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SOIL FERTILITY STUDIES WITH COMPOST AND IGNEOUS PHOSPHATE ROCK AMENDMENTS IN MALAWI

J. C.V.B. Nyirongo, S.K. Mughogho and J.D.T. Kumwenda¹
University of Malawi, Bunda College of Agriculture, P. O. Box 219, Lilongwe, Malawi
¹Chitedze Agricultural Research Station, P. O. Box 219, Lilongwe, Malawi

Oxide mineralogy of acid tropical soils contributes to the problem of phosphorus (P) deficiency due to P sorption. The objectives of this research were to evaluate composting methods for improving the availability of phosphorus in igneous low grade (low reactivity) Tundulu phosphate rock and to test their effect on maize (Zea mays L.) yield. The treatments included two composting materials (cattle manure and maize stover) which were applied at 3 t ha^-1. Phosphate rock, was composted at three levels: 50, 75 and 100 kg P₂O₅ ha^-1. Epigeic earthworms were introduced into phospho-composts of manure. Sole manure and crop residue composts were also prepared. Three levels of phosphorus were also applied as direct application of phosphate rock. Single super phosphate (SSP) was used as a standard treatment at the rate of 20 kg P₂O₅ ha^-1. There were 16 treatments in total. The experiment was arranged as a Randomised Complete Block with 3 replications. Soil analysis for P has shown no significant differences among treatments in improving the status of P in the soil at both Lunyangwa and Bembeke experimental sites both at seedling and harvest stages of maize, and among sampling times during the first season (1997/98). All P values were below the critical value for P in the soil and therefore maize yield realised was far below the potential for all the treatments.

Key Words: Earthworms, fortified compost, P deficiency, soil acidity, Tundulu rock phosphate

La mineralogie des oxides de sols tropicaux contribue au problème de déficience du P due à sa fixation. Les objectives de cette recherche étaient d’évaluer les méthodes de compostage pour améliorer la disponibilité du phosphore dans l’indigène roche phosphatée de Tundulu à faible réactivité et de tester son effect sur le rendement du maïs (Zea mays L.). Les traitements comprenaient deux materiels de compostage (fumure de vache et les résidus du maïs) étaient applîqués à un taux de 3 tones ha^-1. La roche phosphatée, qui était compostée était à trois niveaux: 50, 75 et 100 kg P₂O₅ ha^-1. Les verres de terre étaient incorporées dans 3 additionnelles fumures de composte de phosphore. Seule la fumure et les compostes de résidus de culture ont été préparés. Trois niveaus de phosphore ont été aussi appliqués comme une application directe de la roche phosphatée. Le superphosphate simple (SSP) a été utilisée comme un traitement standard à un taux de 20 kg P₂O₅ ha^-1. Les traitements étaient 16 au total. L’essai était conduit dans un bloc complètement randomisé. L’analysis du sol pour le P a montré qu’il n’y avait pas de difference significative entre traitements dans l’amélioration de la situation du P dans le sol des sites experimentaux de Lunyangwa et Bembeke à la germination et au stade de recolte du maïs, et entre les périodes d’echantillonnage pendant la première saison (1997/98). Toutes les valeurs du P étaient en dessous de la valeur critique du P dans le sol et ainsi le rendement du maïs obtenu était beaucoup inférieur au rendement potentiel pour tous les traitements.

Mots Clés: Verre deterre, compost fortifié, Malawi, déficience en P, acidité du sol, phosphate de Tundulu

One of the major problems that has inhibited the development of economically successful agriculture in many areas of the tropics is the poor soil fertility for crop production (Chien and Hammond, 1988 ) and Malawi is no exception. Malawi has for long depended on imported fertilisers to meet the escalating demand for phosphorus, but this is a drain on the foreign exchange reserves of the country. However, Malawi contains a deposit of over 1,250,000 tons of phosphate rock (PR) with an average P concentration of 8.7% (20% P₂O₅) (Carter and Bennett, 1973) at Nathace Hill, Tundulu in Phalombe district. This may offer an inexpensive source of phosphorus, although Tundulu phosphate rock has been classified as low grade in terms of its reactivity (Lowell et al., 1994).

Many researchers in India and other areas have used phosphate rock by composting it with different kinds of organic compounds to improve its agronomic effectiveness (Mishra et al., 1982; Singh et al., 1983; Singh, 1985; Singh and Amberger, 1991). Tundulu RP is composed of igneous apatite and is much less soluble in citric acid than Minjingu RP (van Kauwenburg, 1991) being mined in neighbouring Tanzania and used in nutrient replenishment efforts in western Kenya (Woomer et al., 1997) Earthworms which live and feed on litter and manure produce casts and faecal pellets. These have a proportionately higher exchangeable and water extractable inorganic P in casts than in non-ingested control materials (Lavelle et al., 1994). It is possible that earthworms may assist in solubilising PR. The objective of the present study is to evaluate composting methods for improving the availability of phosphorus in Tundulu phosphate rock (TPR). The hypotheses are that: (a) increasing the ratio of TPR to crop residues and manure will increase the amount of available phosphorus, (b) the low pH in the Lunyangwa and Bembeke soils will increase the dissolution of TPR, (c) composting of TPR with crop residues and manure respectively will enhance the dissolution of the TPR, and (d) introduction of epigeic earthworms to compost will have an effect on the dissolution of TPR.

