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There are two components to a seed pellet: bulking (or coating) material and binder. The bulking material can be either a mixture of several different mineral and/or organic substances or a single component. The coating material is the "work-horse" of the duet. The coating material changes the size, shape and weight of the seed. Desirable characteristics of a good coating material include: uniformity of particle size distribution, availability of material, and lack of phytotoxicity. The second component, the binder, holds the coating material together. Binder concentration is critical because too much binder will delay germination. Too little binder will cause chipping and cracking of pellets in the planter box, which can cause skips and/or wide gaps in the plant rows. Many different compounds have been used as binders, including various starches, sugars, gum arabic, clay, cellulose, vinyl polymers (Halmer, 1987) and even water (Burgesse, 1949).

Seeds of various sizes are commercially pelleted, from relatively large seeds like onion and tomato to very small seeds like Begonia sp. For onion, the seed can increase in weight 6-fold due to pelleting; there are approximately 230 raw seed per gram, and after pelleting the diameter may be 13.5/64th of an inch (0.54cm). The volume for 1000 propagules is 3.7 cm³ for raw seed compared to 18.0 cm³ after pelleting. The smallest seed that Seed Dynamics  pellets is Begonia. Median seed weight for raw begonia is 88,000 seeds g ¯1. After pelleting, the seed count can average 857 seeds g¯1, an increased mass of over 100-fold.

In the coastal and desert valleys of California and southwest Arizona (as in other areas of the world), the most popular planter for field singulating pelleted vegetable seeds is the Stanhay. The Stanhay meters seeds with a continuous belt with pre-drilled holes. The diameter of the hole accepts only one pellet at a time, while the space between holes determines the seed spacing within the rows. This planter was first introduced in the US in the early 1970s. Stanhay's popularity is due to the ability to plant one or more rows per planter box, the flexibility to plant different sized pellets at different spacings, and the versatility of planting either pelleted or non-pelleted seeds. Several other planters can be used for pelleted seed such as the Milton and the Gramor. Even the Gaspardo vacuum planter (designed for raw seed) does an excellent job in planting pellets.

Historically, the increased usage of pelleted seed occured with the outlawing of the short-handled hoe in California in the early 1970s. This legislative change caused an increased demand for pellets because only with pelleting could lettuce seed be adequately field-singulated for thinning with a long-handled hoe. Several other methods of precision planting lettuce seeds have become commercially available over the years. Products like the seed tape (Gurley, 1970) and seed tablets (Robinson and Johnson, 1970); Sharples and Gentry, 1980) have been commercially available and tested on a large scale. However, the vegetable industry has retained seed pelleting as its preferred precision planting method.

As the demand for pelleted seeds increased, so did the number of companies that produced pelleted seeds. Increased competition in the pelleted seed market has fostered the development of more effective pellets with greater capabilities and wider planting characteristics. Pellet improvements over the last ten years include:

• increased oxygen penetration/availability
• wider pellet density range
• pellet loading
• better field visibility

These improvements and their significance will be discussed, except for better field visibility.

INCREASED OXYGEN PENETRATION
Historically, the primary obstacle for pellet use has been slow and erratic emergence primarily associated with insufficient oxygen supply to the seed. Clay coatings, for example, have been shown to be a barrier to oxygen for the seed (Sachs et al., 1981). Even sand and diatomaceous earth coatings have been shown to limit oxygen supply (Sooter and Miller, 1978).
Sooter and Miller (1978) found that common pelleting materials like silica can extract dissolved oxygen from water as the liquid moves through the pellet. Increased oxygen supply for pelleted seed, especially in over-saturated soil conditions, has been achieved through the use of oxygen-donating chemicals (Sladdin and Lynch, 1983) and the development of splitting-pellet technology. The development of a splitting-pellet like the SDI High-Density® or the SDI Medium-Density® lettuce pellets have been especially beneficial to growers that plant lettuce under saturated soil-water conditions. Saturated soil conditions are caused by irrigation after sowing. A pellet that can split open upon hydration allows oxygenated water to move directly to the seed.

WEIGHT DELINEATION
The development of different pellet weights and density have been spurred over the years by growers' needs to "fine-tune" plantability and ease-of-handling. For example, the greenhouse industry sows pepper seeds for transplanting in plug-trays using a vacuum-drum. Greenhouse managers prefer a light, smooth pellet that permits rapid adherence with a tight seal to the vacuum planting drum. Pellets such as the SDI Gro-Vac® have a 50% lower weight per pellet than conventional pellets like those used for field seeded onion. This lighter, smoother pellet is exclusively utilized in the greenhouse industry. Pellet weight is not important because there is little or no pellet "drop" during planting. Drop refers to the distance from the sowing metering device to the soil or greenhouse media. The best example of pellet weight delineation to suit particular planting requirements is found with lettuce. Tractor speeds during planting in the Yuma and Imperial Valleys can be twice as fast as speeds used in the coastal valleys of California due to larger fields and tighter planting schedules. Growers using the higher tractor speeds prefer a heavier-weighted pellet. The higher the pellet weight, the better the pellet "drop". Straighter drops during planting produce less bouncing in the seed furrows, thus field singulation and uniform plant spacing are maintained. Lighter pellets weigh less per box or pail (boxes and pails are packed by seed number), so shipping costs are lower and handling is easier. Grower preferences in different regions of the US have initiated the development of different pellet weights (Table 1).

All pellets have the same volume; only the weight, and thus the density, of the pellet is different.

PELLET LOADING
Because of the large increase in volume obtained when seeds are pelleted, pellets have been shown to be effective carriers of plant protectants (pesticides). The same plant protectants that are often deleterious if applied diretly to the seed can be "carried" in the seed pellet. The act of applying a plant protectant in a band within the pellet is known as "pellet loading". The pellet either acts to "dilute" the negative impact of a plant protectant as it moves through the pellet to the seed, or acts as a barrier to prevent direct seed contact. Active products can thus be "loaded" onto the seed while minimizing adverse seed germination effects. The total amount of "toxicants" applied per acre is less than with in-furrow or other soil applications.

For example, in Europe, Gaucho® is added to sugar beet pellets at concentrations that would be near phytotoxic (90 g a.i. per 100,000 seeds) if applied directly to the seed. By applying the plant protectant to the pellet, the plant will be protected against insects such as thrips and leaf-hoppers. There is no need for conventional foliar applications and the amount of overall pesticide usage per acre is lower than the conventional application methods (requiring 4.5 - 6.8 kg of pesticides for the equivalent of 100,000 seeds). Other commercial examples of pellet loading include Pro-Gro® and Trigard® applied to onion pellets. These chemicals have been shown to slow down or even inhibit onion seed germination. When applied in the pellet, minimal adverse effects are noted and the seedlings are protected against smut and maggots, respectively.
In summary, chemical loading of pellet provides a means to treat the seed directly. When possible, application of active ingredients to the seed is considered better than other applications such as in-furrow, foliar or broadcast because of the following reasons:

• precise placement on the target
• minimum toxicant used
• minimum environmental impact
• minimum wildlife and beneficial organism exposure

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