- P Interaction on Phosphorus Adsorption Levels of the Two Soils

The phosphate adsorption isotherms of extremely and the strongly acidic soils were determined by plotting the amounts of phosphate adsorbed (X) against the equilibrium concentrations of phosphate (C). Comparing the highest amount of P adsorbed extremely acidic

soils; it is evident that soils treated with different levels of phosphorus had different capacities to adsorb P (Fig. 1). The results of the study showed that maximal P adsorption decreased with increasing fertilizer P application. Soils supplied with 0 mg P kg⁻¹ had the highest value of maximum adsorption (1967 mg kg⁻¹) while soils supplied with 3 g P kg⁻¹ had the lowest maximal P adsorption (960 mg kg⁻¹).

<figure><img src="image_4.png" /><figcaption>Fig. 1. Phosphate adsorption isotherm of extremely acidic soil treated with different phosphate fertilizer rates (Error bars inserted)</figcaption></figure>
Maximal P adsorption decreased with increasing fertilizer P application in strongly acidic soils (Fig. 2). Soils supplied with 0 mg P kg⁻¹ had the highest value of maximum adsorption (1216 mg kg⁻¹) while soils supplied with 2.5 g P kg⁻¹ had the lowest maximal P adsorption (889 mg kg⁻¹).

Lime application significantly influenced the capacity of soils to adsorb P (Fig. 3). The results of the study showed that maximal P adsorption decreased with increasing liming rates. Soils

supplied with 0 tonnes ha⁻¹ lime had the highest value of maximum adsorption (1568 mg kg⁻¹) while soils supplied with 7.4 tonnes ha⁻¹ lime had the lowest maximal P adsorption (1242 mg kg⁻¹) in extremely acidic soils. Similarly, soils supplied with 0 tonnes ha⁻¹ lime had the highest value of maximum adsorption (1376 mg kg⁻¹) while soils supplied with 7.4 tonnes ha⁻¹ lime had the lowest maximal P adsorption (804 mg kg⁻¹) in strongly acidic soils (Fig. 4).

<figure><img src="image_5.png" /><figcaption>Fig. 2. Phosphate adsorption isotherm of strongly acidic soil treated with different phosphate fertilizer rates (Error bars inserted)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Phosphate adsorption isotherm of extremely acidic soil treated with different rates of lime (Error bars inserted)</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4. Phosphate adsorption isotherm of strongly acidic soil treated with different rates of lime (Error bars inserted)</figcaption></figure>

Phosphorus significantly reduced the standard phosphorus requirement (SPR) of the tested acid soils (Table 8). Use of 3.0 g P kg⁻¹ reduced SPR by more than 40% compared to 0 g P kg⁻¹ in extremely acidic soils. Similarly, 7.4 tonnes ha⁻¹ of lime significantly reduced SPR by >81% compared to 0 tons ha⁻¹.

Phosphorus and lime use significantly reduced SPR in strongly acidic soils (Table 9). Use of 2.5g P kg⁻¹ significantly reduced SPR by 73%

while 4.5 tonnes ha⁻¹ reduced SPR by 84% compared to the control.

A significant negative relationship was observed between standard phosphorus requirement of tested acid soils, lime application, langmuir adsorption constants (bL) and langmuir constant related to binding energy (kL) (Table 10). Significant relationships were also observed between lime application, bL and kL.

<table><caption>Table 8. Interactive effect of lime and phosphorus on standard phosphorus requirement (SPR) in extremely acidic soils</caption><thead><tr><th>Phosphorus (g kg<sup>-1</sup>)</th><th>SPR</th><th>R<sup>2</sup></th><th>Lime (tonnes ha<sup>-1</sup>)</th><th>SPR</th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td>0.00</td><td>308.71</td><td>0.93</td><td>0.00</td><td>409.63</td><td>0.84</td></tr><tr><td>0.15</td><td>256.41</td><td>0.94</td><td>2.20</td><td>279.25</td><td>0.90</td></tr><tr><td>0.30</td><td>220.87</td><td>0.92</td><td>5.20</td><td>162.13</td><td>0.96</td></tr><tr><td>0.59</td><td>184.08</td><td>0.85</td><td>7.40</td><td>75.34</td><td>0.98</td></tr></tbody></table>

