Fertiliser is any mixture, organic or synthetic, used to make soil more fertile. American variety grapevines require a balanced fertiliser nutrient ratio and a pH below 5.5, while European grapevines require a balanced fertiliser ratio and a pH level below 6.5, according to the University of Connecticut. The best fertiliser for grape vines largely depends on the available soil nutrients and vine age.
All balanced fertilisers, organic or synthetic, contain a mixture of nitrogen, phosphorus and potassium, as well as a variety of secondary nutrients and additives, depending on the brand and fertiliser type. The Ohio State University recommends 227grs. of balanced 10-10-10 fertiliser per grapevine during the first year for optimal growth. The 10-10-10 or N-P-K ratio represents the ratio of nitrogen, phosphorus and potassium levels contained in the fertiliser. Second year plants require 0.454kg. of the 10-10-10 fertiliser, while third year and beyond plants require 0.68kg. of the mixture. The University of Connecticut recommends a first-year fertiliser spread of 4 to 5 feet in diameter, a second year spread of 4 to 5 feet and a third year and older spread of 3 to 5 feet. The balanced ratio is best for fertile soils because too much of any nutrient can cause severely weakened growth.
Soil tests done by university-based or private labs determine the quantity of available nutrients present in soil. If phosphorus and potassium, two of the three primary nutrients found in fertiliser, are below optimum levels for the grape species grown, additional phosphorus or potassium-based fertilisers are required. According the University of Connecticut, if phosphorus levels are below optimum for the species of grapevine, apply phosphate-based fertiliser, such as a 0-45-0 fertiliser, at a rate of 0.227kg. per vine. Phosphate-only fertiliser may contain nutrient levels lower than the 0-45-0 ratio, therefore the amount of application varies depending on the concentration. For example, a 0-20-0 ratio requires 0.567kg. of fertiliser per vine because of the decreased nutrient content per pound. In the instance of potassium levels bellow optimum, add potassium sulphate fertiliser, 0-0-43 analysis, at a rate of 0.34kg. per vine. Avoid adding additional nitrogen, as the nutrients can cause poor vine hardening, which leaves vines susceptible to disease and pests.
Soil with depleted nutrient levels requires the use of starter fertiliser with added nutrients for optimal grapevine growth. The Ontario Ministry of Agriculture recommends using a starter fertiliser solution, such as 10-52-10 or 20-20-20, for an extra nutrient boost. The nutrient ratio depends on available nutrients in the soil before fertilisation. According to the Ontario Ministry of Agriculture, in the case of an abundance of nitrogen, cover crops, such as the Italian ryegrass, absorb excess nitrogen and leave adequate nutrient levels for grapevines during the growing season.