The interpretation of Olsen extractable P values in relation to recommendations for the fertilizer requirements of crops

A greenhouse experiment, with Okra (Abelmoschus esculentus L.) as the test crop, was conducted on twenty-one soils ranging in Olsen's extractable phosphorus from 1.8 to 15.5µg Pg^–1 soil. The experiment was conducted at Punjab Agricultural University, Ludhiana, India. The soils were nonsaline with pH ranging from 7.7 to 8.6. A critical level of 2.55µg Pg^–1 soil was predicted by Cate and Nelson's (1971) statistical procedure. Because of a wide range in relative yields, this value did not accurately predict response to applied P. An approach to compute minimum response to applied fertilizer, which is likely to be obtained at a particular Olsen P level, has been presented. It involves calculation of lower 60 percent confidence limits for relative yield and fitting log_e-linear regression to the transformed data. The regression was tested on a published data set and was found to hold well.     

Critical levels for soil pH,available P,K, Zn and Mn and maize ear-leaf content of P,Cu and Mn in sedimentary soils of South-Western Nigeria

In the sedimentary soils of South-western Nigeria, actual and expected relative yields of maize were plotted against soil physical factors, soil avalilable nutrients and ear-leaf content of maize. These were used to set critical ranges of these factors for optimum production. Regression equations were obtained for each of the soil and plant factors for predicting yield, thereby making possible yield prediction with levels of each of these factors in these soils if all other factors are constant.The critical range concept combined with the soil physical and chemical properties and plant nutrient content could be a useful diagnostic tool for soil ammendment in crop production. Critical ranges were set as follows:pH, 6–6.5; available P (Bray's Pl), 10–16 mg Kg^–1; Exchangeable K, 0.6–0.8 me K100g^–1; available Zn, 5–10mg kg^–1; available Mn, about 25 mg Kg^–1; Ear-leaf P, 2.5–3.0%; Ear-leaf Cu, 10–20 mg Kg^–1; Earleaf Mn, about 50 mg Kg^–1.     

Evaluation of chromium release during the decomposition of leather meal fertilizers applied to the soil

Greenhouse studies of 14 soils, having a range in DTPA extractable Mn, were made to determine the critical deficiency level of Mn in ustochrepts for predicting response of green gram to Mn application. Soil Mn was significantly related with Bray's per cent dry matter yield (r = 0.68^**). Soil application of 20 mg Mn kg^–1 soil significantly increased the yield. Both graphical and statistical models of Cate and Nelson indicated the critical level to be 2.9 mg kg^–1 soil of DTPA extractable Mn. The critical deficiency level in youngest matured terminal leaf (YML) of 40 day green gram plants was 19.0µg g^–1. The predictability of soil and plant critical Mn level was 93 per cent.     

Critical levels of Mn in coarse textured rice soils in India for predicting response of barley to Mn application

Green house studies of 20 soils, having a range in DTPA extractable Mn, were made to determine the critical deficiency level of Mn for predicting response of barley to Mn application. Soil Mn was significantly related with both Bray's per cent dry matter yeild (r = 0.70^**) and Mn uptake (r = 0.65^**). Soil application of 25 mg Mn kg^–1 soil significantly increased yield. Both graphical and statistical models of Cate and Nelson indicated the critical level to be 2.05 mg kg^–1 soil of DTPA extractable Mn. The critical Mn deficiency level in 45 day barley plants was 18.6 mg kg^–1 dry matter. The predictability of soil and plant critical Mn level was 91 and 80 per cent respectively.     

