Before applying fertilizers in your garden, you need to read the labels which are usually inscribed on the packaging bag. Many farmers have made losses because of failing to effectively apply fertilizers to their gardens.

Fertilizer labels list three numbers; for example, 15-30-15 indicate the relative amounts of nitrogen, phosphorus and potassium, or N-P-K. Choose a fertilizer based on the ratio of these nutrients. For instance, a 15-30-15 fertilizer has a 1:2:1 ratio of N-P-K with one part nitrogen, two parts phosphorus, and one part potassium.

Flowering plants thrive with a 15-30-15 ratio, while a typical lawn fertilizer may use a ratio of 10-1-1, depending on your soil test. Fertilizers with a 1-1-1 ratio are considered all-purpose. NPK fertilizer uses in your garden depend on the soil fertility and the needs of your plants. A soil test will reveal the best NPK fertilizer ratio for your garden. Purchase test kits at most home and garden centers.

Nitrogen Fertilizer for Plants

Nitrogen is an essential nutrient for numerous plant functions and is one of the elemental components of chlorophyll. Nitrogen fuels vegetative growth, particularly the foliage, stems and branches. Plants can deplete the soil of nitrogen, leaching from the soil faster than any other element. If your plants have yellowing leaves or appear stunted with poor growth, the soil may lack nitrogen.

Phosphorus for Plants

A stimulant for root growth, seed and flower formation, phosphorus is most available to plants when the soil pH is between 5.5 and 7. The pH refers to the acid or alkaline level of the soil, and can range from 0 to 15 with 7 regarded as neutral. Different from nitrogen, phosphorus tends to remain in the soil; add it only for new growth. Root vegetables, such as carrots, benefit from a high phosphorous fertilizer as they use large amounts of phosphorus, particularly during early growth.

Potassium for Plants

Potassium promotes root and bud growth and the ripening of fruit. It enhances disease resistance as well as tolerance to drought, heat and freezing. This element is essential for all plants to thrive, particularly in changing weather conditions. Potassium also has a tendency to remain in the soil, but is heavily used by growing vegetables, so should be supplemented as needed, based on a soil test.

Additional Fertilizer Nutrients

Along with nitrogen, phosphorus and potassium, fertilizers contain additional nutrients in much smaller quantities. Calcium (Ca) helps to build a plant’s cell walls. Iron (Fe) plays an active role in the creation of chlorophyll and in photosynthesis. Magnesium (Mg) aids in the creation of components of chlorophyll as well as in a plant’s growth and repair process. Sulfur (S) helps to build proteins essential to plant growth and the maturation of fruit and seeds.

Fertilizer Application Decision

Based on the growth stage of your plants, the fertilizer needs will determine the npk ratio. Before you start a farm, it is advisable to do your soil test first. However many farmers I know of doesn’t do soil test and your fertilizer application will be based solely on your plant growth stage. 

Transplanting Stage

Before transplant or after transplant, the best fertilizer to use is a high phosphorus fertilizer. The best is DAP.

Growth Stage

Like 2 weeks after applying DAP, you will need a high nitrogen fertilizer. Example is 20-10-10. However, DAP is very expensive for farmers in Nigeria, so you can just start by applying 20-10-10 one week after transplant and spray 30-10-10 foliar fertilizer.

Flowering (Bud Stage)

At this stage you need a phosphorus based fertilizer both foliar and granular. You can buy 15-30-15. However this is not available in Nigeria, so you can buy preferably 15-15-15 and spray 11-46-14 foliar fertilizer. At this stage, you mix the fertilizer to Calcium Nitrate Fertilizer.

Fruiting (Half way fruiting and flowering) Stage

Two weeks after the former application, you will need a potassium High fertilizer. Here you can add 12-12-17 with potassium Nitrate and apply plus a foliar spray of 15-15-30.

Harvest Stage

You can apply 20-10-10 granular fertilizer. 

If you count the number of time to apply fertilizer, you discover that you apply four times to have better yield.

Fertilizer Application Method

You can apply fertilizer using the following methods:
Application in solid form (granular application)
Application in liquid form (foliar and drip Irrigation-fertigation application)

Application of Fertilizers in Solid Form:

Broadcasting: Evenly spreading the dry fertilizer over the entire field and incorporating by cultivation is termed as broadcasting.

Basal application: Evenly spreading of solid fertilizers over the entire filed before or at sowing or planting is called basal dressing.
Top dressing: The broadcasting of the fertilizer on closely sown standing crops is called top dressing. Generally, nitrogenous fertilizers are used for top dressing. N- fertilizer should not apply on wet leaves.

Placement:

Plough sole placement. Fertilizer is placed in a continuous band on the bottom of the furrow during the process of ploughing.
 Deep Placement: Usually nitrogenous fertilizer (urea) is placed in a reduction zone (deep in the soil), where it remains in ammoniac form and is available to the crop slowly.
 Sub-soil placement: Fertilizers like phosphate and potash are placed in sub­soil. This method is recommended in humid and semi-humid regions where many subsoil are strongly acidic.
Localized Placement: Fertilizers are applied close to the seed or plant.
Drill Placement or Contact Placement: Fertilizers are drilled along with the seed, using suitable equipment (e.g., fertilizer drills or seed-cum-fertilizer drill).
Band Placement: This is a system of localized placement of fertilizer in the hill or along the row of crops.
Side Dressing: The fertilizer applied after the crop has been seeded in spaced rows. This may be done either as a continuous band near the crop row or dibbled in between the plants. Side dressing is a very common practice in long duration crops such as sugarcane.
Spot Placement: Sometimes the fertilizer is placed in between the plants in pinches at the base of each pair of plants.
Ring Placement: In this method, fertilizers are applied around the plants in ring (basin).

Application of Fertilizers in Liquid Form

Foliar Application

Some fertilizers in liquid form can be applied by spraying directly on the leaves of crop plants. Several nutrient elements are readily absorbed by leaves. Urea (N- fertilizer) and micronutrients like zinc sulfate, magnesium sulfate, etc, are most common. Since sulfate is an organic form of nitrogen, it causes the least injury. It can be sprayed at the rate of 2-4 percent. Three kilograms of urea in 500 liters of water has been reported safe.

Advantages

This method may be suitable where soil application of fertilizers is not possible. For example hills and sandy soil etc.
Micro-nutrients are required in small quantities and their solution is not strong, their application by foliar spray has given better results compared to soil application.

Disadvantages

Plant nutrients are absorbed through the leaves only in limited quantities,
In several cases of plant nutrient deficiencies, it will generally, take more than one spray application to correct the deficiency, a strong solution may cause leaf injury,
it is a costly method.

Fustigation

Some fertilizer (soluble granular) can be applied by mixing it with water and applying it through drip Irrigation.

Principles involved in selecting the Correct Method of Fertilizer Application

Nitrogen fertilizer is easily soluble in water and moves rapidly in all directions from a place of application. Nitrogen applied on the soil surface reaches the plant root easily and rapidly. As such, these fertilizers are broadcast on the soil surface just before sowing.

Leaching and volatilization loss are very high in nitrogenous fertilizers. So that N- fertilizers are suitable for top dressing and side dressing.

To reduce the fixation of phosphate, phosphate fertilizers should be placed so that they come into minimum contact with the soil particles and are close to the plant roots. In other words, localized placement of phosphate fertilizers near the seed or seedling roots should be practiced. Phosphate fertilizers are not suitable for top dressing.