Weil: Soil–plant Influences on Phosphate Rock Efficacy

of nutrients, especially phosphate rock (PR) in subSahara Africa (McClellan, 1989; Buresh et al., 1997). Affordable technologies are needed to allow smallholder farmers Substantial deposits of PR in many sub-Sahara Afrito effectively use the phosphate rocks (PRs) found in many African countries. A pot study was conducted in Tanzania using two PRs can countries are too low-grade to justify commercial (Panda and Minjingu) and two soils (an Alfisol and an Andisol) to production of refined fertilizer products, but could be assess responses of several types of crops to these PRs and to determined and ground using labor intensive methods with a mine whether changes in crop responses to PR with time are due to minimum of capital expenditure to produce inexpensive crop sequence or merely contact time with soil. The Panda PR had sources of potentially useful nutrients (Sheldon, 1982). no effect on growth or tissue P content in maize (Zea mays L.), bean However, since many of the PR deposits in sub-Sahara (Phaseolus vulgaris L.), and pigeon pea [Cajanus cajan (L.) Millsp.], Africa are both low grade and low reactivity (Kurtanjek but it nearly tripled these parameters for cabbage (Brassica oleracea and Tandy, 1984), untreated PR often does not produce L.) on the Alfisol. Freshly applied Minjingu PR only slightly stimua plant growth response, even on acid soils (Lüken and lated maize and pigeon pea, but nearly tripled cabbage yield in both Blümel, 1984; Ngatunga et al., 1989). soils. Previous crop had a greater effect than previously applied PR on second crop maize. Yields and P content of maize were always Tanzania has two principal deposits of PR that could lowest following cabbage and highest following pigeon pea. Minjingu potentially be used to boost agricultural production and PR, but not Panda PR, had residual benefits on maize. Severe Mn enhance food security in Tanzania and neighboring toxicity occurred in all crops on the unamended Andisol. The calcarecountries (Harris, 1961). In northeast Tanzania, along ous Minjingu PR, but not the Panda PR, increased yields dramatically the shores of Lake Manyara, a sedimentary PR occurs on the Andisol, partly by raising the soil pH in water enough (from at Minjingu Hill. This material has been mined sporadi4.6 to 5.6) to alleviate Mn toxicity. Future P fertility work in Africa cally with and without various degrees of beneficiation should pay adequate attention to the effects of crop sequences and since the early 1960s. After removing clay impurities, soil biological properties. the Minjingu PR has a total P content of 115 to 160 g kg21 and a high content of carbonates. It is considered a reactive PR because of its high solubility in citric acid T densely settled humid and subhumid regions in and moderate solubility in neutral ammonium citrate East Africa are suffering high rates of soil fertility (Table 1). Minjingu’s location relative to Tanzanias depletion (Smaling et al., 1993). These areas are domitransport infrastructure makes it exportable to Kenya nated by Alfisols, Oxisols, and Andisols—the productivand Uganda as well as available for use in the northern ity of which is commonly limited by deficiency of P (Weil part of Tanzania. et al., 1991; Jama et al., 1997). Because the majority of Panda Hill near Mbeya in southern Tanzania contains African farmers cannot afford to adequately fertilize a residual PR derived from igneous rock, a carbonatite, their cropland with imported manufactured fertilizers, with a total P content of about 46 g kg21 in the raw ore there is much interest in developing indigenous sources and up to 85 g kg21 in ore that has been concentrated by simple magnetic removal of magnetite. The Panda Department of Natural Resource Sci. and Landscape Arch., Univ. of PR is well located for distribution throughout the wellMaryland, College Park, MD 20742-4452. Received 13 Dec. 1999. *Corresponding author ([email protected]). Abbreviations: PR, phosphate rock; TSP, triple superphosphate; AM, arbuscular mycorrhizae. Published in Agron. J. 92:1167–1175 (2000). 