Effectiveness of recovered magnesium phosphates as fertilizers in neutral and slightly alkaline soils

Magnesium phosphates such as struvite (MgNH4PO4 · 6H2O) can be recovered from municipal, industrial, and agricultural wastewaters. However, limited information is available on the beneficial reuse of these recovered products; research has focused on low pH soils. Th is study determined whether recovered struvite and dittmarite (MgNH4PO4 · H2O) were effective P fertilizers in neutral to slightly alkaline soils. In addition to commercially available triple superphosphate (TSP) and certified organic rock phosphate (RP), recovered struvite, dittmarite, and a heterogeneous recovered phosphate were evaluated in a laboratory dissolution study and as fertilizers for spring wheat (Triticum aestivum L.) in a greenhouse study. Struvite and dittmarite were much more soluble than RP, but less soluble than TSP. Laboratory dissolution kinetics were fast, with most materials nearing equilibrium within 7 to 14 d. At a soil pH of 6.5, both dittmarite and struvite increased the average plant P concentration over the control. Struvite and dittmarite performance was similar to TSP. There were no significant differences in plant dry matter (DM) production or total P uptake at pH 6.5. In the limed soil (pH 7.6), many treatments had plant P concentrations significantly lower than the control, but most fertilizers increased DM production over the control; all fertilizers generally performed similarly to one another. These findings support previous work showing recovered Mg phosphates to be effective in acidic soils, and provide evidence that they are also effective in slightly alkaline soils. Recovered Mg phosphates could become a useful alternative for P fertilization in arid and semiarid environments

Low seed phosphorus concentration depresses early growth and nodulation of narrow‐leafed Lupin (Lupinus angustifolius cv. Gungurru)

Narrow‐leafed lupin (Lupinus angustifolius L.) seed with phosphorus (P) concentrations of 0.21, 0.26 or 0.43% were graded to uniform size (128 ± 5 mg) and grown in two glasshouse experiments to examine the effects of seed P concentration on early shoot, root and nodule growth, and its response to external P supply. Four days after imbibing seeds for a solution culture experiment, whole plant fresh weight (FW) of plants grown from low P seed (0.21%) was depressed compared to that from medium (0.26%) or high (0.43%) P seed. This depression in whole plant FW from growing low P seed persisted to final harvest at day 32 for levels of solution P supply ranging from nil to luxury levels. However, at adequate and luxury levels of solution P relative growth rates of plants grown from low P seed recovered from day 6 onwards to equal those of plants grown from medium and high P seed. In deficient‐P treatments, low P in seed strongly depressed root length, especially that of lateral roots which it did largely by decreasing their number. Low P in seed depressed nodule number and mass at all levels of external P supply but to a greater extent where P supply was deficient for growth. In these deficient‐P treatments, N concentrations in shoots were decreased slightly in plants from low P seed. Seed P may be specifically involved in the early stages of nodule development in lupins. In a P‐deficient loam and a P‐adequate sand, low P in seed depressed early shoot and root growth and nodule formation as it did in solutions. We conclude that low P concentration in lupin seed may limit successful crop establishment of lupins in the field, especially when P is deficient for plant growth or seedlings are subject to early stresses

Increased risk of zinc deficiency in wheat on soils limed to correct soil acidity

Addition of lime to ameliorate soil acidity has been observed to induce zinc (Zn) deficiency for wheat in sandy soils of south-western Australia, reducing grain yields. The implications of widespread use of lime to treat acid soils for the residual value of Zn in these soils are not known. In a glasshouse experiment, using a Zn-deficient sand from south-western Australia, 3 levels of finely powdered calcium carbonate were added and incubated in moist soil for 6 weeks at 22°C to produce three different pH values (1 : 5 soil : 0.01 m CaCl2): 4.9 (original soil not treated with calcium carbonate), 5.8, and 7.4. Five amounts of Zn, as solutions of Zn sulfate, were then incubated in moist soil for 0, 30, 60, 120, and 180 days before sowing spring wheat (Triticum aestivum L.). The residual value of the applied Zn was determined using yield of dried shoots, Zn content in dried shoots, and soil test Zn (DTPA extraction). This was done by calculating the effectiveness of the incubated Zn for all 3 soils relative to the effectiveness of Zn applied just before sowing wheat (0 day incubation, freshly applied Zn) for the soil not treated with calcium carbonate. As measured using yield of dried shoots, Zn content of dried shoots, or soil test Zn, the residual value of the incubated Zn decreased with increasing soil pH and with increasing period of incubation of Zn with moist soil before sowing wheat. The critical Zn concentration, associated with 90% of the total yield of dried wheat shoots, was (mg Zn/kg) 13 in the youngest mature growth (apex and youngest emerged leaf), and 20 for rest of dried shoots. These values were similar to current critical values for unlimed soils. The relationship between yield of dried shoots and DTPA soil test Zn was similar for unlimed and limed soils, so similar critical soil test Zn was applicable on the sandy soil regardless of soil pH. Critical DTPA soil test Zn, the soil test Zn that was related to 90% of the maximum yield of dried shoots, was 0.14 mg Zn/mg soil. To combat the increased risk of Zn deficiency on soils limed to ameliorate soil acidity, fertiliser Zn needs to be re-applied to the soil when soil and plant tests indicate a high likelihood of deficiency