factors including C/N ratio, temperature, soil moisture and aeration (Mupambwa and
Mnkeni 2018). Organic fertilizer quality can be widely variable depending on
several factors such as parent organic materials, vermicomposting technique and
maturity, as well as the interactions between these factors. Such variations in quality,
hence yield response are a major reason why farmers have at times regarded organic
fertilizers as an undependable source of nutrients (Mupambwa et al. 2019).
Materials low in certain essential plant nutrients will not create an organic
fertilizer rich in those nutrients. For example, organic fertilizers derived from
livestock feeding on a depleted veld tend to be poor in nutritional quality, especially
P. On the contrary, organic fertilizers derived from composted feedlot wastes
typically contain more P because the base ration ingredients are high in P. In fact,
the quality of organic fertilizers that are based on animal manures may also vary
considerably depending on the type of animal, its age and physical condition.
Handling methods, application technique and timing, including field conditions
post application, are also critical factors that influence crop response to organic
fertilizers. In the sub-sections below, we review variables that influence quality of
organic fertilizers and crop response to provide standards for production and application of organic fertilizers.
18.5.1 Application Rates for Organic Fertilizers
Plants require a proper balance of all the essential nutrients for normal growth and
optimum yield. Organic fertilizers usually contain all the nutrients required for plant
growth, albeit in varying quantities. Prior to application onto the soil, it is important
to collect a sample that accurately represents the quality of the organic fertilizer and
have it properly analysed. The nutrient availability of an organic fertilizer needs to be
determined so that these organic resources can be effectively utilized. Application of
excessive amounts could result in environmental pollution and plant toxicity,
whereas under-application results in poor crop performance. It should be noted
that not all the nutrients in organic fertilizers are plant-available. Some need to be
mineralized in the soil before being absorbed by plants. The plant-availability
spectrum of nutrients may increase due to mineralization or decrease due to immobilization, depending on soil properties. Organic fertilizers with finer texture, or
those more matured, tend to be characterized by lower C/N ratios, as well as faster
decomposition and nutrient release. Farmers with limited access to soil laboratories
could consider a greyish black colour, fine texture, longer composting time and
earthly smell as indicators of better organic fertilizer quality.
Nutrient levels in organic fertilizers do not precisely match crop nutrient needs.
For example, crops generally require N and P in a ratio of 10:1. The N:P ratio in most
organic fertilizers is often much less, and this often translates into an excess
application of P when the N requirements are met solely through composts.
Hence, applying an organic fertilizer to meet the crop’s needs for one nutrient may
lead to over- or under-application of other nutrients. Getting the maximum value out
312
H. A. Mupambwa et al.
Mnkeni 2018). Organic fertilizer quality can be widely variable depending on
several factors such as parent organic materials, vermicomposting technique and
maturity, as well as the interactions between these factors. Such variations in quality,
hence yield response are a major reason why farmers have at times regarded organic
fertilizers as an undependable source of nutrients (Mupambwa et al. 2019).
Materials low in certain essential plant nutrients will not create an organic
fertilizer rich in those nutrients. For example, organic fertilizers derived from
livestock feeding on a depleted veld tend to be poor in nutritional quality, especially
P. On the contrary, organic fertilizers derived from composted feedlot wastes
typically contain more P because the base ration ingredients are high in P. In fact,
the quality of organic fertilizers that are based on animal manures may also vary
considerably depending on the type of animal, its age and physical condition.
Handling methods, application technique and timing, including field conditions
post application, are also critical factors that influence crop response to organic
fertilizers. In the sub-sections below, we review variables that influence quality of
organic fertilizers and crop response to provide standards for production and application of organic fertilizers.
18.5.1 Application Rates for Organic Fertilizers
Plants require a proper balance of all the essential nutrients for normal growth and
optimum yield. Organic fertilizers usually contain all the nutrients required for plant
growth, albeit in varying quantities. Prior to application onto the soil, it is important
to collect a sample that accurately represents the quality of the organic fertilizer and
have it properly analysed. The nutrient availability of an organic fertilizer needs to be
determined so that these organic resources can be effectively utilized. Application of
excessive amounts could result in environmental pollution and plant toxicity,
whereas under-application results in poor crop performance. It should be noted
that not all the nutrients in organic fertilizers are plant-available. Some need to be
mineralized in the soil before being absorbed by plants. The plant-availability
spectrum of nutrients may increase due to mineralization or decrease due to immobilization, depending on soil properties. Organic fertilizers with finer texture, or
those more matured, tend to be characterized by lower C/N ratios, as well as faster
decomposition and nutrient release. Farmers with limited access to soil laboratories
could consider a greyish black colour, fine texture, longer composting time and
earthly smell as indicators of better organic fertilizer quality.
Nutrient levels in organic fertilizers do not precisely match crop nutrient needs.
For example, crops generally require N and P in a ratio of 10:1. The N:P ratio in most
organic fertilizers is often much less, and this often translates into an excess
application of P when the N requirements are met solely through composts.
Hence, applying an organic fertilizer to meet the crop’s needs for one nutrient may
lead to over- or under-application of other nutrients. Getting the maximum value out
312
H. A. Mupambwa et al.