Springer 2005
Nutrient Cycling in Agroecosystems (2005) 72: 299–308 DOI 10.1007/s10705-005-6081-7
0
Hard Minjingu phosphate rock: an alternative P source for maize production on acid soils in Tanzania M.M. Msolla1, J.M.R. Semoka1 and O.K. Borggaard2,* 1
Department of Soil Science, Sokoine University of Agriculture, P.O. Box 3008, Morogoro, Tanzania; Department of Natural Sciences, Royal Veterinary and Agricultural University, 40 Thorvaldsensvej, DK-1871 Frederiksberg C, Copenhagen, Denmark; *Author for correspondence (e-mail:
[email protected]; fax: +4535282398) 2
Received 26 July 2004; accepted in revised form 16 April 2005
Key words: Agronomic effectiveness, Andisol, Apatite, Grain yield, Oxisol, Triple superphosphate, Ultisols
Abstract The phosphate rock (PR) deposit at Minjingu in northern Tanzania consists of two forms, locally called hard Minjingu phosphate rock, MPR (4.8 · 106 Mg with 10.6% P) and soft MPR (3.3 · 106 Mg with 13.3% P). Extensive chemical and instrumental analyses have shown that the two MPRs differ mainly in consistency, while their reactivities are comparable. Soft MPR in direct application has been extensively evaluated with good results under greenhouse and field conditions whereas hard MPR has only been tested in a greenhouse experiment. The agronomic value of directly applied hard MPR on maize growth was therefore tested under field conditions on four acidic soils low in available Ca and P at Magadu (Ultisol), Mlingano (Oxisol), Nkundi (Ultisol) and Sasanda (Andisol). The treatments tested were hard MPR, soft MPR, triple superphosphate (TSP) and a control. Each P source was applied at a rate of 80 kg ha1 P at Magadu, Mlingano and Nkundi but 160 kg ha1 P at Sasanda. Other nutrient deficiencies were corrected in each soil with appropriate fertilizers that were applied in all three years of experimentation. The soils gave significant positive responses to application of the three P sources but TSP application resulted in significantly higher P concentrations in leaves and grain yields than MPR addition in the first year. However, in the second and third years the performance of MPRs approached that of TSP and the relative agronomic effectiveness (RAE) of MPRs increased from 50 – 70% in the first year to 80 – 95% in year three. Moreover, crop performances on hard MPR-treated plots and soft MPR-treated plots were not significantly different. This important result suggests that hard MPR can replace soft MPR and even TSP on acidic soils low in available Ca and P. However, more testing is needed to ensure confident delineation of soils that respond to direct application of hard MPR from non-responding soils. The rather poor performance of MPR for the first year must also be improved.
Introduction Phosphorus is considered a main limiting plantgrowth factor in highly weathered tropical acid soils (Rajan et al. 1996). These soils are characterized by low total and available phosphorus content, and high phosphate retention capacities
(Friesen et al. 1997; Borggaard and Elberling 2004). In Tanzania, phosphorus deficiency is a widespread fertility constraint of many acid soils (Mnkeni et al. 1991; Szilas 2002), i.e. application of plant available P is essential for sustaining and increasing crop production. Unfortunately, the use of imported phosphate by the resource poor
300 farmers has not been possible due to high prices. However, direct application of local phosphate rock (PR) can reduce dependency on expensive, imported processed fertilizers (Semoka et al. 1992; Butegwa et al. 1996; Sikora 2002). In Tanzania, the PR deposit at Minjingu (MPR) is the most promising for direct application (Semoka et al. 1992; Van Straten 2002). MPR occurs in two forms, locally called soft MPR and hard MPR (Klo¨ckner Industrie 1970; Van Kauwenbergh 1991; Szilas 2002). Both MPRs are sedimentary deposits and contain carbonate apatites but they differ in consistency and fabric as well as in accessory minerals. The estimated reserves of the two MPR forms are 4.8 and 3.3 million tons (Mg) of hard and soft rocks, respectively (Mwambete 1991). Due to its harder consistency, which has been assumed to be synonymous with low reactivity, hard MPR has not been properly tested but merely treated as waste material. Only soft MPR has to date been evaluated extensively for direct application in soils (Semoka and Kalumuna 2000; Szilas 2002). These tests have shown that soft MPR can be an effective P fertilizer of acid soils low in P and Ca (Mnkeni et al. 1991; Semoka et al. 1992; Szilas 2002; Van Straten 2002). Despite hard MPR constitutes about 60% of the deposit at Minjingu, evaluations have been restricted to soft MPR until recently, where a greenhouse experiment showed similar performance of hard and soft MPRs regarding crop yield and P uptake of maize (Msolla et al. 2005). These results need, however, to be confirmed under field conditions with different soils. This paper reports the results obtained in a comparative field study on maize crop responses, i.e. leaf P concentration and grain yield, to direct application of hard and soft MPRs on four different acidic soils covering important agronomic areas in the more humid part of Tanzania. MPR responses were compared with performance after triple superphosphate (TSP) application, and the relative agronomic effectiveness (RAE) of the MPRs calculated.
