Organic matter and nutrient status of soils under cultivated fallows: an example of Gliricidia sepium fallows from South Western Nigeria

Agroforestry Systems 10: 23-32, 1990. ~) 1990 Kluwer Academic Publishers. Printed in the Netherlands. 23 Organic matter and nutrient status of soils under cultivated fallows: an example of Gliricidia sepium fallows from South Western Nigeria J.O. A D E J U W O N and F.A. A D E S I N A Department of Geography, Obafemi A wolowo University, lle-lfe, Nigeria Key words: Cultivated fallows, tropics, nutrient immobilization; organic matter. Gliricidia sepium, Nigeria Abstract. This paper examines the dynamics of organic matter and nutrients under planted fallows of Gliricidia sepium in South Western Nigeria, as an investigation into the behaviour of soils under such fallows. Data on soil characteristics were collected from thirty fallow fields ranging in age from one to eight years. The data sets were described using the descriptive statistics. Furthermore, the relationships between the soil properties and the age of fallow were evaluated with simple bivariate correlation analysis. Compared with published data and trends in natural fallow, the study shows that the progress of the planted fallow leads to greater organic matter build-up and increase in nitrate-nitrogen and potassium concentration in the soil. It also shows that the biomass retain greater amount of nutrients during the fallow. Introduction The dynamics o f organic matter and nutrients under self-regenerating fallows o f varying durations in the humid and s u b - h u m i d tropics have been well studied over the years [e.g. Bartholomew et al. 1953; Nye and Greenland, 1960; Aweto, 1985; Ekanade, 1988]. These studies have shown that in general, organic matter content and the concentration o f nutrients in the soil increase as the fallow progresses. A major function o f the fallow is therefore, to raise the fertility status o f the soil. Studies like those referred to above have, however, not been conducted with respect to cultivated fallows. One logical explanation for this is that such fallows have not yet become popular, although it is known that farmers in the tropics have long encouraged certain species o f tree on their abandoned land to hasten soil fertility restoration [Gourou, 1966; Benneh, 1972]. Furthermore, it is also probably assumed that since cultivated fallows are in m a n y ways similar to natural fallows, their effects on soil properties would be comparable. F o r instance, earlier writers on the ecology o f the lowland rain forest o f Nigeria have suggested that the clearance o f forests for the 24 establishment of tree crops such as cocoa and rubber, is actually not a resource degradation but rather, a replacement of one form of forest with another [Keay, 1959; Smyth and Montgomery, 1962]. However, investigations into the effects of tree crops on soils (e.g. Ekanade, 1985; Adejuwon and Ekanade, 1987), have shown that for varying reasons, soils under cultivated trees deteriorate in quality over time, and that this in part, is responsible for the die-back of these tree crops especially cocoa, now being widely experienced in the humid parts of the tropics. As cultivated fallows become more relevant because of the need to maintain soil productivity under shortening fallows [Agboola et al; 1982; Raintree and Warner, 1986; Adesina, 1988a], it is necessary to understand the dynamics of soil properties under such fallows. In this way, it will be possible to determine their efficiency and develop suitable management techniques to ensure that maximum benefit is derived from them. The objective of this paper is therefore, to assess the changes in organic matter content and nutrient status of soils under fallows cultivated to Gliricidia sepium (Jacq.). Compared with natural fallows in which the development of trees is random, and where such trees become significant members of the fallow community only from about the fourth or fifth year, the cultivated fallows of G. sepium have regularly occurring and greater number of trees. These trees form a main group of fallow plants right from the beginning of the fallow. G. sepium is a popular fallow tree in some parts of Western Nigeria. It usually becomes an active member of the plant community on the farm plot when its poles are staked as support for training yam vines. As it is capable of propagating vegetatively, the staked poles coppice within a short time and unless uprooted, go into the fallow with the land as live trees, at the end of the cropping cycle. The available knowledge on the tree [e.g. Falvey, 1982; Adesina, 1988a], shows that it has many characteristics which make it a promising fallow tree in the humid and sub-humid tropics. It is likely that its usage in this respect will increase in the future. Thus, it is relevant to assess the dynamics of soil properties under its fallow both as a means to understanding its effect on soil properties, and as an example of cultivated fallows. The stud)' area The study was conducted in a part of the lowland rain forest areas of South Western Nigeria as mapped and described by Keay [1959] (see Fig. 1). The area has an annual rainfall of about 1300mm and a mean annual tern- 25 I 8oN. NO ff~ern Guineo soy ;5;552 .