Phosphorus Availability as Affected by the Application of Phosphate Rock Combined With Organic Materials to Acid Soils in Western Kenya

Abstract

Most of the agricultural lands in the highlands of western Kenya are depleted of plant nutrients, particularly phosphorus. This depletion has resulted in a continued decline in crop production in the area. Recent experiments, in which direct application of indigenous phosphate rocks were evaluated, have yielded variable results, depending on the relative reactivity of the rock phosphate tested. The effectiveness of rock phosphate was generally low compared with fertilizers such as triple super phosphate. This is attributed to the relatively low solubility of the rocks as opposed to the readily water soluble phosphorus fertilizers. Phosphate rocks are available locally and are cheaper than triple super phosphate. If the solubility of these phosphate rocks could be improved, the resource-poor smallholder farmers in western Kenya would have an affordable source of phosphorus for their crops. Greenhouse and field experiments were conducted to evaluate the effects of combining different on-farm organic materials with the Minjingu rock phosphate (from Tanzania) on the availability of phosphorus to maize (Zea mays) in western Kenya. The greenhouse results indicated that there were significant positive linear relationships between rock phosphate application rates and (i) the Olsen-extractable soil phosphorus in the soil samples taken four weeks and nine weeks respectively after the application of the treatments; (ii) the dry matter yield, and (iii) phosphorus uptake. Results of the field experiments showed that rock phosphate combined with farmyard manure or crop residues (maize stover) generally increased the Olsen-extractable soil phosphorus, maize yields and phosphorus uptake, particularly in the first season when both the Minjingu rock phosphate and organic materials were applied, but the effectiveness of the materials and their combinations varied between the two sites. Combining Minjingu rock phosphate with the organic materials improved its relative agronomic effectiveness for maize.

Effects of Organic and Inorganic Materials on Soil Acidity and Phosphorus Availability in a Soil Incubation Study

“…The findings of the present study are in contrast to others (e.g., [2,4,36]) who reported synergism when OMs such as manures were combined with PRs. These authors combined PRs with OMs of diverse composition and concluded that due to acidifying effect organic acids produced during the decomposition of the OMs, the solubilization of PRs was enhanced thus leading to the higher extractable P values in treatments where PR was combined with OMs than from application of PR alone.…”

“…This coupled with concerns for environmental protection and sustainability has renewed interest in the use of alternative cheaper locally available materials. The use of phosphate rocks (PR) and organic materials has in particular received increased attention in recent years in eastern Africa [2][3][4]. In addition to provision of P, PRs have Ca and Mg which makes them assume a significant role as a potential tool for sustaining soil productivity by reducing soil acidity through its liming effect [5].…”

White Lupin (Lupinus albus L. cv. Amiga) Increases Solubility of Minjingu Phosphate Rock, Phosphorus Balances and Maize Yields in Njoro Kenya

“…This can be attributed to insolubility of MPR (Waigwa et al, 2003). Various factors could be responsible for P availability to crop plants.…”

The Effect of Minjingu Phosphate Rock and Triple Superphosphate on Soil Phosphorus Fractions and Maize Yield in Western Kenya

“…Much research on soil fertility management in western Kenya has therefore been devoted to testing nutrient inputs that are thought to be inexpensive, locally available, and sustainable, as alternatives to conventional fertilizers. The use of organic materials (OMs) and phosphate rocks (PR) has in particular received considerable research attention in recent years [3][4][5]. This has been given impetus by a paradigm shift in soil fertility management towards the integrated soil fertility management (ISFM).…”