The best way to know how much, and at what proportion, to add nutrients to the soil is with a soil test. Simple soil tests can be purchased at garden supply centers. Once you determine what nutrients are lacking or in abundance, you can amend the soil to correct most problems.
There are four main nutrients that are most likely to be a problem in the soil: nitrogen, phosphorus, potassium, and calcium. The first three nutrients are found in most mixed fertilizers, and calcium can be purchased separately in the form of limestone. Nitrogen is necessary for new cell formation in all parts of a plant. Compared to other nutrients, nitrogen is typically the most lacking. A symptom of a shortage of nitrogen is yellow-green stunted growth. Potassium (potash) is necessary for strong roots and stems as well as deep flower color. A symptom of potash deficiency is weak stems and yellowing or browning leaf tips and edges. Phosphorus is necessary for development of roots and stems. This nutrient also stimulates fruit and seed production. A symptom of Phosphorus deficiency is red or purple discoloration of leaves. Because phosphorus can become fixed to soil particles, it is important to place it close to the roots.
Fertilizer needs to be somewhat soluble; available to plants soon after application. The nutrients in organic plant foods, such as compost, manure, bone meal, and blood meal are not readily available to plants. These materials must breakdown, which make them slow acting. The nutrients of inorganic plant foods are in soluble form, which are readily available to plants. Inorganic plant foods are not long lasting, therefore, frequent fertilizing may cause the chemicals to destroy the plant. If applied in concentrated form, do not allow the fertilizer to come in direct contact with foliage and roots as the plant may be damaged or killed.
The ratio of nutrients is indicated on the fertilizer container. The numbers indicate the percentage of nitrogen, phosphorus, and potash respectively - the higher the first number, the more nitrogen, etc. An inorganic fertilizer labeled as 20-20-20 indicates equal portions of nitrogen, phosphorus and potash and typically used in gardens with little nutrient discrepancies. Due to the difference between organic and inorganic plant foods, a combination of the two may produce the best results.
|PLANT TYPE||WHEN TO FEED||COMMENTS|
|Annuals||Before planting||Spread fertilizer before turning soil. Feed again when plants are divided.|
|Bulbs||Early spring of fall||Add food to planting hole. Cover food with a light layer of soil so bulbs are not sitting directly on top of food.|
|Evergreens||Early spring of fall||If pruned, feed again in fall. Use an acid food.|
|Fruit Trees||Fall or spring||Supplement with nitrogen in early spring in addition to annual feeding.|
|Hedges||Spring||If pruned, feed again in fall.|
|Perennials||When new growth appears||Feed again when flower buds appear.|
|Roses||Spring and summer||Do not feed in fall as newly encouraged growth may be damaged by cold weather.|
|Shrubs||Spring or fall||For mature plants, one feeding per year should suffice.|
|Trees||Spring||Feed again in fall if tree is damaged, diseased, or stressed.|
|Tubers||Early spring of fall||Add food to planting hole. Cover food with a light layer of soil so tubers are not sitting directly on top of food.|
|Vines||Spring or fall||Prior to establishment, feed in spring and fall. Once established, feed once a year.|