Reprinted from the New Organic Grower by Eliot Coleman copyright (c) 1995 used with permission from Chelsea Green Publishing Co., White River Junction, Vermont (www.chelseagreen.com)
Chapter 14 - Soil Blocks
It is always satisfying to find a technique that is simpler, more effective, and less expensive than what existed before. For the production of transplants, the "soil block" meets those criteria. The Dutch have been developing this technique for some 80 years, but the human experience with growing plants in a cube of "soil" goes back 2,000 years or more. The story of how cubes or rich mud were used to grow seedlings by the Aztec horticulturalists of the chinampas of Xochimilco, Mexico, makes fascinating reading.(Click here for the full story) A related technique is the old market gardener's practice of using 4-5 inch cubes of partially decomposed inverted sod for growing melon and cucumber transplants.
How Soil Blocks Work
A soil block is pretty much what the name implies --a block made out of lightly compressed potting soil. It services as both the container and the growing medium for a transplant seedling. The blocks are composed entirely of potting soil and have no walls as such. Because they are pressed out by a form rather than filled into a form, air spaces provide the walls. Instead of the roots circling as they do upon reaching the wall of a container, they fill the block to the edges and wait. The air spaces between the blocks and the slight wall glazing caused by the block form keep the roots from growing from one block to another. The edge roots remain poised for rapid outward growth. When transplanted to the field, the seedling quickly becomes established. If the plants are kept too long in the blocks, however, the roots do extend into neighboring blocks, so the plants should be transplanted before this happens.
Despite being no more than a cube of growing medium, a soil block is not fragile. When first made, it is bound together by the fibrous nature of the moist ingredients. Once seeded, the roots of the young plant quickly fill the block and ensure its stability even when handled roughly. Soil blocks are the answer for a farm-produced seedling system that costs no more than the "soil" of which it is composed.
The best thing about the soil-block system is that everything that can be done in small pots, "paks", trays, or plugs can be done in blocks without the expense and bother of a container. Blocks can be made to accommodate any need. The block may have a small depression on the top in which a seed is planted, but blocks can also be made with a deep center hole in which to root cuttings. They can also be made with a large hole in which to transplant seedlings. Or they can be made with a hole precisely the size of a smaller block, so seedlings started in a germination chamber in small blocks can be quickly transplanted onto larger blocks.
Blocks provide the modular advantages of plug trays with the problems and expense of a plug system. Blocks free the grower from the mountains of plastic containers that have become so ubiquitous of late in horticultural operations. European growers sell bedding plants in blocks to customers, who transport them in their own containers. There is no plastic pot expense to the grower, the consumer, or the environment. In short, soil blocks constitute the best system I have yet found for growing seedlings.
The Soil-Block Maker
The key to this system is the tool for making soil blocks --the soil-block maker or "blocker". Basically, it is an ejection mold that forms self-contained cubes out of a growing medium. Both hand and machine models are available. For small-scale production, hand-operated models are perfectly adequate. Motorized block-making machines have a capacity of over 10,000 blocks per hour. But they way overscaled for a 5-acre vegetable farm.
There are two features to understand about the blocker in order to appreciate the versatility of soil blocks: the size of the block form and the size and shape of the center pin.
Forms are available to make 3/4-inch blocks (the mini-blocker), 1-1/2-inch blocks, 2-inch blocks, 3-inch blocks, and 4-inch blocks (the maxi-blocker). The block shape is cubic rather than tapered. Horticultural researchers have found a cubic shape to be superior to the tapered-plug shape for the root growth of seedlings.
Two factors influence choice of block size --the type of plant and the length of the intended growing period prior to transplanting. For example, a larger block would be used for early sowings or where planting outside is likely to be delayed. A smaller block would suffice for short-duration propagation in summer and fall. The mini-block is used only as a germination block for starting seedlings. Obviously, the smaller the block, the less potting mix and greenhouse space is required (a 1-1/2-inch block contains less than half the volume of a 2-inch block). But, in choosing between block sizes, the larger of the two is usually the safer choice. Of course, if a smaller block is used, the plants can always be held for a shorter time. Or, as is common in European commercial blocking operations, the nutrient requirements of plants in blocks too small to maintain them can be supplemented with soluble nutrients. The need for such supplementary fertilization is an absolute requirement in plug-type systems, because each cell contains so much less soil than a block. The popular upside-down pyramid shape, for example, contains only one-third the soil volume of a cubic block of the same top dimension.
