research.

Hellooo, did some more research just then.

Apparently: (this is all from http://www.agr.state.nc.us/cyber/kidswrld/plant/nutrient.htm)

Soil Texture (the amount of sand, silt, clay, and organic matter in the soil)

Soil texture affects how well nutrients and water are retained in the soil. Clays and organic soils hold nutrients and water much better than sandy soils. As water drains from sandy soils, it often carries nutrients along with it. This condition is called leaching. When nutrients leach into the soil, they are not available for plants to use.

An ideal soil contains equivalent portions of sand, silt, clay, and organic matter. Soils across North Carolina vary in their texture and nutrient content, which makes some soils more productive than others. Sometimes, the nutrients that plants need occur naturally in the soil. Othertimes, they must be added to the soil as lime or fertilizer.

Maybe this is a reason why plants should grow faster in more organic soil such as potting mix because the texture affects how well nutrients and water can be kept in the soil rather than with backyard soil where the texture is not thick enough to absorb all the nutrients and water necessary.

more infoooo on compost soil: (http://www.uvm.edu/vtvegandberry/factsheets/pottingmix.html)

Compost is rarely used by itself as a potting medium. Compost alone does not have the optimal water holding characteristics, and soluble salt levels are often higher than optimal for potting mix. Plus, high quality compost can be relatively expensive so it makes sense to dilute it with other ingredients like peat. Organic potting mixes are typically made with 20% to 50% compost by volume, depending on the type of crop that will be grown in the mix, the container size, and the growing conditions.

This should explain the slower growth of the sunflowers in compost. I always thought that the more nutrients it had, the faster it would grow. That is true in some cases but not in others; just like here where water cannot be held strongly enough. ooooh, nice, I like it when things make sense. (: