The Oil palm (Elaeis guineensis Jacq), widely known for its red palm oil1, is cultivated on a wide range of soils, notably amongst which are the so called acid sands, which have been classified as Ultisols or Alfisols depending on their base saturation percent2. These acid sands dominate soils of the Nigerian Institute for Oil Palm Research (NIFOR) main station2 and are developed on coastal plain sand parent materials3. Ultisols is a prominent soil order within the tropics4-6 and form bulk of the soils under oil palm at the main station of the Nigerian Institute for Oil Palm Research. These soils vary in their nutrient content, particularly potassium, which undoubtedly is the most required element for fresh fruit bunch production by the oil palm7. Assessment of potassium uptake by plants according to Zharikova8 must be done using both the intensive parameters (p-values) and extensive parameters, i.e., the contents of potassium and calcium in soils. These two factors characterize two aspects of ion status in soils and their complementary relationships can be interpreted by use of the potential buffering with respect to potassium (PBCK). The PBCK is related to sorption-desorption processes acting in the soil. The range of its values is divided into very low (<20), low (20-50), medium (50-100), elevated (100-200) and high (>200).
Labile-K is the K that is adsorbed on unspecific sites (p-positions) in the soil exchange complex. These positions are of relatively low and even energy of bonds that are believed to be associated with exchangeable cations9. In addition, K is present in positions with higher but uneven energy of bonds and projected parts of crystal surfaces, i.e., on specific exchange positions also constitute part of the labile-K pool.
The intensity which is related to the K in solution is defined as the activity ratio given by:
ARk = aK/(aCa+aMg)1/2
assuming Ca and Mg to be the dominant cations in the soil10. In the equation, aK, aCa and aMg refer to the activities of K, Ca and Mg ions respectively in the soil solution9-11. These parameters (PBCK, Labile-K and ARK) have been related to cropping activities by various workers10, found a reduction in labile-K and activity ratios, ARK with increased PBCK after cropping on the soils while11 found a good relationship between activity ratio which considered aluminium for the acid Nigerian soils known as unified activity ratio, ARu and yield response to K in oil palm field experiments.
In this present study, attempt has been made to determine the relationship between yield parameters of oil palm (Number and weight of fresh fruit bunches) and PBCK, labile-K and ArK.
MATERIALS AND METHODS
Soil sampling procedures: Eighteen soil samples were obtained in 2015 from 3 profile pits sunk at field 14 dominated by Orlu series at the Nigerian Institute for Oil Palm Research (NIFOR) main station to cover slight differences in the location. This research project was conducted from September, 2015 to November, 2018. Profiles I, II, III were sited on 164 m ASL on N 06°32159.711, E 005°37115.811, 159 m a.s.l (N 06°32159.911, E 005°37118.511 ), 160 m a.s.l (N 06°33100.711, E 005°37117.311). The soil samples were taken to the laboratory, air-dried and sieved through a 2 mm mesh after which further analysis were carried out.
Determination of PBCK and labile K: 2.5 g of the soil samples were put in 25 mL solutions of 0.01M CaCl2 that contained potassium concentrations of 0, 4, 8, 16 and 32 mg L1 and shaken for 24 h at 25+1°C to achieve equilibration. The contents were filtered using Whatman No 42 filter papers. The concentration levels of potassium in the filtrate were measured using a flame photometer12.
Adsorption isotherms were constructed using the method described by Kenyanya et al.13. The amount of K adsorbed was obtained by subtracting the amount found in filtrate from the initial amount that was in solution as shown in Eq. 1:
where, ΔK is the change in amount of K (Quantity factor (Q)) in solution and represents amount of K adsorbed, CKi and CKf are the initial K concentrations added and final equilibrium concentrations of K in solution respectively. V and M are the solution volume and mass of the soil used. The K adsorption data were fitted into the Freundlich linearised adsorption equation as suggested by Kenyanya et al.13, given as follows:
where, x/m is the mass of adsorbed K per unit mass of soil (mg kg1, C is the equilibrium K concentrations of solutions (mg L1), a and b are constants obtained from the intercept and slope, respectively. The ionic strength of the soils was calculated by a formula proposed by Griffin and Jurinak14:
where, EC is electrical conductivity of soil pastes in dS m1.
Activities of potassium, calcium and magnesium ions were tabulated as the product of their activity coefficients (fi) and their concentrations (Ci) as shown in Eq. 4:
The fi of the ions were determined using the extended Debye and Huckel15 equation cited by Al-Zubaidi et al.9 as shown in Eq. 5:
|| Valency of ion
|| 0.508 for water at 298 Kelvin
|| 0.328×108 at 298 Kelvin
|| Effective size of hydrated ions
|| Ionic strength of cation
Determination of activity ratio of potassium (Ark): The activity ratio of potassium ions were calculated as suggested by Beckett16 and Zubaidi et al.9:
Statistical analysis: Data obtained were fitted into a simple linear regression with yield parameters taken as the dependent variable (Y) while values of PBCK and Labile-K obtained from the isotherms as well as computed values of the activity ratios taken as the independent variable (X).