My preference is always for the larger block, first because I believe it is a false economy to stint on the care of young plants. Their vigorous early growth is the foundation for later productivity. Second, I prefer not to rely on soluble feeding when the total nutrient package can be enclosed in the block from the start. All that is necessary when using the right size block and soil mix is to water the seedlings.
Another factor justifying any extra volume of growing medium is the addition of organic matter to the soil. If lettuce is grown in 2-inch blocks and set out at a spacing of 12 by 12 inches, the amount of organic material in the blocks is equivalent of applying 5 tons of compost per acre! Since peat is more than twice as valuable as manure for increasing long-term organic matter in the soil, the blocks are actually worth double their weight in manure. Where succession crops are grown, the soil-improving material added from the transplanting alone can be substantial.
The pin is the object mounted in the center of the top press-form plate. The standard seed pin is a small button that makes an indentation for the seeds in the top of the soil block. This pin is suitable for crops with seeds the size of lettuce, cabbage, onion, or tomato. Other pin types are dowel- or cube-shaped. I use the cubic pin for melon, squash, corn, peas, beans, and many other seeds of those dimensions. A long dowel pin is used to make a deeper hole into which cuttings can be inserted. Cubic pins are also used so a seedling in a smaller block can be potted on to a larger block; the pin makes a cubic hole in the top of the block into which the smaller block is placed. The different types of pins are easily interchangeable.
The 3/4-inch block made with the mini-blocker is used for starting seeds. With this small block, enormous quantities of modular seedlings can be germinated on a heating pad or in a germination chamber. This is especially useful for seeds that take a long time to germinate, because a minimum of space is used in the process.
Mini-blocks are effective because they can be handled as soon as you want to pot on the seedlings. The oft-repeated admonition to wait until the first true leaves appear before transplanting is wrong. Specific investigations by W.J.C. Lawrence, one the early potting-soil researchers, have shown that the sooner young seedlings are potted on, the better is their eventual growth. (Click here for the full story)
The 1-1/2 -inch block is used for short-duration transplants of standard crops (lettuce, brassicas) and as the seed block for cucumbers, melons, and artichokes by using the large seed pin, When fitted with a long dowel pin it makes excellent for rooting cuttings.
The 2-inch block is the standard for longer-duration transplants. When fitted with the 3/4-inch cubic pin, it is used for germinating bean, pea, corn, or squash seeds and for the initial potting on of crops started in mini-blocks.
The 3-inch block fitted with a 3/4-inch cubic in offers the option to germinate many different field crops (squash, corn cucumber, melon) when greenhouse space is not critical. It is also an ideal size for potting on asparagus seedlings started in mini-blocks.
The 4-inch block fitted with a 1-1/2-inch or 2-inch cubic pin is the final home of artichoke, eggplant, pepper, and tomato seedlings. Because of its cubic shape, it has the same soil volume as a 6-inch pot and can grow exceptional plants of these crops to their five to eight-week field transplant age.
Other Pin Options
In addition to the pins supplied with the blocker, the grower can make a pin of any desired size of shape. Most hard materials (wood, metal, or plastic) are suitable, as long as the pins have a smooth surface. Plug trays can be used as molds and filled with quick-hardening water putty to make many different sizes of pins that allow the integration of the plug and block systems.
When transplants are grown, whether in blocks or pots, their rooting area is limited. Therefore the soil which they grow must be specially formulated to compensate for these restricted conditions. For soil blocks, this special growing medium is called a blocking mix. The composition of a blocking mix differs from ordinary potting soil because of the unique requirements of block-making. A blocking mix needs extra fibrous material to withstand being watered to a paste consistency and then formed into blocks. Unmodified garden soil treated way would become hard and impenetrable. A blocking mix also needs good water-holding ability, because the blocks are not enclosed by a nonporous container. The bulk ingredients for blocking mixes are peat, sand, soil, and compost. Store-bought mixes can also work, but most will contain chemical additives not allowed by many organic certification programs. If you can find a commercial peat-perlite mix with no additives, you can supplement it with the soil, compost, and extra ingredients described below.
In the past few years commercial, preformulated organic mixes with reasonably good growth potential have begun to appear on the market. However, shipping costs can be expensive if you live far away from the supplier. To be honest, I have yet to find any of these products that will grow as nice seedlings as my own homemade mixes.
