Monthly communications by the International Potash Institute, Berne (Switzerland) Subject 12 Industrial crops No. 4/1979 21st suite O Fertilization of the cotton on some soils of the States of Portuguesa and Barinas, Venezuela (Fertilicaci6n del algod6n en algunos suelos de los Estados portuguesa y Barinas) 1 , , 2 L. Avilln Rovira L. Vilain2 J. Abreu2 and R. Salas From: Agronomia Trop., Vol. 26, no. 5, 425-439 (1976), Venezuela. Summary During the years 1969 and 1973, CENIAP, CIARCO and ANCA carried out several experiments in the States of Portuguesa and Barinas with the purpose of. determining the effects of nitrogen, phosphorus and potassium applications on the production of cotton. A randomised block design with fourfold replication of each treatment was employed. The plant varieties tested were Delta Pine Smooth Leaf and Delta Pine 16. Soils were chosen so as to represent three groups, classified according to physical and chemical characteristics: First group: Much weathered soils, acid, of low natural fertility, well drained. Second group: Lightly weathered soils, moderately acid, of medium to high natural fertility, well drained, mainly of medium texture. Third group: Lightly weathered soils of medium to high fertility, having problems of drainage, with fine textures predominating throughout the profile. The major yield increments were due to nitrogen and phosphorus in soils of the first group, with responses to potassium less pronounced. The best treatment was 60-60-40. In the second group of soils significant effects were not recorded except for nitrogen; the best treatment was 60-30-40. In the third group nitrogen and potassium gave the major effects. The best treatments were 60-60-80 and 80-40-40. Differences between forms of nitrogen (ammonium sulphate and urea), tested on the soils of the third group were not apparent. Introduction The cotton is one of the more important enterprises in the agricultural sector of the economy of the States of Barinas and Portuguese, where it occupies 45 per cent of the area in the whole country to this crop. This, in the year 1974 was 64688 ha giving a mean yield of 1076 kg/ha (M.A.C., 1975). With the aim of Instituto de Investigaciones Agrlcolas Genemales (CENIAP), Maracay, Venezuela. * CIARCO, Araure, State of Portuguesa, Venezuela. 1.. establishing a rational fertilizing programme and so improving the low yields observed, experiments have been carried out during some years past by the Ministry of Agriculture and Animal Production through the Centro Nacional de Investi- gaciones Agropecuarias (CENIAP) and the Centro de Investigaciones de la Regi6n Centro Occidental (CIARCO), in collaboration with the Asociaci6n Nacional de Cultivadores de Algod6n (ANCA). This paper reports the results of some of these experiments. Survey of the literature Westin (1962) in accordance with the 7th Approximation', classified the soils of the Barinas Series as Ochrultic Typudalfs. They are soils of alluvial origin over- lying accumulated material, always distinctly sandy and distinctly clayey, and situated on terraces and alluvial basins at the bases of mountains. Nelson (1967) working with grain sorghum, noted the low fertility of these soils. He reported positive effects from treatments with the fertilizing elements nitrogen and phos- 0 phorus, indicating further that the response to nitrogen depends on the level of available phosphorus. Rodriguez and Aviln (1969), and Aviln and Rodriguez (1972) obtained similar results for the cotton and for maize, and reported further that the element potassium was a less effective element in raising yields. Nelson (1967) and Avildn and De Rojas (1975) pointed to sulphur as another limiting factor in the productivity of these soils. The soils of the Guanare Series were classified by P&ezetal. (1968) in accordance with the 7th Approximation as Ustifluvents. They find their place amongst the soils suited to cotton cultivation (Strebin and Tua, 1966), permitting a vigorous development of the root system (Dumith et al, 1964). In soils of Barinas State with similar physical and chemical characteristics, Rodriguez and AviMn (1969) have reported responses of maize and cotton to nitrogen fertilization, though not as marked as those observed on the Barinas Series. Materials and methods The experiments were located in various places, representative of the soil units most commonly cultivated in the cotton growing region. They were conducted in the years 1969 and 1973. A randomized block design with fourfold replication of treatments was used. The experimental plots consisted of three strips 10 m wide with spaces of 1 m between them. For the yield data the central strips were recorded and the figures refer to raw cotton. Density