An attempt to evaluate phosphorus fertilizer requirements of western Nigeria savannah soils

A greenhouse fertilizer trial was carried out on 60 surface soils of the western Nigeria savannah derived from basement complex rocks. Bray's P₁ available P in the soils varied between 1 and 112µg ml^–1. There was maize response to P addition and a critical P level of 12.7µg ml^–1 was calculated for the soils.For 22 of the soils, a laboratory incubation technique was used in evaluating changes in Bray's P₁ extractable P at various rates with time. The initial rapid decline in soil available P was completed between 28 and 84 days of incubation. A fertilizer factor, calculated from extracted P in treated and untreated soils varied between 1.5 and 16.7µg ml^–1 and was significantly correlated with soil pH and citrate-dithioniteextractable oxides of Fe and Al.Fertilizer rates based on critical soil P, available soil P and fertilizer factor, correlated significantly with greenhouse estimates for optimum yield obtained with the linear response plateau model (r = 0.91,p < 0.001). At ten field locations varying in available P content, response was only to P applications lower than 60 kg ha^–1 and the calculated P rates using a mean fertilizer factor of 3.0µg ml^–1 corresponded to P rates at which maximum yields were obtained in the sites.     

Critical manganese deficiency level for soybean grown in Ustochrepts

A greenhouse study with 15 soils, having a range in DTPA extractable Mn, was conducted to determine the critical deficiency level of Mn in Ustochrepts for predicting response of soybean to Mn application. Soil application of 10 mg Mn kg^–1 soil significantly increased the dry matter yield in deficient soils. Soil Mn was significantly related with Bray's per cent yield (r = 0.72^**) and Mn uptake (r = 0.75^**). Both graphical and statistical models of Cate and Nelson indicated the critical level to be 3.3 mg kg^–1 soil of DTPA extractable Mn. Critical Mn deficiency level in recently matured terminal leaflet blade at V6 growth stage in soybean plant was 22.0µg g^–1 dry matter. The predictability of soil and plant critical Mn level was 87 per cent.     

Critical level of DTPA extractable Zn for wheat in alkaline soils of semiarid region of Punjab,India

Field experiments were conducted at 32 locations, chosen for their wide range in DTPA extractable Zn, to determine the critical deficiency level of Zn for predicting response of wheat to Zn application. Soil application of 5.6 kg Zn ha^–1 significantly increased the grain yield in deficient soils. Soil extractable Zn was significantly related with per cent grain response and absolute grain yield. Both the graphical and statistical methods of Cate and Nelson indicated the critical level to be 0.75 mg kg^–1 soil of DTPA extractable Zn. This level gave a predictability value of 82 per cent.     

Response of chickpea (Cicer arietinum) to zinc fertilization and its critical level in soils of semi-arid tropics

In a greenhouse experiment the response of chickpea (Cicer arietinum) to zinc fertilization was examined using 27 soils from the semi-arid tropics. The critical level of DTPA extractable soil Zn was evaluated. Zinc additions to the soil increased the dry matter yield of six weeks old plant shoot, grain and straw significantly at the 5 mg kg^–1 level, but tended to decrease it at the 10 mg kg^–1 level.The DTPA extractable Zn of the soils ranged from 0.28 to 1.75 ppm and was negatively correlated at 1 per cent level with pH (r = – 0.81) and positively with organic carbon (r = 0.79) and Olsen's P (r = 0.63). The per cent yield increase or decrease over zero zinc ranged from 67 to – 16 in respect of grain yield and was positively correlated with available Zn (r = 0.86^**). Zinc concentration in plants was greatly increased with the application of Zn and accumulation of Zn was higher in grain than straw. The critical level of available zinc in soil below which plant response to Zn fertilization may be expected was 0.48 mg Zn kg^–1 soil. Soils between 0.48 to 0.70 mg kg^–1 of DTPA extractable Zn appear boarderline and a negative response to applied Zn was observed in soils of high Zn category. The results show the suitability of DTPA soil test for demarcating soils on the basis of plant response to zinc fertilization.     

