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The analytical methodology of choice for plant analysis is emission spectrophotometry. Many laboratories around the country analyze plant tissue on a daily basis. Almost all of these are listed in the publication entitled "Soil and Plant Analysis Laboratory Registry for the United States and Canada" (Council on Soil Testing and Plant Analysis, Georgia Univ. Station, Athens, GA 30612-0007; about $15/copy). This provides analytical services offered, contact person, phone and fax numbers. Be sure to check with the laboratory before sending them a sample. Each lab has different recommendations for plant sampling, drying, and shipment. The lab should be able to provide you with an analysis of nutrient toxicities and deficiencies. J. Benton Jones article in the 1993 HSA Proceedings more thoroughly explains details associated with plant sampling and analysis.

As an example of the typical cost of analysis, the 1995 analytical charges at the Soil and Plant Analysis Laboratory at Utah State University are as follows:
ICP-emission spectrophotometry for 22 elements: $15
Kjeldahl or LECO Total Nitrogen analysis: $8
nitrate-N analysis: $6
Total nitrogen plus ICP-ES elements (package discount): $20

pH monitoring and control

Is pH control important?

Most people assume pH control is essential, but there is considerable misunderstanding about the effect of pH on plant growth. Plants grow equally well between pH 4 and 7, if nutrients do not become limiting. This is because the direct effects of pH on root growth are small, the problem is reduced nutrient availability at high and low pH. The recommended pH for hydroponic culture is between 5.5 to 5.8 because overall availability of nutrients is optimized at a slightly acid pH. The availabilities of Mn, Cu, Zn and especially Fe are reduced at higher pH, and there is a small decrease in availability of P, K, Ca, Mg at lower pH. Reduced availability means reduced nutrient uptake, but not necessarily nutrient deficiency.

Unfortunately, hydroponic systems are so poorly buffered that it is difficult to keep the pH between 4 and 7 without automatic pH control. Phosphorous (H2PO4 to HPO4) in solution buffers pH, but if phosphorous is maintained at levels that are adequate to stabilize pH (1 to 10 mM), it becomes toxic to plants. Plants actively absorb phosphorous from solution so a circulating solution, with about 0.05 mM P has much less buffering capacity than the fresh refill solution that is added to replace transpiration losses. Figure 2a is a titration curve of fresh refill solution compared to the recirculating solution. Six mmoles of base were required to raise the pH of fresh solution from 5.8 to 8, but only 1 mmole of base raised the pH of the circulating solution to 8. Figure 2b shows the slopes (derivatives) of the lines in Figure 2a. Figure 2b clearly shows poor buffering of the circulating solution between pH 5 to 9; small amounts of acid or base rapidly change the solution pH. The fresh refill solution is buffered by phosphorous, which has its maximum buffering capacity at pH 7.2. This point is called the pKa of the buffer and it is the point at which half of the phosphorous is in the H2PO4 form and half is in the HPO4 form. In other words, the phosphate ion absorbs and desorbs hydrogen ions to stabilize the pH. Unfortunately, phosphorous is quickly removed from the solution.

We were surprised to find that the circulating solution was better buffered below pH 5 than the fresh solution. The reasons for this are unclear, we cannot identify compounds in the refill solution that provide buffering capacity at pH 4. We are preparing to repeat these measurements and are investigating this finding.

How important is maintaining pH 5.8?

We control the pH at 4 to study root respiration (to eliminate bicarbonate in solution). We compared growth at pH 4 and pH 5.8 with wheat and were not able to find a significant difference in root growth rate or root metabolism. We now routinely grow wheat crops at pH 4 during the entire life cycle. However, although there is usually a broad optimum pH, it is still best to maintain pH at about 5.8 to optimize nutrient availability. pH levels below 4 may start to reduce root growth, in one study our pH control solenoid failed just after seed germination and the pH went to 2.5 for 48 hours. The roots turned brown and died, but new roots quickly grew back and the plants appeared to make a complete recovery.

An automated pH Control System.

Although organic pH buffers can be used to stabilize pH (Bugbee and Salisbury, 1985), in the long run it is better and less expensive to use an automated pH control system that adds acid or base to the solution. These systems require 3 components: a pH electrode, a pH controller, and a solenoid. We have had 7 pH control systems in continuous operation at the Utah State University Crop Physiology Laboratory during the past 8 years. It is useful to pass on our experience with the system components.

pH electrodes. We have not found that expensive electrodes last any longer than cheap electrodes (about 2 years per electrode) so we use cheap electrodes. We currently use a general purpose pH electrode from Omega (model PHE-4201; $49). It appears to be important to avoid rapid flow of solution across the tip of the electrode. Rapid response time is not important and the high flow appears to greatly decrease electrode life and also causes significant calibration drift. We check the calibration of the electrode every 2 to 3 months and adjust it if necessary.

pH controller. In about 1987 a new, digital-display pH controller became available (model 3671, $225., Whatman Lab Sales, Hillsboro, OR, 1-800-942-8626). This controller has been excellent in our laboratory - we have yet to have a controller fail. Automatic temperature control is completely available with the controller for another $65. but it is unnecessary.

When the pH increases to 5.8, the controller opens a solenoid that allows nitric acid (HNO3) to flow into the bulk solution. When nitrate nitrogen is used the solution pH increases as the nitrate is absorbed so only one solenoid is necessary. The acid inlet should be in close proximity to the tip of the pH electrode so that frequent small additions of acid occur and the bulk solution pH is stable.

Acid/base solenoid. A peristaltic pump can be used to add acid or base, but a solenoid is less expensive. Proper solenoid selection is important because common solenoids quickly deteriorate from acid corrosion. We use a shielded core acid solenoid from The Automatic Switch Company (ASCO, model D8260G56V or G53V; about $76.). These solenoids do not corrode, but in our experience, about 50% of the diaphragms in the valves failed in less than 2 years in continuous use. The valves are rated for a million cycles so they should last at least 10 years. We are currently working with ASCO to determine the cause of the premature failure. We previously used ASCO valve number D8260G54V, but this valve is not shielded core and corrodes in less than a year, even with 0.1 molar acid. Most plumbing suppliers sell ASCO solenoids, it pays to shop around for good price and quick delivery. Many other companies sell acid resistant valves that may be suitable, but some require a transformer for 24 volt operation.

The total cost (1995) of a pH control system as described above is $350. to $400. depending on availability of system components.


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Why add silicon to nutrient solution?

Although silicon has not been recognized as an essential element for higher plants, its beneficial effects have been shown in many plants. Silicon is abundant in all field grown plants, but it is not present in most hydroponic solutions. Silicon has long been recognized as particularly important to rice growth, but a recent study indicated that it may only be important during pollination in rice (Ma et al. 1989). The beneficial effects of silicon (Si) are twofold: 1) it protects against insect and disease attack (Cherif et al. 1994; Winslow, 1992; Samuels, 1991), and 2) it protects against toxicity of metals (Vlamis and Williams, 1967; Baylis et al. 1994). For these reasons, I recommend adding silicon (about 0.1 mM) to nutrient solutions for all plants unless the added cost outweighs its advantages.


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Experiences with phythium control in hydroponic solution

The phythium fungus has been the only serious disease we have encountered in our systems, and disease problems have been relatively rare, particularly when all parts of the system are kept covered to keep dust and dirt particles away from the solution. Every plant pathologist on the planet recommends sanitation as the best control procedure for phythium, yet many hydroponic systems are not as well sealed as they should be.

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