Materials and methods Sites and soils The investigation was carried out at four different sites in four agro-ecological zones (de Pauw 1984)
in eastern and southern Tanzania (Figure 1). At Magadu the soil is developed on alluvium from the Uluguru Mountains consisting of hornblendepyroxene granulites and mica-bearing quartzofeldspathic gneisses, at Mlingano the soil is formed on pyroxene granulites on the end of Usambara Mountains and the Nkundi soil is developed on mixed schists, gneisses and amphibolites on the peneplain of the Ufipa terrain, while the Sasanda soil is formed on volcanic pumiceous ash most probably from Mt. Rungwe (Szilas 2002). The present soil moisture regime is ustic, and the soil temperature regime ranges from isohyperthermic at altitudes below 1700 – 1900 m above mean sea level (Magadu, Mlingano, Sasanda) to thermic above this altitude (Nkundi). At each site a soil pit was excavated and the soil profile described and sampled according to the FAO guidelines (FAO 1990). Classification of the soils according to the Soil Taxonomy system (Soil Survey Staff 1999) is included in Table 1. The soil samples were airdried and the <2-mm fraction used. Some soil characteristics are shown in Table 1. A 1:2.5 soil:water suspension was used for pH determination. Texture was determined by combined sieving and pipette method after removal of organic matter by concentrated H2O2 (ISRC 1995). Organic carbon was determined by dry combustion at 1250 C in oxygen (ELTRA 1995). Total P was determined by extracting a soil sample heated to 550 C for 1 h with 6 M H2SO4 (Szilas 2002). Available P was determined by the bicarbonate method (Olsen and Sommers 1982). Exchangeable bases (Ca, Mg, K, Na) and CEC at pH 7 was carried out by the silver thiourea method (ISRIC 1995). Duplicate analyzes were performed throughout.
Minjingu phosphate rock (MPR) The Minjingu phosphate deposit is located in the Eastern Rift Valley (Figure 1), near Lake Manyara around the footslopes of a small inselberg that served as resting/nesting place for a large bird colony sometimes during Pleistocene (Szilas 2002). The deposit consists of locally called hard and soft PRs as described above. The two MPRs are formed similarly but due to its lower position in the landscape, soft MPR was submerged in saline Lake Manyara water for longer periods than hard
301
Figure 1. Map of Tanzania showing location of the four test soils and the Minjingu mine together with the physiographic regions according to de Pauw (1984): C: Coastal zone; E: Eastern Plateau and Mountain blocks; H: Southern Highlands; N: Northern Riftzone and volcanic Highlands; P: Central Plateau; R: Rukwa-Ruaha Riftzone; S: Inland sedimentary Plateau; U: Ufipa Plateau; W: Western Highlands.