Phosphate sorption approach for determining phosphorus requirements of wheat in calcareous soils

Five field experiments involving P application rates from 0 to 66 kg P ha^–1 were conducted on irrigated wheat at Tandojam, Pakistan. The soils belonged to two great soil groups, Torrifluvent and Camborthid. All soils were calcareous. Olsen-P contents ranged from 3.5 to 6.3 mg P kg^–1. Phosphate sorption curves were developed for soils from control (no P) plots at each site. Concentrations of P in solution established by fertilization in the field as estimated from the sorption curves ranged from 0.008 to 0.16mg P L^–1. Actual grain yields were converted to relative grain yields and plotted against corresponding concentrations of P in solution. Yield response to P application was obtained in each experiment. Control plot yields ranged from 57 to 89% of maximum yield of respective experiments. Phosphorus requirements of wheat were 0.032 mg L^–1 for 95% yield as determined from a composite yield response curve. Predicted quantities of P required to attain 0.032 mg P L^–1 ranged from 18 to 29 kg P ha^–1. The results of the study suggest that the P sorption approach can be used as a rational basis for making P fertilizer recommendations for various soil-crop combinations.     

Fertilizer requirement of rice-wheat and maize-wheat rotations on coarse-textured soils amended with farmyard manure

Field experiments with rice-wheat rotation were conducted during five consecutive years on a coarse-textured low organic matter soil. By amending the soil with 12t FYM ha^–1, the yield of wetland rice in the absence of fertilizers was increased by 32 per cent. Application of 80 kg N ha^–1 as urea could increase the grain yield of rice equivalent to 120 kg N ha^–1 on the unamended soil. Although the soil under test was low in Olsen's P, rice did not respond to the application of phosphorus on both amended and unamended soils. For producing equivalent grain yield, fertilizer requirement of maize grown on soils amended with 6 and 12 t FYM ha^–1 could be reduced, respectively to 50 and 25 per cent of the dose recommended for unamended soil (120 kg N + 26.2 kg P + 25 kg K ha^–1). Grain yield of wheat grown after rice on soils amended with FYM was significantly higher than that obtained on unamended soil. In contrast, grain yield of wheat which followed maize did not differ significantly on amended or unamended soils.     

Phosphate sorption isotherms in relation to P nutrition of wheat (Triticum aestivum)

The phosphate sorption isotherms are needed to explain differential plant responses to P fertilization in soils. Laboratory and greenhouse experiments investigated the use of phosphorus sorption isotherms in relation to P fertilizer requirement of wheat in ten benchmark soils of Punjab, India. The modified Mitscherlich Equation (3) was used to describe plant response observed in different soils. Maximum obtainable yield (MOY) ranged from 11.6 g pot^–1 in Gurdaspur (I) sandy clay loam to 7.0 g pot^–1 in Nabha sandy clay loam. Response to P applied @ 25 mg P kg^–1 soil was maximum (77%) in Bathinda sand and minimum in Chuharpur clay loam (33%). The response curvature varied from 3.74 × 10^–2 in Nabha sandy clay loam to 4.43 × 10^–2 in Kanjli sandy loam. The soil solution P required to produce optimum yield (90% MOY) varied from 1.61 µg ml^–1 in Bathinda sand to 0.10 µg ml^–1 in Sadhugarh clay. Dry matter yield obtained at 0.2 µg ml^–1 solution P concentration ranged from 55% in Bathinda sand to 85% of MOY in Gurdaspur (II) clay loam. At the same solution P concentration (0.1 µg P ml^–1), dry matter yield was 91% in Sadhugarh clay, 80% in Gurdaspur (II) clay loam and, 43% of MOY in Bathinda sand and eventually coincided with the decreasing maximum buffer capacity (MBC) in these soils. At the same level of sorbed P (100 mg P kg^–1 soil) the yield was observed to be inversely proportional to MBC. The study, therefore, concludes that, soils should be grouped according to their P sorption characteristics and MBC before using critical soil solution P as a criterion for obtaining optimum yields.     

