Nutrient Solutions for Greenhouse Vegetable Culture

Nutrient Availability

Employing soluble fertilizers provides readily available nutrients to plant roots.  However, whether the nutrients remain in solution for any length of time depends upon several factors; one of which is the makeup of the media as to being calcareous or non-calcareous.  For example, sand culture for tomatoes in Texas utilizes mostly calcareous sand which will remove non-chelated copper from the nutrient solution quickly.  With a drip irrigation system, the nutrient solution is applied to the same spot each time so that the feeder roots multiply under the emitter in sizeable mass.  Studies have shown that when the water needs of tomato plants were supplied in eight or more irrigations during the daylight period, sufficient copper was taken up for good plant growth and fruit yield but not enough to prevent the cracking or splitting of ripening fruit in hot weather.  A chelated copper instead of copper sulfate or copper chloride is suggested as the copper carrier in the nutrient solution for calcareous sand culture during harvest in hot weather.

Acidity

The pH of the rooting media in the range of 6.0 to 7.0 is very satisfactory for most vegetables.  When employing a nutrient solution through a drip irrigation system, it is necessary to maintain the nutrient solution below pH 7.0 to prevent calcium phosphate crystals from precipitating in the lines.  When the raw water contains more than 180 ppm of calcium, the total calcium in the water can be deleted from the formula when preparing the nutrient solution if the pH of the solution is maintained at 5.8 to 6.0.  If the pH is maintained at 6.6 to 6.8, only about half the calcium in the water can be considered available to the plants.

The nutrient solution in a recycled system such as NFT lettuce culture is so highly aerated that unless the pH is adjusted daily to about 5.8 to 6.0, calcium precipitates out of solution as insoluble calcium carbonate (limestone) and the pH rises.  As a result, calcium deficiency is exhibited on the lettuce as dark brown to black specks like black pepper on the edges of the new leaves.  When this occurs, the breakdown of the tissue is an excellent location of the Botrytis fungus to infect the lettuce.  Similarly, NFT tomatoes can become calcium deficient as exhibited by blossom-end-rot on the fruit.

Sulfuric acid is probably the best acid with which to lower the pH.  However, it is dangerous and extreme care should be taken with its use.  When using acids, always add the acid to the water, do not add water to the acid which can cause acid to splatter.

Muriatic acid as well as nitric acid could also be used to lower pH.  The fumes of both are highly toxic.

Soluble Salt Concentrations

The concentration of salts in solution and the roots can have a marked influence on fruit quality.  When salt levels are low, especially during the winter and early spring, the tomatoes can be mealy in texture and have a bland taste.  High salts generally result in fruit being sweet and tart.  As the salts become progressively higher, the growth of the tomato plants slows, the main stem becomes thin near the growing point, the fruit ripen at smaller than normal size, the fruit show very dark green shoulders, the plants wilt in the heat of the day, and the lower leaves die.

A dissolved salt testing meter (ds meter) is very useful for testing salt levels in the media to determine when leaching is needed to remove excess salts, especially if the raw water contains significant amounts of extraneous ions such as sodium, chlorides, and sulfates.  Growers employing a recycled nutrient solution can use a ds meter to determine the day-to-day usage of the total nutrients by the plants and the amount to add periodically to maintain the level desired.  By monitoring the drainage water from plots of different media, it was found that the yields of tomatoes decreased with increasing salt levels above 3500 ppm.  However, with a drip irrigation system, plants can produce excellent crops when the bulk of the roots are under an emitter in a low salt environment even though high salts occur in the drainage water.  Such a case was observed in soil culture irrigated with a nutrient solution through a drip system.  The plants showed no evidence of salt damage even though white salts appeared on top of the soil in the middle of the rows and the drainage water tested over 5000 ppm of dissolved salts.

Visual inspection of the plants daily in regard to vigor of foliage growth and fruit set and size is probably as important as many tests on the solution, the media, or the plant.

