fertigation, the viticultural practice of mixing soluble fertilizers with irrigation water for direct application to vines. The technique is most often used with drip irrigation systems, for which each vine has a water outlet. Fertilizers are placed in a tank through which the irrigation water passes, and so the vine is fed with appropriate amounts of water and nutrients as the growth proceeds. Some nutrients such as nitrogen are readily available in a soluble form (urea); others such as phosphorus require a relatively expensive formulation to render them immediately soluble. Some vineyard additions such as gypsum and lime are quite insoluble and so require special formulations.