°5? 6°E I __ Fig. l. Vegetation map of Western Nigeria after Keay (1959). perature of about 26.5 °C [Nigerian Meteorological Observations, 1962]. Its soils belong to the Egbeda Association [Smyth and Montgomery, 1962], and Orthic Luvisols [FAO/UNESCO, 1974]. They are derived from fine-grained biotite gneisses and schists of the Basement Complex. The rural economy involves the production of cash and food crops. The main cash crops are Theobroma cacao (Linn.) (Cocoa) and Cola spp. (Kolanut), while Dioscorea spp. (yams), Xanthosoma sagittifolium (Linn.) (cocoyam), and Manihot esculenta (Crantz) (Cassava), are the main food crops. Rotational bush fallowing is still widely used both for the establishment of tree crops and the production of food crops. Methodology Thirty fallow fields whose tree component is made up of, or dominated by G. sepium, were selected for investigation. The ages of these fields ranged between one and eight years, and were all of similar cultural histories. The owners of the plots provided the information which helped to establish the ages of the fields and the sequence of crops raised on them before they were left to fallow. In order to reduce to the minimal, the effect of variable ground drainage on results, all the chosen fallow fields were on sites of similar drainage properties, all being on slopes not greater than 2 o. On each field, a quadrat 20 by 20 metres in dimension was demarcated. 26 Each of these was subdivided into sixteen 5 by 5 m quadrats, five of which were randomly selected for the collection of soil samples. The soil samples were taken at two depths; 0-15 cm and 15-30 cm. These are referred to as topsoil and subsoil respectively. The samples for each plot were pooled, mixed thoroughly and a sufficient quantity of each mixture taken to the laboratory for immediate analyses. The soil properties evaluated were particle size distribution by hydrometer method [Bouyoucos, 1926]; bulk density by the graduated cylinder method; water holding capacity by saturating soil samples with water, allowing them to drain for 24 hours and then oven drying them at 105 °C; soil pH determined potentiometrically in 0.01 M calcium chloride solution using soil to solution ratio of 1:2; organic matter by the Walkley-Black's [1934] dichromate method; nitrate-nitrogen, using 2, 4, phenoldisulphonic acid; available phosphorus by the Bray No. 1 method [Bray and Kurtz, 1945]; exchangeable calcium, sodium and potassium with a flame analyser, and magnesium with an atomic absorption spectrophotometer; exchange acidity by the Residual Carbonate method; CEC by summation method; % base saturation by relating exchange acidity to CEC; extractable copper and zinc by spectrophotometry; and boron by colorimetry. The latter three properties were determined for the topsoil only. Descriptive statistics including mean, standard deviation and coefficient of variation were used to describe the data. The simple bivariate correlation was also used to assess the nature and strength of the relationships between fallow length and each of the soil properties. Results The descriptive statistics of the soil properties and the strength of the relationships of the properties with the length of the fallow are given in Tables 1 and 2 for the topsoil and subsoil respectively. For the topsoil, the coefficient of variation is significantly high (being greater than 33% e.g. Areola, 1982) for all the soil properties with the exception of pH, soil acidity, CEC, % base saturation and copper. Four of the properties including exchangeable potassium, base saturation organic matter and nitrate-nitrogen have significant correlations with the length of the fallow. Only potassium, organic matter and nitrate-nitrogen have both high coefficients of variation and significant correlations. Thus, the significantly high variabilities displayed by the other properties referred to above do not