<table><caption>Table 9. Interactive effect of lime and phosphorus on standard phosphorus requirement (SPR) in strongly acidic soils</caption><thead><tr><th>Phosphorus (g kg<sup>-1</sup>)</th><th>SPR</th><th>R<sup>2</sup></th><th>Lime (tonnes ha<sup>-1</sup>)</th><th>SPR</th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td>0.00</td><td>293.06</td><td>0.86</td><td>0.00</td><td>352.84</td><td>0.85</td></tr><tr><td>0.13</td><td>243.02</td><td>0.88</td><td>1.40</td><td>241.83</td><td>0.90</td></tr><tr><td>0.26</td><td>153.33</td><td>0.94</td><td>3.20</td><td>135.53</td><td>0.96</td></tr><tr><td>0.51</td><td>77.83</td><td>0.97</td><td>4.50</td><td>54.85</td><td>0.98</td></tr></tbody></table>
# 4. DISCUSSION

## 4.1 Chemical and Physical Properties of the Soils

Levels of exchangeable Ca (Ca<4.0) and CEC< 15 cmolkg⁻¹ soil such as identified in this study are low [35]. Soils which are extremely (pH <4.5) to strongly acidic (pH 4.5-4.9) and have aluminium levels (> 2.0 cmolkg⁻¹) have also been reported by [29,36] to be toxic for most plants. Acid soils with such high Al³⁺ ions, low base cations and CEC are characteristics of highly weathered soils, which have lost most of the base cations due to high rainfall and extensive weathering. As a result, the soils have high levels of Fe and Al sesquioxides that may lead to high P fixation resulting to low levels of extractable P [35,37,38]. In addition to leaching of bases, parent material mineralogy also contributes to the soil acidity [38] and P fixation.

<table><caption>Table 10. Relationship between standard phosphate requirement, lime application and Langmuir equation constants of the acid soils</caption><thead><tr><td>Pair</td><td>R</td><td>P-value</td></tr></thead><tbody><tr><td>SPR and lime application</td><td>-0.90**</td><td>0.00</td></tr><tr><td>SPR and Bl</td><td>-0.57**</td><td>0.00</td></tr><tr><td>SPR and KI</td><td>-0.68**</td><td>0.00</td></tr><tr><td>Lime and Bl</td><td>0.54**</td><td>0.00</td></tr><tr><td>Lime and KI</td><td>0.68**</td><td>0.00</td></tr></tbody></table>

SPR-Standard lime requirement; bl- Langmuir equation adsorption maxima (mg P kg⁻¹); kL- Langmuir equation constant related to binding energy; **Significant at P<.001 level

## 4.2 The Effect of Lime - Phosphorus Interaction on Soil Chemical Properties and P-sorption

The increase in soil pH and extractable P and decrease of P adsorption levels and SPR after application of both lime and P could be attributed to increased base saturation leading to replacement of hydrogen ions on the soil surface by bases like Ca²⁺ [4,39]. Neutralization of H⁺ results in a pH increase and a shift in the ratio of basic cations adsorbed onto soil particles and in solution [4,10]. Aluminium and iron ions held by negative sites on soil particle surfaces are displaced into soil solution and transformed back into the solid state sesquioxides (e.g Al(OH)3, Fe(OH)3) [4,40]. The Ca²⁺ could either come from lime (CaO) or released upon dissolution of Ca (H₂PO₄)₂ [41]. The Ca²⁺ displacement leads

to reduced Fe³⁺ and Al³⁺ in solution consequently due to the formation of Al (OH₃) and Fe (OH₃) hence increased P availability in the exchange complex [38-40]. Additionally, pH increase plays a role in stimulating mineralization of P from soil organic phosphorus [16]. Similar findings on reduction of nutrient toxicities and corresponding increase in nutrient availabilities as a result of lime and P have been reported [15,39-42].

# 5. CONCLUSION

Lime-Al-P interaction significantly increased soil pH levels, extractable P, and reduced exchangeable aluminium, P adsorption and SPR of the acid soils. Combined application of 7.4 tonnes lime ha⁻¹ with either 0.30 g P kg⁻¹ or 0.59 g P kg⁻¹ soil in extremely acidic soils and 4.5 tonnes lime ha⁻¹ with either 0.26 g P kg⁻¹ or 0.51 g P kg⁻¹ in strongly acidic soils significantly reduced SPR and exchangeable Al. It can therefore be concluded that lime-Al - P interactions is important in the management of phosphorus in acid soils of the Kenya Highlands. However, more research is required to ascertain optimal P levels that can be applied to achieve optimal lime-Al- P interactions under field conditions.

# ACKNOWLEDGMENTS

The authors express their appreciation to Waruhiu- farmers training centre for provision of research facilities and the Alliance for Green Revolution Africa (AGRA), for funding the research work.

# COMPETING INTERESTS

Authors have declared that no competing interests exist.

# REFERENCES

1. Olibone D, Rosolem CA. Phosphate fertilization and phosphorus forms in an Oxisol under no-till. Scientia Agricola. 2010;67:465-471.