Peat is a partly decayed, moisture-absorbing plant residue found in bogs and swamps. It provides the fiber and extra organic matter in a mix. All peats are not created equal, however, and quality can vary greatly. I recommend using the premium grade. Poor-quality peat contains a lot of sticks and is very dusty. The better-quality peats have more fiber and structure. Keep asking and searching your local garden suppliers until you can find good-quality peat moss. Very often a large greenhouse operation that makes its own mix will have access to a good product. The gives "body" to a block.
Sand or some similar granular substance is useful to "open up' the mix and provide more air porosity. A coarse sand with particles having 1/8- to 1/16- inch diameter is the most effective. I prefer not to use vermiculite, as many commercial mixes do, because it is too light and tends to be crushed in the block-making process. If I want a lighter-weight mix I replace the sand with coarse perlite. Whatever the coarse product involved, adequate aeration is the key to successful plant growth in any medium.
Compost and Soil
Although most modern growing mediums no longer include any real soil, I have found both soil and compost to be important for plant growth in a mix. Together they replace the "loam" of the successful old-time potting mixtures.
[Loam is made by stacking layers of sod from a grass field upside-down to decompose for a year or two. The development of the old loam-based mixes is well covered in Seed and Potting Composts by W.J.C. Lawrence and J. Newell (London: Allen & Unwin, 1939). I based my earliest mixes on modifications of their formulas by using soil and compost to replace the loam.]
In combination with the other ingredients, they provide stable, sustained-release nutrition to the plants. I suspect the most valuable contribution of the soil may be to moderate any excess nutrients in the compost, thus giving more consistent results. Whatever the reason, with soil and compost included there is no need for supplemental feeding.
Compost is the most important ingredient. It is best taken from two-year-old heaps that are fine in texture and well decomposed. The compost heap must be carefully prepared for future use in potting soil. I use no animal manure in the potting-mix compost. I construct the heap with 2- to 6-inch layers of mixed garden wastes (e.g., outer leaves, pea vines, weeds) covered with a sprinkling of topsoil and 2 to 3 inches of straw sprinkled with montmorillionite clay (Montmorillionite clay is an expanding-lattice clay that has been determined to have both biotic and abiotic effects in aiding the conversion of organic matter into stable humus. The resulting clay humus fraction that develops is very beneficial to soil fertility and plant growth.). The sequence is repeated until the heap is complete. The heap should be turned once the temperature rises and begins to decline so as to stimulate further decomposition. There are no worms involved in our composting except those naturally present, which is usually a considerable number. (I have purchased commercial worm composts [castings] as a trial ingredient, and they did make an adequate substitute for our compost.) Both during breakdown and afterwards the heap should be covered with a landscape fabric. I strongly suggest letting the compost sit for an additional year (so that it is one and a half to two years old before use); the resulting compost is well worth the trouble. The better the compost ingredient, the better the growth of the plants will be. The exceptional quality of seedlings grown in this mix is reason enough to take special care when making a compost. Compost for blocking mixes must be stockpiled the fall before and stored where it won't freeze. Its value as a mix ingredient seems to be enhanced by mellowing in storage over the winter.
Soil refers to fertile garden soil that is also stockpiled ahead of time. I collect it in the fall from land which onions have just been harvested. I have found that seedlings (onions included) seem to grow best when the soil in the blocking mix has grown onions. I suspect there is some biological effect at work here, since crop-rotation studies have found onions (and leeks) to be highly beneficial preceding crops in a vegetable rotation. [T.E. Odland, R.S. Bell and J.B. Smith, "The Influence of Crop Plants on Those Which Follows: V." Bulletin No. 309. (Kingston, RI: Rhode Island Agricultural Experiment Station, 1950).] The soil and compost should be sifted through a 1/2-inch mesh screen to remove sticks, stones, and lumps. The compost and peat for the extra-fine mix used either for mini-blocks or for the propagation of tiny flower seeds are sifted through a 1/4-inch mesh.
Lime, blood meal, colloidal phosphate, and greensand are added in smaller quantities.
Lime. Ground limestone is added to adjust the pH of the blocking mix. The quantity of lime is determined by the amount of peat, the most acidic ingredient. The pH of compost or garden soil should not need modification. My experience, as well as recent research results, has led me to aim for a growing-medium pH between 6 and 6.5 of all the major transplant crops. Those growers using different peats in the mix may want to run a few pH tests to be certain. However, the quantity of lime given in the formula below works for the different peats that I have encountered.
