Crops nutrients removed

In small-scale farm production of ethanol a possible environmental impact could occur through removal of crop residues for use as a boiler fuel. Crop residues are important because they help control soil erosion through their cover and provide nutrients, minerals, and fibrous material which help maintain soil quality. However, not more than one-third to one-half of the residues from a grain crop devoted to ethanol production need be used to fuel the process. Also, there are several methods, such as crop rotation and winter cover crops, which lessen the impact of crop residue removal. ... 

Nandwa and Bekunda (1998) report that in order to replace the nutrients removed by a 2-t crop of maize, 1 - 2 t of poultry manure or 7 t of low-quality ruminant manure need to be applied per ha. Depending on manure quality, 2-8 cattle are needed to supply the necessary amount. In the miombo woodlands of southern Africa, 14 - 42 ha of grazing land are needed to source 10 t of low-quality manure containing 96 kg N (Nandwa and Bekunda, 1998). Sandford (1989) accordingly estimates that in semiarid West Africa, 16 - 47 ha of grazing land may be required to sustain one ha of a maize crop producing 1 - 3 t grain yield ha 1. 

Crop Yield level (gm-2) Nutrients removed annually (dg m - ) ... 

Estimated annual nutrient removal by major crops — harvested portion (USDA, 1967a.)... 

In livestock farming, where all manures and wastes are returned to the soil, and the only product removed is the animals, it is obvious that the maintenance of fertility is a comparatively simple matter. In contrast, in grain farming most of the product is removed as grain and sold on the market. Any direct comparison between the two systems is unfair and may be misleading. The nutrients removed in harvested crops must necessarily be returned in part as fertilizers, if the fertility of the soil on such areas is to be kept at the same level as where livestock is the only marketed product. 

Wetland values are dependent on social perceptions. The valued functions have historically included water storage, flood control, erosion control, sediment control, nutrient removal, protection of general water quality, habitat for crops and fisheries, recreation, and wildlife... 

Soil nutrients removed in farm products must be-re-plenished or the soil becomes impoverished of those nutrients, and plant growth and production from succeeding crops is decreased. From the very beginning of sedentary agriculture, farmers have recognized the basic problem of nutrient depletion. On all continents slash... 

Nutrient removal by the crop is the largest factor accounting for nutrient depletion from well-managed crops. Fertilization practices are generally targeted to replenishing these nutrients consistent with economic considerations. 

In order to appreciate the function of artificial fertilizers it is instructive to look at plant nutrition and some natural cycles which replace nutrients removed from the soil by growing crops. 

Nutrients removed in grain, leaf, and wood biomass should be replaced via organic and inorganic fertilizers otherwise the system ceases to be sustainable. On recently deforested sites, additional nutrients may not be required for several years before productivity declines set in. Abandoned pastures, however, show a combination of soil chemical and physical constraints to crop production and will require inputs to alleviate soil compaction and nutrient deficiencies (principally phosphorus and calcium). 

A ready reckoner for the amount of N, P and K removed by certain representative crops is shown in Table 5.2. The requirement for P and K may be expressed in terms of the element rather than the oxide (P205 or K20). P205 contains 0.43 units of P K20 contains 0.83 units of K. The depletion of N, P and K from the grain of wheat, barley and oats is pro rata for yield, but the nutrient composition of the straw is different, oat straw containing very much more potassium than wheat or barley straw. Potatoes and kale are very much more exhaustive of N and K than the cereal crops. 

A plant accumulates nutrients from the soil as it grows. Such accumulation depletes the amount of nutrient remaining in the soil so, harvesting an arable crop, such as maize, barley or com, removes nutrients from the field. A farmer needs to replenish the nutrients continually if the land is not to become exhausted after a few seasons. 

Greenland (1997) has compiled realistic average annual nutrient balances for wetland ricefields pre- and post-1960 from probable inputs and outputs. Inputs come from rainfall, R, irrigation and floodwater, F, sediments, S, nitrogen fixation, N, and manures and fertilizers, M. Outputs are due to crop removals in... 

Figure 7.2 Typical annual nutrient balances for irrigated rice soils pre- and post-1960 calculated from probable inputs (left side of each graph) and outputs (right side) (data from Greenland, 1997). Inputs come from R = rainfall, F = floodwater and irrigation, S = sediments, N = nitrogen fixation and M = manures and fertilizers. Outputs are due to removals in Cg = rice grain, Cs = rice straw. Cl = legume crop, S = seepage and percolation and G = gaseous emission...

Ecosystems subjected to oxidant air pollutants must be carefully observed and described individually if we are to understand and predict the complex consequences of chronic injury. Woodwell has summarized some of the expected effects of air pollutants on ecosystems elimination of sensitive species and reduction of diversity in numbers of species selective removal of larger overstoiy plants and a favoring of small plants reduction of the standing crop of organic matter, which leads to a reduction of nutrient elements held within the living system and increase in the activity of insect pests and in some diseases that hasten producer mortality. Many other effects can be suggested. 

Chloride differs from other nutrient elements present in native rocks because it is not fixed by colloids it is repelled by negatively charged clay surfaces, and all chloride compounds formed in soils are highly soluble. In addition to leaching, chloride is lost from soils through crop removal. 

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