The 4R approach to fertilizer application, identifying nutrient deficiencies, and crop scouting

Backgrounder

Introduction

Fertilizers are synthetic or organic materials added to soil to increase its fertility. They provide at least one of the essential nutrients (nitrogen, phosphorus, and potassium) plants need for healthy growth and good yields.

There are different types of fertilizers and different methods for applying them. The two main types of fertilizers are organic and inorganic fertilizers, which can either be liquid or solid (granular), and both are used by Ghanaian farmers. Fertilizers play a major role in crop production in Ghana.

Incorrect use of synthetic fertilizers can have harmful effects on the climate and the natural environment. Agriculture is estimated to be the third largest contributor to Ghana’s greenhouse gas (GHG) emissions, and a significant portion of agricultural emissions are due to fertilizer use, especially incorrect use. Applying fertilizers correctly is key to reducing GHG emissions and fostering a safe environment and climate for all, as well as achieving optimum yields. Therefore, the Ministry of Food and Agriculture in Ghana promotes the 4R method for applying fertilizers.

The 4R approach to applying fertilizers is a framework for intensive and sustainable crop productivity based on the 4Rs: using the Right Source of nutrients, applied at the Right Rate, at the Right Time, and in the Right Place.

The 4Rs are guidelines for using fertilizers effectively and efficiently to receive good crop yields.

What are the 4Rs?

Attention to the factors listed below will ensure adequate nutrition for good crop production while minimizing the risk of loss of nutrients to the environment.

1. Right source: what the fertilizer contains.

Factors to consider when determining the right source include:

a. Crop nutrient requirements

b. Soil conditions

c. Transportation and storage

d. Environmental risks related to fertilizer use

e. Price

2. Right rate: how much fertilizer the crop and soil needs.

Factors to consider when determining the right rate include:

a. Type and solubility of fertilizer

b. Soil type

c. Existing nutrients in the soil

d. Crop nutrient requirements

e. Stage of plant growth

3. Right time: the best time to apply fertilizer

Factors to consider when determining the right time to apply fertilizer include:

a. The composition of nutrients in the fertilizer

b. The type and growth stage of the crop

c. Moisture level of soil

d. Type of equipment used to apply fertilizer

e. Plant spacing

f. Tillage practices

4. Right place: how and where to apply the fertilizer

Factors to consider when determining the right method and place to apply fertilizer include:

a. The composition of nutrients in the fertilizer

b. The type and growth stage of the crop

c. Moisture level of soil

d. Type of equipment used

e. Plant spacing and planting practice

f. Tillage practices

Fertilizer application methods

There are several methods for applying fertilizer. Here are some common methods:

1. Broadcasting: Application of fertilizer to a piece of land by scattering, either by hand or by machine. In Ghana, this is mostly done by hand.
2. Drilling: Application of fertilizer close to seeds in planting holes with drilling machines.
3. Ringing: Placement of fertilizer in a circle around a plant, with each point in the circle equidistant from the plant.
4. Side dressing: Applying fertilizer at the sides of a crop’s base. The fertilizer is uncovered but slightly dug in.
5. Spraying: Applying liquid fertilizers to crop leaves with a sprayer.

Fertilizer application methods must ensure that growing plants have access to the quantity of nutrients they require at key growth stages. Most small-scale farmers in Ghana use the broadcasting and spraying methods of applying fertilizer.

In Ghana, farmers are encouraged to use the drilling method. If farmers lack a drilling machine, they can use the “drill and cover” method, where they dig a hole close to the seed with a stick (instead of a drill seeder), apply the fertilizer, and cover it. The drill and cover method is effective because the fertilizer stays in place and provides the seed/plant with the required nutrients. Unlike broadcasting, there is little risk of fertilizer drying up, evaporating, or being washed away by the rains.

Recommendations for fertilizer application

• Fertilizer recommendations for specific crops, for example maize, depend on the agro-ecology, current soil fertility status, cropping history of the field, and the target yields for the current season.
• The recommended application rate differs between agro-ecologies. * It is therefore important that a farmer knows the rates applicable in his or her agro-ecology.
  • For example, in maize grown in northern Ghana, under guinea savanna agro-ecology, the recommended application rate for nitrogen is 60–120 kg per hectare, while the recommended rate of phosphorus and potassium is 30–60 kg per hectare. These recommendations should be adjusted based on the soil fertility of a field, and on past cropping history.
• Recommended fertilizer application rates can also be adjusted based on expected or targeted
• Fields that have been cultivated for a long time with minimal application of fertilizer or manure are often less fertile and require adequate fertilizers.
• Fields that have only recently been converted to farmland or have usually received large quantities of manure are often more fertile and require lower quantities of fertilizer.
• Where possible, farmers should collect soil samples from their fields and submit them for fertility analysis at soil laboratories such as those at the Savanna Agricultural Research Institute and the Soil Research Institute of the CSIR. The results can help determine the right fertilizer recommendations.
• Farmers are best-positioned to determine the right fertilizer to use after identifying nutrient deficiencies in crops. However, this may delay application and could cause economic loss.