MPR, while the hard MPR must have been most exposed to weathering. To concentrate and improve the MPRs, they were beneficiated before use. Beneficiation of soft
MPR involves drying, comminution, hand-picking, drying and size classification, while hard MPR beneficiation involves crushing, grinding and size classification. Furthermore, the beneficiated
Table 1. Classification and selected characteristics of the upper horizons of four soils used in the field experiments. Site/horizon/ depth (cm)
Clay (%)
Org.C (%)
pHH2O
Exchangeable cationsa (cmolc kg1) Ca
Magadu: Very-fine, kaolinitic, isohyperthermic Kanhaplic Haplustult Ap (0 – 10) 55 1.2 4.9 2.9 Bo (10 – 47) 65 0.7 4.8 1.4 Mlingano: Very-fine, kaolinitic, isohyperthermic Rhodic Kandiustox Ap 0 – 25) 54 1.2 5.3 0.7 Bto (25 – 52) 66 0.5 5.1 0.8 Nkundi: Fine, kaolinitic, thermic Acrustoxic Kandiustult Ap (0 – 29) 23 1.3 5.5 1.1 Bto1 (29 – 62) 34 0.5 5.2 0.2 Sasanda: Medial, mixed, isohyperthermic Dystric Haplustand Ap (0 – 32) 35 3.4 5.5 1.2 Bw1 (32 – 65) 28 1.5 6.0 0.1 a
Exchangeable Na £ 0.1 cmolc kg1.
PTotal (mg kg1)
POlsen
Mg
K
CEC7
1.7 1.4
0.6 0.2
11.6 15.0
420 220
31 1
0.9 0.7
0.2 0.1
7.3 8.5
180 140
2 1
0.6 0.1
0.2 0.1
4.8 4.3
200 210
3 1
0.6 0.5
0.5 0.5
10.1 10.1
430 250
1 <1
302 MPRs were granulated with water before use. The granulation was done by drum granulation using a 1:1 mixture of MPR and water, followed by drying, crushing and sieving to obtain a final particle range of 0.5 – 1 mm. A previous study of soft MPR (Szilas 2002) has shown no significant difference in agronomic effectiveness between MPR powder and granulated MPR of this granule size used. The granulated MPR was almost free of dust, and hence much more user-friendly than the very dusty non-granulated MPR. Selected characteristics of beneficiated and granulated hard and soft MPR are shown in Table 2. Total P was determined as described in AOAC (1990). The fluoride content was determined by means of the F selective electrode as described by Evans et al. (1970). MPR dissolution in neutral ammonium citrate (NAC) and water was carried out as described in AOAC (1990) measuring dissolved phosphate by means of the molybdenum blue method (Murphy and Riley 1962). Calcium carbonate equivalent and pH were determined as described in ISRIC (1995). The mineral composition of MPRs was assessed by X-ray diffraction analysis using unoriented mounts and CoKa radiation on a Siemens D500 diffractometer Table 2. Selected characteristics of beneficiated hard and soft Minjingu phosphate rock (MPR). Characteristic
Hard MPR
Soft MPR
e
NC PR
f Nauru PR
pHH2O 7.9 8.9 – – Total P (%) 10.6 13.3 13.0 16.9 Ca (%) 24.3 31.0 33.6 37.8 F (%) 1.8 3.5 3.5 2.5 Ca:P-ratio 2.3 2.3 2.6 2.2 F:P-ratio 0.17 0.26 0.27 0.15 CaCO3 (%) 5.7 6.9 – – Si (%) 5.5 3.2 0.8 – a NAC-P (%) 1.8 2.6 2.8 – b H2O-soluble P (%) 0.02 0.05 – – c a-cell (nm) 0.9370 0.9356 0.9328 0.9375 c c-cell (nm) 0.6890 0.6892 0.6888 0.6890 Crystal size (nm) 164 108 250 400 d CO2-index 0.58 0.62 0.59 – Data for soft North Carolina PR and hard Nauru PR are included. a Neutral ammonium citrate-soluble phosphate. b Water-soluble phosphate. c Crystallographic parameter determined by X-ray diffraction. d Determined by FTIR. e North Carolina PR. f Nauru PR data from Smith and Lehr (1966).
equipped with a graphite crystal monochromator. The CO2-index, i.e. the ratio of intensities of the C– O and P–O absorptions, was determined from Fourier Transform Infrared (FTIR) spectra using the KBr pellet method (0.35 mg PR and 300 mg KBr compressed at 150 MPa in an evacuated die) and a Perkin-Elmer FT-IR2000 instrument. Scanning electron microscopy (SEM) using a microprobe technique with energy dispersive X-ray (EDX) was used to obtain information about arrangement, composition, size, shape and texture of mineral constituents.
