Soil response to impacts of pesticide and fertilizer over time

Lazarus Dawa

University of Sriwijaya, Palembang, Indonesia
Email: ldawa@live.com.au
September, 2013

Introduction

The world population had increased at annual rate of 1.76 from 1950 to 2000 and it was predicted for the next 50 years the average growth rate will be at 0.77 percent per annum (UN,2004).  This directly implies that the current demand for food and meat products will increase and the land and environment must be utilized to meet these growing human needs. In addition to population growth, increase in income and changes in diet pattern especially to more livestock products consumption will also be the main drivers of increase food production (FAO, 2012).  In order to produce more food, the agriculture practices have to be adjusted to increase production quantity.  Land is under pressure from increase human activities and there will be competition for land use for infrastructure development and other non-food uses versus agriculture utilization.  Decrease in land available for agriculture and deterioration of soil quality from continuous cultivation will require intensification process to grow crops by utilizing fertilizers, pesticides, irrigation and other technologies.  Over use of fertilizers and pesticides can have negative effect on soil qualityies.  Application of nitrogen fertilizer in wheat and rice over period of 20 years was found to reduce yield in both crops (Kumar & Yadav, 2001).

Therefore to maintain long term sustainability and productivity of the soil, it is better to choose the best practice of applying fertilizers and pesticides while ensuring food productions are still maintained.
The purpose of this paper is to discuss the various impacts of continuous fertilizer and pesticide use on the soil quality over time.  It will also extend to suggest some of the best options that can be taken to reduce the negative impact of fertilizer and pesticide on the soil.

Effect of pesticides on soil

Pesticides refer to any substance or mixture of substances intended to control, inhibit or destroy any pests that interfere with crop production.  The use of synthetic pesticides has both beneficial and negative effects.  The primary benefit of pesticide use is protection of crops from diseases and pests which reduce losses and preserve crop yields.  Pesticides if manage and use efficiently is safe however over use of it can cause health hazards, interfere with soil and natural bio-ecosystem which can impede food production.
Pesticides use are known to affect soil microorganisms which carry out important soil function such as decomposition of organic matter, supplying of nutrients to soil and maintaining soil structure.
It was found that fumigation of soil with commercial isothiocynate (ITC) based fumigant (metham sodium) had inhibited microbial activity, reduced biomass of all microbial groups and create consistent changes on the structure of the ascomycete community (Omirou et al, 2011).  Ascomoycetes fungus is important for biodegradation process of organic matter through secretion of enzymes that act on degradation of lignin in woods of dead plants (Martinez, et al 2005).  When ascomycetes population in soil is lowered, the degradation process of organic matter will be slowed thus affecting the nutrient profile of the soil especially in terms of carbon and other minerals like phosphorous and nitrogen which are released from the humus as it decay.

While Ahemad and Mohammad (2012) study on rhizobium bacteria exposed to pesticides stress showed reduce production of plant growth producing substance.  The rhizobium bacteria are group of organisms that live freely in soil and colonize the root.  They are usually referred to as plant growth promoting bacteria (PGPR).  The PGPR operate by synthesizing of particular compounds for the plants, facilitates uptake of certain nutrients from the soil and lessening or preventing the plants from disease (Hayat et al, 2010).
Increasing concentration of arsenic pesticide use in soil decrease biomass, shoot height and root length, all of which are important traits of showing plant growth (Quazi et al 2011).  Arsenic being one of the non-essential substance for plant growth, it interferes with metabolic process and inhibit plant growth and death at high concentration.

Effect of fertilizers on soil

Chemical fertilizers are key inputs to increased agricultural food production.  The overall world food supply has been sufficient to meet the population need, the global average per capita food available for direct consumption was 2770 kilo calories/person/day in 2005 and 2007. It is also projected that fertilizer use will increase from 166 million tonnes in 2005 and 2007 to 263 million tonnes in 2050.    Much of this achievement in general food production can be attributed to the use of fertilizers and developing countries account for 70 percent of global fertilizer consumption (FAO, 2012).  Fertilizers improve and buffer soil fertility by providing essential nutrients such as nitrogen, potassium, phosphorous, and other minerals which the plant needs to sustain growth and production. Although increased fertilizer use will boost crop production, it is also crucial here to identify some negative impact of continuous fertilizer application on the soil health.

Continuous cropping with nitrogen fertilizer application for 25 years in acidic soil was found to reduce the yield of maize and wheat significantly.  The use of nitrogen fertilizer alone had greater impact on reducing soil pH from pH of 5.8 to 4.7 during that period. (Sharma & Subehia, 2003). Nitrogen fertilizers are not acidic but are acid forming due to the presence of ammonia group (NH4) the ammonia oxidizing arachea and ammonia oxidizing bacteria in the soil converts the ammonia to nitrite and then to nitrate by nitrite oxidizers (Alam et al, 2013).

Soil pH is an important aspect of agronomy since it dictates the response of metal ion solubility and availability of nutrient to the varying pH level.  When soil pH is low below the neutral point of 7, the metal ion become more soluble and there is increased tendency of uptake by plants which can lead to mineral toxicity where as some nutrients are bounded in acidic conditions depriving the plants from its nutrients demand.  The response to declining pH level varies among crop types.  For example corn grows in pH between 5.8-7.0, wheat from 6.3-7.0 and cowpea can grow between pH 5.5-7.0.
In long term cropping observation, applying of only nitrogen fertilizer had no increase effect on the soil organic carbon level (Liu et al, 2013).  Organic carbon in soil is a reliable indicator of the state of soil organic matter which the latter is important for improving the soil physical and biological properties to sustain crop production.

Practices to improve soil quality

Based on the few reviewed study on impact of fertilizer and pesticides use on soil which in turn determines the crop health and growth it is important to identify and promote good agricultural practices that is beneficial to the soil as well as the surrounding ecosystem.

1. Altering pesticides and fertilizer usage
Soil quality can be improved by changing the agricultural practices that involve use of fertilizers and pesticides. As observed, it can be seen that use of only one fertilizer affect soil quality and by combining more than one fertilizer can alleviate the negative effect of continuous fertilizer use.  Integrated nutrient management by combining manure and inorganic fertilizer has shown to improve soil organic matter in degrading agriculture land (Liu et al, 2013).  This will provide long term sustainable food production and also contribute to reducing of green house gases from nitrogen fertilizer use.

2. Organic farming
Organic farming represents a promising solution to maintain current soil quality while meeting world food demand.  Shifting from conventional to organic farming eliminates the need for fertilizer and pesticides by utilizing organic means such as manure, cover crops and biopesticides.  In that way, many negative impacts of chemical inputs can be reduced and allow for natural cycles to occur.  Although the question of whether organic farming can meet future food demand exist, a study by Badgley et al (2006) showed that organic farming could produce enough food to meet the current and even expanding world population without increasing the land area. It was also found that leguminous cover crops could produce enough nitrogen to replace the amount of synthetic nitrogen fertilizers currently in use.

Conclusion
The need to feed the world in the next 20 years will depend largely on soil and water availability and qualities.  It is important that these resources must be used in sustainable way by limiting use of chemicals.  Some pesticides are known to affect soil microorganisms that are beneficial to soil health in sustaining plant growth. Fertilizer overuse can also decrease soil quality which creates the problem for the soil to be more reliant on fertilizer and continuous use can eventually lead to unproductive soils.  By date, organic farming remains the best options for long term food production and preservation of soil.