To meet these challenges fertilizers recommendations to farmers become a common practice worldwide generally optimizing fertilizer doses to sustain a desired yield without a load to the environment. Consequently, soil P recommendation systems are widely used around the world to ensure good soil management and nutrient efficiency promoting agricultural sustainability. However recommendation systems differ considerably among countries. Not many systems can be found as peer reviewed literature; however a brief overview on those available is hereby given. Phosphorus recommendation systems are commonly used in Brazil, in a country where soils are generally nutrient poor. The Brazilian recommendation systems are based on quantitative analyses of soil input variables. The input variable consists of the following factors; cation exchange capacity (CEC), base saturation (BS), base sum, exchangeable aluminium (Al), calcium/magnesium (Ca/Mg), potassium (K) and P levels, sodium (Na) saturation and electrical conductivity. The output variable of the system is the amount of fertilizer to be applied. This is mainly based on 4 classes, low, medium, high to very high (Palhares et al., 2001). While Brazil follows a detailed set of variables when recommending P fertilizer levels, the agronomists at Kansas University – who, among other land grant Universities in the United States, provide single rate recommendation for nutrients such as P – are developing a fertilizer recommendation system that gives growers the flexibility to choose a soil management practice suitable for their needs. This flexibility included choosing from 2 systems, the “nutrient sufficiency recommendation system” which is developed to provide a 90–95% maximum yield for the year, and the “build maintenance fertility program” based soil test values over a planned period of time, usually 4–8 years, for both immediate crop needs and build up levels to a non-limiting value (Leikam et al., 2003). In West Africa, a framework to optimize soil fertility management in rice production is in use were the yield potential is estimated by an ecophysiological model based on weather conditions, cultivar species and sowing date. This yield potential is used as an input into a static model together with field specific data such as recovery efficiency of applied N, P and K, indigenous NPK supply and maximum NPK accumulation. Outputs of the framework include, required fertilizer doses to obtain different yield targets depending on yield potential and the soil nutrient supply (Haefele et al., 2003).
classes
4
nutrient sufficiency recommendation system
maximum yield for the year
90–95%
%
soil test values
period of time
4–8 years
years