Clay soils cropping

Figure 8 The effect of fertilizer application and rainfall and the emissions of NO and N,0 from clay loam soils cropped with ryegrass cut for silage in South Scotland.

Seed germination reduced 50% (Kulshrestha etal. 1982) Effectively controls weeds in wet sandy soils some damage to crop possible in dry clay soils (Amor et al. 1987)... 

Haraldsen TK, Asdal A, Grasdalen C, Nesheim L, Ugland TN (2000) Nutrient balances and yields during conversion from conventional to organic cropping systems on silt loam and clay soils in Norway. Biol Agric Hort 17 229-246... 

Kahnt, G., Pfleiderer, H. and Hijazi, L.A. 1986. Effect of amelioration dosed of rock powder and rock sand on growth of agricultural plants and on physical characteristics of sandy and clay soil. Agronomy and Crop Science 157 169-180. 

Beck, A.J. and K.C. Jones (1996). The effects of particle size, organic matter content, crop residues and dissolved organic matter on the sorption kinetics of atrazine and isoproturon by clay soil. Chemosphere, 32 2345-2358. 

Crops show a higher tolerance to vemolate than to EPTC. It is used in soybean, tobacco, sweet potatoes and peanut. Investigations of Bourke and Fang (1968) showed that the selective applicability of vemolate in soybean can be attributed to the fact that it is rapidly metabolised to inactive compounds in soybean and other tolerant dicotyledonous crops. Its half-life in moist clay soil is approximately 1.5 weeks. 

The condition for uptake of the pesticide by the plant is given by its persistence or system characteristics. The most persistent pesticides are particularly organometallic (most of them organomercury) compounds or derivatives of chlorinated hydrocarbons. Depending on the concentration in the soil, they can be identified especially in root crops, carrot, radish, beet, potatoes and plants giving oil. The rate of uptake of these substances by plants are different even in the case of substances with a similar persistence [29, 30]. The uptake can also be different in the case of identical plants [31]. Solid pesticides penetrate more easily into agricultural products from sandy and clay soils [32]. 

Molybdenum (Mo) deficiencies have been reported from many countries around the world, mainly in acidic soils. Sandy soils are Mo-deficient more often than are loam or clay soils. Most Mo deficiencies are associated with legume crops, because Mo is an essential constituent of enzymes necessary for fixation of nitrogen (N) by bacteria growing symbiotically with legumes. Molybdenum is also required in other enzyme systems in all plants. 

Research is also handicapped by the fact that most of the released phosphoric acid is likely to be fixed immediately as insoluble and largely unavailable salts of calcium, iron and aluminum. Any crop present will assimilate a portion of the phosphoric acid as it is released but in clay soils most of it is fixed in inorganic forms. In the case of actively decomposing plant materials in soil the phosphorus content of the material undergoing decomposition is a major factor in determining what happens to the released phosphorus. There is a critical C P level here just as there is a critical C N level in the case of decomposing nitrogenous substances. 

Herbicide use. Some soil textures are more prone to herbicide leaching, particirlarly of some soil applied residrrals. Recommended application rates of these cherrricals are affected by the soil texture in order to avoid any risk of crop damage. On clay soils rates may be higher as some chemicals are adsorbed onto the soil particles and are then unavailable for weed uptake. 

Many factors affect the mechanisms and kinetics of sorption and transport processes. For instance, differences in the chemical stmcture and properties, ie, ionizahility, solubiUty in water, vapor pressure, and polarity, between pesticides affect their behavior in the environment through effects on sorption and transport processes. Differences in soil properties, ie, pH and percentage of organic carbon and clay contents, and soil conditions, ie, moisture content and landscape position climatic conditions, ie, temperature, precipitation, and radiation and cultural practices, ie, crop and tillage, can all modify the behavior of the pesticide in soils. Persistence of a pesticide in soil is a consequence of a complex interaction of processes. Because the persistence of a pesticide can govern its availabiUty and efficacy for pest control, as weU as its potential for adverse environmental impacts, knowledge of the basic processes is necessary if the benefits of the pesticide ate to be maximized. 

Soils properties are very sensitive to the type of exchangeable ions. Calcium imparts favorable physical properties to the soil, while adsorbed sodium causes clay dispersion and swelling. It is generally recognized that an exchangeable sodium percentage of 10 is sufficient to cause soil dispersion, reduction of soil permeability and impaired growth of some crop plants. On the other hand, excess salt concentration prevents the dispersive effect of adsorbed sodium. 

O2 availability, water content. Structural - soil type, porosity, clay content, fertility. Agricultural - Fertilizer addition, herbicide pesticide application, tillage, cropping systems etc. 

For studies involving test substance application to soil, there may be a requirement for more soil information than for studies where applications are made to foliage of established crops. The study protocol should describe any specific requirements relative to soil type selection and how to confirm the soil characteristics for the study. Most studies simply require that the soil be identified by its name (e.g., Keystone silt loam) and composition (e.g., percent sand, silt, and clay). This information can typically be acquired from farm records, a soil survey of the local area, or a typical soil analysis by a local soil analysis laboratory. In some instances, a GLP compliant soil analysis must be completed. The study protocol must clearly define what is needed and how it is to be obtained. Unless specified in the protocol, non-GLP sources are adequate to identify the soil and its characteristics. The source of the soil information should be identified in the field trial record. 

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