Blood Meal. I find this to be the most consistently dependable slow-release source of nitrogen for growing mediums. English gardening books often refer to hoof-and-horn meal, which is similar. I have also used crap-shell meal with great success. Recent independent research confirms my experience and suggests that cottonseed meal and dried whey sludge also work well. [B. Gagnon and S. Berrourard, "Effects of Several Organic Fertilizers on Growth of Greenhouse Tomato Transplant." Canadian Journal of Plant Science, vol. 74, no. 1 (1994), pp. 167-68.]
Colloidal Phosphate. A clay material associated with phosphate rock deposits and containing 22 percent P2O5. The finer the particles the better. Greensand. Greensand contains some potassium but is used here principally as a broad-spectrum source of micronutrients. A dried seaweed product like kelp meal can serve the same purpose, but I have achieved more consistnet results with greensand.
The last three supplementary ingredients --blood meal, colloidal phosphate and greensand --when mixed together in equal parts are referred to as the "base fertilizer."
Blocking Mix Recipe
A standard 10-quart bucket is the unit of measurement for the bulk ingredients. A standard cup measure is used for the supplementary ingredients. This recipe makes approximately 2 bushels of mix. Follow the steps in the order given.
- 3 buckets brown peat
- 1/2 cup lime. Mix.
- 2 buckets coarse sand or perlite
- 3 cups base fertilizer. Mix.
- 1 bucket soil
- 2 buckets compost
- Mix all ingredients together thoroughly.
The lime is combined with the peat because that is the most acidic ingredient. Then the sand or perlite is added. The base fertilizer is mixed in next. By incorporating the dry supplemental ingredients with the peat in this manner, they will be distributed as uniformly as possible throughout the medium. Next add the soil and compost, and mix completely a final time.
To use this recipe for larger quantities, thin of it measured in "units". The unit can be any size, so long as the ratio between the bulk and the supplementary ingredients is maintained. A "unit" formula would call for:
- 30 units brown peat
- 1/8 unit lime
- 20 units coarse sand or perlite
- 3/4 unit base fertilizer
- 10 units soil
- 20 units compost
Mini Block Recipe
A different blend is used for germinating seeds in mini-blocks. Seeds germinate better in a "low-octane" mix, without any blood meal added. The peat and compost are finely screened through a 1/4-inch mesh before adding them to the mix.
- 16 parts (4 gallons) brown peat
- 1/4 part (1 cup) colloidal phosphate
- 1/4 part (1 cup) greensand (If greensand is unavailable, leave it out. Do not substitute a dried seaweed product in this mix.)
- 4 parts (1 gallon)compost (well decomposed)
Sterilizing the Mix
In more than 20 years of using homemade mixes, I have never sterilized them. And I have not had problems. I realized early on that damping-off and similar seedling problems, which are usually blamed on unsterilized soil, are actually a function of cultural mistakes like overwatering, a lack of air movement, not enough sun, overfertilization, and so forth. Good, fertile garden soil and well prepared compost contain many organisms that benefit seedling growth. If you "sterilize" these ingredients you lose the benefits of a live mix without gaining the advantages that are achieved through proper seedling management. Recent university studies agree and emphasize the specific value of finished compost as a disease-suppressing ingredient in growing mixes.
[Harry A.J. Hoitink, "Bases for the Control of Soilborne Pathogens with Composts," Ann. Rev. Phytopath, vol. 24 (1986), pp. 93-114.]
[H.A.J. Hoitink, Y. Inbar, and M.J. Baehun, "Status of Compost-Amended Potting Mixes," Plant Disease (Sept 1991), pp. 869-73]
With certain crops (mostly the more delicate bedding-plant flowers) there may be a further consideration. Where organic sources of nitrogen like blood meal or the old-time hoof-and-horn meal are included in a mix, the mineralization of the nitrogen by biological processes and the consequent production of ammonia can inhibit plant growth for a period of time after the mix is made, especially if moisture and temperature levels are high. [A.C. Bunt, Modern Potting Composts (University Park, PA: Penn State University Press, 1976).] If you use a dried seaweed product instead of greensand this consideration probably applies as well. To avoid this reaction, make up the mix fresh as you need it and never store it for more than three weeks. To my knowledge I have never been bothered by this problem, but I feel it is worth mentioning. In my experience, when the mix is stored for more than three months, it actually gets better, as all the ingredients seem to mellow together.