RESULTS AND DISCUSSION
PBCK, Labile-K, activity ratio and yield of oil palm: The values of the PBCK and labile-K are indicated in Fig.1-6. The values of the PBCK ranged from 0.70-1.82 cmol kg1 mol L1 (Fig.1-6). These are extremely low values using10 classifications as follows: very low (<20), low (20-50), medium (50-100), elevated (100-200) and high (>200).
Freundlich adsorption isotherm for 0-15 cm NIFOR soils
Freundlich adsorption isotherm for 15-30 cm of soils of NIFOR
Freundlich adsorption isotherm for 30-45 cm of soils of NIFOR
Freundlich adsorption isotherm for 45-60 cm of NIFOR soils
Freundlich adsorption isotherm for 60-90 cm NIFOR soils
Freundlich adsorption isotherm for 90-120 cm soil of NIFOR
Relationship between bunch weight and PBCK of oil palm
These very low values indicated that the soils are poor in their ability to resist changes with respect to potassium. In concrete terms, it signifies that the dynamics of potassium in the soils might be in one direction only. Simply put, if K is often leached in the soils due to high rainfall, high total porosity and the presence of low activity clays, this might continue for a long time because the soils lack what it takes to prevent this phenomenon. Furthermore, the extremely low PBCK values indicated the lack of inputs in the soils with respect to potassium fertilizers. The values of the labile-K were just as low and ranged from 0.016 -0.733 cmol kg1 (Fig. 1-6). This is the K held in unspecific sites and ready to be taken up by the palms or leached. The low values of the labile-K were due to the low values of the PBCK .The computed activity ratio values ranged from 0.18 -0.24 (M L1)1/2 (Table 1) and compare well with values obtained by earlier workers9-11. The yield parameters of oil palm averaged over a fifteen year period were indicated in Table 1.
Relationship between yield parameters of oil palm, PBCK, labile-K and activity ratios of the soils: The fitted regression equations relating the yield parameters with PBCK, Labile-K and activity ratios respectively are indicated in Fig. 7-11. The relationship between yield parameters of oil palm (number of bunches and bunch weight of fresh fruit) and PBCK are shown in Fig. 7 and 8. There was no relationship between PBCK and bunch weight of oil palm (Fig. 7) as shown by the values of coefficient of determination R2 since only 4.7% of the bunch weight were accounted for by the labile-K content of the soils.
Yield parameters and computed activity ratios of Oil Palm at NIFOR main station
Source of yield records: Harvesting division, Nigerian institute for oil palm research (NIFOR)
Relationship between bunch number and PBCK of oil palm
Relationship between bunch weight and labile K of oil palm
Relationship between bunch number and labile K of oil palm
Relationship between bunch number and activity ratio of oil palm
Relationship between bunch weight and activity ratio of oil palm
There was however a very strong relationship between number of bunches and labile-K content of the soils (Fig. 10) as 73.1% of the variations in number of bunches of oil palm were accounted for by the labile-K content of the soils. The labile-K content of soils is the K that can be taken up by plants or leached when not taken up. There were no relationships between bunch weights, number of bunches and activity ratios of the soils (Fig. 11, 12) as only 17.1% of the variations in bunch weights and number of bunches were accounted for by the activity ratios of the soils. It has been shown that oil palm yields may fluctuate with a 3-5 years cycle17. To overcome this, Tinker averaged oil palm yield records for at least 4 years11. In this study, oil palm yield records were averaged for 15 years.
The relationship between labile-K and number of bunches showed that 73.1% of the variations in number of bunches were accounted for by the labile-K content of Ultisols under oil palm while 53.1% of the variations in number of bunches were accounted for by the PBCK. The lack of relationship between bunch weight, number of fresh fruits of oil palm and activity ratios of the soils showed that potassium fertilization for oil palm yield improvement need not be made in consideration with Ca and Mg minerals as earlier suggested.
The corresponding author is grateful to the Executive Director of the Nigerian Institute for oil palm Research (NIFOR) for funding this research and Harvesting Division for providing the yield data.
This study has discovered that there exists a relationship between labile-K (soil solution K) and number of bunches of oil palm grown on Ultisols. It has shown that potassium fertilizers made to improve oil palm can be applied without a corresponding application of Ca and Mg minerals. This study will help the researcher to uncover the critical areas of potassium fertilization for yield improvement in the oil palm. Thus a new theory detailing the mechanism of action of potassium fertilization on PBCK and number of fresh fruit bunches may be arrived at.