Crops, as the basic source of essential substances and nutrients, do not always contain sufficient amounts of these essential nutrients to meet dietary requirements. In this review paper, we discussed the effects of fertilization and other agronomic measures on the nutritional quality of cereal, oilseed and protein crops, tuber plants and vegetables. Research indicates that application of N, P, K and S fertilizers generally increases crop yield as well as nutritional quality. For example, fertilizer increased protein concentration in cereals and pulses, oil concentration in oilseed crops, starch concentration in tubers, and concentration of essential amino acids and vitamins in vegetables. However, excessive fertilizer application, especially N fertilizer, can result in undesirable changes such as increases in nitrate, titratable acidity and acid to sugar ratio, while decreasing the concentration of vitamin C, soluble sugar, soluble solids, and Mg and Ca in some crops. Other agronomic measures, such as tillage and crop rotation, organic farming, soil moisture management, and crop breeding and genetic engineering can also have a large effect on food crop quality, though the potential benefits of these measures for improving crop quality has not been fully exploited. Research literature on this subject suggests that more information is needed in order to achieve an increase in the concentration of essential microelements, prevent accumulation of toxic levels of elements such as Cu, Mo, Zn, Ni, Se and nitrate, and other dangerous or toxic substances and elements in crops. Copyright © 2007 Society of Chemical Industry

Extraction of soil nitrate nitrogen (NO₃ ⁻-N) and ammonium nitrogen (NH₄ ⁺-N) by chemical reagents and their determinations by continuous flow analysis were used to ascertain factors affecting analysis of soil mineral N. In this study, six factors affecting extraction of soil NO₃ ⁻-N and NH₄ ⁺-N were investigated in 10 soils sampled from five arable fields in autumn and spring in northwestern China, with three replications for each soil sample. The six factors were air drying, sieve size (1, 3, and 5 mm), extracting solution [0.01 mol L⁻¹ calcium chloride (CaCl₂), 1 mol L⁻¹ potassium chloride (KCl), and 0.5 mol L⁻¹ potassium sulfate (K₂SO₄)] and concentration (0.5, 1, and 2 mol L⁻¹ KCl), solution-to-soil ratio (5:1, 10:1, and 20:1), shaking time (30, 60, and 120 min), storage time (2, 4, and 6 weeks), and storage temperature (−18 ^oC, 4 ^oC, and 25 ^oC) of extracted solution. The recovery of soil NO₃ ⁻-N and NH₄ ⁺-N was also measured to compare the differences of three extracting reagents (CaCl₂, KCl, and K₂SO₄) for NO₃ ⁻-N and NH₄ ⁺-N extraction. Air drying decreased NO₃ ⁻-N but increased NH₄ ⁺-N concentration in soil. Soil passed through a 3-mm sieve and shaken for 60 min yielded greater NO₃ ⁻-N and NH₄ ⁺-N concentrations compared to other treatments. The concentrations of extracted NO₃ ⁻-N and NH₄ ⁺-N in soil were significantly (P < 0.05) affected by extracting reagents. KCl was found to be most suitable for NO₃ ⁻-N and NH₄ ⁺-N extraction, as it had better recovery for soil mineral N extraction, which averaged 113.3% for NO₃ ⁻-N and 94.9% for NH₄ ⁺-N. K₂SO₄ was not found suitable for NO₃ ⁻-N extraction in soil, with an average recovery as high as 137.0%, and the average recovery of CaCl₂ was only 57.3% for NH₄ ⁺-N. For KCl, the concentration of extracting solution played an important role, and 0.5 mol L⁻¹ KCl could fully extract NO₃ ⁻-N. A ratio of 10:1 of solution to soil was adequate for NO₃ ⁻-N extraction, whereas the NH₄ ⁺-N concentration was almost doubled when the solution-to-soil ratio was increased from 5:1 to 20:1. Storage of extracted solution at −18 °C, 4 °C, and 25 °C had no significant effect (P < 0.05) on NO₃ ⁻-N concentration, whereas the NH₄ ⁺-N concentration varied greatly with storage temperature. Storing the extracted solution at −18 ^oC obtained significantly (P < 0.05) similar results with that determined immediately for both NO₃ ⁻-N and NH₄ ⁺-N concentrations. Compared with the immediate extraction, the averaged NO₃ ⁻-N concentration significantly (P < 0.05) increased after storing 2, 4, and 6 weeks, respectively, whereas NH₄ ⁺-N varied in the two seasons. In conclusion, using fresh soil passed through a 3-mm sieve and extracted by 0.5 mol L⁻¹ KCl at a solution-to-soil ratio of 10:1 was suitable for extracting NO₃ ⁻-N, whereas the concentration of extracted NH₄ ⁺-N varied with KCl concentration and increased with increasing solution-to-soil ratio. The findings also suggest that shaking for 60 min and immediate determination or storage of soil extract at −18 ^oC could improve the reliability of NO₃ ⁻-N and NH₄ ⁺-N results.

The recycling of livestock manure in cropping systems is considered to enhance soil fertility and crop productivity. However, there have been no systematic long-term studies of the effects of manure application on soil and crop macro- and micro-nutrients, heavy metals, and crop yields in China, despite their great importance for sustainable crop production and food safety. Thus, we conducted field experiments in a typical cereal crop production area of the North China Plain to investigate the effects of compost manure application rates on wheat yield, as well as on the macro-/micro-nutrients and heavy metals contents of soil and wheat. We found that compost application increased the soil total N and the available K, Fe, Zn, and Mn concentrations, whereas the available P in soil was not affected, and the available Cu decreased. In general, compost application had no significant effects on the grain yield, biomass, and harvest index of winter wheat. However, during 2012 and 2013, the N concentration decreased by 9% and 18% in straw, and by 16% and 12% in grain, respectively. With compost application, the straw P concentration only increased in 2012 but the grain P generally increased, while the straw K concentration tended to decrease and the grain K concentration increased in 2013. Compost application generally increased the Fe and Zn concentrations in straw and grain, whereas the Cu and Mn concentrations decreased significantly compared with the control. The heavy metal concentrations increased at some compost application rates, but they were still within the safe range. The balances of the macro-and micro-nutrients indicated that the removal of nutrients by wheat was compensated for by the addition of compost, whereas the level of N decreased without the application of compost. The daily intake levels of micronutrients via the consumption of wheat grain were still lower than the recommended levels when sheep manure compost was applied, except for that of Mn.