This experiment was conducted at Lunyangwa and Bembeke Research Stations in Malawi. These stations were selected because they are P deficient sites and are characterised by low soil pH. Maize stover was chopped into approximately 15 cm length and weighed. Cattle manure was also collected and weighed. The chopped maize stover and manure were composted with three levels of Tundulu Phosphate Rock as indicated below. An epigeic earthworm, Eisenia setida, was introduced into the other three combinations of cattle manure with TPR. Composting proceeded for 90 days. Samples from the compost heaps were collected and analysed for labile P (Melhich-3 Method) and total P by digestion in Selenium/Sulphuric acid mixture. The treatments were as follows: (1) Control; (2) Compost at 3 tons ha^-1; (3) Manure at 3 tons ha^-1; (4). 0.5 kg TPR (50 kg P₂O₅ ha^-1); (5) 1.0 kg TPR (75 kg P₂O₅ ha^-1); (6) 2.0 kg TPR (100 kg P₂O₅ ha^-1); (7) 0.5 kg TPR (50 kg P₂O₅ ha^-1) + 13.5 kg composted stover (CS); (8) 1.0 kg TPR (75 kg P₂O₅ ha^-1) + 13.5 kg CS; (9) 2.0 kg TPR (100 kg P₂O₅ ha^-1) + 13.5 kg CS; (10) 0.5 kg TPR (50 kg P₂O₅ ha^-1) + 13.5 kg manure (M); (11) 1.0 kg TPR (75 kg P₂O₅ ha^-1) + 13.5 kg M; (12) 2.0 kg TPR (100 kg P₂O₅ ha^-1) + 13.5 kg M; (13) 0.5 kg TPR (50 kg P₂O₅ ha^-1) + 13.5 kg M + earthworms (EW); (14) 1.0 kg TPR (75 kg P₂O₅ ha^-1) + 13.5 kg M + EW; (15) 2.0 kg TPR (100 kg P₂O₅ ha^-1) + 13.5 KG M + EW; and (16) Single superphosphate (SSP) at 20 kg P₂O₅ ha^-1.

The plots were arranged at both locations in a Randomised Complete Block design with three replicates.

All the phospho-composts were applied at 3 tons ha^-1 by the banding method. In all the treatments 0.12 kg^-1 plot gypsum was added to supply sulphur at the rate of 8 kg S ha^-1. All plots received 60 kg N ha^-1 as Urea and 20 kg K20 ha^-1 as KCl. Each plot consisted of 5 ridges 0.9 m apart and 6 m in length with sample areas of 3 ridges of 4 m in length. In the second year, the same plots were also planted to maize at Lunyangwa and Bembeke research stations to test the residual effects of the treatments.

Soils were sampled at 0-15 cm and 15-30 cm from the experimental plots at the seedling stage and at harvest of maize. The soils were air-dried and sieved to pass through a 2 mm sieve before analysis. The soils were analysed for pH in 0.01 M CaCl₂, phosphorus by Melhich-3 Universal Soil Extraction Method, exchangeable acidity (i.e., % Aluminum saturation) by the Titration Method which uses 1 M KCl as an extractant, and P adsorption isotherms were developed. Plant samples were collected at seedling stage of maize as whole plants sampled above soil depth, ear-leaf samples at silking stage, and grain samples at harvest of maize. The samples were then oven-dried, ground and digested in a Selenium/Sulphuric acid mixture to determine P concentration in the samples.

Data collected were subjected to analysis of variance using the SAS statistical package.

Availability of P in the soils. Data from soils sampled at the 0-15 cm soil depth at seedling stage of maize showed no significant (P<0.05) differences in P among treatments (Fig. 1 ). The range was 3.5 mg P kg^-1 in the control treatment to 7.2 mg P kg^-1 in the treatment where P was applied at 100 kg P₂O₅ ha^-1 as direct application of PR (Fig. 1 ) and 12.5 mg P kg^-1 (control) to 18.1 mg P kg^-1 (treatment 12) (Fig. 2) at Lunyangwa and Bembeke, respectively. The critical value for available P by the Melhich-3 method is 31.5 mg P kg^-1 (Chilimba, 1997). These P values are too low to support a crop especially at the critical seedling stage. This led to crop failure in most of the plots especially in the control plots. The P status of the soil at seedling stage was also reflected in the yields at the end of the season in that maize yield was far below the potential yield of 5,500 kg ha^-1 (Table 1).

