All of the nutrients in the soil aren't free floating, amorphous bodies just waiting to be sucked in by a hungry plant root. There's an intricate choreography going on all the time in which nutrients are changed from forms that are acceptable to plants into forms that can be readily assimilated. For, soil phosphorus is locked up in insoluble compounds, and soil potassium is bound up in mineral form. As is, neither of these nutrients are in forms that allow them to be taken up by plant roots and put to use. Organic matter is the key that unlocks both of these nutrients and makes them available. In the case of phosphorus, the organic matter promotes active colonies of soil bacteria to secrete acids that spur the breakdown of the insoluble phosphorus compounds into a form that the plants can use. Some soluble potassium is available in the soil for the plants to draw upon, but most is locked away in a mineral form. The soluble nutrient is not a dependable supply, for it can be leached away quickly. Organic matter acts as a sponge and holds the soluble potassium right where the roots can get to it and helps transform mineral potassium into a form acceptable to the plants.
Soil type and texture will dramatically affect how available nutrients are to plants. Clay particles and organic particles act like magnets, holding minerals on their surfaces so they aren't washed away. When plant roots absorb some of the soluble minerals from the soil water, minerals attached to the clay or organic particles are released to replenish the mineral content of the water.If your soil is lacking in organic matter and/or in clay content, it won't hold onto minerals very well and will be low in fertility. Overall, organic matter is better at holding onto minerals than clay, and it will release the minerals back into the soil water more readily than clay will.
Using Fertilizers in the Garden
Many organic gardeners shy away from using chemical fertilizers because they know these materials don't contribute to the structure and overall health of the soil, but they aren't entirely sure just what their shortcomings are. We are convinced that overdependence on chemical fertilizers leads the unwitting gardener into a self-defeating cycle that can be described in the following way. When you use chemical fertilizers, nutrients are available to your plants for only a short time. Only a portion of the minerals are held by the soil particles, while the rest are washed away. If you continue to use only chemical fertilizers year after year, the natural processes of erosion, water movement, and plant growth use up whatever organic matter was in the soil to start with . Each season there are fewer organice particles in the soil to retain the minersals you add, so you have to add more and more fertilizer each year to sustain the plants' growth. As the organic material decreases, our soil becomes subject to erosion, because there are fewer microorganisms producing cementing materials. You find that you have to watre more frequently because there's less organic matter to absorb watre and hold it in the root zone. Perhaps worst of all, chemical fertilizers make the soil more and more acid and promote salt buildup to the point where your plants may actually have trouble growing.
Organic gardeners avoid this unpleasant scene by turning to rock powders, compost, manure and other organic materials to boost the soil's fertility. There is a possible drawback to organic fertilizers, however, in that most of them are very slow action. If tyou are starting out with poor soil, most organic fertilizers simply will not get your plants off to a fast start. You can make up for this pause between application and availability by planning ahead and adding slow-acting fertilizers to the soil well before you plan to plant so the nutrients will be there when needed. For example, slow-release potassium fertilizers like granite dust and greensand are best applied in the fall for spring.crops or in the spring for crops that will be grown later in the season. This rule of thumb also applies to the application of slow-release phosphorus fertilizers like phosphate rock and bone meal.
There are some organic fertilizers that release their nutrients at a relatively faster rate. These should not be applied too early, or all the nutrients will be leached away and wasted. Apply these quick release fertilizers right before planting for best results. Relatively fast-acting potassium sources include green plant residues, seaweed, and wood ashes. (A word of warning: On contact, wood ashes burn seeds, stems, and root hairs of young plants. To avoid any mishaps, work the ashes thoroughly into the top 8 inches of soil before planting.) Fast-acting nitrogen sources include dried blood or fish emulsion. If you take time to understand the mechanics of nutrient availability and apply organic fertilizers so that they will be releasing nutrients when needed, you'll seldom have to make supplemental feedings through the season, since there won't be any nutrient deficiencies to remedy.