Growth chamber study of phosphorus applied as drilled granules or as seed coatings to wheat sown in soils differing in P-sorption capacity

The effect of seed coatings or drilled granules containing phosphorus (P) on the early growth of wheat was evaluated in a pot trial in two soils differing in P-sorption capacity. P as monocalcium phosphate (MCP) was applied in seed coatings (C) at rates of 0 (inert coating of diatomaceous earth), 5 and 10 kg P ha^–1 whereas drilled rates (D) of P were applied at 0, 5, 10, 20 and 40 kg P ha^–1 to wheat sown at the equivalent of 60 kg ha^–1. The soils used were an aquic haplustalf with low (LS) and a gibbsiorthox with high (HS) P-sorption capacities respectively. Both soils were deficient in P, having 7.3 and 13.1 ppm available P in the LS and HS soils respectively. Emergence was recorded and plant height was measured at regular intervals. Shoot and root dry weights and P contents were determined in the final harvest at 27 days after sowing.Emergence was significantly delayed only by C10. Cumulative plant height per pot of C5 was significantly greater than that of C0, C10, D0, D5 or D10 from day 17 onwards. The only drilled treatments to produce a significant increase in plant height over D0 were D20 and D40. Dry weights increased in the LS soil up to 20 kg P ha^–1 and up to 40 kg P ha^–1 in the HS soil. Whilst C10 suffered some injury during emergence which also reduced the early growth of that treatment, seed coating at 5 kg P ha^–1 resulted in more effective use of P than an equivalent drilled rate in both soil types, particularly with respect to plant height and root weight.     

Influence of soil organic carbon on the interpretation of soil test P for wheat grown on alkaline soils

Organic carbon is known to alter crop response to applied phosphorus (P) but that fact has not been incorporated in soil test interpretations. To achieve this objective, field experiments with wheat were conducted for four years on alkaline soils of Punjab, India. The experimental soils ranged from loamy sand to loam in texture, 7.4 to 9.6 in pH, 0.16 to 0.75% in organic carbon (OC) and 2 to 40 mg Olsen extractable P kg^–1 soil. Response of wheat to fertilizer phosphorus application was related to the combined effect of Olsen P and soil OC content. At a given Olsen P level, wheat yield was a function of soil OC content. Multiple regression analysis of the data showed that OC content <0.2% did not affect yield significantly. At values >0.6%, OC along with Olsen P accounted for 97% of the variation in yield and there was no response to applied fertilizer P. Yield isoquants for 4 and 5 tons grains ha^–1 showed that for a given Olsen P level, as OC content increased the amount of fertilizer P required to achieve a yield target decreased. It was shown that OC may be used to approximate the contribution of organic P mineralization to plant available soil P during a growing season. The reliability of fertilizer recommendations based on Olsen P may be improved on some alkaline soils by consideration of soil OC content.     

Nitrogen use efficiency by maize as affected by a mucuna short fallow and P application in the coastal savanna of West Africa

Maize is the primary food crop grown by farmers in the coastal savanna region of Togo and Benin on degraded (rhodic ferralsols), low in soil K-supplying capacity, and non-degraded (plinthic acrisols) soils. Agronomic trials were conducted during 1999–2002 in southern Togo on both soil types to investigate the impact of N and P fertilization and the introduction of a mucuna short fallow (MSF) on yield, indigenous N supply of the soil, N recovery fraction and internal efficiency of maize. In all plots, an annual basal dose of 100 kg K ha^–1 was applied to the maize crop. Maize and mucuna crop residues were incorporated into the soil during land preparation. Treatment yields were primarily below 80% of CERES-MAIZE simulated weather-defined maize yield potentials, indicating that nutrients were more limiting than weather conditions. On degraded soil (DS), maize yields increased from 0.4 t ha^–1 to 2.8 t ha^–1 from 1999 to 2001, without N or P application, in the absence of MSF, with annual K application and incorporation of maize crop residues. Application of N and P mineral fertilizer resulted in yield gains of 1–1.5 t ha^–1. With MSF, additional yield gains of between 0.5 and 1.0 t ha^–1 were obtained at low N application rates. N supply of the soil increased from 10 to 42 kg ha^–1 from 1999 to 2001 and to 58 kg N ha^–1 with MSF. Application of P resulted in significant improvements in N recovery fraction, and greatest gains were obtained with MSF and P application. MSF did not significantly affect internal N efficiency, which averaged 45 kg grain (kg N uptake)^–1. On non-degraded soils (NDS) and without N or P application, in the absence of MSF, maize yields were about 3 t ha^–1 from 1999 to 2001, with N supply of the soil ranging from 55 to 110 kg N ha^–1. Application of 40 kg P ha^–1 alone resulted in significant maize yield gains of between 1.0 (1999) and 1.5 (2001) t ha^–1. Inclusion of MSF did not significantly improve maize yields and even reduced N recovery fraction as determined in the third cropping year (2001). Results illustrate the importance of site-specific integrated soil fertility management recommendations for the southern regions of Togo and Benin that consider indigenous soil nutrient-supplying capacity and yield potential. On DS, the main nutrients limiting maize growth were N and probably K. On NDS, nutrients limiting growth were mainly N and P. Even on DS rapid gains in productivity can be obtained, with MSF serving as a means to allow farmers with limited financial means to restore the fertility of such soils. MSF cannot be recommended on relatively fertile NDS.     

