No, you did not encourage the weeds with the Osmocote fertilizer. The weed seeds were in the leaf mold/compost that you placed on top of the plants. If you make your own compost it often is full of weeds. Compost often does not heat up enough to kill weed seeds. There is no reason to stop using your own compost, however. Just bury the compost near the roots of your plants as the weed seeds need sunlight to sprout. Also, when you dig in any soil you expose new weed seeds to sunlight so they sprout. This is common with new gardens but if you weed in early spring you can get ahead of the problem. The best way to improve sandy soil and keep down weeds is to use a weed-free mulch on top of the soil. This would be bark chips, gravel, oyster shells or even sawdust. You can also use composted dairy manure or compost in bags from a garden center that has been made at a high temp so it is weed free. Don't be discouraged as you will have less weeds next year as your soil improves and as the plants grow and shade the soil over time. The weed-free mulch may only be needed for a few years until you get ahead of the problem. You can also hoe the soil on a sunny day to uproot the weed seedlings. Keep growing, Marianne Binetti