The four best management practices, commonly referred to as the 4Rs or four ‘rights’ of fertilizer management are to apply the right source of nutrient at the right rate, at the right time and in the right place to meet crop demand. These 4Rs help to improve the recovery fraction of fertilizer and therefore contribute to improved agronomic efficiency.
Right fertilizer product (Source)
Right fertilizer product means matching the fertilizer source and product to the crop’s needs and the properties of the soil. Fertilizer can be applied as straight fertilizers that provide one nutrient or compound fertilizers and bulk blends that provide more than one nutrient. As mentioned above, compounds provide several nutrients in one product and therefore offer some convenience to the farmer over the use of straight fertilizers. They are often more costly, however, and should be used if they are more cost effective than straight fertilizers. In the end the farmer’s choice will be affected by the local availability of fertilizer materials.
It is important to be aware of interactions between nutrients. For example, the application of P and K fertilizer may be required in order to achieve a full response to N fertilizer. So-called ‘balanced fertilization’ therefore is an important aspect of increasing fertilizer use efficiency. • The choice of fertilizer will depend on the particular crop, current and past use of manure, as well as soil properties and climate conditions. For example, where soils have a low buffering capacity (e.g., sandy soils), it would be unwise to use ammonium sulphate as a source of N due to its soil-acidifying potential, while in areas with very heavy rainfall during the cropping season it is better to avoid nitrate-based fertilizers because they are more prone to leaching than ammonium-based fertilizers such as urea. • Several methods are used to identify which nutrients are deficient in the soil, and these include soil analysis, nutrient omission trials and nutrient deficiency symptoms observed on crops. • Even though some nutrients may not be deficient over the short term, it may be worthwhile to apply small amounts to avoid depleting soil nutrient stocks that lead to nutrient deficiencies over the long term. Some soils have large reserves of particular nutrients that can be exploited for many years without any negative effects. Soil analysis and omission plots are required to determine whether there are sufficient stocks of particular nutrients to sustain crop production without fertilizer application. • Not all fertilizer products available on the market are of good quality. If a farmer buys and uses adulterated or poor-quality fertilizer, it will not increase yields as expected as it does not contain the correct amounts of the required nutrients. For fully soluble fertilizers like urea, ammonium sulphate, ammonium nitrate, KCl, TSP and DAP, farmers can find out if the fertilizer has been adulterated with sand or brick dust by adding 100 g fertilizer to 1 l of water. Unadulterated fertilizers will dissolve in water, cause a decrease in water temperature and leave only a very small undissolved residue.
Right fertilizer rate
Right fertilizer rate means matching the amount of fertilizer applied to the crop’s needs.
• Fertilizer rates are site- and crop-system specific and are estimated after considering:
• the nutrient requirements of the crop;
• the soil’s capacity to supply nutrients (measured by soil analysis and omission plots);
• the amount of nutrients applied in crop residues and farmyard manure;
• the amount of nutrients applied to previous crops;
• the target yield;
• the attainable yield under local climatic conditions; and
• the cost of fertilizers and the value of crop products.
Applying too much fertilizer leads to waste of nutrients not taken up by the crop and possible contamination of the environment. On the other hand, applying too little fertilizer results in less yield and crop quality and less crop residues to protect and build the soil or for use as animal fodder. • In many areas in SSA published fertilizer rates for a particular locality are out of date and are geared towards maximizing yield rather than the farmer’s economic returns. It is very important to assess the farmer’s goals and attitude to risk before recommending fertilizer application rates.
Right time for fertilizer application
Right time for fertilizer application means making nutrients available when the crop needs them. • Nutrients are used most efficiently when their availability is synchronized with crop demand. Basal fertilizer application is done at or just after planting to supply N, P, K, and other nutrients required for early crop growth. • Because fertilizer N is highly mobile and easily lost from the soil due to leaching, some fertilizer N should be applied as a ‘top dressing’ at key stages during crop development, usually when the crop is growing fastest. • Top-dressed fertilizer N can be applied as several split applications to improve fertilizer use efficiency. Top-dressing rates can be adjusted according to how well the crop is developing and the expected price of crop products. • Top-dressings produce good agronomic results if the crop is developing well under favourable climatic conditions and good economic results if high crop prices are expected. If the crop has developed poorly because of poor rainfall and the price of crop outputs is expected to be low, top dressings can be cancelled and the fertilizer set aside for the next planting season. • Application timing (pre-plant or split applications), controlled release technologies, stabilizers, inhibitors and product choice are examples of practices that influence the timing of nutrient availability. • Leaf colour charts or chlorophyll meters are available on the market to guide the application of N, based on crop demand. • Slow-release N fertilizers and deep placement of fertilizer N improve the match between nutrient release and crop demand (sometimes referred to as synchrony). • Look-up tables are also available to guide decision making on the timing of fertilizer application.
Right placement of basal fertilizer
Right placement of fertilizer means applying fertilizer where the crop can access the nutrients contained in the fertilizer. The choice of application method by the farmer will depend on the labour required. • Application methods should be selected based on the particular crop or cropping system and soil properties. It is usually best to incorporate basal fertilizer in the soil at or before planting to achieve efficient fertilizer use.
Produced by the Africa Soil Health Consortium 44
• There are four main fertilizer placement methods: • Broadcasting. Fertilizers are applied uniformly to the soil surface. This is done either before sowing or in the standing crop. The method is easy to implement and has low labour requirements. N fertilizer top dressings are usually broadcast in irrigated rice fields. • Banding. Fertilizers are placed in a band at a depth of 5–8 cm below the soil surface and covered by the soil. Seeds are planted above the covered fertilizer. Banding is the most common method of placement for basal fertilizer applications. • Spot application. Fertilizers are applied in small amounts either at planting in each plant hill together with the seed or close to each plant station during the crop growing season. Spot application is preferred where plants are widely spaced and where soil and climate conditions increase the risk of nutrient losses due to leaching. Spot application is becoming popular amongst farmers because it is more cost effective than broadcasting. • Deep placement. Slow-release N fertilizers are placed in the soil in flooded fields.