Essential plant nutrients

Plants obtain carbon, hydrogen, and oxygen from water, air, and sunlight to make food for growth. They also require the following nutrients for healthy growth:

Macronutrients

These include nitrogen, phosphorus, potassium, sulphur, calcium, and magnesium and are required by plants in relatively large amounts.

Micronutrients (also known as trace or minor elements)
These include copper, manganese, zinc, iron, boron, and molybdenum, and are required by plants in smaller quantities.

Nutrient deficiencies occur when a plant lacks enough of an essential nutrient required for growth. Without adequate essential nutrients, plants will not grow well and will show various symptoms to express the nutrient deficiency.

Symptoms of crop deficiency

Common symptoms of nutrient deficiency include yellowing of leaves, shortened internodes *, and abnormal colouration such as red, purple, or bronze leaves. These symptoms appear on different plant parts because of nutrient mobility * in the plant.

In maize, nutrient deficiencies do not immediately result in visible symptoms. Plant growth can be stunted before symptoms are visible. These invisible deficiencies are called “hidden hunger,” and are far more common than visible deficiency symptoms. By the time deficiency symptoms first appear, maize has already lost yield and quality.

Identifying nutrient deficiencies in maize

Nitrogen (N) deficiencies

• Maize can easily become nitrogen (N) deficient. Plants are stunted and have small cobs.
• Young plants may be uniformly pale green in colour.
• Lower leaves begin to turn yellow.
• Leaf yellowing progresses from the tip along the midrib * to the base. Eventually the entire leaf can become brown and die.

Rapidly growing maize takes up large amounts of N from the soil. After maize is about knee-high, N demand increases quickly until tassels appear. If an N deficiency is detected early, applying additional N fertilizer can correct the problem.

Phosphorus (P) deficiencies

• Symptoms usually first appear on the edges of older leaves, especially when plants are young.
• Purple-coloured leaves are a distinct symptom of phosphorus deficiency in maize.
• Weak and thin stalks, small and twisted cobs.

Deficiencies sometimes appear in young plants due to cool soil temperatures or excessively wet or dry conditions, all of which interfere with P uptake. Plants may be able to overcome this deficiency if soil conditions improve.

Steady uptake of phosphorus is required for most of the growing season. Where there is an increased risk of phosphorous deficiency, it is recommended to apply P fertilizer before or at the time of planting (formerly April-May, but as late as June-July due to the changing climate). This prevents P deficiencies during the growing season.

Potassium (K) deficiencies

• Symptoms first appear as yellowing along the edges of lower leaves.
• Over time, the edges of lower leaves become brown and yellowing continues towards the midrib. Eventually, the yellowing and browning advances towards the higher leaves.
• The nodes on the inside of the stalk turn dark brown. The entire plant becomes shorter as the space between the nodes is reduced.
• Stalks of K-deficient maize tend to break late in the growing season due to poor stem strength.

Potassium plays a role in managing plant water use; thus, drought has a greater impact when there is a deficiency in K.

Sulfur deficiencies

• Pale green or yellow colouration first appears on younger leaves, and then the entire leaf becomes pale green or yellow.
• The entire plant may become pale if the deficiency persists.
• The entire plant becomes stunted and thin.

Sulfur deficiencies are most common in maize growing on sandy soils with low levels of organic matter.

Magnesium (Mg) deficiencies

• Symptoms first appear as light striping along lower leaves.
• Since magnesium is mobile in plants, it is easily transported from the older leaves to young growth.
• As the Mg deficiency progresses, the underside of the leaf may develop a purplish colour. The leaf edges may also begin to turn yellow and die.

Deficiencies of Mg often occur in maize growing in acid soils. Applying liming materials that contain Mg (such as dolomite) is typically recommended in these conditions. When there are Mg deficiencies in non-acidic soils, fertilizers that contain magnesium can be applied to correct deficiencies.

Zinc (Zn) deficiencies

• Symptoms of Zn deficiency are frequently seen during the early growth period.
• Younger leaves first develop symptoms of yellowing, which appear along the sides of the midrib from the stalk to the middle of the leaf. The midrib and margins may continue to be green.
• While older leaves appear healthy, emerging young leaves may be pale green.
• The stalk internodes become stunted, resulting in shorter plants and a bunching of the leaves at the top of the stalk.

An adequate supply of Zn is important for healthy plant development and the proper maturation needed for a timely harvest.

Boron (B) deficiencies

• B deficiency symptoms are seen at the growing points and on the youngest leaves.
• Leaves become twisted and develop yellow or white spots.
• In severe cases, there can be barren cobs with few kernels.

The major periods of B uptake are in the early vegetative stage and about one month after silking.