Field experiments Field experiments were carried out at Nkundi, Sasanda, Magadu and Mlingano (Figure 1) for three years from 1999/2000 to 2001/2002 using maize as the test crop. Year 1 (1999/2000) The experimental fields were ploughed by tractor and fine seedbeds were prepared by hand hoe. Plots of 6 m by 4 m with one meter alley were made to which four treatments were applied, namely control (no P application), hard MPR, soft MPR and TSP. Only one rate of P was tested, that was 80 kg P ha1 at Nkundi, Magadu and Mlingano but 160 kg P ha1 at Sasanda because this soil has a very high P fixation capacity (Szilas 2002). The treatments were replicated four times and arranged in a complete randomized block design at all sites. To ensure optimum of nutrients, deficient nutrients were applied to all soils as required. Application rates at the different sites were as follows: potassium (100 kg ha1) as K2SO4 was applied at Nkundi, while zinc (10 kg ha1) as ZnSO4Æ7H2O and copper (5 kg ha1) as CuSO4Æ5H2O were applied at Nkundi and Sasanda together with a first split of nitrogen (15 kg ha1) as (NH4)2SO4 at all sites during planting. The other fertilizers were broadcasted and thoroughly mixed in each plot by hand hoe. It has been found that for PRs to be most effective, they should be broadcasted onto the soil surface and incorporated into the soil (Khasawneh and Doll 1978). Maize (Zea mays L.) was planted at a spacing of 75 cm between rows and 30 cm within rows with maize seeds being put approximately 5 cm deep. Planting
303 was done on the same day as the fertilizers were applied in order to optimize plant competitiveness for P. Two or three maize seeds (variety TMV2+145 for Nkundi and Sasanda and variety staha for Magadu and Mlingano) were sown. Thinning was carried out one week after emergence leaving one plant per hole giving 120 plants per plot. Three additional doses of nitrogen (30, 40 and 15 kg N ha1) were applied 30, 58 and 80 days after planting at Sasanda, after 30, 50 and 80 days at Nkundi and after 30, 55 and 65 days at Mlingano and Magadu. Routine management practices such as weeding, pest control were carried out as need arose. Plant leaf analysis and grain harvesting: Ear leaves of the four inner rows of maize plants were sampled when plants have attained almost 50% flowering or at silking stage with five plants sampled per row giving 20 leaves per plot. Plant sampling at Mlingano and Magadu sites was carried out at 55 days from the planting date, whereas Sasanda and Nkundi sites were sampled 90 days after planting because of a slightly cooler climate. The leaves were cleaned with a moist cloth and dried at 65 C to constant weight. After grinding in a Cyclotec 1093 mill, the samples were analyzed for P after digestion using a dry ashing procedure (Chapman and Pratt 1961). Phosphorus in the digests was measured by the molybdenum blue method as described by Murphy and Riley (1962). Maize was harvested after 150 days from planting at Mlingano and Magadu and 210 days after planting at Nkundi and Sasanda. Grain yields were calculated for a net plot size of 17.1 m2 and expressed in tons ha1 at 12.5% moisture content. Crop residues for each plot were evenly spread over the plot and were ploughed under before planting next crop.
was applied at Mlingano. Due to nitrogen deficiency symtoms seen the first year, the N doses were changed to 25 (during planting), 30, 30 and 15 kg N ha1 applied at approximately same intervals as in the first year. The maize variety grown at Magadu and Mlingano was changed to TMV-1 so as to minimize moisture stress which was experienced during parts of the critical growth period in the first year as TMV-1 has a shorter growing period (approximately 110 days) as opposed to 120 days for staha. Plant leaf analysis and grain harvesting were the same as described for the first year. Crop residues were left in the plots and ploughed under before planting next crop.