One of the European organic farms I visited actually processed their mix for a whole year before using it by layering the ingredients --horse manure on the bottom, then leaf mold, then compost --in a cold frame and growing first a crop of cabbage, followed by melons, then mache. After the manure, leaf mold, an compost have been "processed" for a year by the roots of those crops, they become the basis for the mix. Seven parts of the processed ingredients are mixed with three parts of peat, rock powders are added, and the mix is ready. Another grower of my acquaintance uses only pure compost for growing seedlings in flats and plug trays. I relate those stories as examples of the extremely wide variety of answers that different growers have found to the potting soil question. The formulas I have given above the answers that worked well so far for me. They are not the only answers.
Moistening the Mix
Water must be added to wet the mix to blocking consistency. The amount of water varies depending on the initial moisture content of the ingredients. On average, to achieve a consistency wet enough for proper soil block-making, the ratio of water to mix by volume will be about 1 part water to every 3 parts mix. A little over 2-1/2 gallons of water should be added to every cubic foot of mix. For successful soil block-making, be sure to use a mix that is wet enough. Since this will be much wetter than potting mixes used for pots or flats, it takes some getting used to. The most common mistake in soil block making is to try to make blocks form a mix that is too dry. The need to thoroughly wetten the mix is the reason why the mix requires a high percentage of peat, to give it the necessary resiliency.
Handling Soil Blocks
Many large soil block-making operations set the newly formed and seeded blocks by the thousands on a plastic sheet on the floor of the greenhouse. When they are ready to go the field, the blocked seedlings are lifted with a broad, fine-tined fork and slid into transport crates. These crates have high sides so they can be stacked for transport without crushing the seedlings. In lieu of these special crates, two other options are practical for small-scale production.
Simple three-sided wooden flats work well for soil blocks. The inside dimensions are 18-3/4 inches long by 8 inches wide by 2 inches high. Three-quarter-inch stock is used for the sides and 1/2-inch stock for the bottom. One flat holds 60 of the 1-1/2 blocks, 36 of the 2-inch blocks, or 18 of the 3-inch size. These soil block flats are efficient to use in the greenhouse, because the benches need to be no more than 2 x 4s spaced to hold two rows of flats side by side. Low-sided flats such as these are not stackable when filled with plants. For transport, a carrying rack with spaced shelves is required.
The flats have only three sides so the blocked seedlings can be easily removed from the open side one at a time as they are being transplanted in the field. The flat is held in one hand by the long side while soil blocks are quickly placed in holes in the soil with the other. Similar three-sided flats (half as wide and only 3/4 inch high at the sides) are used for mini-blocks. Since they are th same length as the others, they fit two to a space on the greenhouse bench for modular efficiency. Each of these flats holds 120 mini-blocks.
When handling greater quantities of blocks, you can use the large plastic-mesh bread trays seen in bread delivery trucks. They can generally be bought used at a reasonable cost from regional bakeries. Since the sides on these trays are higher than all except the tallest seedlings, they can be stacked for transport. Bread trays vary in size, but on average each tray can hold 200 of the 1-1/2inch blocks and proportionally few of the larger sizes. Results are excellent with bread trays. What with the open mesh sides and bottom plus the air spaces between the blocks, the roots of the seedling remain poised at all five potential soil-contact surfaces. The bread trays are not as easy to handle for field transplanting as the smaller three sides flats, but they become manageable with practice.
Making Soil Blocks
The wet mix should be spread on a hard surface at a depth thicker than the blocks to be make. The soil-block maker is filled by pressing it into the mix with a quick push and a twisting motion to seat the material. Lift the soil blocker, scrape off any excess mix against the edge of a board, and place the blocker on the three-sided flat, the bread tray, the plastic sheet, a concrete floor, or other surface. The blocks are ejected by pressing on the spring-loaded handle and raising the form in a smooth, even motion. (Occasionally, if the mix is shade too moist, some blocks may fall out when you lift the blocker. If you first tip the blocker slightly with a quick motion before lifting it, you break the moist suction between the soil blocks and the surface beneath them, ensuring that the blocks remain inside the blocker.) After each use the soil blocker is dipped in water to rinse it. A surprising rate of block production (up to 5,000 per hour using the 1-1/2-inch commercial-scale model) will result with practice.
