Loss of Extraction Capacity of Mehlich-1 and Monocalcium Phosphate as a Variable of Remaining P and Its Relationship to Critical Levels of Soil Phosphorus and Sulfur

Abstract

the Mehlich-1 (M-1) extractant and Monocalcium phosphate in acetic acid (Mcpa) have mechanisms for extraction of available p and s in acidity and in ligand exchange, whether of the sulfate of the extractant by the phosphate of the soil, or of the phosphate of the extractant by the sulfate of the soil. in clayey soils, with greater p adsorption capacity, or lower remaining p (rem-p) value, which corresponds to soils with greater phosphate buffer capacity (pbc), more buffered for acidity, the initially low ph of the extractants increases over their time of contact with the soil in the direction of the ph of the soil; and the sulfate of the M-1 or the phosphate of the Mcpa is adsorbed by adsorption sites occupied by these anions or not. this situation makes the extractant lose its extraction capacity, a phenomenon known as loss of extraction capacity or consumption of the extractant, the object of this study. twenty soil samples were chosen so as to cover the range of rem-p (0 to 60 mg L -1 ). rem-p was used as a measure of the pbc. the p and s contents available from the soil samples through M-1 and Mcpa, and the contents of other nutrients and of organic matter were determined. for determination of loss of extraction capacity, after the rest period, the ph and the p and s contents were measured in both the extracts-soils. Although significant, the loss of extraction capacity of the acidity of the M-1 and Mcpa extractants with reduction in the rem-p value

Soil phosphorus availability, dynamic and speciation in long-term high fertilized soils

“…Two factors may have contributed to these results. The first one was the elevation of soil pH after liming, what decreases the dissolution of rock phosphate , and increases the loss of extraction capacity by M1 (Camêlo et al, 2015;Novais et al, 2015), and M3 , due to their sensitivity to soil PBC . The second one is the fact that the IER also works at a Ca 2+ sink, what does not occur when only the Anionic Exchange Resin is used .…”

“…The second one is the fact that the IER also works at a Ca 2+ sink, what does not occur when only the Anionic Exchange Resin is used . The Anionic Exchange Resin was the first method used in laboratories in the State of São Paulo , however, with the use of mixed resin (anionic + cationic), this soil test also extracts forms of P-Ca, but in a smaller proportion than M1 Novais et al, 2015).…”

“…However, this behavior was not observed for Prr-IER, which suggests the low sensitivity of this extractant to soil PBC. In most studies comparing the available P content by M1 with other extractants, generally, the available P content obtained with M1 is lower, as are the Prr-M1 Novais et al, 2015;. This is owing to the fact that in most of correlation works acidic soils with low levels of available P are used, as there is a necessity to establish response curves for P fertilization .…”

“…Another mechanism that promotes loss of M1 extraction capacity is the decrease of its acidity . It occurs when M1 is used in soils with pH around 6, and its initial pH of 1.2 is raised to pH values close to the soil pH and, thus its mechanism of extraction by acid dissolution is smaller Novais et al, 2015). However, in the set of soils of the present study, the mechanism of loss of anion exchange capcaity seems to be more effective than due to the decrease in acidity, since on average, the soils have a pH above 5.5 and yet M1 had similar recovery rates as the ones obtained with IER ( Table 2).…”

“…The use of IER instead of Anionic Exchange Resin, initially used in the State of São Paulo , aimed to increase the efficiency in laboratories, determining beyond P, also Ca 2+ , Mg 2+ , and K + . However, because IER acts as a Ca 2+ drain, it can, as well as M1, overestimate in some conditions the P availability in soils rich in forms of P bonded to Ca (P-Ca) due to liming and phosphate fertilizers application Novais et al, 2015).…”