Year 2 (2000/2001) The same plots used during the first year of the experiment were used in the second year. Phosphorus and other deficient nutrients were applied at the same rates as in the first year. Plant analysis made in the first year identified some other deficient nutrients which were corrected for the second year as follows: magnesium (28.2 kg ha1) as MgSO4.7H2O and boron (2 kg ha1) as Na2B10O16Æ10H2O were applied at Sasanda, zinc (10 kg ha1) as ZnSO4Æ7H2O was applied at Magadu and potassium (100 kg ha1) as K2SO4
Properties of the soils
Year 3 (2001/2002) The same plots used in the previous two years were again used in year 3 and all agronomic and fertilizer application strategies were as in the first/second year, except for the following necessary changes: at Nkundi, maize variety hybrid 6304 was used instead of TMV-2+145, which was not available and magnesium nitrate (as Mg(NO3)2Æ6H2O applied at a rate of 50 kg/ha) was used at Sasanda as magnesium sulphate was not available. Plant leaf analysis and grain harvesting were the same as described for the first year. Statistical analysis The MSTATC statistical package was used for analysis of variance and comparison of means for leaf P concentrations and grain yields was done by Duncans New Multiple Range Test (DMRT).
Results and discussion
The four soils used in the study are highly weathered and low to very low in essential plant nutrients (Table 1). Only data for the upper 50 – 60 cm of the soil profiles are shown in Table 1 as almost all roots occurred in this layer; subsoil data down to 180 cm can be obtained from the authors on request. The soils are strongly acidic to slightly acidic with pH in the various horizons ranging from 4.8 to 6.0 with increasing pH down the profiles. The clay contents are high in the Magadu
304 and Mlingano soils and moderate in Nkundi and Sasanda soils. The Magadu and Nkundi soils have clay-enriched subsoils (Borggaard and Elberling 2004) and very low base saturations (BS7, i.e. base saturation on the basis of CEC at pH 7) in the B horizons resulting in their classification as Ultisols. Although the Magadu and Mlingano soils seem very similar with low BS7, comparable clay contents and substantial clay increases from A to B horizons, their classifications are different, Magadu is an Ultisol while Mlingano is an Oxisol. The classification difference is due to different CECs of the clay fractions in the B horizons, the Mlingano CEC is smaller whereas the Magadu CEC is larger than the critical limit of 16 cmolc kg1 of clay (Soil Survey Staff 1999), although both soils have a kaolinitic mineralogy class (Table 1). Exchangeable Ca is very low in all four soils and decreases with soil depth, whereas exchangeable Mg is rather high in Magadu, moderate in Mlingano and Sasanda and very low in Mlingano. Except for the Ap horizon in Magadu, all nonvolcanic soils are low to very low in exchangeable K. In contrast, the soil on volcanic material (Sasanda) contains substantial amounts of available K, and hence a high K:Mg ratio, which may lead to K-induce Mg deficiency in the plants (Marshner 1995), and can explain the observed necessity to add magnesium fertilizer to the Sasanda soil. Although all soils have substantial total phosphorus contents, available phosphate is low to very low, especially in subsoils as assessed by the Olsen method (Table 1) indicating severe P deficiency in these soils. The phosphate adsorption capacities of Ap horizons (not shown) exhibit a rather broad range from low in the Nkundi soil to very high in the Sasanda soil (Szilas 2002). The high phosphate adsorption capacity of the Sasanda soil is due to its volcanic parent material and is consistent with very low available P in this soil. The soils seem suitable for testing effectiveness of direct application of PRs as they have low but different pH values, as well as low to very low exchangeable Ca and available phosphate, which are the soil properties that accelerate PR dissolution rate, and hence PR effectiveness as P fertilizer (Kanabo and Gilkes 1987; Bolan et al. 1990; Rajan et al. 1996; Van Straaten 2002; Guidry and Mackenzie 2003).
