How to calculate the actual amount of lime needed

Whether you use the lab's or the table's recommendation, you and the farmer will need to make adjustments for the neutralizing value, fineness, and Purity of the actual material being used.

Types of Liming Materials and their Neutralizing Value

There are 5 general kinds of liming materials:

1. Limestone I calcium carbonate): Usually the cheapest of all, since it's taken directly from the ground and crushed without further processing. It's non-caustic.

2. Dolomitic limestone (dolomite): Contains both calcium and magnesium carbonates. Often recommended if available, since liming with straight limestone can produce a magnesium deficiency. However, you can also supply magnesium in fertilizer form such as epsom salts (magnesium sulfate) which doesn't affect soil pH.

3. Burned lime or quicklime: Made by heating limestone or dolomite in a kiln to drive off the carbon dioxide to form calcium oxide (or calcium and magnesium oxide>. It's very cuastic but has the highest neutralizing value and is also more rapid-acting than limestone. It tends to form flakes or granules unless thoroughly mixed with the soil.

4. Hydrated or slaked lime (calcium hydroxide): Made by burning limestone or dolomite in the presence of steam. Like burned lime, it's rapid-acting but isn't used much by farmers due to its higher cost. Also very caustic.

5. Miscellaneous materials

· Where available, coral sand can be used as a liming material; it is basically calcium carbonate with varying amounts of magnesium. The Pacific Agric. Trade School on Ponape island in Micronesia recommends 2 shovelfuls (4.5 kg) per 9 sq. meters, but this dosage would vary considerably with the initial soil pH and the soil's buffering capacity.

· Wood ashes are a potent liming material; in fact, they can easily raise soil pH too high if used indiscriminately (much more than 300-450 cc/sq. meter yearly).

NOTE: Gypsum (calcium sulfate) is not a liming material; it is a neutral salt and will not raise pH. The use of gypsum to improve alkali soils is explained in Chapter 12.

As shown by Table 11-2, the neutralizing value of a liming material varies with type.

TABLE 11-2 Neutralizing Value of Some Liming Materials (Based on 100% strength and compared to limestone)

Using Table 11-2: As an example, it would take 3580 kg of limestone to equal the neutralizing effect of 2000 kg of burned lime (2000 kg x 179% = 3580 kg).

FIGURE 11-1 The time required for lime to affect soil pH is greatly influenced by its fineness. In this greenhouse experiment at Oregon State University, it took more than a year for 20-30 mesh lime to raise soil pH just 0.5 units, but 100 mesh lime raised the pH by 1.5 units in just 2 weeks!

Fineness of Liming Materials is Important!

The time it takes a liming material to react with the soil depends a lot on its Particle size. The finer the material, the more rapid the reaction. Note that even fine-textured materials may take 2-6 months to produce a significant rise in soil pH. Good-quality burned lime and hydrated lime are naturally fine, but crushed limestone and dolomite are often relatively coarse and will react more slowly.

Any liming material contains a mixture of different particle sizes. As shown in Figure 11-1, limestone passing through a 100 mesh sieve (holes are about 0.17 mm square or 1/150th of an inch) will react with soil acids in just 4-6 weeks if thoroughly mixed with the topsoil. Material passing though a 40-50 mesh screen may take 12-18 months to react completely. Material in the 20-40 mesh range will have reacted only 60% in 3 years and 10-20 mesh material only 30%. (See Figure 11-1).

TABLE 11-2

Approximate Amount of Finely-Ground, Pure Limestone Needed to Raise the pH of an 18 cm (7") Layer of Soil as Indicated'

The amounts below are for true "tropical"-type soils. Since they have less buffering capacity than "temperate" soils of similar texture, the amount of lime needed is lower and might have to be increased as much as 50-60% for "temperate" soils. Both types of soils are found in the tropics (see Chapter 1).

Amount of pure fine limestone needed per hectare to raise the soil pH from:

Purity of liming materials: Unless the material has a label guarantee, it's hard to judge purity. Most developed countries have regulations that require purity and fineness guarantees for liming materials, but not many Third World nations do. A soil lab may be able to evaluate locally available liming materials.

How to Estimate the Amount of Lime Needed

If you don't have access to a reliable soil's lab, you can roughly estimate the amount of lime needed using Table 11-3. Check the soil periodically starting about a month or two after application to measure the effect. Lime won't react much with the soil during the dry season once the topsoil dries out. If a farmer is lucky enough to be using a liming material with very fine particle size (fast-reacting), she can try test-liming a small area and having pH checked after a month or two (1000 kg/hectare of lime = 100 grams/sq. meter).

PRACTICE PROBLEM: Suppose a communal vegetable garden project needs to raise the pH of its clay loam soil from 4.5 to 5.6. The table says that about 3350 kg/ha of limestone are needed. What amount per hectare is needed if they're using burned lime estimated to be about 80% pure and the soil appears to be "tropical"?

SOLUTION: The neutralizing value of burned lime is 179%, compared to 100% for limestone so:

Kg of limestone needed/Neutralizing value of material = Amount of material needed (100% purity)

3350 kg/ha/1.79 = 1870 kg/ha pure burned lime needed

Since the burned lime is only 80% pure, a further adjustment is needed:

1870 kg/ha/0.8 = 2340 kg/ha needed (234 grams/sq. meter)