Increasing fertilizer use efficiency and content of available phosphorus in soil are the
key issues of dry farming and modern agriculture. To solve the problem of soil phosphorus
accumulation and fixation in calcareous soil, extraction experiments and incubation
experiments were conducted to investigate the effect of low molecular weight (LMW)
organic acids on phosphorus fraction and availability in different fertile calcareous soils.
The main conclusions were drawn as follows:
The organic acids could significantly stimulate the mobilization of soil phosphorus in
all three soils tested. When in the same concentration, the three organic acids followed an
order of oxalic acid> citric acid> malic acid in the capability of activating soil phosphorus
in the low fertility and moderate fertility soil; in the high fertility soil, the three followed
the order of citric acid> oxalic acid> malic acid. Low concentration of organic acids (≤2
mmol/L) could significantly restrained soil phosphorus activity in low and moderate
fertility soil, especially for the malic acid. LMW organic acids could decline the pH of soil
extract significantly, and the citric acid showed the strongest declining capacity.
Meanwhile, for the same kind of organic acid, the extent of phosphorus activating
decreased with the increasing of soil pH.
The LMW organic acids could significantly stimulate the soil available phosphorus.
For calcareous soil, the ability of oxalic acid in mobilizing soil available phosphorus was
higher than that of malic or citric acid; citric and malic acid could significantly restrained
soil available phosphorus activity in calcareous soil. The content of soil available
phosphorus decreased with the lengthened of the incubation time. Moreover, with the
increase of oxalic acid concentration, or the decrease of citric and malic acid
concentrations, the content of soil available phosphorus increased. After disposed by LMW
organic acids, the contents of liable organic phosphorus (L-OP), moderate liable organic
phosphorus (ML-OP), and moderate stable organic phosphorus (MS-OP) increased, while
the content of high stable organic phosphorus (HS-OP) decreased in calcareous soil. As a conclusion, LMW organic acids can improve the soil phosphorus availability by affecting
the organic phosphorus fractions.
The effect of organic acids on soil inorganic phosphorus fraction was obvious. After
treatment with oxalic, citric and malic acid, the content of Ca2-P, Ca8-P and O-P increased,
while the content of Al-P, Fe-P and Ca10-P decreased. The general tendency observed was
that organic acid application promoted the transformation of phosphates in the form of less
or unavailability to higher availability. These promoting ability was varying with the
varieties and properties of organic acids, among which oxalic acid>citric acid>malic acid.
Organic acids can improve soil phosphorus availability by affecting the inorganic
phosphorus fraction and mobilizing sparingly soluble phosphate. The ability of organic
acids transformation of inorganic phosphorus fraction was varying with the soil. In the
general, the application phosphorus fertilizer soils had lower activation rate, but had higher