Seeding the Blocks
Each block is formed with an indentation in the top to receive the seed. The handmade blocks are usually sown by hand. With the motorized blockers, the sowing as well as the block-forming is mechanized. An automatic seeder mounted over the block belt drops one seed into each indentation as the blocks pass under it. These motorized models are too large and expensive for the small-scale grower, but if a group of growers get together, there is a role for one of them in a specialized seedling operation. Small farmers always benefit from such cooperative arrangements and should consider participating whenever the opportunity arises.
One seed is sown per block. There is a temptation to use two (just to be on the safe side), but that is not necessary. Germination is excellent in soil blocks because of the ease with which ideal moisture and temperature conditions can be maintained. The few seeds that don't germinate are much less of a problem than the labor to thin all those that do. Of course, if th seed is of questionable vitality, it is worth planting more than one seed per block, but obviously it pays to get good seed to begin with. Seeding can be done the fingers for large seeds such as cucumber, melon, and squash. Finger-seeding is also possible for small seeds that have been pelleted, although pelleted seeds are not easily available in most varieties, and naked seeds are more commonly used. The small seeds can be more accurately handled by using a small thin stick, a sharpened dowel, a toothpick, or similar pointed implement. Seeds are spread on a dish. The top of the stick is moistened in water and touched to one seed. The seed adheres to the tip and is moved to the seed indentation in the top of a soil block and deposited there. The solid, moist block has more friction than the tip of the stick, so the seed stays on the block.
Another obvious technique is to crease one side of a seed packet or use any other V-shaped container and tap out the seeds by striking the container with the fingers or a small stick. The Park Seed Company sells small seeds in packets made of a heavy metal foil. If you take a pair of scissors and cut and crease an empty packet, as shown at left, the resulting "seed tapper" works exceptionally well, even for tiny seeds. Put only enough seeds in the packet at one time so they can be tapped out in a single row without bunching up.
Commercial seeding aids are available that aim to either wiggle, click, or vibrate th seeds out one by one. There are electrically operated vacuum seeders for the small-scale grower than can be adapted to seeding soil blocks. I have experimented with a nonelectric, homemade vacuum seeder specifically for mini-blocks which gets its suction from the return stroke of a foot-powered pump for rubber rafts. I haven't quite perfected the suction tips yet, but I will get it right one of these days. Instructions for a homemade, multipoint vacuum seeder are given in a recent issue of HortScience.[J.L. Townsend, "A Vacuum Multi-Point Seeder for Pots," HortScience, vol.22, no.6 (1987), p. 1328] Growers should try such aids and decide for themselves whether they are worth it. Whichever method is used though, the seeding should be done carefully to ensure that the seeds are accurately planted in each block.
In practice, these planting techniques quickly become efficient and precise. Remember that for many crops the soil-block system avoids all intermediate potting on. Crops are started in the block and later go directly to the field. That savings in time alone is worth effort required to become proficient at single-seeding.
I never cover the seeds planted in mini-blocks. Oxygen is important for high-percentage seed germination. Thus, even a thin covering of soil or potting mix can lower the germination percentage. I find that to important for all small flower seeds also. If the sowing instructions suggests the seeds need darkness to germinate, I cover the flats temporarily with a sheet of black plastic. I keep the moisture level high during the germination period by misting frequently with fine spray of water. For the larger-seeded crops in the larger blocks, I get sturdier seedlings if I cover the seeds. I do that by sprinkling a thin layer of potting soil over the top of the blocks.
The third key to high-germination percentage, in addition to air and moisture, is temperature. Ideal temperature for germination can best be maintained by using a thermostatically controlled soil-heating pad under the blocks. Although there new low-watt models composed of a thin metal heating grid between layers of plastic, I still prefer the old heavy-duty model consisting of a 1/4-inch-thick red-rubber pad with heating cables embedded within. The temperature is controlled at the desired setting by a remote thermostatic probe inserted into the potting soil or in the gap between the soil blocks. I use a temperature of 70-75 degrees F. (21-24 degrees C.) for most crops. For asparagus, cucumber, eggplant, melon, pepper, and squash, I use a setting of 75-80 degrees F. (24-27 degrees C.).
Although I have stressed the wisdom of sowing only one seed per block, there is an important exception to that rule --the multiplant soil block. In this case 3 to 12 seeds are deliberately planted in each block with no intention of thinning. Many crops grow normally under multiplant conditions, and transplant efficiency is enhanced by putting out clumps rather than single plants.