Effect of poultry litter and composts on soil nitrogen and phosphorus availability and corn production

Environmental problems associated with raw manure application might bemitigated by chemically or biologically immobilizing and stabilizing solublephosphorus (P) forms. Composting poultry litter has been suggested as a means tostabilize soluble P biologically. The objectives of this study were to assessthe nutrient (N, P) value of different-age poultry litter (PL) compostsrelativeto raw poultry litter and commercial fertilizer and determine effects ofpoultrylitter and composts on corn (Zea mays) grain yield andnutrient uptake. The research was conducted for two years on Maryland'sEastern Shore. Six soil fertility treatments were applied annually to aMatapeake silt loam soil (Typic Hapludult): (1) a check without fertilizer, (2)NH₄NO₃ fertilizer control (168 kg Nha^–1), (3) raw poultry litter (8.9 Mgha^–1), (4) 15-month old poultry litter compost (68.7Mg ha^–1), (5) 4-month old poultry litter compost(59 Mg ha^–1) and (6) 1-month old poultry littercompost (64 Mg ha^–1). We monitored changes inavailable soil NO₃-N and P over the growing season and post harvest.We measured total aboveground biomass at tasseling and harvest and corn yield.We determined corn N and P uptake at tasseling.Patterns of available soil NO₃-N were similar between raw PL-and NH₄NO₃ fertilizer-amended soils. LittleNO₃-N was released from any of the PL composts in the first year ofstudy. The mature 15-month old compost mineralized significant NO₃-Nonly after the second year of application. In contrast, available soil P washighest in plots amended with 15-month old compost, followed by raw PL-amendedplots. Immature composts immobilized soil P in the first year of study. Cornbiomass and yields were 30% higher in fertilizer and raw PL amendedplotscompared to yields in compost-amended treatments. Yields in compost-amendedplots were greater than those in the no-amendment control plots. Corn N and Puptake mirrored patterns of available soil NO₃-N and P. Corn Puptakewas highest in plots amended with 15-month old compost and raw PL, even thoughother composts contained 1.5–2 times more total P than raw PL. There wasalinear relationship between amount of P added and available soil P, regardlessof source. The similar P availabilities from either raw or composted PL,coupledwith limited crop P uptake at high soil P concentrations, suggest that raw andcomposted PL should be applied to soils based on crop P requirements to avoidbuild-up of available soil P.     

Phosphorus fertigation of trickle-irrigated potato

A 3-year field study, on Pellic Vertisol, investigated the response of trickle-irrigated potato (Solanum tuberosum L.) to four P levels applied with the irrigation water. Waters supplied with 130 and 120 mg l^–1 of N and K, respectively, and with P levels of 0, 20, 40 and 60 mg l^–1, were applied when the soil water potential was between 0.03 and 0.04 MPa. The water applied at each irrigation was equivalent to 0.8 of pan evaporation from a screened USWB Class A pan. With the application of 40 mg P l^–1 no P accumulation deeper in the soil profile occurred, whereas P in petioles was at levels recommended for high yield of good quality. At this P concentration in irrigation water, removal of P from soil by tubers was 22 kg/ha/year. The highest yield of good quality was obtained at 40 mg P l^–1.     