1168 AGRONOMY JOURNAL, VOL. 92, NOVEMBER–DECEMBER 2000 Table 1. Chemical composition of Minjingu and Panda phosphate be important because taking this delay into account rocks. The data are for various samples of PR from the batches might lead to more efficient use of PR in cropping sysof ground rock used in the pot study. Except for total P content, tems by planting a high-P demanding, staple crop later the data were determined by Kamasho et al. (1992), Floor and in the rotation after applying the PR. Also, since poor Kimambo (1989), and IFDC (personal communication, 1994). farmers look for an immediate return on their investConcentration ment, this delayed action of PR could discourage them Parameter Minjingu Panda from using even relatively reactive PRs. It has generally g kg21 been assumed that the increased effectiveness of PR Total P 150 80 with time in acid soils is due to a slow reaction by which Neutral ammonium citrate soluble P 13 7 soil acids dissolve a certain fraction of the P from the Citric acid–soluble P 60 22 Ca 330 19 PR over a period of several months to years (Sanchez Mg 20 8 and Salinas, 1981; Rajan et al., 1996). However, observaNa 6 14 tions from some pot studies (Mughogho and Weil, unK 11 30 Si 48 224 published data, 1993) have indicated that the PR beneF 28 17 fits a second crop much more than the first crop, even Al 12 80 Fe 7 87 if the first crop was grown for only about 1 mo. An Carbonate as CO2 53 32 alternative hypothesis is that improved effectiveness over time is related to the action of the particular plants watered and densely settled Southern Highlands region growing in the soils, and not simply to the time of of Tanzania, where soils are generally acid and deficient soil–PR contact. in P (Chesworth et al., 1988). The solubility of Panda Most studies of crop response to direct application PR (Table 1) is much lower than that of Minjingu PR, of PR in Africa have used maize as the test crop (e.g., and the iron content is much higher (van Straaten et Bromfield et al., 1981; Mnkeni et al., 1991; Buresh et al., 1992). Yield responses to Panda PR, relative to triple al., 1997). However, research has shown that some superphosphate (TSP), have been even more erratic plants are better able than others to use PR as their than to Minjingu PR (Kamasho et al., 1992; van Straaten source of P (Bekele et al., 1983; Flach et al., 1987; Floor et al., 1992). Panda PR generally failed to increase crop and Kimambo, 1989; van Diest, 1991; Haynes, 1992). yields when used in direct application field trials on acid Further, arbuscular mycorrhizae (AM) infection is well soils of the region (J.A. Kamasho, Uyole Agriculture known to greatly improve P uptake by many plants in Research Station, personal communication, 1994). Parlow P soils (Islam et al., 1980; Flach et al., 1987). Crop tial (50%) acidulation of the Panda PR significantly strategies for P uptake may be grouped into four types. improved its effectiveness in field trials (van Straaten First, many monocots have extensive fibrous root syset al., 1992); however, the partial acidulation process tems, generally with AM infection, that allow these may be nearly as costly as importing TSP (Sanchez and plants’ roots to have very intimate contact with PR Salinas, 1981). Therefore, direct application of PR particles in the soil, and may give them some advantage would be much preferred, if it can be done in a way in PR use (Menon et al., 1995). Second, despite a less that effectively alleviates crop P deficiencies. extensive root system, annual legumes have also been An intriguing phenomenon reported in many longershown to use PR efficiently (Aguilars and van Diest, term experiments is a delay in crop response to PR such 1981; Bekele et al., 1983). Because of their N2 fixation that the first crop grown after PR application gives little ability, legumes generally use such low amounts of nior no response, but the second and third crops respond trate that the plants take up an excess of cations over well (Anderson, 1965, 1970; Bromfield et al., 1981; anions. This imbalance leads to an excretion of H ions, Gichuru and Sanchez, 1988; Kimbi, 1991; Haque and which acidify the rhizosphere and release soluble P from Lupwayi, 1998). For example, Bromfield et al. (1981) Ca phosphates such as found in PR. Third, pigeon pea used Minjingu PR at 50 kg P ha21 and found a nonsignifi[Cajanus cajan (L.) Millsp.] has been shown to excrete cant 29% increase in maize (Zea mays L.) yield compiscidic acid, which specifically complexes with Fe to pared with the control in the first harvest after PR appligreatly increase the availability of Fe-bound P (Ae et cation, but there was a significant 73% increase in the al., 1990). Fourth, it is well known that crops in the third harvest. Understanding such a delayed response to PR would Cruciferae family, such as rape (Brassica napus L.) and Table 2. Selected properties of the two soils used in the pot experiment. Both soils were sampled from the Ap horizon (0–15 cm). Water Soil Bulk holding Total Total Bray 1 Total DTPA Langmuir# Langmuir# name Suborder Clay Sand density capacity C† N† pHw pHKCl CEC‡ P Mn§ Mn¶ 1/b k g kg21 Mg m23 L L21 g kg21 cmolc kg21 mg kg21 Ihanda Kandiustalf 300 630 0.97 0.35 22.1 1.4 6.3 5.60 18.4 3.