By David Wall
Many gardeners prefer synthetic fertilizers because they work so quickly, by directly feeding plant roots. Unfortunately, they don't improve the soil, are subject high leaching, and over time actually degrade soil by destroying beneficial organisms, thus hindering vegetable growth and fruiting. Organic fertilizers work slowly, because they have to be converted to a form the soil can use. BUT, they feed the soil and allow the soil mycorrhiza to feed your plants. They provide nutrients at a rate soil mycorrhiza can use, so very little of the fertilizer is lost to leaching. Virtually 100% of organic fertilizers are good for the soil. Synthetic fertilizers only have a small percentage of useful nutrients. A 15-5-10 fertilizer, for example, means than only 30% of the contents are useful to soil.
To keep nitrogen in a useable form, partially decomposed organic matter (humus) is required. Without something to hold it in place, nitrogen can revert back to a gas and be lost or be washed into underground waterways by watering or rains. In a low organic soil content, more nitrogen fertilizer will be lost than used by your vegetables. Later after your vegetables die, nitrogen will be lost unless there is sufficient organic matter to hold it.
While nitrogen is important for above ground growth, below ground roots and above ground flowerings need phosphorous. There is a problem, however, because while phosphorus doesn't readily leach, it is tightly held, even if large quantities are available. Only a tiny portion is available for your vegetables. Any remaining overabundance, however, prevents plant uptakes of both iron and zinc.
Potassium is needed by your vegetables for overall plant growth, good water use, fighting diseases, resisting pests, and increased drought resistance. Potassium shortages relate to slower growth and yellow leaves. Basically, potassium assists with all plant functions.