a mixture of mineral salts that are essential for plant nutrition. Nutrient solutions are used in the pot method of studying plants (seePOT METHOD).
Experiments to select the composition of nutrient mixtures were begun in the mid-19th century, when it was established that normal plant development requires nitrogen, phosphorus, sulfur, magnesium, calcium, potassium, and iron. In the 20th century it was discovered that nutrient mixtures must also contain trace elements, including manganese, boron, copper, zinc, cobalt, and nickel. One of the first nutrient solutions, Knop’s solution, was prepared in 1859 by the German agrochemist W. Knop and is used in water cultures. Per liter of water, it contains 1 g Ca(N03)2, 0.25 g KH2P04, 0.125 g KCl, 0.25 g MgS04, and either traces of FeCl3 or a small quantity of freshly precipitated FeP04. In 1883 the German scientist H. Helriegel suggested using the same salts, but in different proportions and in smaller concentrations. Per liter of water or kilogram of sand, Helriegel’s solution contains 0.492 g Ca(N03)2, 0.136 g KH2P04, 0.075 g KCl, 0.06 g MgS04, and 0.025 g FeCl3; it is used mainly for sand cultures.
A disadvantage of both these solutions is the instability of the pH, which is due to uneven absorption of the cations and anions in the salts over the course of the plant’s development. For normal plant development the pH of the solution should be neutral or slightly acidic. When growing plants on these solutions, pH must be systematically checked and brought to the required level by adding acid, for example, H2S04, or base, for example, NaOH.
From 1900 to 1926, D. N. Prianishnikov worked on finding solutions with more stable pH; he proposed adding buffering salts to the nutrient solution. In his laboratory he produced solutions with relatively stable pH that varied within a specified, narrow range during the entire growth period in permanent water cultures. For example, a nutrient solution that maintains a pH of 5 includes, per liter of water, 0.334 g NH4NO3, 0.166 g KNO3,0.70 g Ca3(PO4)2, 0.25 g Fe2(SO4)3, 0.614 g KCl, 0.50 g MgSO4 · 7H20, and 0.50 g CaS04.2H20. This nutrient solution is considered the best for growing wheat, barley, buckwheat, soy, rye, millet, corn, sorghum, and oats; it is unsuitable for peas and flax.