In soil, there is a complex mixture of different ingredients containing all the nutrients for healthy plants. Plants extract these nutrients through their root system, many of these nutrients are trace elements, meaning that they are needed in such minute quantities that they are not likely to be depleted. Others need to be replaced to keep the soil productive.
Of the 16 elements known, three come from the air and water, these are carbon, hydrogen and oxygen. The rest are taken from the soil itself. As stated earlier, many elements are so micro that they most likely will not be depleted but others will need to be replaced. The three elements that are used up the fastest are, nitrogen, phosphorus and potassium, these nutrients are the chief ingredients.
Nitrogen is one of the building blocks required continually as the plant is growing as the plants need it for the plant cells. Nitrogen stimulates vegetable/plant growth as it is part of both protein and chlorophyll molecules. If your soil is lacking nitrogen your plants will produce yellow or light green leaves, depending on how low your nitrogen is in the soil. Leaves may turn brown and drop off. Usually, the lower leaves will be the first to show signs of this. Nitrogen is water soluble and is usually the most deficient element. When purchasing fertilizer, nitrogen is the first number in the three digits located on the bag/bottle.
Phosphorus aids in the plants root, flower and fruit development. Having a high content of phosphorus at planting time will favor rapid root and stem growth. Phosphorus is not soluble, so it needs to be worked into the soil so the roots can absorb it. It can be hard to tell if you are lacking phosphorus but if you notice your plants are stunted this could be an indication of low phosphorus, also leaves can be very dark green which later can turn purple or red on the underside. On fruit trees, the leaf stems will be purple. When purchasing fertilizer, phosphorus is the second number in the three digits located on the bag/bottle.
Potassium is required for the movements of sugars and nutrients within the plant. Potassium will form stronger stems and roots and will deepen the flower color. Potassium is soluble and will wash out slowly after watering or a rain. Having a shortage of potassium will cause yellowing of the leaves, similar to that of a shortage of nitrogen, except that the yellowing occurs on the margins of the leaves at first. The margins will then turn brown and the leaves will curl under. When purchasing fertilizer, potassium is the third number in the three digits located on the bag/bottle.
Other nutrients in soil are calcium, sulfur, magnesium, iron, manganese, zinc, copper, boron, molybdenum and chlorine, these are usually the trace elements found in soil and most times will not have to be replenished. Some plants prefer to have a higher content of some of the trace elements, for example tomatoes enjoy having extra calcium in the soil.
Having your soil tested is the only true way of learning what your soil contains. Check with your provincial agricultural office to find out if this is available, or you can purchase a soil testing kit yourself, available at most garden centers.