Phosphate Fertilization Management in Israel: Where Irrigation and Water-Treatment Technologies Meet Prof. Ran Erel Agricultural Research Center (ARO), Volcani Center,  Israel

Phosphorus is often the second most limiting nutrient for plant and therefore, P fertilization is required to maintain high productivity. Phosphate fertilizers are typically applied through the soil and react with clay minerals, metal oxides or Ca near the point of fertilizer placement. Consequently, a substantial fraction of the applied P may be adsorbed, fixed or precipitate as insoluble complexes. Due to its slow mobility and high fixation, P fertilizer-utilization efficiency (PUE) is typically low.

The Israeli agricultural sector is characterized by: (i) low arable lands and highly  dense population, (ii) high evaporative demand and low, infrequent, precipitation and, (iii) demand for premium products. These unique circumstances were the drive for highly intensive and water efficient agro-system using drip irrigation with marginal water as the main practice (“necessity is the mother of invention”). Utilization of the irrigation system for fertilizer application (i.e. fertigation) affecting water and solutes distribution, root system, microbial activity which expected to have implication on PUE. Solid P fertilizer is commonly applied once, before sowing. In such case, P is destined to undergo fixation before being took up by plants and therefore farmers increases application rates. Fertigation enables application of low P dose at high frequency, occasionally at daily basis. Increasing application frequency was shown to accelerate P mobility and elevate its availability. Furthermore, under fertigation, the limited soil volume is allowing to maintain higher soil moisture level (in the root zoon) and therefore P mobility is increases. Yet, at P fixing soils, P mobility is still limited to the upper soil layer while root length density tends to be higher in deeper moist layers. We therefore speculate that subsurface fertigation will ensure that P is supplied at the right time and the right place.

To overcome the severe water deficient, Israeli agro-system vastly utilizes treated wastewater (TWW) which are roughly 50% of the available irrigation water. TWW may contain considerable amounts of valuable nutrients including P (average of 4-5 mg l-1). These “recycled” nutrients often have large annual variations depending on the source of sewage water, treatment and precipitation. Therefore continuous monitoring of P level is needed for accurate P management. In some cases, all of the crop requirement can be supply from the TWW only. Numerous recent studies on fruit trees demonstrated that P fertilization is sometimes underestimated since P deficiency symptoms are rare and develop long after productivity was impaired.

The modern Israeli agro-system, designed to save water, was shown to increase PUE mainly due to: (i) application of P through fertigation, (ii) high application frequency at the time and place P is needed and (iii) recycling P in the TWW and manure.