The distribution of phosphorus fractions and desorption characteristics of some soils in the moist savanna zone of West Africa

The fractionation of soil P into various organic and inorganic pools with differing levels of bioavailability, coupled with knowledge of the P adsorption and desorption characteristics of the soils, provides insights into management strategies that enhance P availability to crops. Sequential soil P fractionation was conducted on samples from 11 soil profiles and different experimental fields selected from the derived savanna (DS) and northern Guinea savanna (NGS) zones of the West African moist savanna to assess the influence of soil characteristics and management on soil P pools. Phosphorus adsorption and desorption studies were conducted on samples from the surface horizon of the soil profiles. The total P content varied within and among the soil profiles and tended generally to decrease as depth increased. The total P content in topsoil varied from 90 to 198 mg kg^–1 of which about 30% was organically bound P. The resin P fraction was generally low (mean = 5 mg kg^–1, topsoil) and decreased with depth. These low resin P levels indicate low P availability. Within the DS, where the organic resource (OM) was Senna siamea residues, the effects on soil P fractions of OM and soluble P fertilizer (PF), whether sole or in combination, were site-specific. While resin P was significantly increased by OM in some sites, no significant differences were observed in others. In the NGS fields, farmyard manure (organic resource, OM) combined with PF and PF applied alone increased the inorganic P (Pi) fractions extractable with resin, bicarbonate, and NaOH by about 400% but had no significant effect on the organic P (Po) pools and the more stable Pi forms. The P sorption capacities were low, with the adsorption maximum deduced from the Langmuir equation ranging from 36 to 230 mg kg^–1. The amount of P sorbed to maintain 0.2 mg l^–1 in solution ranged between 0.6 and 16 mg kg^–1. Phosphorus desorption with anion exchange resin differed among the soils, with the recovery of added P ranging from 17 to 66% after 96 h. On average, more of the applied P was recovered in the DS soils than in the NGS soils. Because of the relatively low sorption capacity and the relatively high percentage recovery, small additions of P to most of the soils studied might be adequate for crop growth. In essence, quantities of P fertilizer needed in these soils might be estimated based on considerations of P uptake by crops rather than on sorption characteristics.     

Fertility status of soils of the derived savanna and northern guinea savanna and response to major plant nutrients, as influenced by soil type and land use management

