Short- and Long-term Effects of Sparingly Soluble Phosphates on Crop Production in Two Contrasting Nigerian Alfisols

The effectiveness of Sokoto and Ogun rock phosphates (RP) as substitutes for the conventional phosphorus (P) fertilizer sources in the production of tomato (Lycopersicon esculentum) and okra (Hibiscus esculentus) were evaluated using single super-phosphate (SSP) as reference. Their residual effects were also investigated using rice (Oryzae sativa) and soybean (Glysine max). The two experiments were 2 × 2 × 4 factorial combinations with completely randomized design (CRD) involving two crops, two soils, three P-fertilizer sources and a control. The treatment combinations were replicated four times to give a total of 64 pots, each containing 5 kg of either a slightly acid (pH 6.3) alfisol (Typic Paleudalf) A, from Abeokuta (rainforest ecological zone) or a medium acid (pH 5.9) alfisol (Oxic Paleustalf) B, from Zaria (southern Guinea savanna zone). The soils represent a wide range of Nigerian soils with medium to low P-availability and cultivated to various food and economic crops. The rock phosphates (particularly SRP) were more efficient than SSP in soil A but had less than 50% relative agronomic efficiency (RAE) in soil B, especially when tomato was the test crop. They also had higher RAE in soil A than in soil B when okra was grown, though the efficiency was not as high as that of the reference fertilizer. In the slightly acid alfisol (A), ORP was less efficient than SRP whereas it had 47.8% RAE compared with 34.9% RAE for SRP in the medium acid soil. In the second cropping, the soil from the rainforest zone still produced greater biomass than soil B while soybean gave more biomass than rice. The results confirmed that, apart from crop species to be grown, organic matter and clay contents as well as pH of soils should be considered for efficient utilization of the sparingly soluble phosphates for both shortand longterm effects in crop production. Introduction Phosphorus (P) is an important limiting nutrient element in the production of vegetables, cereals and leguminous crops. It stimulates root growth, flower development and seed formation besides enhancing rapid and vigorous as well as early maturity in these crops. With inadequate P supply, leaves are often small in size and greenish red, reddish brown, purple or bronze-like colours while fruits and seeds are small and have reduced quality (Denton & Swarup, 1981; Al-Wandawi, 1983). The occurrence of excess available P is rare in soils since it is often adsorbed, sometimes strongly through fixation by colloidal (inorganic clay and organic materials) complexes formed with Al, Fe and Ca. It can also be lost through erosion, leaching and crop removal (Aduayi & Ekong, 1981). Crops have become so expensive to grow that nutrient deficiencies should not be allowed to limit their yields. However, this goal is far from reality. The use of phosphatic fertilizers is beyond the reach of peasant farmers due to procurement difficulties, especially in developing countries of the world. This necessitates research to investigate alternative materials that can be used as P fertilizer sources. One of the possibilities is the direct use of sparingly soluble rock phosphates (RP). Yet, it is not all soils that are suitable for such direct applications (Obigbesan & Akinrinde, 2000). The objective for this work was, therefore, to investigate the direct and residual effects of two rock phosphate materials, namely Ogun rock phosphate (ORP) and Sokoto rock phosphate (SRP) applied directly as Pfertilizer sources in sequential cropping of okra and tomato, as well as rice and soybean in two contrasting soils. Materials and methods Two soils, A and B, were used for the experiments. Soil A was a slightly acid alfisol (Typic Paleudalf) collected from Abeokuta in the rainforest zone of south-west Nigeria (Soil Survey Staff, 1975; Agboola & Ogunkunle, 1993) while soil B was a medium acid alfisol (Oxic Paleustalf) collected from Zaria in the southern Guinea savanna zone of northern Nigeria (Soil Survey Staff, 1975; Akintunde et al., 2000). Their physical and chemical properties were determined using laboratory methods described by Udo & Ogunwale (1981). The soils were crushed, passed through a 2-mm sieve and 4 kg filled into 32 polyethylene bags for each soil type. A 2 × 2 × 2 factorial completely randomized design with four replications was adopted, giving a total of 64 experimental pots. The treatments used were an absolute control and 100 mg P205/kg soil each of Ogun rock phosphate, ORP (31.4% P205), Sokoto rock phosphate, SRP (34.2% P205) and single super phosphate, SSP (18% P205). Each fertilizer was thoroughly mixed with the soils, watered to 60% field capacity (FC) and allowed to equilibrate for 3 days. Tomato (Lycopersicon esculentum var. Ibadan local) and okra (Hibiscus esculentus var. V-35) were planted as first crop followed by rice and soybean. Two seedlings of the test crops were allowed to grow in each pot for 10 weeks before the aerial portions (shoot, fruits and root) were harvested and weighed. Prior to this, however, the height of plants was measured and the leaves counted at 2, 4 and 6 weeks while flowers and fruits were counted at weekly intervals between the 7th and 10th weeks of growth. In order to compare the residual effects of the P sources in the two soil types, the growth and forage yields of rice (Oryza sativa L.) and soybean (Glycine max) were also evaluated after air-drying and sieving of the first cropping soil, but only for a 7-week-growth-period. The use of rice and soybean was in conformity with the cropping practice of the farmers around the experimental soil locations. The relative agronomic efficiencies (RAE) of the P sources were computed as the ratios of the yield responses with test fertilizer (GRP-ground rock phosphate) to the respective yield responses of the reference (SSP) fertilizer at the same rate (100 mg P205/kg soil), i.e. RAE = [(yGRP – yCONTROL / ySSP – yCONTROL) × 100]%, where y = yield. In all cases, analysis of variance (ANOVA) was performed based on the completely randomized design, CRD (Steel & Torrie, 1981), using the statistical analysis system (SAS, 1985) computer software. Treatment effects and the magnitude of interactions were determined and the F-LSD (Carmer & Swanson, 1971) was used to detect differences between treatment means at 5% significant level. Results and discussion The soils investigated represent a wide range of Nigerian soils with medium to low acidity and P availability. They are soils on which a wide range of food and economic crops such as oil palm, kola nuts, cassava, yam, maize, sweet potato, tomato and okra are cultivated. Soil properties The physical and chemical properties of the soils are given in Table 1. The Typic Paleudalf (soil A) had a higher proportion of sand (712 g/kg) than the Oxic Paleustalf (soil B) with 523 g/kg. On the other hand, soil B had higher silt (354 g/kg) and clay (114 g/kg) contents than soil A. Silt and clay contents of soil A were 214 g/kg and 74 g/kg, respectively. The two soils were, however, sandy loam in texture. Soil A had higher pH than soil B (i.e. 6.3 against 5.9). Soil A also had a higher available P than soil B. Based on the 6–20 mg/kg critical range (Agboola & Adeoye, 1985), soil A could be said to be adequately furnished with available P than soil B.