Will farming and soil quality collide?

Will farming and soil quality collide?

Better management practices will avoid conflicts between productivity and soil qualityWorld Agriculture and the Environment is an important new book addressing the fear that increasing demand for food and fiber is on a “collision course” with soil quality.

This article is in two parts. In Part One, some of the main issues discussed in the book are reviewed. Part Two then explains how more than 50 years of research and practical use have shown that controlling weeds with paraquat can help provide improved and sustainable crop management practices to improve soil quality.

Part One: What ‘World Agriculture and the Environment’ says

In World Agriculture and the Environment authorJason Clay (World Wildlife Fund-US vice president, Center for Conservation Innovation) reviews the production and environmental impact of 21 of the world’s major food commodities. The main threats to the environment posed by crops, fish and meat are identified and explored, as well as the trends that shape those threats.

Major Issues

A fundamental acknowledgement in the book is that low intensity cropping can not support current, let alone future, levels of world population. There are two underlying reasons. First, using more land for farming destroys natural forests and grasslands. Second, greater use of external inputs and mechanization has greatly reduced labor requirements – to expect a mass return of people to plant seeds, hoe weeds and harvest produce by hand is hardly realistic.

However, Jason Clay cites a number of areas in which agriculture - as the planet's largest industry, providing work for over one billion people and with an annual output worth more than one trillion dollars - has critically important impacts on the environment. These include:

  • Degradation of soil

Soil erosion has serious implications for productivity, depleting nutrients and reducing water holding capacity. Off-site, eroded soil can silt-up reservoirs and water courses making problems for irrigation and hydroelectric power generation. In tea, for example, annual soil losses of much as 75 t/ha have been recorded in Sri Lanka, and in coffee:

“One of the most degrading forms of coffee cultivation for soils is the use of herbicides to produce ’clean’ fields free of other vegetation. …weed-free fields … on the slopes of coffee farms, particularly those at high elevations, is (sic) one of the major causes of soil exposure and erosion.” (page 84 of the book)

  • Contamination of water by run-off


    The occurrence of surplus liquid (like rain) which originates up-slope and is collected beyond the ability of the soil to absorb it. The surplus liquid then flows away over the surface to reach the nearest surface water (pond, lake, river).

    Authoritative On-line References and Resources

    US Geological Survey's Water Science School

    and leaching


    The natural process by which water soluble substances are carried downward through the soil into groundwater.

For example, in cotton and tea:

“ … the most important production impacts [in cotton] are the use of agrochemicals ... The quality of soil and water and the impact on biodiversity in and downstream from the fields are also major concerns.”(page 292 of the book)

“The chemical inputs applied on tea plantations have had a deadly effect on soil biodiversity while simultaneously polluting river water, killing fish, and harming the animals and people who depend on the rivers for water.” (page 103 of the book)

  • Loss of habitats and biodiversity

The UN Food and Agriculture Organisation estimate that more than 13 million hectares of forest are cleared for farming in developing countries every year. Apart from the loss of habitat and biodiversity this entails, further losses come from intensive crop monocultures:

“By providing vast areas of food and eliminating (the) diversity … agriculture has reduced the number of species … while allowing a few to become dominant” (page 49 of the book)

Towards Better Management Practices

For each crop Clay points out where better management practices could help to increase sustainability. As the demand for food has intensified over the past 50 years or so pressures have increased on the ability of the soil to sustain production. Erosion, and soil structure, its health and fertility have become serious problems in many parts of the world. The availability of fertilizers and agrochemicals has encouraged trends away from rotations towards monocultures with sometimes serious issues for biodiversity through loss of habitats and water due to leaching and pollution.