Copper (Cu) deficiencies

• Young leaves develop a yellow colour and may be twisted in a spiral.
• Deficiencies are mainly observed on younger growth as light streaks between the leaf veins and on the tips.

Many African soils are low in plant-available Cu and deficiencies are becoming more common. Good maize production in these kinds of soils requires supplemental application of Cu and other micronutrients, which successfully addresses the problem.

Other visible signs of maize problems include the following:

• Drought causes maize plants to have a greyish-green colour; leaves may roll up to the size of a pencil.
• Disease infections start as small spots and gradually spread across the leaf.
• Chemical damage can sometimes burn tips of edges of leaves at the point of contact. The tissue dies and the leaf becomes bleached.

Identifying crop deficiencies through scouting

Identifying nutrient deficiencies helps farmers determine the right fertilizer to use at the right rate, right time, and right place. Regular crop scouting is the best way for a farmer or agronomist to detect deficiencies in standing crops.

Early detection of problems is key to success as it provides more time to respond and allows the crop a chance to recover.

Tips for scouting

1. Watch for unusual patterns—compare problem areas in the field with normal, healthy areas.
2. Walk through your farm or field every day, but thoroughly inspect fields several times during the season. Take notes on a field map if possible.
3. Make detailed notes of what you see and where you see it in the field. Use a camera to document problem areas. Visual records allow farmers to describe both the symptoms and field conditions as they observe them.
4. Pull or dig up some plants in normal and in problem areas. The differences you see will provide useful diagnostic information.

Important scouting questions

Ask yourself:

1. Is a physical limitation like soil compaction or poor drainage impacting growth?
2. Is there too much or too little water in the field?
3. Do you see signs of a nutrient deficiency?
4. Do you see signs of a pest infestation or of disease?
5. Is the soil shallow or acidic?
6. Are weeds robbing nutrients and water from the crop?

Other points to note when scouting:

• When a deficiency is detected early in a crop season, supplemental fertilizer can solve the problem.
• Carefully inspect the roots, split the stalks, and examine maize cob development. Watch for diseases, insect pests, or obvious damage to leaves, stalks, cobs, and roots. Tissue testing is especially useful to detect “hidden hunger” or help explain differences in growth between areas.
• Harvest is another opportunity to check the crop. Poorly filled, deformed maize cobs and barren stalks may indicate nutrient shortages that can be corrected before the next crop is planted.
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The importance of good farm practices

For best results, it is important to use good agronomic practices in combination with the 4R approach to applying fertilizers. The following practices are appropriate for maize crops in Ghana.

• Site selection: Look for gently sloping, well-drained, loose, and well-aerated fields, for example, sandy loam. These allow easier cultivation and good crop growth.
• Land preparation: Start at the onset of early rains, Plough and harrow across the slope to prevent runoff.
• Timely planting: Plant only when there is adequate moisture.
• Use good seed/varieties: Acquire seed from approved sources only.
• Recommended planting density: Talk to an extension agent on how to identify the recommended planting density.
• Pest and disease control: To effectively manage pests and diseases, farmers need to regularly visit their maize field during the growing season and scout for pests and diseases. Farmers should walk through the field in a zig zag pattern every week and closely inspect plants for indications of pests and diseases, including:
  • Perforated leaves
  • Larvae or egg clusters
  • Large groups of insects
  • Discoloured leaves.

After identifying specific pests and diseases, farmers should consult their local extension agent for advice on the best pest and disease control measures.

Disease and pest control should be carried out in good time to allow the crop to recover.

• Timely weed management: To manage weeds before crop emergence, apply recommended, selective pre-emergence pesticides 2–3 days after planting.
  • If there are weeds in the field at planting, apply non-selective herbicides.

About six weeks after planting, a second weeding may be required. At this stage, hand weeding can be conducted, particularly in small farms. The second weeding should be conducted just before top-dressing to ensure that weeds do not compete with maize for the nutrients in the fertilizer.

• Timely harvesting: All crops must be harvested in good time, and using recommended methods. Maize should be harvested as soon as it is dry, detaching cobs from the stalk. Harvested cobs should be placed on a clean, dry surface and dried to reduce the moisture content of the grain. Grains should be shelled from the cobs using locally available methods.

Avoid placing cobs and grain in direct contact with soil during drying to avoid infestation with aflatoxin-producing fungi. Place cobs and grains on a clean tarpaulin during drying.

Treat dried maize grain with recommended storage chemicals and store in jute sacks lined with polythene bags.

• Using manure and other organic resources: When available, manure and other organic resources such as maize and groundnut residues should be incorporated into the soil prior to planting.

Definitions

Agroecologies: Geographical areas with similar climatic conditions that determine their ability to support agriculture.

Internodes: The interval between two nodes on a stem.

Nutrient mobility: The ability of nutrients to move within a plant.

Midrib: The central vein of a leaf.