Characteristics of the MPRs The MPRs differ in physical properties. While soft MPR has a fine texture and friable consistence, hard MPR is massive and occurs as sandy conglomeratic and silicified rock. The SEM/EDX analyses showed soft MPR to consist of a very porous matrix of more or less altered bone fragments partly surrounded or imbedded in a mass of clay. In addition, quartz, K-feldspar, calcite and dolomite were identified as accessory minerals in soft MPR. Hard MPR consists of minerals grains (quartz, various feldspars) and bone fragments embedded in a matrix of clays, calcite and colloidal silica resulting in the cemented, hard and conglomeratic appearance. A comparison of bone fragments from the two ores showed those from hard MPR to be more changed than bone fragments from soft MPR indicating a more advanced weathering stage of hard MPR in accordance with its more exposed (higher) position in the landscape. SEM/EDX data as well as FTIR and X-ray spectra/diffractogrammes are not shown here but may be seen elsewhere (Szilas 2002). Chemical, mineralogical and reactivity characteristics of hard and soft MPRs are shown in Table 2 together with similar data for a ‘standard’ soft (reactive) PR from North Carolina and a ‘standard’ hard (unreactive) PR from Nauru. The dominant phosphate minerals of these guano-derived deposits are carbonate fluorapatites (francolites); the phosphate in hard MPR is carbonate substituted fluorine deficient francolite, while that of the soft ore is carbonate substituted fluorine excess francolite (McClelland and Van Kauwenbergh 1990; Szilas 2002). The data in Table 2 indicate, however, anomalous behavior of the MPR francolites, especially that of the soft ore. The high CO2-indices corresponding to a high degree of carbonate substitution are in contrast to the a-cell values, which indicate very low carbonate substitution. Moreover, the high a-cell values indicate very low reactivity, which is contradicted by the measured high NAC solubility. In fact, as the efficiency of NAC is partly ‘masked’ by the CaCO3 (Bolan et al. 1990), the solubility of the MPRs would undoubtedly be much higher if tested by the commonly used acidic 2% citric acid or 2% formic acid extraction methods. Furthermore, except for the a-cell the MPRs resemble the very reactive North Carolina PR
305 (Table 2) indicating that hard and soft MPR can be categorized as medium and highly reactive, respectively. The small crystal size of the MPR apatites, which is even smaller than that in North Carolina PR, is noteworthy in relation to reactivity (Borggaard and Elberling 2004). High reactivity of soft MPR, and hence its suitability for direct application as P fertilizer, has been confirmed in numerous field and laboratory studies (Semoka and Kalumuna 2000; Szilas 2002). In contrast very little has been done with hard MPR (Msolla et al. 2005), although the similarity between hard and soft MPR in chemical and structural characteristics suggests that direct application of hard MPR may be a promising alternative to use of soft MPR.
Maize yields and leaf P concentrations Results obtained from the first year experiment showed that all P sources produced significantly (P £ 0.05) higher grain yields than control plots (Table 3), which clearly demonstrates the beneficial effect on crop production of P fertilization of all four soils. The P concentrations in leaves from the control plots ranged from 0.05 to 0.16% of dry matter, which is considerably lower than the range of 0.3 – 0.5% for optimum plant growth (Marschner 1995). However, even after addition of rather high P doses (3 · 80 kg ha1 to the Magadu, Mlingano and Nkundi soils and 3 · 160 kg ha1 to the Sasanda soil) of MPR or TSP over three years, the P concentrations in leaves are still below the optimal lower limit of 0.3% of dry matter. This indicates that continued P application is needed to completely alleviate P deficiency on these soils with very low inherited available P pools (Table 1) and moderate to very high phosphate adsorption capacities (Szilas 2002). In the first year, TSP gave significantly (P £ 0.05) higher grain yields and P concentrations in leaves than both MPRs on all soils (Table 3). TSP application resulted in P concentrations from 0.20 to 0.23% compared to 0.13 – 0.20 for the MPRs and yields ranging from 2.0 to 3.0 tons ha1 for TSP compared to 1.8 – 2.6 ton ha1 for MPRs. The different responses to TSP and MPRs can undoubtedly be ascribed to faster dissolution of TSP compared to MPRs resulting in more readily available P after TSP than after MPR