One general approach to sustainability applicable to all cropping systems is the concept of conservation tillage. In conservation tillage systems soil disturbance is minimised and much of the soil is covered with plant material throughout the year. In ‘no-till’ systems, there is no soil cultivation at all. Clay cites the US Dept of Agriculture’s findings that:

“Conservation tillage saves on fuel, labor and depreciation of farm equipment while improving soil structure and fertility” (page 423 of the book)

Weeds or a cover crop such as rye are controlled by a burndown herbicide, but as Clay points out:

“While no-till has been shown to reduce the use of some agrochemicals, even some of the most harmful ones, it does seem to lead to an increased reliance on glyphosate.” (page 197 of the book)

Clay recognises that good yields and profitability are paramount in encouraging farmers to adopt practices that protect soil and water and enhance biodiversity. The judicious use of appropriate agrochemicals can help alleviate the pressure to exploit new land. For example, in discussing the destruction of upland wilderness and forests to plant coffee, Clay states that:

“…there is no reason to clear pristine habitat to plant coffee. With the agrochemicals available today and with improved overall production and management practices, much previously degraded land can be brought back into production.” (page 86 of the book)

The ultimate goal of the book is to suggest better management practices to reduce the damage to ecosystems and improve efficiency and profitability. Clay does not endorse the use of specific products, but by highlighting the issues which really matter, provides a sound basis for the world farming industry to formulate practical solutions.

World Agriculture and the Environment: a commodity-by-commodity guide to impacts and practices by Jason A Clay. Published by Island Press (2004). ISBN No. 1-55963- 370-0

Part Two: How paraquat can help

In this section, the opportunities to improve crop management practices by using paraquat are described. A number of practical case studies are used to illustrate the benefits.

Protecting Soil

Paraquat can be used as the basis of a weed management program which, rather than keeping fields clean and totally weed-free, allows the presence of a non-competitive weed cover for at least part of the time. A vegetative cover over the soil protects it by dispersing the impact of rain drops, which is a primary cause of erosion. Not cultivating soil to control weeds helps to prevent soil erosion which removes nutrients and organic matter, and decreases water holding capacity.

There's an awful lot of coffee in BrazilIn Brazil, establishing grass strips between coffee rows has been found to reduce soil losses from over 4 tonnes/ha each year to 0.2 tonne/ha and rainfall run-off by 90% (May et al., 1993). Paraquat can be safely sprayed to manage the weed flora along the crop rows between the grass strips without fear of damaging the coffee bushes. It is immobile in soil and cannot move to the roots and up into the shoots.

Bark cannot be penetrated by paraquat meaning that it can be sprayed right up to the base of the bushes. Even if paraquat drifts on to coffee leaves there is little or no damage because paraquat does not move through plants systemically like glyphosate.

In tea grown in the mountainous upper reaches of the Yangtze River in China, tilling the soil to control weeds has resulted in serious soil erosion caused by surface run-off of water.

Scientists from the Sichuan Academy of Forestry found that using paraquat to control weeds is twice as effective as glyphosate at reducing erosion compared to conventional tillage.In trials in tea gardens with slopes of 8º–18º, losses of nutrients and organic matter along with the soil were 80-100% less when using paraquat rather than glyphosate.

No-till plots treated with paraquat lost less soil

This is because glyphosate leaves the soil exposed for longer than paraquat, which allows a non-competitive green cover to regenerate.

The presence of vegetation disperses the impact of rain and weed roots provide an anchorage for the soil.

Protecting Water and Biodiversity

Paraquat cannot leach into water because it is so tightly bound to soil – far more so than other agrochemicals. This fundamental property means that paraquat can not move in to water courses by surface run-off or leaching. Nor, by the same token, can it affect soil fauna, flora and microorganisms because it is not accessible to them.

Used as recommended, paraquat is not hazardous to fish or invertebrates because, besides its immobility, even if paraquat spray should drift onto ponds, rivers or ditchwater, it is rapidly removed by adsorption to plants and sediments, and by microbial degradation. Extensive ecological studies have shown that paraquat is not a risk to aquatic environments.

