**Abstract**

Rice is the second most important cereal in Ghana after maize. However, current production levels are about 47% of the country's requirements resulting in huge annual imports of the crop. One major constraint to production has been low soil nutrients and poor nitrogen management. Nitrogen is not only a major nutrient but also most often the most limiting nutrient element in lowland ecologies. With the introduction of improved soil and water management ("sawah" system) for lowland rice production, a study was conducted to determine the optimum nitrogen rates required. A randomized complete block design arranged in a split plot consisting of five levels of nitrogen as main treatments and three improved rice varieties as sub-treatments was adopted. Results showed that the total number of tillers per m<sup>2</sup> increased significantly with increasing levels of N as was total dry matter production. However, total number of panicles did not show the same relationship. Total biomass yield increased significantly and linearly with increasing levels of N. Paddy yield significantly increased from 1.7 t ha<sup>−</sup><sup>1</sup> (control) to a maximum of 9.4 t ha<sup>−</sup><sup>1</sup> (90 kg N ha<sup>−</sup><sup>1</sup> ) before declining to 5.8 t ha<sup>−</sup><sup>1</sup> (150 kg N ha<sup>−</sup><sup>1</sup> ) in the order 0 < 30 < 60 < 150 < 120 = 90 kg N ha<sup>−</sup><sup>1</sup> , respectively. This result significantly and positively reflected on grain harvest index (GHI) in the order 0.27 < 0.38 < 0.46 < 0.47 < 0.57 < 0.68 for 0, 30, 60, 150, 120 and 90 kg N ha<sup>−</sup><sup>1</sup> , respectively. Nitrogen at 90–120 kg ha<sup>−</sup><sup>1</sup> was therefore recommended. These rice varieties in addition to other improved ones will also perform well in other environments with similar biophysical characteristics across the country.

**Keywords:** grain yields, mineral fertilization, "sawah" technology, soil nutrients

## **1. Introduction**

Poor and declining soil fertility remains the most important biophysical (abiotic) stress that accounts for the decline in agricultural productivity particularly in rice-growing environment in sub-Saharan Africa and in Ghana in particular [1–13]. Another notable and critical factor contributing to low agricultural productivity especially rice in Ghana is the low use of fertilizers [10, 14]. In highly weathered soils with low clay content and low activity clay minerals [13] as those of West Africa including Ghana, technology development for increased and sustainable

nutrient management under improved soil and water management is very paramount. In Ghana where over 80% of rice farmers are poorly resourced, rice production levels will continue to be low unless technology development for increased and efficient use of inputs such as fertilizer is critically and urgently promoted.

Rice consumption has been on the increase in Ghana over the past few decades. According to the Ministry of Food and Agriculture, Ghana [15] rice has become the second most important staple food after maize, and its consumption keeps increasing. This has led to large annual imports of the crop as production constantly falls short of demand. On the average, annual rice import for Ghana is about 400,000 tons. The self-sufficiency ratio of rice in Ghana declined from 38% in 1999 to 24% in 2006. Rice yields in Ghana average about 2 tons per hector due to inherent poor soils and improper soil management practices [5, 6, 8]. With a potential available lowland area of over 700,000 ha, rice is cultivated across all the agroecological zones of Ghana. However, there are significant differences in the production potential (area available and suitability) among these ecosystems due to differences in soil, climate, and economic conditions.

The high rain forest and semi-deciduous rain forest agroecological zones have a comparative advantage due to their good rainfall and better water availability throughout a greater part of the year. While the impact of fertilizer use for crop production is considered large in regions of extremely low soil fertility (particularly N and P), the application of chemical fertilizer to crops in Ghana is one of the lowest in West Africa [3, 16]. Rice is grown within these zones with very little or sometimes no application of mineral fertilizers. There are also no structures put in place to manage water. Efficiency of applied fertilizer is therefore very low due to poor water control. With the recent introduction of improved soil and water management, rice framers' yield of at least 4.0 t ha<sup>−</sup><sup>1</sup> is ensured [4, 7]. However, for higher yields and improved/sustained productivity, mineral fertilization is necessary. Previous studies have shown that rice responds to mineral fertilization in these lowlands [5–7]. Hence, for site-specific management and the bulk of rice growers being resourcepoor small-scale farmers, it is necessary to develop technologies (optimum levels of critical nutrients such as nitrogen) that are easily transferrable and adaptable.

