Availability and utilization of soil and fertilizer phosphorus, with focus on starter fertilizer in spring cereals

Instituttt for plante- og miljøvitenskap

Summary

Phosphorus (P) as mineral fertilizer is a relatively cheap input factor from the farmer’s point of view, but its uncritical use can give dramatic consequences for catchments with a high proportion of agricultural land. Further, P in mineral fertilizer is derived from rock phosphate, which is a limited, non-renewable resource, whose utilization efficiency is low as P quickly becomes immobilized in soil. Therefore, it is desirable to adjust the fertilizer application according to crop requirement, and to develop fertilizer methods that increase the plants’ ability to compete with soil particles for soluble P.

The main objectives of this thesis were to gain increased insight into the availability and utilization of soil and fertilizer P in cereal production under both favourable and unfavourable conditions for plant and root growth. Factors that were considered included soil texture, soil available P status, moisture and temperature regimes and soil compactness. Special attention has been paid to the starter fertilizer method, in which some fertilizer, mostly P, is placed near the seed, with the aim of increasing its utilization efficiency and improving crop establishment.

Field studies were performed in southeast Norway in the years 2000-2003. Spring barley and spring wheat were fertilized with band placed P and varying amounts of P and/or N as starter fertilizer. Trials were performed on a silty clay loam, a silt and a loam, thus representing typical soil types used for cereal production in Norway. Two pot experiments were conducted in a phytotron, in order to study the effects of various climatic and edaphic factors on P uptake and growth under controlled environmental conditions. Finally, the results of the field trials were used for validating the mechanistic model PHOSMOD, developed at Horticulture Research International, Wellesbourne, England. This model simulates daily growth and P concentration in plants, dependent on the P supply to plant roots, as affected by fertilizer supply and placement, and soil P dynamics.

The results of the field trials showed that starter fertilizer gave more vigorous plants and increased P uptake early in the growing season. The responses were not so obvious later, and they differed between the soil types. Starter fertilizer was clearly beneficial on the silt soil, whilst responses were more variable on the silty clay loam. In the case of the loam soil, there was little response to P, neither when given as starter fertilizer nor when band placed.

It was first assumed that the difference in responses between soil types was mainly due to variations in soil temperature. However, after examination of soil temperature measured in the field, we rejected this hypothesis. Rather, the results indicated that soil structure and the actual amount of plant available P in the soil had important consequences for the crop responses to fertilizer application. In the pot experiments, at both high and low P-AL levels, the highest biomass production was found on loam soil. On the other hand, plants grown on silt soil at low P-AL level had distinct symptoms of P deficiency throughout the whole growth period and the P uptake and biomass production were lower than on the loam soil, regardless of the level of P-AL. On the clay loam soil, the biomass production was intermediate between that of the other two soils. The results indicate that a need exists for greater differentiation between soil types in the advice given on P fertilizer requirements, even when soils contain the same P-AL levels.

Strong soil compaction in the pot experiment was found to reduce the total root length (RL) by 59 %, and the length of the finest roots (diameter < 0.5 mm) was especially affected. Both P uptake per unit RL and above-ground biomass production per unit RL were greater with increased soil compaction. Nevertheless, this was not enough to fully compensate for the drastic reduction in total RL that was found in the most compacted soil. The results show that satisfactory soil structure is important for the uptake of soil P.

The plant DM yields simulated by PHOSMOD corresponded well with field measurements, both with the use of starter fertilizer and with band placed P. The model predicted responses to P fertilizer application on the silt soil, but not on the silty clay loam and loam soils. The model was found to be very sensitive to the soil water content at field capacity, and to the amount and distribution of precipitation. Thus the model reflects what is found in reality in soil plant systems. The P diffusion rate, which is crucial for determining the P supply to plant roots, is strongly dependent upon soil water content. In one of the pot experiments, we observed large increases in yield and P uptake when the water supply was optimal, compared to conditions with more restricted supply. This was especially marked on soil with a high P-AL level. At the lowest P-AL level, on the other hand, the results indicated that the P supply was probably more limiting for growth, than was the water supply.

In conclusion, the results show that starter fertilizer can be recommended as a method for optimizing P fertilizer application under some conditions. Further, provided good system knowledge by the user and careful interpretation of results, PHOSMOD was found to be a suitable tool for estimating the need for P fertilization to spring cereals on different soil types, and for assessing alternative fertilizer placement strategies.


Oppdatert: 30.10.05
Utskriftsvennlig versjon : Arkiv : Tips en venn