Recommended Nutrient Solutions

The basic Steiner nutrient solution for the major elements (as modified by increasing the phosphorus to 48 parts per million) is presented in Table 1.  One method of preparing the Steiner nutrient solution is given in Table 2.  Whether this nutrient solution is a universal one for most plants or primarily for tomatoes depends on the concentration of micronutrients.  The solutions in Table 4 list the micronutrient concentrates for a universal solution as well as for tomatoes.

Many nursery, foliage and flowering plants will tolerate the higher micro-nutrients as listed for tomatoes for a short time or for an occasional irrigation.  If used over an extended period, some species of plants are likely to exhibit some micronutrient toxicity.  This might be shown by a burn, spotting or chlorosis of the new leaves except for boron toxicity which exhibits a uniform burn around the edge of the older leaves.  Cucumbers and lettuce seem to tolerate the higher micronutrients as given for tomatoes.

For a single long tomato crop with fruit set and harvest through the winter, a few tomato growers have had to increase the manganese 1 ppm with a maximum of 2 ppm in the nutrient solution in order to have bloom development.  If it is necessary to increase the iron 1ppm with a maximum of 4 ppm in the nutrient solution.

Potassium magnesium sulfate (K-mag, Sul-Po-Mag or sulfate of potash-magnesia) is a much less expensive source of magnesium than Epsom salts.  Epsom salts can be concentrated over 1000 times more than needed to supply the magnesium in Table 2 and is quickly soluble.  The potassium magnesium sulfate can be concentrated about 150 times, but requires 12 or more hours of continuous stirring to dissolve.  One grower solved the slow solubility problem by adding the amount for 100 gallons of dilute solution in 1 gallon plastic jugs once or twice a week, the fertilizer dissolved in 2 to 3 weeks.  Whenever his 600-gallon feed tank needed refilling, the grower emptied 6 jugs in the tank for the potassium magnesium sulfate requirement.

The fertilizer grade monocalcium phosphate (0-46-0) is normally the least expensive source of a soluble phosphate.  It also requires more time to dissolve than most other phosphorus sources.  It is not suitable for use with most proportioners since it will not concentrate much over 50 times the phosphorus requirement given in Table 1.  Considerable unwanted sludge is obtained after the soluble material is dissolved.  The sludge need not be a problem when allowed to settle and the clear solution filtered.

The pH of the raw water employed for the nutrient solution can have a bearing on the phosphorus source used.  It would be desirable with an alkaline water of about pH 7.5 to 8.0 or above to have phosphoric acid as the phosphorus source since it may lower or help to lower the pH of the nutrient solution below 7.0.  A non-acid phosphorus source such as monocalcium phosphate would be best for raw water with pH of 6.0 to 7.0.

For calcareous sand media, the pH of the nutrient solution applied by drip irrigation should range from 6.6 to 6.8 in order to have minimal rock formation in the sand under the emitter.  The first sand culture crop with the nutrient solution delivered at pH 6.0 to the emitters resulted in a sand conglomerate about 6 to 7 inches in diameter.  A hammer was required to break the rock back to sand grains.  Since that first crop, there have been very few rocks with a nutrient solution of pH 6.6 to 6.8.

Preparing the Nutrient Solution

Predissolving each nutrient carrier in plastic barrels at 50 to 100 times the concentration called for in Table 2 can be very helpful in refilling the feed tank with a dilute solution.  When the nutrient carriers are once dissolved, the solution should not be agitated before using since any sludge that has settled out will be added to the feed tank where it can contribute to emitter plugging.  Likewise, the feed tank solution should not be agitated before or during the irrigation cycle.
The complete Steiner solution cannot be concentrated more than 4 to 5 times that given in Table 2 without some precipitate of calcium phosphate or calcium sulfate forming, therefore, in order to avoid precipitates when adding calcium nitrate first in preparing a dilute solution, do not add a second ingredient until the tank is at least one-third full.  If the calcium nitrate is added last, the tank should be at least one-third full with all ingredients dissolved before adding the calcium nitrate.