TABLE 1. Grain yield (kg ha^-1) of maize at Lunyangwa and Bembeke (1997/98 season) as affected by different soil fertility management strategies

Treatment	Lunyangwa	Bembeke

	Grain yield (kg ha^-1)

1	195	1865
2	813	2610
3	841	2012
4	664	1650
5	587	1905
6	187	1719
7	591	2953
8	397	2380
9	951	2336
10	933	1800
11	1259	2122
12	452	1934
13	757	2507
14	500	2208
15	1306	2240
16	929	1235

Mean	710.10	2092.32
S.E.±	291.54	386.06
C.V.%	82.11	36.90

Lack of significant differences of soil P between treatments and the amount of P in composts is a reflection of the low degree of PR dissolution in the different compost combinations. It can be assumed that there were no significant differences in PR dissolution between the different composted materials. This also meant that the ratio of TPR to composting materials (i.e., maize stover and cattle manure) did not have a noticeable effect as far as soil extractable P at harvest is concerned. This may reflect that the TPR was very non-reactive and, therefore, the extractable P in the soil may only have come from the composting materials with insignificant contribution from the relatively insoluble TPR.

Soils sampled at harvest of maize showed significantly higher levels of available P at Lunyangwa than soils sampled at the seedling stage. The P values ranged from 7.7 mg P kg^-1 in the control to 11.8 mg P kg^-1 and 12.4 mg P kg^-1 to 16.8 mg P kg^-1 at Lunyangwa and Bembeke, respectively. However there were no significant differences in P estimated between treatments (Figs. 1 and 2). At the seedling stage, the maize plants in the single superphosphate treatments displayed zinc deficiency symptoms. This may be attributed to high soluble P availability, which may have resulted in the formation of P-Zn complexes and thereby making the zinc less available to the maize plants. This is termed phosphorus induced zinc deficiency (Fig. 3).

Concentration of P in the maize plant tissue. A young plant test could be a useful tool to evaluate P availability early during the growing period of maize, a period that is especially critical for P nutrition (Mallarino et al., 1992). The concentration of P in plant tissues reflects the amount of P that was assimilated by the plant. Whole plant samples from the Lunyangwa experimental site analysed for P showed no significant differences in the levels of P in the plant tissue between treatments at the seedling stage of maize. The test values of P ranged from 0.13% to 0.18% (Fig. 4).

Okalebo et al. (1993) reported that the adequate level of P in maize tissues at the seedling stage is 0.3-0.5%. This, therefore, shows that the crop took up very little P. This data agrees with the soils analytical data which showed a maximum value of 7.2 mg P kg^-1 at seedling stage (Fig. 1 ) which is far below the critical value.

Plant analysis often is used as a diagnostic tool to evaluate the nutrient status of crops. The most commonly used P tissue test for maize is based on the P concentrations of the ear-leaf blades sampled at the silking stage (Mallarino et al., 1992). The concentration of P in the ear-leaf showed a range of 0.25% to 0.41% (Fig. 4). There was no significant difference between treatments in the levels of P in the ear-leaf samples. However, the concentration of P in the ear-leaf showed significantly higher values than P concentration at seedling stage (P< 0.05). P in the soils was generally higher when sampled at maize harvest than in soils sampled at the seedling stage. Under normal circumstances, whole plant samples at seedling stage contain higher P concentration than that in the ear-leaf, because at silking, ear-leaf P is preferentially translocated to the process of grain formation. The situation at Lunyangwa was the reverse in that P was lower at seedling stage than at harvest stage probably due to the fact that the crop was deprived of P early in the season.

Residual phosphorus of the soils. The P test values during the second season at seedling stage of maize at both Lunyangwa and Bembeke sites were not significantly different from values obtained during the first season. The values ranged from 3.9 mg P kg^-1 to 8.6 mg P kg^-1 and 3.5 mg P kg^-1 to 7.1 mg P kg^-1 (Fig. 5 and 6) at Lunyangwa and Bembeke, respectively. There were no significant differences in P between treatments at both locations. The trend at Bembeke was that the P values declined if compared to those obtained at harvest of the first season. This may reflect the cumulative effects of phosphate sorption by oxide minerals.

Tundulu Phosphate Rock (TPR) was found to be non-reactive and composting with maize stover and/or cattle manure did not substantially improve its solubility. This was reflected in soil P at both sampling times (i.e., seedling stage and harvest stage of maize) and also maize yields in that no significant differences were detected between treatments. Inclusion of earthworms in composting TPR with manure did not have any effect on solubilisation of TPR. The ratio of TPR to composting material had no effect on the solubilisation of TPR. The results suggest that the potential of Tundulu rock phosphate for direct application, even in combination with organic inputs, is rather limited in terms of realising immediate benefits. According to the results of this research, composting TPR may not be an effective method in improving availability of P. Progressive application of compost (every year) may reduce Al toxicity in acid soils because there was generally a better vegetative growth of maize in all plots where compost was applied than where TPR was directly applied without composting it. Acidification of Tundulu phosphate rock with sulfuric acid should be examined and the cost effectiveness as a P source studied as an alternative to the unsatisfactory addition of untreated rock P.

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