Perlite-grown plants can be irrigated by a starter-tray set-up like rockwool. We have had good success using the start-tray and similar management schemes to those for rockwool. Several (40 to 50) small slits are made in the bottom of a perlite bag and the bag formed into the start tray.

The approach to fertilizer and water management, with either timer or start tray, is to apply enough water and enough nutrients at the correct time of crop requirement. Usually we start early in the season with nutrient solutions low in N concentration. Frequent, short irrigations will supply enough total nutrients to the crop. If a timer is used and irrigations are infrequent (once or twice per day), then a more concentrated nutrient solution might be needed.

Fully grown tomato plants may use two to three pints of water per day in the winter (including that for leaching) and three to five pints on warm spring days.

Watering frequency is the key to balancing the amount needed by the crop with the total needed for the crop and leaching. A general rule-of-thumb is to leach about 10 to 15%. Leaching is needed to minimize salt buildup in the media.

What about media salt content? High salts in the media can damage plant roots and upset nutrient and water uptake by roots. Tomatoes can tolerate fairly high soluble salt content in the root zone. As water is absorbed by plants, some salts are left behind in the media. These salts are mostly carbonates and sulfates, e.g. calcium sulfate, calcium carbonate (lime), and magnesium carbonate. If you are applying a nutrient solution of 1.0 decisiemens/meter electrical conductivity (EC), you can tolerate a media EC of 1.5. If you are applying a solution of 2.0 E.C. (full-grown plants), then you can tolerate a media BC of 2.5 to 2.8. The key is to watch the EC trends and begin corrective measures if it continues to climb. Climbing EC indicates the need to increase frequency of irrigation (more water) by raising the probe setting of the starter tray or increasing run time. Remember the idea is to balance the amount of water needed by the crop with that needed by crop plus leach. Maintaining the EC of the media slightly above the delivered solution shows that you are a good manager of nutrient solution delivery.

It is a good idea to have a few milk jugs positioned around the house with an extra emitter punched in. Paint the jug black to reduce algae growth and then scratch a clear line from top to bottom of the jug so you can see the solution level. Calibrate the mark by sequentially pouring in pints of water and marking each pint level. The jug helps tell you that the system came on that day and it can tell you how much water was applied. With experience, you can tell how much solution should be in the jug on a cloudy day versus a sunny day or on a winter day versus a spring day. Several jugs about the house can help you get an idea of emitter flow rate uniformity. You might have problems if variance among emitters is greater than 15 percent. Get into the habit of performing the "bag slap test." Pick up the corner of one bag and flop it up and down a couple of times to get the feel for its weight. Check a random six bags per house every day or so. Lightweight bags indicate not enough irrigation or clogged emitters.

Be sure the emitter spaghetti tube length (of the emitter plugged in for the jug), is the same length and size as those for the plants. Otherwise, different flow rate could result. Also, be sure to position opening of emitter, or tube in the milk jug, so that the opening is above the level of the irrigation line in between the rows. Otherwise, siphoning (forward or backward) could result in false jug accumulations.

Check emitter flow rate and uniformity periodically. Use a graduated cylinder or a measuring cup. Collect solution as it is delivered for several emitters about the house and check for uniformity. They should be within 15% of each other. Also measure flow rate - amount of solution delivered in 15 seconds, for example. This can help determine how long to run the system each cycle. Remember the idea is to achieve about 10 to 15% of leach and to manage the media EC slightly higher than the delivered EC. It takes time and experience to get all the pieces together. Early season irrigation programs are different from full-grown plants. Young seedlings do not have large root systems in the media. Therefore, young seedlings (up to about first blossom) will need to be irrigated on a timed basis instead of by the starter tray. Irrigate at least three times a day with enough solution to wet the young root system to encourage roots into the media. After roots become established in the perlite, then control can be switched over to the starter tray.

What about starter tray probe setting? Generally, shallow setting is for frequent irrigations and deeper setting is for less frequent irrigations. Probe setting is one way to adjust the irrigation cycles to achieve the desired leach rate, to achieve the desired amount of water for the plant, and to manage the soluble salt level in the media.

Sometimes, a salt deposit builds up on the probe point. This should be removed regularly as it in effect lengthens the probe. Also, algae and bacterial slime might build up in the probe reservoir. Slimes might create a strand from the reservoir to the probe, thus maintaining contact between solution and probe, thus reducing irrigation cycle frequency.

Also, roots can grow into the reservoir and should be trimmed back.

A starter tray should be placed in an area of the house representative of the house, generally at least 1/4 of the distance from end walls. Place tray in an alley where it will be observed daily. It is amazing how many growers do not observe their tray frequently. The tray is the heart of the irrigation system, therefore, it is the heart of your profits! Check it daily!

RECORD KEEPING