Although the fertility status of soils in the West African moist savanna is generally believed to be low, crop yields on farmers' fields vary widely from virtually nil to values near the potential production. The soil fertility status was evaluated for a number of farmers' fields selected at random in 2 villages (Zouzouvou and Eglimé) representative for the derived savanna (DS) benchmark area and in 2 villages (Danayamaka and Kayawa) representative for the Northern Guinea savanna (NGS) benchmark area. The relation between soil fertility status and soil type characteristics and fertilizer use was explored. In an accompanying missing nutrient greenhouse trial, the most limiting nutrients for maize growth were determined. While soils in the DS villages were formed on different geological units, soils in the NGS villages could be differentiated according to their position on the landscape. Generally, soils in the DS contained a smaller amount of silt (104 vs. 288 g kg^–1), a larger amount of sand (785 vs. 584 g kg^–1), C (9.3 vs. 6.3 g kg^–1), N (0.7 vs. 0.5 g kg^–1), Olsen-P (10.7 vs. 5.4 mg kg^–1), and had a higher CEC (7.0 vs. 4.8 cmolc kg^–1) than soils in the NGS villages. The large silt content of the soils in the NGS is a reflection of the aeolian origin of the parent material. Within the benchmark areas, general soil fertility characteristics were similar in the villages in the NGS, except for a larger amount of particulate organic matter in Kayawa than in Danayamaka. This may also have led to a significantly larger amount of ammonium-N content in the 0–20 and 20–40 cm soil layers in Kayawa compared to Danayamaka (42 vs 24 kg N ha^–1 in the 0–20 cm soil layer). Differences in topsoil soil characteristics between the DS villages were a reflection of differences in clay quality (kaolinitic vs. 2:1 clay minerals) of the parent material and past fertilizer use. The Olsen-P and exchangeable K contents were observed to increase with increased fertilizer application rate in both benchmarks, while fertilizer application rate had no significant effect on the organic C or total N content of the soil nor on its ECEC. The response of maize shoot biomass production to applied N was similar for both benchmarks (biomass accumulation in the treatment without N was, on average, 55% of the biomass production in the treatment which received all nutrients), while soils in the NGS responded more strongly to applied P than soils in the DS (37% vs 66% of biomass production in the treatment which received all nutrients). The more favourable P status of soils in Eglimé (DS) was attributed to the more intense use of P fertilizers, as a result of government-supported cotton production schemes. Response to cations, S or micronutrients were neglegible. A significant linear relationship was found between the soil Olsen-P content and the response to applied P up to levels of 12 mg kg^–1 in the topsoil. Above this level, a plateau was reached.     

Effect of NPK fertilization on the mineral nutrition and yield of three coconut genotypes

Annual application of NPK fertilizers over a 18 year period to coconut on red sandy loam soils resulted in a minimal increase in mineralisable N, but in a marked increase in available P and K. Plant N levels, however, reflected the improved N nutrition but did not reach sufficiency levels found elsewhere. An available P status of 15 ppm in the control plots kept leaf P at sufficiency levels. P fertilizers did not increase the P content of leaves. K fertilizers raised the K leaf content to sufficiency levels. Doubling the M₁ fertilizer rates of 500 g N, 220 g P and 830 g K per palm per year had no effect on N, P and K levels in the palm leaves.Changes in K levels of the leaves had antagonistic effects on leaf Mg (r = – 0.68^**) and leaf Na (r = – 0.87^**). As this effect brings leaf Mg close to deficiency values palms receiving K might need additional Mg as well.The findings and interpretation of soil and leaf analysis data were confirmed by large yield responses to application of NPK fertilizers. Genetic differences between palms in their response to levels of nutrient supplies were apparent. The CDO × WCT hybrid outyielded the high yielding WCT variety especially when NPK was given at the M₁ level. The response in yield to applied fertilizers was linear for WCT and curvilinear for the hybrids CDO × WCT and WCT × CDO.     

Fertility characterization of soils at six research sites in NW Cameroon

Fertility capability of surface (0–20 cm) soils was evaluated at six sites in the North-West Cameroon highlands. Two main soil groups, designated as Classes A and B, were identified based on elevation. The Class A soils from low elevations (600–1178 m) had higher Ca, Mg, K, pH, sorbed less P and were lower in organic carbon and sesquioxides than the highland (> 1200 m) soils. Soil acidity (Al saturation > 30%) and high P sorption appeared to be the most limiting factors to crop production especially on the Class B soils where the Standard P Requirement exceeded 500 mg kg^–1. Phosphorus sorption data were best described by the Freundlich equation. Amorphous aluminium was the most important determinant of solution P concentration (r = 0.85,p < 0.001) followed by soil organic carbon, (r = 0.80,p < 0.001) at high P rates. Nitrogen deficiency symptoms of maize were pronounced on the Class B soils. Consequently, crop growth and yield were lower on Class B than on Class A soils despite the high organic carbon in B. We hypothesize that the supply of high quality organic material (high in N and low in lignin and polyphenols) at site B through agroforestry and related cropping systems, would improve the fertility of the soil and crop yield.This article is a contribution from the IITA-IRA-NCRE, USAID-supported National Cereals Research and Extension Project in Cameroon.