The concept of the multiplant block is based on spatial rather than linear plant distance in the field. For example, say the average ideal in onion spacing is one plant every 3 inches in rows spaced 12 inches apart. Multiplants aim at an equivalent spacing of four onions per square foot. The difference is that all four onions are started together in one block and grow together until harvest. Since it is just as easy to grow four plants to the block as it is to grow one, there is now only one-quarter the block-making work and greenhouse space involved in raising the same number of plants. A similar advantage is realized when transplanting the seedlings to the field. When four plants can be handled as one, then only a quarter as many units need to be set out. Although bunched together, the plants will have extra space all around them. The onions grow normally in the clump, gently push each other aside, attaining a nice round bulb shape and good size.
Not only bulb onions but scallions (green onions) thrive in multiple plantings. Scallions are seeded 10-12 per block and grow in a bunch ready to tie for harvest. Weeding between the plants in the row is no longer a chore, since the wider spaces allow for easy cross-cultivation with a hoe. Obviously, multiplant blocks must be transplanted to the field a bit sooner (at a younger age) than single-plant blocks because of the extra seedling competition in the limited confines of the block. Multiplant blocks can be sown either seed by seed or in bunches. For counting out seeds, one of the wiggle, click, or vibration seeders have a place here in speeding up the seeding operation, though at the sacrifice of some accuracy. When I need to be precise, I first tap the number of seeds required into a 1/4 teaspoon measure so I can be sure of the count before sowing the soil block. Tiny scoops or spoons or other small-volume measures can be fabricated by the grower to hold 5, 12, or whatever number of seeds. These are used to scoop up the seeds and dump them in each seed indentation. This method is not as accurate as counting, but it is a lot faster.
Multiplant soil blocks are an efficient option for a number of crops. In my experience onions, scallions, leeks, beets, parsley, spinach, corn, pole beans, and peas have been outstandingly successful in multiple plantings. Spinach, corn, pole beans, and peas, which are rarely transplanted, even for the earliest crop, become a much more reasonable proposition when the transplant work can be cut by 75 percent. European growers claim additional good results with cabbage, broccoli, and turnips planted at three to four seeds per soil block.
Soil blocks are made in a moist condition and need to be kept that way. Their inherent moisture is what makes them such an ideal germination medium. It is therefore most important that blocks are not allowed to dry out, which can result both in a check to plant growth and difficulty rewetting. When blocks are set out on a bench or greenhouse floor, the edge blocks are the ones that are most susceptible to drying. A board the same height as the soil blocks placed along an exposed edge will help prevent this. Since the block has no restricting sides, the plants never sit in too much water. The block itself will take up no more water than it can hold. To prevent erosion of the soil block, watering at first should be done gently with a very fine rose. If the rose is not fine enough, the mini-blocks should be misted rather than watered. Once the plants in blocks are growing, water can be applied through any fine sprinkler. Extra care in attention to watering is a general rule successful soil block culture. It will be repaid many times over in the performance of the seedlings.
Potting on is the practice of starting seeds in smaller blocks and then setting those soil blocks into larger soil blocks for further growth. Since most crops benefit form the bottom heat to ensure and speed up germination, this practice makes efficient use of limited space in germination chambers or on heating pads. For example, 240 mini-blocks fit into the same space as only 36 of the 2-inch blocks.
Potting on blocks is quickly accomplished in one-third the time required for potting on bare-root seedlings. The smaller block easily fits into a matching size hole in a larger block. The mini-blocks are usually potted on to 2-inch soil blocks and those, in turn, to 4-inch maxi-blocks.
The 2-inch blocks are easily potted on using the fingers. For the mini-blocks, some form of transplant tool for lifting the blocks and pressing them into the cavity will be useful. One of the best implements for this job is a flexible artist's pallet knife. It provides the extra dexterity necessary to handle mini-blocks with speed and efficiency.
Keep Them Growing
Potting on should be carried out as soon as the seeds have germinated in the mini-blocks and before the roots begin growing out of the small cubes. The less stress seedlings encounter the better. Crops like tomatoes and peppers need to be given progressively more space as they grow . They produce the best transplants when they are spaced for enough apart so that their leaves never overlap those of another plant.
For ease of moving, the 4-inch blocks can be set on pieces of tile or shallow saucers and moved in that small "flat" as necessary. Or, if they are set directly on a smooth surface, the best tool for handling them is a heavy-duty kitchen spatula. It is easy to slide it underneath, and the blocks can then be safely lifted and moved. There is as much soil volume in a 4-inch maxi-block s there is in a conentional 6-inch pot. It grows a first-class tomato plant.