Improving Soil Fertility with Organic Fertilizers

*Mavis Badu Brempong and Abigail Addo-Danso*

#### **Abstract**

Organic fertilizers with low C:N ratios can be applied to supply both macro and micronutrients to the soil. Aside nutrient supply, they can improve soil structure, texture, water holding capacity and nutrient holding capacity. The mechanisms that may interplay to allow organic fertilizers to affect the soil and crop yields may include improved nutrient synchrony, general improvement in fertility and/or priming effects. The rate, timing and method of organic fertilizer application must be considered to reduce N and P losses during organic fertilizer application. To meet the nutrient requirement of crops, organic fertilizers must be applied in large quantities, so it is more prudent to apply locally available resources. In a case study where sole organic fertilizer, sole inorganic fertilizer and their combinations were applied under rain-fed conditions, it was observed that manure had the potential to hold nutrients longer. This is a positive finding for drought prone areas.

**Keywords:** climate smart agriculture, manure, water holding capacity, nutrient holding capacity

#### **1. Introduction**

Poor soil fertility is the major biophysical factor affecting crop production in the world [1]. It is a major threat to food security considering the ever-increasing growth rate in human population which is projected to reach about 10 billion by 2050 [2]. In times of old, forests and marginal lands were converted to farmlands to meet the food demands of the growing population. This practice caused the extinction and endangerment of many plant and animal species; hence it is frowned upon by many stakeholders. As such, today, it would not be prudent to encroach land reserves and other marginal lands for agricultural purposes. It is therefore imperative that we improve soil fertility and health of the available land, to increase food production and to ensure the world's food security under the current and projected climate change.

Previously, the use of mineral fertilizer was thought of as the most appropriate remedy to soil fertility problems due to its rapid nutrient release [3]. However, mineral fertilizer lacks the ability to improve the soil's physical properties causing fertility improvement by fertilizers alone to be unsustainable. Over-reliance on mineral fertilizer without due diligence to the organics may lead to increased soil erosion, surface and groundwater contamination, increased greenhouse gas emission and reduced biodiversity [4]. In addition, mineral fertilizers are expensive, and many farmers may not have the purchasing power to acquire it [5]. As a result, the attention of various stakeholders has been drawn to use of organic resources [6].

The application of organic fertilizers presents a more sustainable method of food production. There is unending literature reporting the efficiency and effectiveness of organic nutrient sources in maintaining soil quality (physical, biological and chemical properties), improving crop yields and sustaining productivity [6–8]. The benefits of applying organic fertilizers to the soil are elaborated in this chapter.

### **2. Types of organic fertilizers**

Organic nutrient sources are specifically derived from plant and animal origins [9]. They include plant residues, animal wastes and biofertilizers. In this era where climate change and the COVID-19 pandemic has impacted agricultural production and the financial capabilities of all workforces including farmers, farmers could use organic fertilizers available to them for soil fertility purposes because they are cheaper and more environmentally friendly when they are locally available [10].

Organic fertilizers include poultry manure, cattle manure, green manure (often legumes), field crop residues, composts, bone meal, household waste, blood meal, slurry, cocoa pod husks, palm kernel cake, among others. Biofertilizers are products containing single micro-organisms or combinations of them which when applied help fix atmospheric N, solubilize nutrients, mobilize nutrients, or secrete growth promoting substances to aid crop growth. These products do not supply nutrients themselves but enhance the activities of soil microbes to make more nutrients available to crops. They are categorized into N-fixing biofertilizers, phosphorus solubilizing biofertilizers, composting accelerators and plant growth promoting rhizobacteria [9]. Most of the plant and animal residues are often by-products and nuisance to the environment. Using them as nutrient sources would help reduce waste and greenhouse gas emission.

#### **3. Benefits of organic fertilizers to the soil**

#### **3.1 Balanced nutrient supply**

Organic fertilizers supply all essential crop nutrients (N, P, K, S, Ca, Mg, B, Cl, Cu, Fe, Mn, Mo, Ni and Zn) in balanced forms, including micronutrients. This is often not the case for any one inorganic fertilizer. Since all these nutrients make up the biomass of organic residues, they are released during the decomposition process into the soil. The downside to applying organic fertilizers alone is that they contain very minimal amounts of these nutrients and as such must be applied in bulky quantities to meet crop nutrient demands [11]. Also, the fact that only a fraction of the nutrients in organic fertilizers can be released per season must be factored in when applying organic fertilizers. On the average, as a rule of thumb, only about 50% of nutrients in organic fertilizers are mineralized in the first season of application [12]. Usually, the focal nutrient used to calculate the amount of organic fertilizer to apply is its nitrogen (N) concentration. For example, 30% decomposed cattle manure (DCM) contains about 2% N [13]. Assuming a farmer grows maize, which requires about 90 kg/ha N, that means:

$$2\,\text{100 kg DCM} = 2\,\text{kg N}.\tag{1}$$

Therefore, to supply 90 kg N = (90 × 100)/2 = 4500 kg DCM.

*Improving Soil Fertility with Organic Fertilizers DOI: http://dx.doi.org/10.5772/intechopen.103944*

Since the applied DCM will only supply half the amount of N required in a season, the amount must be doubled to make **9000 kg** DCM, thus **9 t** of DCM. [It should be noted that the C:N ratio of the organic material must be below 25 [14] to ensure that all decomposition and mineralization requirements are met. The organic material should also be tested for nutrient concentrations or at least the focal nutrient concentration to use as basis for the calculation].