Table 3. Effects of the various treatments on P concentrations in maize leaves and grain yields at the four test sites over the three years of testing. Site
Leaf P concentration (%)*
Grain yield (Mg ha1)*
Year 1 Year 2 Year 3 Year 1 Year 2 Year 3 Magadu Control TSP Soft MPR Hard MPR Mlingano Control TSP Soft MPR Hard MPR Nkundi Control TSP Soft MPR Hard MPR Sasanda Control TSP Soft MPR Hard MPR
0.163c 0.224a 0.205b 0.205b
0.161c 0.225a 0.220a 0.219a
0.155c 0.220a 0.208b 0.209b
1.507c 2.025a 1.782b 1.778b
1.642c 2.590ab 2.442b 2.465b
0.963c 2.622a 2.280b 2.300b
0.143c 0.213a 0.192b 0.192b
0.150b 0.214a 0.207a 0.210a
0.171b 0.217a 0.215a 0.214a
1.155c 2.438a 1.810b 1.870b
2.027b 3.997a 3.645a 3.505a
0.235c 1.615a 1.393b 1.413b
0.135c 0.225a 0.193b 0.191b
0.160b 0.265a 0.250a 0.251a
0.205b 0.243a 0.241a 0.241a
1.412c 3.007a 2.438b 2.357b
3.247b 4.482a 4.352a 4.325a
2.553b 5.665a 5.402a 5.467a
0.052c 0.201a 0.139b 0.132b
0.104b 0.210a 0.196a 0.193a
0.138b 0.183a 0.180a 0.178a
0.450c 3.435a 2.540b 2.403b
0.443b 5.543a 4.778a 4.709a
0.638b 5.893a 5.543a 5.402a
* For each site and each year, leaf P concentrations and grain yields followed by the same letter are not significantly different at P < 0.05%.
application. Accordingly, several studies have shown that water-soluble P sources dissolve faster than PRs leading to higher soil solution concentration, and hence plant uptake of phosphate (Khasawneh and Doll 1978; Smyth and Sanchez 1982; Rajan et al. 1996). However, already in the second year, P concentrations in leaves and grain yields are not significantly different on plots treated with TSP compared to those treated with MPRs at most sites. This tendency continued and after three treatments (year 3); the differences between performance of TSP and MPRs are no longer significant (Table 3).
Relative agronomic effectiveness The RAE increases over time approaching 100% in year 3 (Table 4). RAE is the plant response (grain yield) to MPR (Ypr) divided by plant response to TSP (Ytsp), both corrected for control response (Yc):
306 Table 4. Relative agronomic effectiveness (RAE) of hard MPR (HMPR) and soft MPR (SMPR) for maize grain yields from the field experiments. Site
Year 1
Year 2
Year 3
SMPR HMPR SMPR HMPR SMPR HMPR Magadu Mlingano Nkundi Sasanda Mean
53.1 51.1 64.3 70.6 59.8
52.3 55.7 59.2 65.4 58.2
RAE ¼
84.4 82.1 89.5 85.0 85.3
86.9 75.0 85.4 83.6 83.2
79.3 83.9 91.5 93.3 87.0
80.6 85.4 93.6 90.7 87.6
Ypr Yc 100 Ytsp Yc
The RAE improved as P application was repeated in the second and third year but the magnitude of change was higher between first and second year than between the second and third year. The increase in RAE suggests that repeated applications of the MPRs were necessary to build up available P reserves to adequate levels as also indicated by the less than 0.3% of P in leaves (Table 3), which is the lower limit of optimum plant growth (Marschner 1995). The little change in RAE between second and third year suggests that the equilibrium level of P under the prevailing conditions was being approached (Khasawneh and Doll 1978; Rajan et al. 1996). The mean RAE ranged from 58.2 – 59.8%; 83.2 – 85.3% and 87.0 – 87.6% for the first, second and third year, respectively. The first year RAE values for grain yield suggest that both soft and hard MPRs would be ranked as having low agronomic effectiveness according to Hammond and Leon (1983). However, the NAC solubility data (Table 2) suggest that hard and soft MPR are of medium and high reactivity, respectively. The discrepancy between chemical reactivity and agronomic effectiveness is thought to be due to slow dissolution of the PRs, which meant that equilibrium concentrations were reached latter in the growing season. This means that plants in the PR treatments might have suffered P stress during early vegetative phase when P demand was higher than supply. There was no significant difference in RAE for grain yield between soft and hard MPR in all the three years (Table 4) suggesting that the two
PRs had comparable agronomic effectiveness. This is further supported by the comparable NaOH–P and available P values (Bray 1-P and Pi-P) obtained after harvesting the first and second crops in a greenhouse experiment (Msolla et al. 2005).
