NPK-fertilizer efficiency — a situation analysis for the tropics
This paper examines the efficiency of applied N, P, and K fertilizers under tropical conditions. To meet their food demands, tropical countries are importing large quantities of fertilizers at an enormous cost. There is a need for improving crop yields at a reduced cost and a better understanding of the factors that contribute to the overall efficiency of applied fertilizers. It is estimated that under tropical condition, the efficiency of applied N is less than 50%, less than 10% for P and for K it is somewhere around 40%. Losses of N are mainly due to leaching, runoff and volatile losses of ammonia. Under flooding and in alternate wetting and drying conditions of rice lands and low lands, dentrification and volatile ammonia losses are considerable. The N losses from these soil could be minimized by proper management such as rate, methods and time of application. The coating of urea with S has shown some improvement in increasing efficiency. Nitrification and urea hydrolysis inhibitors can improve fertilizer efficiency in certain situations provided they are properly used. The efficiencies of these inhibitors depend on the nature of the chemical compounds, soil properties, and method of application. Low efficiency of applied P fertilizer is mainly due to retention of P by soil clay fractions and iron and aluminum hydroxides. Even though retained P is not available to the first crop, it is made available to a certain extent to the succeeding crops. The rate and methods of P applications and forms of P determine the efficiency of applied P fertilizers. The use of native rock phosphate along with P fertilizers on acid soils appears to be an attractive alternative in reducing the fertilizer cost. The loss of K in tropical soils is largely attributed to leaching and runoff. To reduce K loss by leaching, it is more advisible to apply K in split doses than a single dose. Liming has a beneficial effect in retention of K and reducing P fixation in acid soils. 
Controlled-Release NPK Fertilizer Encapsulated by Polymeric Membranes
The commercial granular fertilizer NPK6-20-30 was coated using polysulfone (PSF), polyacrylonitrile (PAN), and cellulose acetate (CA). The coatings were formed from the polymer solutions by the phase inversion technique. Measurements of the thickness and porosity of the prepared coatings and a microphotographic observation of the coatings were performed. The physical properties of the coatings influence the release rate of macronutrients which are present in the core of the coated fertilizer. In the case of PAN coating with 60.45% porosity, prepared from a 16% polymer solution, 100% of NH4+ and P2O5 was released after 4 h of test and 99.7% of K+ after 5 h of test, whereas in the case of coating with 48.8% porosity, 31.8% of NH4+, 16.7% of P2O5, and 11.6% of K+ was released after 5 h. In all experiments, different selectivities of the coatings in terms of the release of components were observed. The release of potassium through the coatings made of PSF and PAN was the slowest. The same tendency was observed for the release of nitrogen through a coating of CA. The release of fertilizer active components was the slowest in the case of PSF. The lowest porosity coating was prepared from the 18% PSF solution. 
The Performance of Zea mays as Influenced by NPK fertilizer Application
Field trials were made to estimate the effect of NPK 15:15:15 fertilizer on the growth and yield of maize were conducted over a two year period. NPK fertilizer applications significantly increase plant height, stem girth, number of leaves, leaf area, leaf area index, dry matter accumulation and yield. The optimum level of NPK 15:15:15 fertilizer for successful production of maize grains was 400 (60 kg N + 27.16 kg P + 49.80 kg K) kg / ha compound fertilizer based on the fact that it had the greatest dry cob yield (12.44 t / ha), grain yield (7.95 t /ha), relative grain yield (2.26) and 100-seeds weight (11.62g). 
Effect of NPK Fertilizer on Fruit Development of Pumpkin (Cucurbita pepo Linn.)
The effect of NPK fertilizer on pumpkin fruit development was studied for two cropping seasons in 2010 at the Teaching and Research Farm, Obafemi Awolowo University, Ile-Ife, Nigeria in 2010. The experiment was a randomized complete block design. The plants were treated with six NPK rates (0, 50, 100, 150, 200 and 250 kg/ha). Data on fruit weight, circumference, length and dry matter were obtained at 7, 14, 21 and 28 days after fruit formation. Increasing NPK fertilizer enhanced the parameters evaluated across the sampling periods. Fresh fruit weight (g/fruit) in control was 39g, 123g, 822g and 1059g and this increased to 80g, 370g, 1350g and 1630g at 7, 14, 21 and 28 days after fruit formation respectively at 100 kg NPK fertilizer rate. Across the NPK levels, pumpkin fruit growth curve was sigmoid. The fruit took approximately 22 days from fruit formation to fruit maturity across all the NPK fertilizer levels. In conclusion, excessive NPK supply did not significantly increase the rate of fruit growth or the fruit size. Fruit growth duration of pumpkin was not influenced by NPK fertilizer application.
Influence of Grasscutter, Chicken Manure and NPK Fertilizer on the Physical Properties of a Chromic Luvisol, Growth and Yield of Carrot (Daucus carota)
A field experiment was conducted to evaluate the effect of grasscutter manure (GM), chicken manure (CM), and NPK on soil physical properties, growth and yield of carrot. The treatments were; no fertilizer or manure (control), 300kgNPK/ha (15:15:15), 10tCM/ha, 3 levels of grasscutter manure (10t, 15t and 20t/ha), laid out in a randomized complete block design with 3 replications. Soil bulk density was highest in the control plot while the sole manure treatments had low soil bulk densities, with the 20t/haGM3 having the lowest value. The 20t/haGM3 treatment also recorded the highest values for the gravimetric moisture content and the total soil porosity. Plant height, number of leaves, root length, root diameter and root yield in the amended treatments were better than the control. Values for the growth and yield parameters of the carrot plant from the 20t/haGM3 treatment were in most cases significantly (P=.05) higher than the rest of the treatments, and the 10tGM/ha treatment had better impact on the parameters than the 10tCM/ha treatment. The highest economic benefit was realized for the 20t/haGM3 treatment. 
 Baligar, V.C. and Bennett, O.L., 1986. NPK-fertilizer efficiency—a situation analysis for the tropics. Fertilizer research, 10(2), pp.147-164.
 Jarosiewicz, A. and Tomaszewska, M., 2003. Controlled-release NPK fertilizer encapsulated by polymeric membranes. Journal of Agricultural and Food Chemistry, 51(2), pp.413-417.
 Law-Ogbomo, K.E. and LAW-OGBOMO, J.E., 2009. The performance of Zea mays as influenced by NPK fertilizer application. Notulae Scientia Biologicae, 1(1), pp.59-62.
 Oloyede, F.M., Agbaje, G.O. and Obisesan, I.O., 2013. Effect of NPK Fertilizer on fruit development of pumpkin (Cucurbita pepo Linn.). Journal of Experimental Agriculture International, pp.403-411.
 Atakora, K., Agyarko, K. and Asiedu, E.K., 2013. Influence of grasscutter, chicken manure and NPK fertilizer on the physical properties of a chromic luvisol, growth and yield of carrot (Daucus carota). International Journal of Plant & Soil Science, pp.197-204.
NPK-fertilizer efficiency — a situation analysis for the tropics