Soil quality must be considered in relation to its use or output, such as rice, wheat, fruits and vegetables.In simple terms, the same agricultural land may not be safe for planting rice, but plant grass can be used for cattle and sheep.

In China, the content of heavy metals in soil is generally on the high side. Taking it as the growth matrix, the heavy metal content of rice, wheat, vegetables and other crops may also exceed the standard.However, a large number of measured data show that there is no stable correspondence between the two.In other words, according to the current monitoring method, the heavy metal content in the soil is high, and the quality of crops growing on it may be qualified.On the other hand, some soils are not rich in heavy metals, but because the crop varieties have strong heavy metal enrichment capacity, the harvest may exceed the heavy metal limit.

Therefore, it is not accurate to talk about soil quality standards without considering the utilization mode or the harvest, especially for agricultural land.Only after repeatedly trying to plant much breed, cannot harvest the produce with eligible quality, then can make sure it lost agricultural use value, should adopt the measure such as rehabilitate management or changing land use property.

Monitoring the total amount of heavy metals has always been the main basis for the preliminary determination of soil quality.However, the analysis results of the original soil showed that except for the combination of cadmium and the solid phase of the soil is relatively loose, the proportion of exchangeable states was relatively high, reaching about 20%, and the migration activity and bioavailability were relatively strong, copper, lead, zinc and other heavy metals were mainly distributed in other forms.That is to say, although the total content of heavy metals such as lead in soil is very high, the proportion that it can be absorbed and utilized by crops is not high, and it is relatively safe. 

The total amount of heavy metals is determined by the strongest acidolysis system, which can completely release all kinds of heavy metal elements. This is a chemical process, aiming to achieve the maximum detection amount.

Currently, the methods for monitoring the effective state of heavy metals mainly include chemical reagent extraction, isotope dilution, rapid biological method and analytical method.Although isotope dilution method, test tube root method and analytical method in rapid biological method are relatively good for the bioavailability of characterization of heavy metals in soil, their operation requires high technology, equipment and other conditions or needs to be further improved.But the chemical reagent extraction method is commonly used in practice.The classical extraction methods of chemical reagents include water extraction, acid extraction, neutral salt extraction, combined reagent or complexing agent extraction, continuous extraction and so on.Due to the different principles of various extraction methods, the extraction efficiency and application are also different, and the results vary greatly.However, some work has shown that the correlation between the effective state of heavy metals and the content of heavy metals in agricultural products is much better than the total statistical results.It can be seen that monitoring the effective state of heavy metals in soil is helpful to further improve the accuracy of judging soil pollution level.

The absorption and utilization of heavy metals in soil by crops is a very complex biochemical process, which is closely related to the effective state of heavy metals and also depends on factors such as the transformation of crop roots to the microenvironment.It is too early to thoroughly explain the relationship between soil heavy metal pollution and accumulation of agricultural products.In view of the immaturity of standardizing such monitoring, systematic studies should be organized to develop standard analysis and evaluation methods as soon as possible.