RAE and soil properties The RAE for grain yield varied slightly among soils. In year 1 the highest average RAE (68%) was found in Sasanda soil, followed by Nkundi soil, while Mlingano and Magadu soils had the lowest RAEs of around 53%. A similar trend was observed during the third year during which Sasanda and Nkundi soils had comparable RAE of around 92%, which was slightly higher than the RAE for Mlingano and Nkundi soils. These data tend to suggest that the PRs had higher efficiency in Sasanda and Nkundi soils. However the two soils contrasted sharply in terms of chemical properties (Table 1) related to P release from PRs and its utilization by plants. Sasanda soil had very high P fixing capacity, while Nkundi soil had low P fixing capacity (Szilas 2002). Of the four soils used in this study, Magadu soil had the most favourable properties for P release from PR namely very low pH, low available P and medium P fixing capacity. Yet this soil had the lowest RAE. Since the study was done under field conditions, factors such as climatic conditions might have influenced RAE of the PRs as well. For instance moisture stress was frequently experienced at Mlingano and Magadu during part of the cropping seasons (Szilas 2002). This might have limited PR dissolution and availability of the dissolved P to plants hence leading to lower RAE values relative to the other two sites with better moisture conditions. Thus, amount and annual distribution of rain is an important factor for the use of PRs as P fertilizers in direct application (Bolan et al. 1990). Nevertheless the data indicate that all the soils were suitable for direct application of the MPRs as they are more or less acidic with low to very low available Ca and P contents (Table 1), which are the soil characteristics that accelerate PR dissolution, and hence P fertilization performance (Kanabo and Gilkes 1987; Rajan et al. 1996; Van Straaten 2002; Guidry and Mackenzie 2003).
307 Perspective The almost equal performances of hard MPR and soft MPR, which have now been demonstrated in pots experiments (Msolla et al. 2005) and field trials, suggest that the currently unutilized hard MPR, which constitutes 60% of Minjingu phosphate deposit can now be utilized in direct application as P fertilizer. Furthermore, since the RAE approached 100% after 3 years, hard MPR (as well as soft MPR) can substitute TSP in the long run. In addition to the P fertilizer effect, application of MPRs and other PRs improves the soil by increasing pH, i.e. the liming effect (Sikora 2002), by decreasing Al toxicity and by supplying Ca (and Mg) as indicated by MPR composition in Table 2. However, in order to improve the initial fertilizer effect of hard MPR, compacted compounds of MPR and TSP or co-granulation of MPR with TSP or other water-soluble fertilizers should be tested in order to improve first-year performance of MPR. This kind of testing is very much needed as are comprehensive field experiments designed to clearly delineate soils that respond by increased yields to direct application of hard MPR from non-responding soils, in order to ensure future sound and reliable advisory of farmers. The suitability for use with other crops than maize should also be tested as the agronomic value of PRs in direct application depends on the crop (Bolan et al. 1990). Furthermore, improvement of the beneficiation technique is needed to increase the P content from the current value of 10.6% to a level comparable to that in other PRs (Table 2).
Conclusions In-field maize production on the four acid test soils situated in more humid Tanzania significantly improved after application of TSP as well as hard and soft MPRs. On all soils and during all three years, crop performances after application of hard MPR and soft MPR were not significantly different. Accordingly, the main difference between hard and soft MPR appears to be different consistency. Although the MPRs were inferior to TSP in the first year, the RAE improved and approached 100% during the following two years. Consequently, hard MPR can replace soft MPR (and
TSP), which more than doubles the potential of the Minjingu deposit, where hard MPR constitutes 4.8 · 106 Mg with 10.6% P and soft MPR 3.3 · 106 Mg with 13.3% P. To benefit from direct application of hard MPR (and soft MPR), soils must be acidic and P-deficient but clear delineation between responding and non-responding soils awaits future studies.
Acknowledgement Financial support from Danida (Danish Development Agency) is greatly acknowledged.
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