Bananas: paraquat can not move into water coursesSyngenta (as its legacy company Zeneca), collaborated with EARTH (Escuela de Agricultura de la Region Tropical Humeda), Del Monte and Dole, and showed how biodiversity can be maintained in commercial banana plantations in Costa Rica. Banana farmers there use paraquat to manage plantation weeds while reducing soil erosion and maintaining biodiversity including fish, amphibians, reptiles and birds.

A well-managed source of water is critical for banana production. Bananas need a constant supply of water – at least 160 mm/month – but cannot stand water-logged soil. In the tropical rainy season, some months may have more than 1000 mm excess rainfall.

Conservation Tillage

Conservation tillage: weed control by paraquat protects soilIn conservation tillage systems for crops like maize, cotton or soybeans, weeds or a cover crop such as rye have to be controlled by a burndown herbicide, generally either paraquat or glyphosate.

Paraquat-based burndown sprays are more reliable than glyphosate in controlling annual weeds and cover crops at earlier growth stages when the weather is cold and rain may fall soon after application.

Under such challenging weather conditions, paraquat will control weeds in a few days compared to glyphosate’s 2-3 weeks, and it is rainfast in minutes rather than hours.

Conservation tillage systems affect weed infestations through combined effects of soil management and herbicide regimes leading to changes in weed flora known as ‘weed shifts’. In the US, intensive use of glyphosate, particularly since the introduction of GM glyphosate resistant varieties, has led to the development of resistant weeds in cotton and soybeans including horseweed (Conyza canadensis), common ragweed (Ambrosia artemesifolia), palmer amaranth (Amaranthus palmeri) and common waterhemp (Amaranthus rudis). In Brazil, ryegrass (Lolium multiflorum) resistant to glyphosate has also recently appeared in soybeans.

Glyphosate is a keystone of world agriculture because it controls perennial weeds cheaply and effectively. It is essential to keep weed shifts and resistance at bay. As an alternative broad spectrum non-selective herbicide to glyphosate, paraquat has a valuable role to play in integrated weed management (IWM) systems. No more than two applications of glyphosate should be applied to any one field over two seasons. Paraquat can provide the alternative means of effective and sustainable weed control.

In Brazil, SIC (Sistema Integrado de Controle de plantas daninhas) is one such IWM system. Paraquat is sprayed within a day either side of soybean emergence as a follow-up to an earlier glyphosate application. This minimizes the risk of weeds which have escaped control by glyphosate becoming a problem and extends the period of weed control such that only one application of post-emergence selective herbicide is needed instead of the usual two.

In perennial crops such as fruit, coffee, oil palm and rubber growing in climates with ample sunshine, heat and moisture, weeds thrive. They compete with crops for space, water and nutrients, shade young crop plants, and hinder crop protection and harvesting operations. However, if they are effectively managed ‘soft’ weeds can help minimise soil erosion. If they are permanently excluded by herbicides such as glyphosate they tend to be replaced by re-invasion of cleared land by noxious weeds which reduce crop yields. Apart from having a virtually unique mode of action (shared only by its analogue, the desiccant diquat) which helps combat weed resistance, paraquat only removes the top growth of well established weeds, allowing them to re-grow after 1-2 months. In this way, soft weeds can be controlled but not eliminated.

Paraquat: A Basis for Better Crop Management Practices

Paraquat is used to control weeds in conservation tillage systems and other uses around farms, and in doing so reduces soil erosion, improves soil health, and avoids the development of resistance to commonly used herbicides like glyphosate. At the same time, water quality is protected because paraquat is adsorbed so tightly to soil that it can not leach. Biodiversity is encouraged because paraquat does not act systemically or residually in the soil, so allowing vegetation to re-grow from underground organs or seeds to maintain habitats and stabilise soil structure.


May, P H et al. (1993). Coffee and cocoa production and processing in Brazil. Geneva: UN Conference on Trade and Development. UNCTAD/COM/17. 27 August 1993.