C. B. Christianson

The effect of crop residue and fertilizer use on pearl millet yields in Niger

A field study was conducted over a 4-year period in Niger, West Africa, to determine the effects of crop residue (CR), fertilizer, or a combination of crop residue and fertilizer (CRF) on yields of pearl millet (Pennisetum glaucum [L.] R. Br.). Despite a decline in yields of control plots (initial yields were 280 kg grain ha−1 declining to 75 kg grain ha−1 over 4 years), yields of fertilizer plots were maintained at 800–1,000 kg grain ha−1. Continued application of CR slowly augmented yields to levels similar to those of the fertilized plots. The effects of CR and fertilizer were approximately additive in the CRF plots. Addition of CR and fertilizer increased soil water use over the control by 57 mm to 268 mm in an average season and helped trap wind-blown soil. These plots tended to exhibit slightly higher soil pH and lower Al saturation than did the fertilized treatments. Return of CR to the soil resulted in significantly reduced export of most plant nutrients, especially Ca, Mg, and K.

Alleviating soil fertility constraints to food production in West Africa: Efficiency of nitrogen fertilizers applied to food crops

An overview is provided of the N efficiency research conducted within the West African Fertilizer Management and Evaluation Network (WAFMEN). Factors such as N rate, mode of N fertilizer application and choice of N sources for different agroecological zones of West Africa are discussed in relation to crop yield response. The interactive effects of cropping density and rainfall on N efficiency and yield are examined with particular emphasis on production of millet in Niger. The potential role of new, slow-release fertilizers as well as urea amended with urease inhibitors is mentioned in relation to present and future fertilizer N requirements in West Africa.

Fate and efficiency of N fertilizers applied to pearl millet in Niger

Field studies were conducted in Niger using 15N-labeled fertilizers to assess the fate and efficiency of fertilizer N in pearl millet (Pennisetum glaucum [L.] R.Br.) production. Total plant uptake of fertilizer N was low in all cases (20%–37%), and losses were severe (25%–53%). The majority of N remaining in the soil was found in the 0- to 15-cm layer though some enrichment at lower depths was found when the N fertilizer was calcium ammonium nitrate (CAN). In a comparison of urea placement methods (band, broadcast, or point placement), no significant differences in 15N uptake or yield were noted though point placement did exacerbate 15N loss. The mechanism of N loss is believed to have been ammonia volatilization. Yields were similar whether urea or CAN was used, but 15N uptake from CAN was higher. A statistical model was developed relating millet yield and N response to midseason rainfall. In drought years, no N response was found, whereas in years of good rainfall a response was found of 15 kg grain for each kilogram of N applied (at 30 kg N ha-1 rate).

Factors affecting N release of urea from reactive layer coated urea

An experimental fertilizer called reactive layer coated urea (RLCU) has been developed by coating urea with a mixture of diisocyanate and polyol in the presence of a catalyst. The hard, durable layer that is formed on the granule conveys slow-release character to the product. A series of soil incubation tests were conducted under simulated upland conditions for periods up to 56 days to study the effect of factors such as temperature, pH, soil moisture, and organic C additions on N release. The N release rate from RLCU was shown to be increased with increasing temperature and decreasing coating thickness. It was unaffected by the addition of lime to raise the pH or organic carbon sources to increase microbial activity. Although a slight effect of soil moisture was noted, it was not pronounced. Urea release tended to be in two stages — a constant diffusive stage in which, it is postulated, urea was still dissolving within the granule and diffusing to the soil at a constant rate and a slower logarithmic stage where the rate of release decreased with time.

Comparison of five soil testing methods to establish phosphorus sufficiency levels in soil fertilized with water-soluble and sparingly soluble-P sources

Field experiments were conducted in Niger with pearl millet (Pennisetum glaucum [L] R. Br.) in which the crop was fertilized with phosphate rock (PR) from two deposits from Niger (Tahoua and Parc W). The PR was applied either as ground rock or as partially acidulated phosphate rock (PAPR) and was compared to water soluble sources (TSP and SSP) in terms of millet yield response. The ability of five soil testing procedures (Bray P1, Bray P2, Mehlich 1, Olsen, and water extraction) to establish P sufficiency levels for millet was tested. The results of all soil testing methods were highly correlated amongst each other for the treatments receiving water-soluble fertilizers or PAPRs. None of the soil testing procedures which were evaluated was able to accurately measure available P when PRs were applied. Sufficiency levels were calculated for the PAPR and water-soluble fertilizers using nonlinear regression analysis and a graphic procedure for each of the P soil testing methods. The Bray P1 method appeared to be the most reliable procedure and was used to study the effect of accumulated total or total water + citrate-soluble P rates on final P availability. A single quadratic function was able to describe this effect when the P rates were expressed as water + citrate-soluble P for both PAPRs and water-soluble fertilizers independently of the P fertilizer source.

