Foliar, soil and nutrient solution analysis are available commercially. These tools are useful in problem solving as well as preventative maintenance. It is often not possible to tell at a glance the cause of leaf discoloration and/or poor plant growth. Research has shown that micronutrients can be lost in a re-circulating hydroponic nutrient solution in less than 1 day or in a NFT system, nutrients (both micro and macro) could be gone by the time the end of the trough is reached (CEA program, unpublished data). If nutrient solution is to be re-used, an addition of nutrients will be necessary and the original concentration of each component is not a good indicator of what is needed to obtain the target nutrient concentrations. It is important to note that the exposure of nutrient solution to light can destroy the chelators which are compounds that hold metal cations like iron in the solution
Two methods of measuring the nutrition available to the crops exist. One is nutrient solution analysis and the other is foliar analysis. Nutrient solution analysis can show the absolute concentration of different elements in the solution. The solution should be kept from bright light and kept cold to limit microbial activity and allow for the most accurate lab results. Nutrient solution analysis is most often used with re-circulating hydroponic systems where the same solution is used for long periods of time with the addition of fresh solution to maintain a steady volume. Foliar analysis can be useful in determining dramatic deficiencies in nutrients however some crops show large variation in mineral composition while being very healthy. Depending on the life cycle of the crop, information from foliar analysis may be obtained too slowly to be of use before the crop is harvested.
A list of laboratories that analyze plant tissue as well as the relevant contact information was published in 1999: "Soil and Plant Analysis Laboratory Registry for the United States and Canada" by the Council on Soil Testing and Plant Analysis, Georgia Univ. Station, Athens, GA 30612-0007 published by the CRC pres. Available here.
Careful attention should be paid to the preparation of the samples for shipment. Each laboratory has a different set of directions which may include specialized packaging material. Once a testing company has been selected the same testing company should be utilized as protocols and therefore results will differ between laboratories.
A discussion of nutrient management for hydroponic plant production may be found in:
Nutrient Management in Recirculating Hydroponic Culture
Bruce Bugbee
Presented at the South Pacific Soil-less Culture Conference
Feb 11, 2003 in Palmerston North, New Zealand
Online here
Reference
Svenson, A., L. Kaj, and H. Bjorndal. 1989. Aqueous photolysis of the iron
(III) complexes of NTA, EDTA and DTPA. Chemosphere 18: 1805–1808.