With the increasing use of lowlands in the forest agroecological zones for rice production, this chapter looks at the relevance and how effective nitrogen can be managed for increased rice grain yields and developing a sustainable production system.

#### **2. Forest agro-ecology and rice-growing soils**

In Ghana, there are two main forest agroecological zones, namely, the high rain forest and semi-deciduous rain forest (**Figure 1**). These agroecological zones lie between latitude 5° N and 7° N and longitude 0° W and 3° W. The agro-ecology covers a total land area of about 3.45 million hectares representing 18.9% of total land area of country. The agroecological zones cover the whole of the Western, Western North, parts of the Ashanti, Central and Eastern regions. Lowlands (inland valleys, floodplains) are spread across the area where rice cultivation is gradually intensifying due to water availability.

Rice is grown mainly in the valley bottoms, valley fringes, floodplains, colluvial foot slopes, and generally hydromorphic sites. Water is readily available throughout greater portions of the year. Rice is mainly grown under partially irrigated and rain-fed conditions. Within very limited areas, however, rice is also grown under irrigation. The main soils of rice-growing sites include valley bottoms (Gleysols), foot slopes, and valley fringes (Humic Ferralsol and Gleyic Lixisols).

**129**

**Table 1.**

*Managing Soil Nitrogen under Rain-Fed Lowland Rice Production Systems in the Forest…*

The site was initially slashed and vegetative cover removed. The area was then ploughed using a mini-tractor (power tiller). The plowed site was demarcated into four main blocks through the construction of bunds. Using a split plot design with nitrogen rate (level) as main treatment and rice variety as sub-treatment, each block was divided into six main plots using minor bunds (100 cm wide and 50 cm high) representing six nitrogen rates (0, 30, 60, 90, 120, 150) kg N per ha. Each main plot was again subdivided to three plots, each measuring 2 m × 2 m for the three rice varieties (Sikamo, Jasmine 85, Marshall). The characteristics of these rice varieties are indicated in **Table 1**. Each subplot was then puddled and manually leveled. A composite soil sample (0–30 cm) was initially collected from the site for laboratory analysis before land preparation. Three-week-old rice seedlings were transplanted to their respective plots using the specified varieties at a spacing of 20 cm × 20 cm and at two seedlings per hill. A uniform level of 60 kg P ha<sup>−</sup><sup>1</sup>

**Av. yield (t ha<sup>−</sup><sup>1</sup>**

Sikamo 130–135 6.5 8.5 Nonaromatic Jasmine 85 120–130 6.5 8.5 Aromatic Marshall 120–130 6.0 8.0 Nonaromatic AGRA 125–130 6.0 8.0 Aromatic Amankwatia 120–125 6.0 8.0 Aromatic CRI-Dartey 120–125 6.5 9.0 Aromatic

**) Yield potential (t ha<sup>−</sup><sup>1</sup> )**

as

**Comments**

*DOI: http://dx.doi.org/10.5772/intechopen.89446*

**3. Materials and methods**

**Rice variety Days to** 

**maturity**

*Characteristics of the varieties used and other existing varieties.*

*Location of the two forest agroecological zones of Ghana.*

**3.1 Experimental setup**

**Figure 1.**

*Managing Soil Nitrogen under Rain-Fed Lowland Rice Production Systems in the Forest… DOI: http://dx.doi.org/10.5772/intechopen.89446*

**Figure 1.** *Location of the two forest agroecological zones of Ghana.*

## **3. Materials and methods**

#### **3.1 Experimental setup**

*Sustainable Crop Production*

in soil, climate, and economic conditions.

framers' yield of at least 4.0 t ha<sup>−</sup><sup>1</sup>

**2. Forest agro-ecology and rice-growing soils**

N and 7°

nutrient management under improved soil and water management is very paramount. In Ghana where over 80% of rice farmers are poorly resourced, rice production levels will continue to be low unless technology development for increased and