The effect of soil tillage and fertilizer use on pearl millet yields in Niger

Farmers in Niger generally do not plow their fields and are therefore unable to incorporate phosphate. Experiments were conducted in Niger to assess the effect of soil tillage, P source, and fertilizer placement on yields of pearl millet (Pennisetum glaucum [L.] R. Br.). Treatments included single superphosphate (SSP) or ground Tahoua phosphate rock (PRT) incorporated into the soil during tillage or SSP surface applied after tillage. In plots which were not tilled, P sources (SSP, PRT, and PAPR-partially acidulated rock) were broadcast on the soil surface with no incorporation. In order to improve P efficiency under zero tillage, P was point placed in the soil near the plant with either broadcast or point-placed urea. Treatments in which tillage was used showed a slight though nonsignificant yield increase over untilled plots. The yield increase did not appear to be due to phosphate incorporation but rather to direct tillage effects on early plant growth. In a comparison of SSP with PRT or PAPR broadcast on soils not receiving tillage, PRT performed poorly relative to the other P sources. SSP outyielded PAPR and PRT in 1986, but in subsequent years, no significant difference was found between PAPR and SSP. Point placement of P or N near the plant did not significantly increase yields over broadcast treatments even though the millet was planted with wide 1×1 m spacing.

Mineralization and nitrification of ureaform fertilizers

In an attempt to produce N materials that exhibit some delayed-release character and yet make all the N available in one growing season, ureaform (UF) fertilizers were prepared using low amounts of paraformaldehyde (PFA) (6% to 15% PFA w/w). Mineralization and nitrification of the various water-soluble components of these UF materials were studied over a period of 35 days by use of15N and high pressure liquid chromatographic techniques. Only the dimethylenetriurea (DMTU) fraction and a fraction tentatively identified as triuret showed any slow-release character, whereas other water-soluble components mineralized rapidly. Less soluble fractions did not mineralize appreciably during the experiment. Due to their reduced solubility, these ureaforms were shown to be less susceptible to volatilization than was urea, and a 37% reduction in loss was found.

Impact on ammonia volatilization losses of mixing KCl of high pH with urea

Ammonia volatilization associated with urea hydrolysis has been shown to be primarily associated with the pH of the soil solution and its buffering ability in the immediate zone of the fertilizer granule. Numerous studies have also shown that these losses can be reduced significantly by the addition of large amounts of KCl with the urea. Because the pH of commercial sources of potash ranges from 6.5 to 9.5, investigations were conducted to determine if the high pH of these K sources had an effect on the ammonia lost from three contrasting soils. Despite large ammonia losses (approximately 50% of N applied) and a significant reduction in loss due to the use of KCl (30%-50% reduction), the experiments showed no effect of potash pH on ammonia loss. It may be concluded that no risk of enhanced ammonia loss can be associated with the use of high-pH potash sources.

Ammonia volatilization from urea nitricphosphate and urea applied to the soil surface

Urea nitricphosphate (UNP) is an N-P fertilizer prepared by solubilizing phosphate ore with nitric acid and conditioning the product with urea. The product is acidic, and its nutrient analysis is 23-12-0. Urea makes up 74% of the N component of this material and the remainder comes from the nitrate added as nitric acid. In volatilization trials, UNP lost significantly less N than did urea in a noncalcareous soil (13 and 31% respectively). In calcareous soils the urea-N component of UNP exhibited loss patterns similar to those of urea. Soil pH remained stable at the center of the granule placement site during UNP hydrolysis, thereby reducing NH3 loss, whereas the pH of the same soil treated with urea rose almost 1.9 units. The urea component of UNP appeared to diffuse from the center of the acidic microsite allowing hydrolysis to take place and permitting limited NH3 volatilization to occur. UNP appears to be an attractive NP fertilizer in terms of nutrient analysis and resistance of the N component to volatile N losses as NH3.

Compaction of metal salt-urea complexes with triple superphosphate

It has been the experience of the fertilizer industry that urea should not be cogranulated or blended with superphosphate because urea reacts with monocalcium phosphate monohydrate (MCP·H2O) in superphosphate to form an adduct. This reaction releases the water of hydration and causes the product to become wet and sticky or severely caked during storage. The objectives of this study were [1] to test the feasibility of preventing or retarding the reaction by complexing the urea with various salt hydrates and [2] to measure ammonia volatilization from metal salt-urea complexes on the soil surface.Three metal salt-urea complexes — Al(urea)6(NO3)3, Fe(urea)6(NO3)3, and Mn(urea)4Cl2 — were prepared and cogranulated by compaction with pure MCP·H2O or triple superphosphate (TSP) at a mole ratio of MCP:urea as 1:2. These materials were then compared with the same material without metal salts in terms of changes in free water content during a storage period of 6 weeks. Without metal salts a rapid and significant increase in free water content of the cogranulated MCP·H2O + urea or TSP + urea was observed. The increases in free water content were found to range from 1.5% to 1.8%, corresponding to approximately 63% and 78% of the added MCP·H2O that reacted with urea in the cogranulated products. On the other hand, little change or only a slight increase (less than 0.5%) in free water content was observed with the cogranulated metal salt-urea complexes.Ammonia volatilization losses from urea on the soil surface were measured in a period up to 14 d with two soils: Windthorst (pH 7.6) and Savannah (pH 7.0). The fertilizer materials used were granular. In Windthorst soil, the amounts of NH3-N lost were 25% for prilled urea, 11% for Mn(urea)4Cl2, and essentially none for Mn(urea)4Cl2 compacted with TSP at a mole ratio of MCP:urea as 1:1 or 1:2. In Savannah soil, the amounts of NH3-N lost were 39% for prilled urea, 24% for Mn(urea)4Cl2, 15% for Fe(urea)6(NO3)3, and less than 6% for each of the two metal salt-urea complexes compacted with TSP. The acidity that resulted from metal complexing of urea reduced NH3 volatilization from hydrolyzed urea in soils, and additional acidity produced from hydrolysis of MCP·H2O further reduced NH3 losses when materials were applied as multicomponent granules (metal salt + urea + TSP).