Determining Mineral Needs

How do we determine which minerals are required for plant welfare? This is done in the same way we determine which vitamins are necessary for human welfare. Namely, we observe what happens when given minerals are absent. The challenge is to arrange. a scenario wherein every required mineral but one is present. This is not easy to do. Very precise methods must be employed to ensure that the mineral being studied is not present, even in minute amounts. Further, some of the required minerals are needed in only micro amounts. Should any of these minerals be present as impurities in a chemical, float into the test on dust from the atmosphere, or be present on the seed when it is planted, testing to determine the consequences of that mineral's absence could be compromised.

Sachs and Knop, two early investigators, learned this lesson.They prepared a nutrient solution containing calcium nitrate, potassium nitrate, potassium di-acid phosphate, magnesium sulfate, and ferric phosphate. They then supplied it to plants growing in solution culture. In seeing no provision of zinc, manganese, copper, or molybdenum in this nutrient solution you might suspect that the plants would show a lack of something. This, however, was not noted. Rather, the plants thrived in good order. Sachs and Knop were thus misled to believe that the minerals provided in the solution were adequate to the need. In actuality, however, other minerals were present as impurities in the chemicals they used. This example illustrates the importance of using the most meticulous methods to ensure that the only elements present are those intended. The water should be twice distilled. Water that has been stored in a glass bottle for a time is not desirable because part of the bottle has dissolved in the water, yielding boron. Water that is distilled but allowed to flow through a metal pipe is also unsatisfactory, having remnants of zinc and copper.

Zinc is needed in amounts of approximately three parts per million of the plant's dry weight. Molybdenum is likewise needed in only trace amounts.