Rice consumption has been on the increase in Ghana over the past few decades. According to the Ministry of Food and Agriculture, Ghana [15] rice has become the second most important staple food after maize, and its consumption keeps increasing. This has led to large annual imports of the crop as production constantly falls short of demand. On the average, annual rice import for Ghana is about 400,000 tons. The self-sufficiency ratio of rice in Ghana declined from 38% in 1999 to 24% in 2006. Rice yields in Ghana average about 2 tons per hector due to inherent poor soils and improper soil management practices [5, 6, 8]. With a potential available lowland area of over 700,000 ha, rice is cultivated across all the agroecological zones of Ghana. However, there are significant differences in the production potential (area available and suitability) among these ecosystems due to differences

The high rain forest and semi-deciduous rain forest agroecological zones have a comparative advantage due to their good rainfall and better water availability throughout a greater part of the year. While the impact of fertilizer use for crop production is considered large in regions of extremely low soil fertility (particularly N and P), the application of chemical fertilizer to crops in Ghana is one of the lowest in West Africa [3, 16]. Rice is grown within these zones with very little or sometimes no application of mineral fertilizers. There are also no structures put in place to manage water. Efficiency of applied fertilizer is therefore very low due to poor water control. With the recent introduction of improved soil and water management, rice

improved/sustained productivity, mineral fertilization is necessary. Previous studies have shown that rice responds to mineral fertilization in these lowlands [5–7]. Hence, for site-specific management and the bulk of rice growers being resourcepoor small-scale farmers, it is necessary to develop technologies (optimum levels of critical nutrients such as nitrogen) that are easily transferrable and adaptable. With the increasing use of lowlands in the forest agroecological zones for rice production, this chapter looks at the relevance and how effective nitrogen can be managed for increased rice grain yields and developing a sustainable production

In Ghana, there are two main forest agroecological zones, namely, the high rain forest and semi-deciduous rain forest (**Figure 1**). These agroecological zones lie

Rice is grown mainly in the valley bottoms, valley fringes, floodplains, colluvial foot slopes, and generally hydromorphic sites. Water is readily available throughout greater portions of the year. Rice is mainly grown under partially irrigated and rain-fed conditions. Within very limited areas, however, rice is also grown under irrigation. The main soils of rice-growing sites include valley bottoms (Gleysols),

W and 3°

W. The agro-ecology covers

N and longitude 0°

foot slopes, and valley fringes (Humic Ferralsol and Gleyic Lixisols).

a total land area of about 3.45 million hectares representing 18.9% of total land area of country. The agroecological zones cover the whole of the Western, Western North, parts of the Ashanti, Central and Eastern regions. Lowlands (inland valleys, floodplains) are spread across the area where rice cultivation is gradually intensify-

is ensured [4, 7]. However, for higher yields and

efficient use of inputs such as fertilizer is critically and urgently promoted.

**128**

system.

between latitude 5°

ing due to water availability.

The site was initially slashed and vegetative cover removed. The area was then ploughed using a mini-tractor (power tiller). The plowed site was demarcated into four main blocks through the construction of bunds. Using a split plot design with nitrogen rate (level) as main treatment and rice variety as sub-treatment, each block was divided into six main plots using minor bunds (100 cm wide and 50 cm high) representing six nitrogen rates (0, 30, 60, 90, 120, 150) kg N per ha. Each main plot was again subdivided to three plots, each measuring 2 m × 2 m for the three rice varieties (Sikamo, Jasmine 85, Marshall). The characteristics of these rice varieties are indicated in **Table 1**. Each subplot was then puddled and manually leveled. A composite soil sample (0–30 cm) was initially collected from the site for laboratory analysis before land preparation. Three-week-old rice seedlings were transplanted to their respective plots using the specified varieties at a spacing of 20 cm × 20 cm and at two seedlings per hill. A uniform level of 60 kg P ha<sup>−</sup><sup>1</sup> as


#### **Table 1.**

*Characteristics of the varieties used and other existing varieties.*

P2O5 using triple superphosphate as phosphorus source, 60 kg K ha<sup>−</sup><sup>1</sup> as K2O using Muriate of Potash as potassium source, and 50% N using urea as nitrogen source was applied to each subplot immediately after transplanting as basal fertilizer. All fertilizer was uniformly broadcasted on the field manually. The remaining 50% N was applied as split, at 25 days after transplanting (maximum tiller formation) and 55 days after transplanting (at panicle initiation) using the same broadcast method. Weed control was manual, mainly by handpicking. Crop growth was then monitored until harvest.
