Soil and hydroponic culture experiments were conducted to investigate the effects of phosphite (Phi) as phosphorus (P) fertilizer via root and foliar applications on the growth and P supply of komatsuna. In both experiments, root P treatments were combinations of Phi and phosphate (Pi) at different Pi:Phi ratios, for a total of high P level (92 mg P pot −1 ; the soil experiment) or low P level (0.05 mM P; the hydroponic experiment). Foliar P treatments were deionized water (control), a Pi solution and a Phi solution at low concentration of 0.05% P 2 O 5 . In both experiments, shoot dry weight of plants significantly decreased as Pi:Phi ratio decreased. In the soil experiment, plants grew abnormally at a Pi:Phi ratio of 25:75 and died when P was applied to soil entirely as Phi form (0:100 treatment). In the hydroponic experiment, no visible damage was found in shoot but root growth was strongly inhibited with severe damage symptoms at low Pi:Phi ratios. Total P concentration in plant decreased significantly with decreasing Pi:Phi ratio, especially in the hydroponic experiment. Foliar application of Phi although greatly increased total P of plants compared to that of Pi in both experiments, it did not improve but further decreased plant growth at low Pi:Phi ratios in the soil experiment and at all Pi:Phi ratios in the hydroponic experiment. The results of this study clearly indicated that Phi could not be used as P fertilizer by komatsuna plants via both application methods and could not substitute P at any rate at either low or high level. No beneficial effect of Phi was detected even when it was applied at low rate or applied in combination with Pi at different ratios. The effects of Phi were strongly dependent on the P nutrition status of plants; and plants that were not sufficiently fertilized with Pi may become vulnerable to Phi even at low levels.

Sorgoleone is a secondary sorghum metabolite released from roots. It has allelopathic properties and is considered to inhibit ammonia-oxidizing archaea (AOA) and bacteria (AOB) responsible for the rate-limiting step (ammonia oxidation) in nitrification. Low activity of these microorganisms in soil may contribute to slow down nitrification and reduce nitrogen loss via denitrification and NO 3 − leaching. The potential nitrification rate (PNR) and the composition of microbial communities were monitored in rhizosphere soil to investigate the growth effect sorghum on biological nitrification inhibition (BNI). A greenhouse pipe experiment was conducted using sorghum lines IS20205 (highsorgoleone release ability), IS32234 (medium-sorgoleone release ability), 296B (low-sorgoleone release ability), and a control (no plants) combined with fertilization application of 0 or 120 kg N ha −1. We applied nitrogen as ammonium sulfate at 16 days (20 N), 37 days (40 N), and 54 days (60 N). We collected soil solutions at 7.5 cm depths every 3 days and measured the pH and nitrate levels. At 1 and 2.3 months, we sampled the bulk and rhizosphere soils and roots in the 0-10 cm, 10-30 cm, and 30-80 cm depths to determine NO 2 , mineral N, total N, total C, sorgoleone, the composition of AOA, AOB, and total bacteria and archaea. Sorgoleone was continuously released throughout the 2.3 months' growth and was significantly higher in IS20205, followed by IS32234 then 296B, which showed shallow levels. The IS2020 5rhizosphere showed lower NO 2 and nitrate levels and significant inhibition of AOA populations. However, we did not find significant differences in the abundance of AOB between plant treatments. Multivariate analysis and Spearman's correlations revealed that sorgoleone as well as environmental factors such as soil pH, soil moisture, NO 3 −-N, and NH 4 +-N shape the composition of microbial communities. This study demonstrated that the release of higher amounts of sorgoleone has great potential to inhibit the abundance of AOA and soil nitrification. The breeding of sorghum lines with the ability to release higher amounts of sorgoleone could be a strategic way to improve the biological nitrification inhibition during cultivation.

Phosphite (Phi) may potentially supply phosphorus (P) nutrition to plants and is widely marketed as a super P fertilizer for many crops. This study investigated the effects of Phi on growth and P nutrition in spinach (Spinacia oleracea L.). High-rate foliar application experiments designed to evaluate the phytotoxicity and P nutritional potential of different Phi formulations by foliar application at two rates (0.15 and 0.3% P 2 O 5 ) showed that all Phi formulations did not improve plant growth under different available P-soils, but rather significantly decreased shoot dry weight (DW) at the higher rate. In two other soil and hydroponic experiments, Phi was foliar applied at a low rate (0.05% P 2 O 5 ) and root P treatments were combinations of Phi and phosphate (Pi) at different Pi : Phi ratios for a high P level (the soil experiment) or a low P level (the hydroponic experiment). In both experiments, shoot DW decreased markedly as the Pi : Phi ratios decreased from 100:0 to 0:100 and Phi foliar application did not improve plant growth. In the soil experiment, plants grew poorest at 0:100, but grew well when both Phi and Pi were applied at a high rate of 115 mg P pot -1 , indicating that at this level Phi had a negative effect on only severely P-deficient plants. Root growth of no Pi-fertilized plants was strongly inhibited by Phi from either root or foliar application. In both experiments, P concentration drastically decreased with decreasing Pi : Phi ratios from 100:0 to 0:100, but increased substantially with foliar application of Phi compared with Pi, suggesting that Phi was absorbed poorly by the roots, but was well absorbed by the leaves compared to Pi. We conclude that Phi cannot be used as a P fertilizer for spinach via either root or foliar applications at low or high levels, and also that Phi has no beneficial effect on the growth of spinach. As Phi is now widely marketed as a P fertilizer for many crops, care should be taken in selecting Phi as a P fertilizer for a given crop.

Nitrogen is one of the most limiting nutrients for crop production in many areas of Africa. One strategy to improve yields is to choose crops with high nitrogen use efficiency (NUE) that can produce economic yields under limited water supply. Little information is available on the comparative performance of pearl millet (Pennisetum glaucum L. R. Br.) and cowpea (Vigna unguiculata L. Walp.) in sole crops and intercrops systems for the NUE of applied fertilizers. This study was conducted under field conditions at the Senegal Agricultural Research Institute from July to October 2001. Two cropping systems were used and included sole crops of pearl millet and cowpea and a millet–cowpea intercrop. 15N‐labeled urea at rates of 20 kg ha−1 (sole and intercrop cowpea) and 41 kg ha−1 (sole millet and intercrop millet) was applied. Sole millet produced 6,041 kg yield ha−1 and accumulated 95.27 kg N ha−1, of which 15.30% was derived from the nitrogen fertilizer and 84.70% from nitrogen mineralized in soil. Therefore, the NUE of the fertilizer was 36.29%. In intercrop millet, sole cowpea and intercrop cowpea, the NUEs were 15.20, 45.33 and 46.00%, respectively, indicating that the use of nitrogen fertilizer significantly decreased in intercrop only for millet. However, no significant difference was observed between sole and intercrop cowpea relative to the nitrogen derived from N2 fixation. The amount of nitrogen immobilized in the soil was significantly higher in sole millet than in sole cowpea and millet–cowpea intercrop. The Land Equivalent Ratio (LER) of grains and total dry matter showed an advantage of the millet–cowpea cropping system over sole crops.

Composition of arbuscular mycorrhizal fungi associated with cassava (Manihot esculenta Crantz) cultivars as influenced by chemical fertilization and tillage in Cameroon