Critical Soil Phosphorus Values for Yield Reduction in Intensive Agricultural Systems

Phosphorus(P)is an essential element for agricultural production.Over-fertilization during decades caused an accumulation of P in soils leading to eutrophication in regions characterized by intensive agriculture.These environmental concerns together with the non-renewability of P resources have led to a more sustainable P use.Knowledge about the P need of crops is essential for a sustainable agriculture thereby minimizing P losses to the environment without lowering the yield substantially.Therefore,in this study,critical soil P values for yield reduction(PCrit)were determined based on fertilizer trials conducted between 1970 and 1988 and more recent fertilizer trials(2016-2017).At rotational level a common PCrit value of 109 mg P/kg dry soil(in an ammonium lactate and acetate extract)was determined.Crop specific PCrit values were also determined for seven crops(potato,winter wheat,barley,rye,maize,sugar beet and temporary grassland).These critical values ranged from 59 mg P/kg dry soil to 164 mg P/kg dry soil with winter wheat the least and maize the most sensitive towards P deficiency.The diversity in PCrit values among crops can mainly be explained by the root intensity but also rooting depth,exudation of organic acids and phosphatases may influence the PCrit value.The soil pH also influenced the P availability significantly.Soils with a favorable pH had a significantly higher availability(i.e.,lower PCrit value)for all crops compared to soils with a suboptimal pH.Critical soil P values might help to set up new or to evaluate current soil P in target zones used for P fertilizer recommendations.

Study on the Yield Reduction Mechanism of Soybean Planted under Continuous and every Second Year Cropping Conditions

Three-year investigation was conducted to demonstrate the mechanism of reduction of soybean yield aroused by continuous and every second year cropping. Compared to normal cropping, there are many unfavorable changes in some major elements of soybean plant and soil environment. Chloropyll content was lower. Phosphorous content of soybean plant was decreased seriously. Potassium content was lower while calcium content was higher. Magnesium content was wot affected and decreased in susceptive varieties. Some deseases and insects of soybean under continuous and wery second year cropping conditions became serious as continuous years prolonged. Organic matter content tended to go down from normal rotation to continuous cropping. Amount of bacteria and antinomyces decreased while amount of fungi increased. The development of symbiotic nitrogen fixation system wsa deteriorated.

Effect of Natural Infection with Onion Yellow Dwarf Virus （OYDV） on Yield of Onion and Garlic Crops in Egypt

Present work highlights, first time in Egypt, the impact of OYDV infection on crops production. Plants grown in the field were individually labelled as either apparently healthy or diseased and were marked for harvesting. The percentage of yield loss due to viral infection was determined, on the basis of the average yield parameters of the healthy plant. The natural infection with OYDV in onion and garlic fields resulted in a substantial reduction in pseudo-stem length, number of leaves, plant weight and weight of bulb as well as number and weight of cloves. Results clearly indicate the impact of natural infection with OYDV on the crop yield of infected onion and garlic plants in comparison with the virus-free （uninfected） ones. The largest reduction in the yield of infected plants occurred in Chinese garlic （Seds40） followed by （Baladi） garlic and onion Giza 20. This information suggests that measures have to be taken to prevent natural infection with OYDV into and within onion and garlic fields in order to avoid yield loss.