Assessing rock phosphate quality

A large number of rock phosphates is available on the market in Southeast Asia. How does the agronomist select a suitable source?

Here are some basic points to keep in mind:

• Whilst we all use P2O5 content as an index of quality it doesn't mean much in terms of the potential effect of the rock phosphate on crops. For example, consider a rock phosphate with quite low total P₂O₅ of 25% and citric acid soluble P of 15%. 1,000 kg rock phosphate yields 16 kg plant available P. By contrast 1,000 kg of a material with 30% total P₂O₅ but citric acid soluble P of only 10% yields only 13 kg P.

P2O5 Concentration	Amount (kg)			2% CA soluble P (%)	Amount available P (kg)
	Rock phosphate	P2O5	P
30	1,000	300	131	10%	13
25	1,000	250	109	10%	11
25	1,000	250	109	15%	16
30	1,000	300	131	15%	20

• Also consider fineness. The finer the material the greater the surface of area of the rock phosphate exposed to the soil solution for dissolution. Most experts consider grinding so that 80% of the material passes through an 80 mesh (0.18 mm) sieve is sufficient.
• Check moisture content of the rock phosphate. You get less 'active' ingredient (P/US$) if the material is damp/wet and damp or wet material is much more difficult to spread in the field.

Buying a quality source of rock phosphate is only one part of the equation. The other important issue is, of course, spreading the material properly in the field and avoiding losses due to erosion and surface water run off by using suitable soil conservation methods. Rock phosphate may be incorporated in the soil, mixed with soil in the planting hole during tree crop establishment or dusted over the soil surface.

If possible, incorporate rock phosphate in the soil to reduce losses due caused by erosion and surface run off. After ploughing the land, broadcast the rock phosphate and plough it in. In this way, the investment in rock phosphate is securely deposited in the 'soil bank'.

If it is not feasible to incorporate the rock phosphate in the soil, spread it as evenly as possible over the soil surface. Don't apply over bare soil because most of the P applied will be lost in surface run-off water and eroded soil. A light cover of weeds will help to reduce losses. Ideally dust rock phosphate over the soil surface and then apply mulch (e.g., empty fruit bunches in oil palm, stover in maize and peanuts) over the soil surface (to prevent surface run-off losses).

Apply rock phosphate in planting holes during tree crop establishment. A large application of rock phosphate (1 kg/tree), thoroughly mixed with the top soil, will provide an adequate supply of P for 2-3 years. Take full advantage of the opportunity to increase the P supply in the zone where root growth and nutrient uptake will be greatest during the early years of tree crop growth!

If necessary install soil conservation measures (terraces and platforms in oil palm, bunds and conservation terraces in other crops).