relate to increasing duration of fallow. They are likely to be related to some biological characteristics of the fallow biomass which are not so 27 Table 1. Chemical properties q[ topsoil Variable Range Mean SD % CV r pH Ca ~ + Mg ~+ K+ Na + 5.55-7.00 1.20-5.70 0.09-0.63 0.14-0.57 0.01-0.32 6.30 3.48 0.34 0.27 0.14 0.32 1.32 0.13 0.14 0.07 5.08 37.93 38.23 51.85 50.00 0.05 0.13 0.01 0.36 + 0.17 H ~ CEC % BS % OM NN AP Cu 0.08-0.16 2.43-6.98 94.24-99.91 0.61-4.30 1.20-9.20 4.50-30.0 5.80-9.30 0.14 4.34 97.09 2.25 5.49 14.57 7.98 0.04 1.34 1.23 1.12 3.10 8.46 0.78 28.56 30.87 1.26 49.78 56.64 58.06 9.77 0.13 0.08 - 0.41" 0.82* 0.57* 0.01 - 0.06 3.30-17,10 0.06-0.46 1-8 8.35 0.19 4.40 3.51 0.09 2.34 42.03 47.37 50.90 -0.17 0.02 1.00 Zn Bo AGE + Significant at 0.05 level. * Significant at 0.01 level r = Correlation coefficient SD = Standard Deviation % CV = % Coefficient of Variation C E C = Cation exchange capacity; % O M = % Organic matter: A P = Available Phosphorus Zn = Extractable Zinc; % BS = Base saturation NN = Nitrate-Nitrogen Cu = Extractable copper Bo = Extractable boron (Cations in meq/100 gin; trace elements in ppm). much affected by the length of the fallow. Such factors may include variations in the intake and retention of nutrients by the fallow plants. For the subsoil, the coefficient of variation is also high for all the soil properties with the exception of pH, exchangeable potassium, soil acidity and base saturation. Only organic matter however, show significant correlation with the length of the fallow. The high variability of most of the other soil properties would be associated with some other factors apart from the length of the fallow. Discussion The results presented above show that the fallow of G. sepium brings about significant enrichment to the soils particularly with respect to organic mat- 28 Table 2. Chemical properties of subsoil Variable Range Mean SD % CV r pH Ca ++ Mg ÷÷ K ++ Na ÷ H+ CEC % BS % OM NN AP AGE 5.20-6.79 0.60-3.50 0.10-0.54 0.13-0.40 0.04-0.21 0.07-0.13 1.33-4.89 92.73-97.79 0.40-2.15 1.32-13.45 0.50-28.80 1-8 6.21 2.36 0.26 0.25 0.13 0.09 3.01 96.62 1.03 5.49 13.34 4.40 0.32 0.99 0.15 0.08 0.06 0.01 1.05 1.31 0.50 3.74 7.77 2.34 5.51 41.94 57.69 32.00 46.15 11.11 33.87 1.36 48.54 58.24 58.24 50.90 - 0.20 0.24 0.20 -0.13 0.24 0.26 0.21 0.01 0.57* 0.03 0.04 1.00 - + Significant at 0.05 * Also significant at 0.01 level r = Correlation Coefficient SD = Standard Deviation % CV = Coefficient of Variation CEC = Cation exchange capacity; % OM = % organic matter; AP = Available phosphorus % BS = % base saturation NN = Nitrate-nitrogen (Cations in meq/100 gin) ter, n i t r a t e - n i t r o g e n and exchangeable p o t a s s i u m within the first 15 cm o f the soil profile. Organic m a t t e r is shown to increase in its concentration with the progress of the fallow even in the subsoil. These results are largely in line with what has been observed in self-propagated fallows [e.g. Nye and Greenland, 1960; Areola et al., 1982; Aweto, 1985]. The litter, particularly the leaves, flowers, fruits and twigs, which are produced as a c o m p o n e n t of the fallow vegetation dynamics, yields the raw material for the generation of organic matter. Provided the foliage cover is maintained in such a way that surface wash is minimised, the organic m a t t e r produced will continue to accumulate even though some of this would normally be lost in the ecosystem [Jordan, 1985]. On mineralisation, some o f the organic m a t t e r is lost t h r o u g h plant use and leaching. G. sepium is capable of generating large quantities o f litter during the dry period when it massively sheds its leaves and flowers. This litter contains high percentage of protein (23%) [Agboola et al., 1982], so that it is capable o f yielding high quantity of organic matter. The tree also produces heavy foliage cover in the wet season which helps to prevent the removal o f the organic m a t t e r regularly added to the topsoil. Thus, if 29 properly established as a fallow tree, a species like G. sepium could bring about accelerated accretion of organic matter during the period of fallow. The significant increment in the concentration of nitrate-nitrogen which the correlation analysis also indicates, should be expected. In the first place, organic matter is a chief source of soil nitrogen. Thus, since the organic matter content of the soil demonstrates significant increment with the length of the fallow, the nitrogen content should normally be expected to show a comparable trend. Besides this fact, G. sepium is a legume with proven ability