Suggested Procedures for Micronutrient Concentrate Preparation

  1. In a 5-gallon plastic bucket (with lid), add 16 liters of water and place on a level surface.  If you have no easy method of measuring 16 liters of water, add 17 quarts of water then remove on-third cupful from the 17 quarts.  That remaining will be very close to 16 liters.  With a waterproof marker or with a file, make a line around the inside of the bucket at the surface of the water.  This line is the 16 liter reference mark to which the bucket will be filled with water after adding and dissolving all the micronutrient ingredients.
  2. Weigh the boron carrier (Borax, Boric acid, or Solulor) and place in bottom of empty bucket.  Add sufficient water to make a thick paste while stirring with a rod made of plastic pipe.  Continue stirring while adding about 3 gallons of water.  Hot water will shorten the time of dissolving the boron carriers.
  3. Weigh the other micronutrient ingredients and add them to the boron solution while stirring.  The fine or small crystal copper sulfate requires considerably less stirring time than the coarse crystals.  There is no special order in adding the other ingredients to the boron solution.
  4. After all the ingredients are apparently dissolved, add about 8 milliliters of concentrated sulfuric acid.  Pour acid slowly into the micronutrient concentrate to avoid any possibility of splashing the acid on yourself.  The acid maintains sufficiently low pH to keep the micronutrient elements in solution.  When agricultural grade manganese sulfate is used, there will always be a slight amount of gray undissolved material settling to the bottom of the bucket.  Ignore this material as it will not dissolve.
  5. Fill the bucket with water to the reference mark as prepared in No. 1 above.  Keep a cover on the bucket and store out of the reach of children.

The 16 liter batch of micronutrient concentrate is sufficient for preparing 8,000 gallons of 100% dilute, ready-to-use, nutrient solution.  If you wish to add the individual micronutrients direct to a 1,000 gallon tank rather than prepare a concentrate, you could weigh one-eighth of each ingredient given in the micronutrient concentrate table and predissolve them in a plastic container before adding them to the tank.

Preparing Concentrates for Proportioners

Multiple-head proportioners are necessary when employing concentrates of a complete nutrient solution.  A twin head proportioner can be employed if the injection ration is one part of concentrate to 100 or less parts of water.  If the injection ration is 1:180, a three-head proportioner is required for a two-part solution with each concentrated 100 times that given in Table 2, it is suggested that one concentrate contain calcium nitrate and iron chelate plus part of the potassium nitrate if the iron chelate will stay in solution.  The second concentrate should contain all other ingredients plus that part of the potassium nitrate if the iron chelate will stay in solution.  The potassium nitrate can be added to either concentrate as far as compatability regarding the formation of no precipitates.

To prepare concentrates for a three-head proportioner with injection ration of 1:180, make one concentrate with calcium nitrate at 180 times that given in Table 2.  It is doubtful that the iron would stay in solution with the high concentration of calcium was obtained which must be made up from some other source.  In checking the Ca column in Table 4, there are three carriers listed other than calcium nitrate.  The first one, monocalcium phosphate, would not supply enough without oversupplying phosphorus.  The next, calcium chloride, would give 74 ppm for 100 grams per 100 gal, which is sufficient and well within the 5% deviation.  However, you will note that it also supplies 169 ppm of chloride ion which in some cases can be a problem, especially if the water is already very high in chlorides.  In this example, 100 grams of calcium chloride pr 100 gal of water was selected to supply the calcium.

Since the potassium nitrate was deleted to help lower the nitrogen, the 65 ppm K formerly supplied by the potassium nitrate must be made up from some other source.  In this example, the potassium sulfate was increased to supply 65 ppm of K.

The calcium deleted with the calcium nitrate could have been supplied by gypsum (calcium sulfate) since the solubility in water alone is about nine grams per gallon.  In the primary nutrient solutions, gypsum was the source of calcium and ammonium nitrate was our nitrogen source.  However, like potassium magnesium sulfate (K-Mag or Sul-Po-Mag), about 12 or more hours of constant stirring was required to dissolve the gypsum.

When substitutions are to be made, the fertilizers could be listed along with the nutrient elements supplied similar to that in Tables 4 and 5, and a formula made to satisfy the total nutrients required for your use.

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