2. Duputel M, Devau N, Brossard M, Jaillard B, Hinsinger P, Gérard F. Citrate adsorption can decrease soluble phosphate concentration in soils: Result of theoretical modeling. Appl. Geochem. 2013;35:120-131.

3. Jorge RA, Arrunda P. Aluminium induced organic acids exudation by roots of
aluminium tolerant maize. Phytochem.
1997;45(4):675-681.

4. Brady CN, Weil RR. The nature and properties of soils, 14th Ed; Pearson Prentice Hall, New Jersey. 2008;975.

5. Mohammadi K. Phosphorus solubilizing bacteria: Occurrence, mechanisms and their role in crop production. Res. & Env. 2012;2:80-85

6. Nduwumuremyi A. Soil acidification and lime quality; sources of soil acidity, effects on plant nutrients, efficiency of lime and liming requirements. Research and Reviews. J. Agri. & Allied Sci. 2013;2(4): 26-34.

7. Foy CD. Physiological effects of hydrogen, aluminium and manganese toxicities in acid soil. In: Pearson RW, Adams F, eds. Soil acidity and liming, 2nd edn. Wisconsin: American Society of Agronomy. 1984; 57-97.

8. Sale PWG, Mokwunye AU. Use of phosphate rocks in the tropics. Fert. Res. 1993;35:33-45.

9. Fageria NK, Baligar VC, Wright RJ. Influence of phosphate rock sources and rates on rice and common bean production in an oxisol. In: Wright RJ, Baligar VC, Murrmann RP, Eds. Plant and Soil Interaction at Low pH, Kluwer Academic Publisher, Norwell. 1991;539-546.

10. Sanchez PA, Uchara G. Management considerations for acid soils with phosphorus fixation capacity. In: The role of phosphorus in Agriculture. F.E. Khasawneh EC. Sample and Kamprath EJ. (eds.) American Society for Agronomy., Wisconsin. 1980;471-514.

11. Martni JA, Kochhann RA, Siquira OL, Borken CM. Response of soybean to liming as related to soil acidity, Al and Mn toxicities, and P in sorne Oxisol of Bnll.il. Soil Sci. Soc. Amer. Proc. 1974;38: 616-620.

12. Janghorbani M, Roberts S, Jackson TL. Relationship of exchangeable acidity to yield and chemical composition of Alfalfa. Agron. J. 1975;67:350-354.

13. Ryan J, Smillie GW. Liming in relation to soil acidity and phosphate fertilizer efficiency. Comm. Soil Sc. & Pl. Anal. 1975;6:409-420.

14. Kisinyo PO, Othieno CO, Gudu SO, Okalebo JR, Opala PA, Ng'etich WK, Nyambati, RO, Ouma EO, Agalo JJ, Kebeney SJ, Too EJ, Kisinyo JA, Opile WR. Immediate and residual effects of lime

and phosphorus fertilizer on soil acidity and maize production in Western Kenya. Experiment. Agric. 2014a;50(1):128-143.

15. Sarker A, Kashem AMD, Osman KT, Hossain I, Ahmed F. Evaluation of available phosphorus by soil test methods in an acidic soil incubated with different levels of lime and phosphorus. Open J. So. Sci. 2014;4:103-108.

16. Haynes RJ. Effects of liming on phosphate availability in acid soils. Pl. & Soil. 1982; 68(3):289-308.

17. Mokwunye U. The influence of pH on the adsorption of phosphate by soils of the Guinea and Sudan savanna zones of Nigeria. Soil Sci. Soc. Amer. Proc. 1975; 39:1100-1103.

18. Roborage WP, Corey RB. Adsorption of phosphate by hydroxyl aluminium species on a cation exchange resin. Soil Sci. Soc. Am. J. 1979;43:481-487.

19. Sims JT, Ellis BG. Changes in phosphorus adsorption associated with aging of aluminum hydroxide suspensions. Soil Sci. Soc. Am. J. 1983;47:912-916.

20. Kamprath EJ. Exchangeable aluminum as a criterion for liming leached mineral soils. Soil Sci. Soc. Am. Proc. 1970;34:252-254.

21. Kisinyo PO, Othieno CO, Gudu SO, Okalebo JR, Opala PA, Maghanga JK, Ngetich WK, Agalo JJ, Opile RW, Kisinyo JA, Ogola BO. Phosphorus Sorption and lime requirements of maize growing acid soils of Kenya. Sust. Agri. Res. 2013;2(2): 116-123.

22. The AW, Calba H, Zonkeng C, Ngonkeu ELM, Adetimirin VO. Response of maize grain yield to changes in acid soil characteristics after soil amendment. Pl & Soil. 2006;284:45-57.

23. Obura PA. Effects of soil properties on bioavailability of aluminium and phosphorus in selected Kenyan and Brazilian soils. Ph.D. Thesis, Purdue University, USA; 2008.

