All fertilizers list three numbers on their label, which represent the percentage of nutrients available in the product. The sequence shown is: (N-nitrogen)-(P-phosphorus)-(K-potassium), or (N-P-K).
Nitrogen is essential to vegetative growth and general plant health. It is particularly important to green, leafy crops such as lettuce, spinach and herbs. Care should be taken around plants such as tomatoes and peppers, where excess nitrogen will promote leaf growth at the expense of fruit production. Synthetic sources of nitrogen flood the plants with a single nutrient, at the expense of others, tend to force soft, fleshy, pest-prone growth and also burn the soil microbes that are responsible for decomposition, nutrient release and disease suppression. On the other hand, organic sources of nitrogen are bound in complex molecules along with other essential macro and micronutrients, and are made available to the plant through their steady breakdown by the soil’ s microbial life. As the nutrients are released, they are either taken up by the plant roots, or bound by organic matter and held until needed by the plant. Thus in an organic system, relatively few nutrients are lost to leaching and are more readily available to plant roots over a longer period of time.
Phosphorus is used by plants for growth, cell division, root growth, flower and fruit set and for disease resistance. Signs of deficiency are reddening of the stems, leaf veins and leaves. The presence of organic matter in the soil is very important to phosphorus release, particularly in heavy soils where phosphorus can be tightly bound. Most organic sources of phosphorus also contain large amounts of calcium, which is necessary in cell development and for seed and fruit set.
Potassium is the major nutritional element necessary for strong root development, sugar production and disease resistance. Soils that contain deficient levels of potassium generally produce stunted, weak plants that show symptoms that one might expect from plants that have a compromised root systems. Many of our heavy, clay soils in the area have adequate potassium content, but the available potassium may be bound up in the soil molecules. By adding organic matter to the soil, and increasing the microbial activity it is possible to free up the held potassium.
In addition to the big three, N,P and K, there are also the trace elements. These are minor mineral nutrients that are needed in extremely small, or “trace” amounts. The importance of these elements can be seen by the fact that while trace elements may constitute a very small percentage of the soil composition, they are often the ingredients that determine the overall health and vigor of growing plants. These are the building blocks of plant life, their availability and presence enhances cell formation, chlorophyll production, photosynthesis, and provides plants with the raw materials that are needed for making amino acids and enzymes. By replenishing these important elements in your garden every year or sower, you will greatly improve both the quality and the quantity of your vegetable, fruit and flower yields.
pH denoting “potential of hydrogen” or “power of hydrogen” is a logarithmic scale used to specify the acidity or basicity of an aqueous solution, with 7 being neutral, the lower numbers being acidic and the higher numbers being alkaline, or ‘sweet’. The pH of your soil is important, as it determines how available the mix of nutrients will be. The optimal range for growing is between 6 and 7, this is the zone in which the majority of essential plant nutrients are the most readily available. Soil sulfur, at 1-2 pounds per 100 sq ft, will lower the pH of your soil, making it more acidic. Fertilizing plants with cottonseed meal will also lower the pH. Liming agents will raise the pH, at 5-10 pounds per 100 sq ft. Liming agents include oyster shell, dolomite lime, colloidial phosphates and bone meal.