to nodulate and fix atmospheric nitrogen. Nodulation has been observed to occur within the first three months of planting with nodules numbering between 50 and 150 per plant [Chardokar, 1982]. Thus, greater concentration of nitrogen in fallows of the tree should generally be expected. There is a difference in the dynamics of nitrate-nitrogen and organic matter during the fallow period between the topsoil and the subsoil. Although organic matter increases with the advancement of the fallow also in the subsoil, the concentration of nitrate-nitrogen which is often associated with the organic matter status of the soil as mentioned above, does not. This implies that the increment in nitrate-nitrogen in the topsoil may relate greater to the nitrogen production properties of G. sepium than the abundance of organic matter. This observation is plausible given the fact that the effects of the tree on soil properties are generally concentrated in the topsoil [Adesina, 1988a]. The significant negative correlation of base saturation with fallow length in the topsoil suggests a decline in the status of this important soil property with the progress of the fallow. Although a reduction is implied, the values recorded for this property as shown in Table 1, with a mean of 97.09% are generally satisfactory for crop production. The status of the property in the subsoil helps to confirm the effect of G. sepium on it. Unlike in the topsoil, base saturation shows little or no change with the progress of the fallow. This suggests that the decline in base saturation observed in the topsoil is probably a real effect of G. sepium on the soil. At all the sites studied, the concentration of nutrients especially the micro-nutrients, in the soil is generally lower compared with values quoted for fallow soils in the various parts of the humid tropics [cfe.g. Aweto, 1985; Jordan, 1985]. In terms of the total nutrient budget of the fallow system, the nutrient available in the soil is normally very small compared to what is held in the biomass [Nye and Greenland, 1960]. The general explanation for this is that the biomass takes large quantities of nutrients from the soil and returns only a fraction of these in the nutrient cycling process [Moss, 1977]. This leaves the soil relatively poor in nutrients. However, the fact that 30 nutrients concentrations in soils under G. sepium are lower than those quoted for self-propagated fallows, may imply that the tree immobilizes greater amount of these elements. It is possible, as Fleming [1981] has shown, that legumes generally retain greater amount of nutrients taken up from the soil in the normal nutrient cycling process. This together with a greater biomass, may be responsible for the lower amount of nutrients in planted fallows of G. sepium compared with self-propagated fallows. A comparative analysis of nutrients stored in the leaf litter and soils of Gliricidia and self-propagated fallows during different times of the year [Adesina, 1988b] in fact, shows that greater amount of nutrients is stored in Gliricidia litter compared to the natural fallow in the same area. Thus, greater nutrient immobilisation rather than an inherent weakness in nutrient cycling would most likely be responsible for lower nutrient status of soils under Gliricidia fallows. Conclusion The key elements of soil dynamics under planted fallows of G. sepium are rapid accumulation of organic matter, increase in nitrate-nitrogen and potassium concentration, a decrease in the base saturation, and immobilisation of many nutrients. Although variations should be expected with respect to the effects of different trees on soil properties [Miles, 1985], it is possible that the results reported here are relevant for some other leguminous trees such as Leucaena leucocephala and Derris indica which are becoming popular for soil management in the various parts of the tropics. An important issue in using trees such as G. sepium as a farm tree, would be that of ensuring that the nutrients held in its biomass are extracted to enrich the soil for cultivation. This could be done by mulching the remains of the tree particularly leaves and twigs, during the preparation of the field for cultivation [Lal, 1987]. Results of experiments carried out at the International Institute of Tropical Agriculture in Ibadan Nigeria (IITA) [Wilson et al., 1986], have shown that mulching of G. sepium litter produces good results in terms of the performances of food crops on the soils mulched with it in the 'alley-cropping' system. 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