of planting was 60 000 plants per hectare. The variety Delta Pine Smooth Leaf was used in the year 1969, and Delta Pine 16 in 1973. Fertilizer dressings were timed as follows: nitrogen was divided, one third at the time of sowing and the rest 35 days later; phosphorus and potassium were entirely applied at sowing time, in the bottoms of the furrows. The sources used, and the application rates in 1969 and in 1973 were as follows. Year Element Rates, kg/ha Sources used 1969 N 0,30, 60, and 90 ammonium sulphate (21%) P.O. 0,30, 60, and 90 triple superphosphate (45%) K.0 0,40 and 80 potassium sulphate (50%) 1973 N 0,40, 60 and 80 ammonium sulphate (21%) P20 0,20 and 40 triple superphosphate (45%) K20 0,60 and 80 potassium sulphate (50%) 2 ... 12121 In the experiments of 1973, two alternative sources of nitrogen were used with the object of testing for differences in the forms of supply of this element. These sources were urea and ammonium sulphate and they were used in the treatment 60-40-40 (kg/ha of nitrogen, phosphoric acid and potash respectively). The topsoils were sampled to a depth of 0-15 cm at each of the experimental units. The analytical tests applied and their results are given in Table 1. The soils of the Barinas Series represent the first group, the characteristics of which were described by Rowles (1969). The terrains occupied by these soils are high, level terraces adjoining the lower slopes of the mountains, with savannah vege- tation, acid in reaction with low exchange capacity, well drained and with low moisture retentivity and low natural fertility (table 1 ). The soils of the second group typified by the Guanare Series, the features of which were described by Prez Materan at al. (1968) are characterised as slightly developed soils of medium texture in all horizons. Internal drainage is normal, reaction moderately acid, exchange capacity medium to high. They generally occupy the first terrace or flat along a river, the natural vegetation being rain forest. They are soils of moderate natural fertility (table 1 ). The soils of the third group, represented by the Maria Series described by Stagno and Granados (1969) are characterised by having a low exchange capacity and medium acidity, with problems of internal drainage associated with their positions on river flats or very tow-lying banks. The results of soil testing, for individual soils and their recognised groups, were analysed statistically using Tukey's significance test according to the procedure given by Pimentel (1966). Table 1 Results of laboratory tests on soil samples representative of the experi- ments on cotton fertilization in different localities. Depth of sampling 0-15 cm. Analysis (4) Locality HDA Apica Est. Exp. Araure Guache Sist. Riego Guanare Texture Ar. (5) F.Ac. F.Ac. Ar. F.Ac. Ar. F. Ar. (1) Phosphorus ppm. 1,5 24 38 13 16 (1) Potassium ppm. 8.5 78 66 139 176 (2) Calcium ppm. 158 462 467 684 741 (2) Nitrate ppm. 19 28 35 74 83 (3) Org. matter % 1.70 4,96 3.90 4,03 4,30 pH, 1:2,5 5,3 5,6 5,00 5.2 5,6 Conductivity 1.5 mmho/cm 25°C 0,05 0,09 0,07 0,07 0,09 (1) Phosphorus and potassium extracted by NaHCO. 0,5 M, pH 8,5. Ratio of soil to extractant solu- tion, 1:20 (2) Calcium and nitrate extracted by CH.COONa 0.125 M. pH 4,2 (3) Organic matter determined by wet combustion according to Walkley and Black (4) Soil Analysis Laboratory I.I.A.G.-CENIAP-MAC. (4) Ar.-sand, F.=silt. Ac.=clay. Results and discussion The results obtained for the different groups of soils studied are presented in table 2. The largest response observed in the first group of soils was that due to nitrogen (yield increment 834 kg/ha), followed by phosphorus (555 kg/ha), whilst potassium had a less marked effect on yields (323 kg/ha). 3 ... Soil analysis (table 1) indicates low available phosphorus contents ( <10 ppm.). Moreover, these soils possess a high power of phosphate fixation, given their high degree of weathering (Rowles, 1969) and the effect of this on the response to phosphate fertilization, despite the low requirement of the cotton for this nutrient (Malavolta et al., 1974) must be such as to give larger increments from phosphatic treatments. In the present case however the response to the element may have been obscured by the high coefficient of variation (25%) observed in the experi- ment, or additionally by an inadequate distribution of the rainfall. Table 2 Effects of N, P and K fertilization on yields of cotton. Field experiments on soils of the first, second and third groups. Treatment Yields of raw cotton (kg/ha) Mean of 5 experiments No. kg/ha Group I Group 2 Group 3 Yield Increase of N POs KO Experi- Experi- Experi- Experi. Experi- kg/ha yield (per ment mont ment ment mont cent) due to Hda. San Guanare Est.