A soil blocker is a specialized, hand-operated tool that compresses a saturated growing medium into free-standing cubes for seed starting. This method eliminates the need for disposable plastic pots or trays, offering a sustainable alternative for gardeners. The compact blocks remain intact, supporting the seedling from germination until transplanting into the garden. A primary benefit of this system is “air-pruning,” where roots reaching the block’s edge stop growing outward, fostering a dense, robust root ball that minimizes transplant shock. Using a soil blocker requires a specific mix and technique to ensure the blocks maintain their structure.
Preparing the Specialized Soil Mix
The success of soil blocking depends entirely on using a specialized medium, as standard potting soil will not hold a compressed shape when wet. A proper mix must contain materials that act as a binder, providing the fibrous structure necessary to maintain the block’s form. This usually involves a significant proportion of peat moss or coconut coir, which offer high water retention and structural integrity. These primary components are combined with a fine, sifted compost to supply initial nutrients.
Mineral additives like perlite or coarse sand are incorporated to ensure adequate drainage and aeration within the dense cube. The most important step is achieving the correct hydration level, which must be significantly wetter than typical seed-starting mixes. The goal is a consistency similar to thick oatmeal or wet cement, where the mix is fully saturated but not soupy, holding its shape firmly when squeezed. This highly saturated state allows the pressure from the tool to fuse the particles into a cohesive, stable block.
Step-by-Step Guide to Forming Soil Blocks
Packing and Ejecting
To begin the process, the pre-moistened, specialized soil mix should be piled into a mound on a flat, waterproof surface. The soil blocker tool is then firmly pressed down into this mound, burying the chambers completely into the mix. This action packs the saturated medium into the tool’s forms, requiring substantial downward pressure and often a slight twisting motion to ensure full compression. Insufficient packing is a common cause of blocks falling apart, so thoroughness is important during this step.
Once the chambers are full, the base of the blocker is scraped clean against the surface to remove excess soil. The filled blocker is then carefully positioned over the planting tray. To release the blocks, the handle or plunger mechanism is pressed down smoothly and steadily, ejecting the compressed cubes onto the tray’s surface.
Seeding
Most soil blockers include a small indentation or dibbler mechanism that creates a shallow depression in the center of each released block. This depression serves as the perfect spot for placing a single seed, which is then lightly covered with a pinch of dry vermiculite or fine soil mix. The block-forming process is then repeated until the tray is filled with perfectly spaced, ready-to-sow soil cubes.
Managing and Hardening Off Seedlings in Blocks
Watering and Nesting
Once the seeds are sown, careful management of moisture is necessary to prevent the erosion of the delicate soil blocks. Overhead watering is avoided because the force of the water stream can quickly break down the exposed soil structure. Instead, the blocks are watered from below by adding a shallow layer of water to the base of the tray, allowing the soil to wick moisture upward through capillary action. This bottom-watering technique maintains block integrity and encourages the downward growth of strong root systems.
If the seedlings outgrow their initial small blocks before transplanting outdoors, a nesting technique can be used. This involves moving the small block directly into the pre-formed depression of a larger, freshly made soil block, allowing the roots to continue expanding without disturbance.
Hardening Off
Before the final move to the garden, seedlings must undergo hardening off, which gradually acclimates them to outdoor conditions. This process typically begins one to two weeks before planting. The seedlings are initially placed outside in a protected, shaded location for just an hour or two, shielded from direct sun and strong wind. Each day, the duration of outdoor exposure is increased, and the plants are slowly moved into brighter, more exposed locations. This gradual exposure encourages the plants to develop thicker cell walls and sturdier stems. This preparation minimizes the stress and shock that occurs when the plants are finally transferred to the garden bed.