Effects of P fertilizer placement and temperature on root hair formation, shoot growth and P content of barley grown on soils with varying P status

Seed placed phosphorus (P) is hypothesized to increase P utilization in plants, particularly in cool climate. A greenhouse experiment was conducted to evaluate the effects of seed placed P at two temperature regimes (7 degrees C night/9 degrees C day or 13 degrees C night/15 degrees C day) within the first weeks of the growing season on root hair formation, shoot growth and P content of spring barley (Hordeum vulgare L.). Three soils (loam, clay loam and sandy silt texture) were used, each with two levels of plant available P (low and high P-AL). In half of the pots, all P (10 mg P kg(-1) soil) was applied 5 cm below the seed. In the other pots, half of the P was applied in the same way, and half was given together with the seeds as starter fertilizer. When plants reached growth stage (GS) 21, two replicates were harvested and temperature adjusted to 15 degrees C night/17 degrees C day for the remaining three replicates, which were harvested at GS 49. Phosphorus fertilizer placement did not interact with temperature, soil P level or soil type, and it had also only few and slight effects on shoot dry matter (DM) and P content. The lower temperature regime delayed growth rate by nine days until GS 21, but shoot DM and P content was not significantly affected. At GS 49, both shoot DM and P content increased by 20% from low to high temperature. The soil P level influenced shoot P content to a high degree, with an increase by 190% at high P-AL compared to low P-AL at GS 21, and by 170% at GS 49. The length of root hairs within treatments was very variable, and no significant differences were found between the treatments. So the ability to adapt morphologically to suboptimal conditions was not great enough to avoid reduced growth because of P deficiency. Placing some P together with the seeds could either inhibit limited P uptake at suboptimal growth conditions in this trial. Seed placement of P is recommended in Norway, especially on silty soils. However, the results from this study support what are observed in field, that the positive effects of seed placed fertilizer are variable. The subject indeed needs further investigation before the effect of seed placed P are fully understood.