24. Opala PA, Okalebo JR, Othieno CO. Comparison of effects of phosphorus sources on soil acidity, available phosphorus and maize yields at two sites in western Kenya. Arch. Agron. Soil. Sci. 2013;59(3):327-339.

25. Kifuko MN, Othieno CO, Okalebo JR, Kimenye LN, Ndungu KW, Kipkoech AK. Effects of combining organic residues with Mijingu phosphate rock on sorption and availability of phosphorous and maize
production in acid soils of western Kenya.
Exp. Agric. 2007;43:51-66.

26. Opala PA. Effect of organic and inorganic phosphorus sources on selected soil chemical properties and maize (Zea mays) yields in acid soil in western Kenya. Ph.D. thesis, Moi University, Kenya; 2009.

27. Nekesa AO. Effects of Mijingu phosphate rock and agricultural lime in relation to maize, groundnut and soybean yield on acid soils of western Kenya. M. Phil. Thesis. Moi University, Eldoret, Kenya; 2007.

28. Kisinyo PO, Gudu SO, Othieno CO, Okalebo JR, Opala PA, Maghanga JK, Agalo JJ, Ng'etich WK, Kisinyo JA, Osiyo RJ, Nekesa AO, Makatiani ET, Odee DW, Ogola BO. Effects of lime, phosphorus and rhizobia on *Sesbania sesban* performance in a western Kenyan acid soil. Afri. J. Agric. Res. 2012;7(18):2800-2809.

29. Kanyanjua SM, Ireri L, Wambua S, Nandwa SM. Acid soils in Kenya: Constraints and remedial options. KARI Technical Note. 2002;11:24.

30. Cochrane TT, Salinas JG, Sanchez PA. An equation for liming acid mineral soils to compensate crop aluminium tolerance. Tropical Agriculture (Trinidad). 1980;57: 33-40.

31. Okalebo JR, Gathua KW, Woomer PL. Laboratory methods of soil analysis: A working manual (2nd ed.). TSBR-CIAT and SACRED Africa, Nairobi, Kenya; 2002.

32. Mechlich AA, Pinkerton RW, Kempton R. Mass analysis methods for soil fertility evaluation. Internal Publication, Ministry of Agriculture, Nairobi; 1962.

33. Fox RL, Kamprath EG. Phosphate sorption isotherms for evaluating the phosphate requirements of soils. Soil Sci. Soc. Am. Proc. 1970;34:902-907.

34. GenStat. The GenStat Teaching Edition. GenStat Release 7.22 TE, Copyright 2008, VSN International Ltd; 2010.

35. Landon JR, ed. Booker Tropical Soil Manual. Longman, Burnt Mill, Harlow, UK; 1991.

36. Landon JR. Booker tropical soils manual: A Handbook for Soil Survey and Agricultural Land Evaluation in the Tropics and Subtropics. New York: John Wiley & Sons; 1984.

37. Sanchez PA, Shephard KD, Soule MJ, Place FM, Buresh RJ, Izac AN, Mokwunye AU, Kwesiga FR, Ndiritu CG, Woomer PL. Soil fertility replenishment in Africa: An investment in natural resource capital. In: Replenishing soil fertility in Africa, SSSA Special Publication Number 51. Buresh, R.J., Sanchez, P.A., and Calhoun, F. (Eds.), pp. 1-46. SSSA, ASA, Madison, Wisconsin, USA; 1997.

38. Van Straaten P. Rocks for crops: Agro minerals for sub-Saharan Africa. ICRAF, Nairobi. 2002;25-28.

39. Tisdale SL, Nelson WL, Beaton JD. Soil fertility and fertilizers. 5th edition. Macmillan, New York, USA; 1990.

40. Kamprath EJ. Crop response to lime in the tropics, In: Adams F (Ed). Soil acidity and liming. 2nd edition. Agronomy Monograph 12. Agronomy and soil science Society of America, Madison Wisconsin, USA. 1984; 349-368.

41. Zhang H, Liu J, Liao W, Zhang Z, Hao X. Effects of phosphate fertilizer and manure on properties of phosphorus sorption and desorption in soils with different phosphorus levels. J. Plant. Nutr. & Fert. 2008;14(2):284-290.

42. Indiati R, Sharpley AN, Izza C, Figliolia A, Felici B, Sequi P. Soil phosphorus sorption and availability as a function of high phosphorus fertilizer additions. Comm. Soil Sc. Plant. Analy. 1995;26(11&12): 1863-1872.

© 2015 Muindi et al.; This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Peer-review history:
The peer review history for this paper can be accessed here:
http://sciencedomain.org/review-history/10546