The role of organic matter on decrease of harmful effects of phosphorous fertilizer overly application in light soils.

Phosphoms (P) feri ilizer recommendations for calcareous-sandy soils low in organic matter need furiher investigation. Therefore, the objectives of this study were to evaluate the effects of P and manure on com (Zea mays L.) growth and chemical compositions and P recommendations for calcareous sandy soils with low organic matter under greenhouse conditions. Treatments consisted of a factorial anangement of P levels (0, 25, 50, and 100 mg kg-1 soil as KH2P04), manure rates (0, 10, 20 and 30 g dried sheep manure per kg soil) and three soils (Soil 1, sandy loam, initial P 10.8 mg kg-1 ; Soil 2, sandy loam, initial P 7.6 mg kg-1 ; and Soil3, loamy sand, initial P 5.5 mg kg-1 ) in a completely randomized design with four replications. Results showedthat P application in Soil 1 decreased com dry matter. However, application of 25 or 50 mg P kg -1 soil increased com yield significantly in Soils 2 and 3, respectively. Maximum com yield was obtained when 30 g kg -1 manure was added to sandy loam soils and 20 g kg-1 to loamy sand soil. Application of P and manure significantly increased plant P concentration and uptake in all three soils. Zinc concentration in plants treated with Phosphoms was higher than in the control in soils 1 and 2. Such a trend was not observed in soil 3, but manure application increased it. Ironconcentration in plants treated with P increased in soils 1 and 2 but was decreased in soil 3; however, manure application increased it in all soils. Plant Mn concentration and uptake responses toP and manure application was not consistent. Applied P, in general, increased plantMn Concentration in soils 1 and 2, but had no effect on plants in Soil 3. Manure effect on plant Mn concentration was not consistent. It seems that addition of manure to sandy soils can improve soil productivity and increase com yield. Due to the low P buffering capacity of sandy soils,application ofhigh rates ofP can increase P concentration to an undesirable level in soil solution. This may depress plant growth and also availability of some micronutrients like Fe and Zn to com plants. Therefore, P feriilizer recommendations for sandy soils should be based on the soil test P level. Manure application is recommended for sandy soils, due to its positive effects on nutrient uptake and plant growth. Prior to any phosphoms feriilizer recommendations for sandy soils the results of this experiment should be verified under field conditions and measuring P concentration in soil solution at different stages of plant growth is highly recommended