To supply same amount of N through mineral fertilizer, a farmer would only need about 200 kg Urea, however, in organic applications, other nutrients are concurrently being applied. Since a large amount of DCM would supply the required N and other nutrients, it must be available to the farmer. Hence advocates of organic fertilizers must emphasize on ways to raise such large amounts of materials for application locally if sole organic production is desired.

#### **3.2 Improving nutrient holding capacity**

Aside the balanced nutrient supply, organic fertilizers add organic matter to the soil if a long-term application is practiced. Organic matter improves the nutrient holding capacity of the soil because it contains organic acids that increase the H<sup>+</sup> ions and surface charge of the soil, causing the soil's cation exchange capacity to increase [15]. Thus, the soil's ability to hold more cations (nutrients) at exchange sites is increased and hence the nutrient holding capacity of the soil is also improved. Organic matter also improves the buffer capacity of the soil and increases the soil's ability to resist a change in pH, which in turn affects nutrient loss or gain to the soil [16]. Organic fertilizers increase microbial activity in the soil, causing increased nutrient mineralization rates for the benefit of crops. They stimulate the activities of aerobic and anaerobic bacteria [17] and arbuscular mycorrhizae fungi that form networks of root extension for extensive nutrient availability to crops. Upon the lysis and decomposition of soil microbes, nutrients retained in their biomass are made available in the soil and to crops.

#### **3.3 Improving water holding capacity**

Soil structure, texture, bulk density, and organic matter content are the controls on soil water holding capacity; therefore, any management practice that improves these soil properties, in turn, improves water holding capacity (WHC) of the soil. Soil moisture content is largely dependent on the specific surface area of the soil and the thickness of films of water surrounding the pores [18]. The addition of organic matter through organic fertilizer application improves soil aggregation and increases the surface area of the soil, presenting the soil with more room for soil particles to be surrounded by films of water. As a result, the soil can hold more water against the pull of gravity which drains water from the soil.

While soil organic matter binds soil particles, it also stimulates the activity of soil microfauna whose movement create micro and macropores in the soil, creating extra room for water infiltration [19]. Thus, soil water holding capacity can be improved by the addition of organic fertilizers. In the wake of climate change, where unexpected droughts may be imminent, improving the water holding capacities of the soil with the application of organic fertilizer is the way to go. Also, the physical presence of organic materials on the soil serves as mulch that reduces evaporation and retains moisture in the soil. It also reduces the speed of runoff water and allows rain or irrigation water to infiltrate the soil at favorable speed, thereby reducing erosion, soil and nutrient loss [19].

#### **3.4 Improving soil texture and structure**

The soil binding properties of organic matter and improvement in soil aggregation helps to improve soil structure [20]. The addition of organic matter also improves soil texture and aeration. Soils with improved structure and texture allow easy air, water, and root movement to support healthy crop growth.

#### **4. Mechanisms underlying organic fertilizer effects on soil**

Many research works have observed extra crop yields with organic fertilizer application compared to when its nutrient equivalents are applied through mineral fertilizer [21–23]. Various mechanisms have been proposed to explain this added crop yields from organic fertilizer application. Some of which include **improved nutrient synchrony, priming effect,** and **general fertility improvement**.

Under the improved nutrient synchrony mechanism proposed by Vanlauwe et al. [23], when organic fertilizers are applied, they supply microbes with energy from the carbon they contain, to drive decomposition processes. This leads to temporal immobilization of soil N [24, 25] to build their body tissues. The immobilized N is made available at a later stage of plant growth when the microbes have decomposed the organic material to make nutrients available and/or some microbes have lysed and released their nutrients to the plant when it needs nutrients most. In effect, the peak of nutrient supply coincides with highest crop nutrient demand point when crops have matured, so that the nutrients are efficiently utilized, and little is lost to the environment. Kapkiyai et al. [26] reported that a combination of organic and mineral nutrient sources has been shown to result into synergy and improved synchronization of nutrient release and nutrient demand and uptake by plants leading to higher yields.

The general fertility improvement mechanism [23] is based on the theory that organic matter, aside its addition of nutrients to the soil, improves other physical properties of the soil that helps to perpetuate the nutrient addition effect in real time. Some of these benefits include the improvement of soil structure, water and nutrient holding capacities as discussed above. It also adds micronutrients which is usually not the focus of inorganic fertilizer application.

Priming effect is another mechanism proposed by Kuzyakov et al. [27], in which organic fertilizers affect additional crop yields. Priming refers to strong short - term changes in the turnover of soil nutrients caused by the addition of easily decomposable organic materials. Changes may be positive or negative depending on whether nutrients are rapidly mineralized or immobilized. Under this mechanism, a sum of nutrients available in the soil after harvest and nutrients in crops from the field are higher than a sum of the initial soil nutrients and nutrients in the organic materials. Thus, the additional unaccountable nutrient is the result of organic fertilizer precursing a more rapid mineralization rate and dissolution of previously unavailable/ fixed nutrients into solution. This is made effective by the improvement in microbial population, diversity and activity affected by the organic material addition.

These mechanisms, though proposed by different authors, all point to the fact that organic fertilizers are beneficial to the soil and consequently, crops.

Despite the benefits of organic fertilizers to the soil, organic resources application is limited by the large amounts required to meet nutrient demand [28]. Hence locally available organic resources must be used to overcome this limitation. In areas where animal production is common, feedlot manure is the most available organic fertilizer resource. Crop residue retention and cash crop- cover crops rotation is an option to increase on-farm residue production. One other option that has proven to be effective is an integrated nutrient management approach where organic and inorganic fertilizers are applied in right quantities [29]. This approach helps to harness the mechanisms underlying the effects of organic fertilizer application on crops, resulting in synergy in terms of crop yields.
