Foliar spraying and fertigation are two occasions when I am comfortable supplementing my organic fertilizers with water-soluble chemical fertilizers.  The best and most expensive brand is Rapid-Gro.  Less costly concoctions such as Peters 20-20-20 or the other “Grows,” don’t provide as complete trace mineral support or use as many sources of nutrition.  One thing fertilizer makers find expensive to accomplish is concocting a mixture of soluble nutrients that also contains calcium, a vital plant food.  If you dissolve calcium nitrate into a solution containing other soluble plant nutrients, many of them will precipitate out because few calcium compounds are soluble.  Even Rapid-Gro doesn’t attempt to supply calcium.  Recently I’ve discovered better-quality hydroponic nutrient solutions that do use chemicals that provide soluble calcium.  These also make excellent foliar sprays.  Brands of hydroponic nutrient solutions seem to appear and vanish rapidly.  I’ve had great luck with Dyna-Gro 7-9-5.  All these chemicals are mixed at about 1 tablespoon per gallon.

Vegetables That: 

Like foliars
Asparagus Carrots Melons Squash
Beans Cauliflower Peas Tomatoes
Broccoli Brussels sprouts Cucumbers
Cabbage Eggplant Radishes
Kale Rutabagas Potatoes

Don’t like foliars
Beets Leeks Onions Spinach
Chard Lettuce Peppers

Like fertigation
Brussels sprouts Kale Savoy cabbage
Cucumbers Melons Squash
Eggplant Peppers Tomatoes

Fertigation every two to four weeks is the best technique for maximizing yield while minimizing water use.  I usually make my first fertigation late in June and continue periodically through early September.  I use six or seven plastic 5-gallon “drip system” buckets, (see below) set one by each plant, and fill them all with a hose each time I work in the garden.  Doing 12 or 14 plants each time I’m in the garden, it takes no special effort to rotate through them all more or less every three weeks.

To make a drip bucket, drill a 3/16-inch hole through the side of a 4-to-6-gallon plastic bucket about 1/4-inch up from the bottom, or in the bottom at the edge.  The empty bucket is placed so that the fertilized water drains out close to the stem of a plant.  It is then filled with liquid fertilizer solution.  It takes 5 to 10 minutes for 5 gallons to pass through a small opening, and because of the slow flow rate, water penetrates deeply into the subsoil without wetting much of the surface.  Each fertigation makes the plant grow very rapidly for two to three weeks, more I suspect as a result of improved nutrition than from added moisture.  Exactly how and when to fertigate each species is explained in Chapter 5.

Organic gardeners may fertigate with combinations of fish emulsion and seaweed at the same dilution used for foliar spraying, or with compost/manure tea.  Determining the correct strength to make compost tea is a matter of trial and error.  I usually rely on weak Rapid-Gro mixed at half the recommended dilution.  The strength of the fertilizer you need depends on how much and deeply you placed nutrition in the subsoil.