| Soils, Crops and Fertilizer Use |
|Chapter 11: Liming soils|
NOTE: The concept of soil pH and the factors influencing it are explained in Chapter 6.
Depending on the crop and OD soil factors, very acid soils may need to be limed to raise their pH and counteract the affects of excessive soil acidity. Very acid soils (below a pH of about 5.0-5.5) may adversely affect crop growth for several reasons:
• Aluminum, manganese, and iron all become more soluble with increasing acidity and may actually become toxic to plants at pH's below 5.5. Many varieties of beans and wheat are especially sensitive to aluminum toxicity, although the true "tropical" soils (see Chapter 1) tend not to release toxic amounts until the pH approaches 5.0. Manganese and iron toxicities can be serious, too, but tend to be more of a problem on soils that are also poorly drained.
• Very acid soils are usually low in available P and have an especially high capacity to tie up added P by forming insoluble compounds with aluminum and iron.
• Although very acid soils usually have enough calcium to supply plant nutrient needs (except for peanuts), they are likely to be low in magnesium, as well as sulfur and available molybdenum (Mo becomes increasingly insoluble as acidity increases).
• Low soil pH depresses the activities of "good-guy" soil bacteria and fungi, such as those that convert the unavailable, organic forms of N, P, and S to available mineral forms. One of the main reasons that soybeans, alfalfa (Lucerne), and many clovers do poorly on acid soils is that their particular types of N-fixing rhizobia bacteria have little tolerance for pH's below 6.0. (On the other hand, many of the rhizobia species associated with the more tropical legumes [e.g. peanuts, cowpeas, and kudzu] can function well at lower pH's.)
Most crops will produce satisfactory yields within a pH range of 5.5-7.5, though micronutrient deficiencies (except molybdenum) become more likely above pH 6.8. Some, such as soybeans and alfalfa, don't grow well below pH 6.0. Others, such as pineapple, rice, coffee, potatoes, sweet potatoes, and watermelon, are more tolerant of acid soils than other crops.
Portable Test Kits
You can check soil pH fairly accurately right in the field using a good quality liquid indicator kit or a portable electric tester Read and follow the instructions carefully and be sure to measure both topsoil and subsoil pH since they're usually different. Even if the field's soil appears uniform in color and texture, check pH at several locations. Readings from the better quality kits like the Hellige-Truog are accurate within 0.2-0.3 pH units. Litmus paper kits don't work well. Soil labs measure pH as a routine part of soil testing.
CALCULATING HOW MUCH LIME IS NEEDED
Role of the Soils Lab
Portable kits won't tell you the full story. They can be be useful for troubleshooting, but if you do find a soil whose pH appears to be too acid, the kit won't tell you how much lime to add or even if it's needed after all. Here's why :
• The amount of lime needed to raise soil pH by one unit varies greatly from soil to soil. In fact, one soil may may require up to 5-10 times as much lime as another to achieve the same rise in pH, even though both have the same initial pH. This buffering capacity varies directly with the amount of negative charge (cation exchange capacity a soil has. Only a soil lab can measure this. (Buffering capacity is explained in more detail below; exchange capacity Was covered in Chapter 6.)
• pH isn't the only criterion for deciding if timing is needed. The soil's content of soluble aluminum (called exchangeable aluminum) is often even more important, and a portable pH test kit can't measure this. The amount of harmful soluble aluminum increases as pH drops. but some soils reach this point at a higher pH than others (i.e. 5.5 instead of 5.0). The soils lab can determine this.
Soil Exchange Capacity and timing Requirements
This is a concept worth understanding so let's go through it step by step:
• Soils have troth an active and a reserve acidity. The active acidity is produced by the those hydrogen ions (H+) floating around free in the soil's water, and it's what a pH test kit measures.
• However, for every free-floating H+ ion, there may be thousands or more H+ ions held (adsorbed) to the soil's clay and humus particles (remember, they're the only soil particles with a negative charge). These H+ ions make up a soil's reserve acidity and can't be measured with a pH test kit.
• As the applied lime neutralizes the free-floating H+ ions (active acidity), the huge reserve of H+ ions held by the clay and humus particles start" supplying H+ ions to take the place of the neutralized ones.
• The higher the amount of clay and humus a soil has, the greater its negative charge and buffering capacity, and the more lime will be needed to obtain a given rise in pH.
• An important point: Don't get confused! In terms of true soil pH (active acidity), clayey or high humus soils aren't any more likely to be acid than sandy soils; but, they're more resistant to changes in pH (either upward or downward), due to their greater buffering capacity.
• "Tropical" v". "temperate" soils: True tropical soils (old, highly-weathered red or yellow soils) usually need less lime than "temperate" soils (see Chapter 1) of the same texture to obtain an equal rise in pH. That's because true tropical clay minerals have a much lower negative charge than temperate types (see Chapter I) and, therefore, less buffering capacity. Remember that both tropical and temperate clay minerals occur in the tropics.
What to Do if No Soil Lab is Available
As explained later in this chapter, applying too much lime to an overly acid soil can be worse than not liming at all, so a farmer needs to be somewhat precise. If reliable soil testing isn't available, there are 2 alternatives:
• Check with your local extension service to see if liming recommendations have been developed for the area's soils based on prior experience, soil type, and lab testing.
• Use a generalized liming recommendation table such as Table 11-3.
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).
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).
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)
• Lime should be broadcast (spread) uniformly over the soil surface and then thoroughly mixed into the top 15-20 cm of soil (normal depth of topsoil) by plowing or hoeing. Harrowing (disking) alone will only move the material down about 5-8 cm and is inadequate.
• When broadcasting lime by hand, divide the amount in half and apply the second portion at right angles to the first. Wear a mask or bandana, and be careful if using burned or slaked lime; these are caustic materials won't burn as long as your skin is dry, but watch your eyes.
• When applying lime over established pastures, it can be spread directly over the pasture without working it in.
• Apply liming materials at least 2-6 months ahead of planting since the reaction time is slow. The caustic forms of lime are unlikely to injure the crop. Note that lime won't react during the dry season (unless the soil is irrigated), because moist soil is required.
• Where lime is expensive or difficult to apply, you can try "spot-liming" the immediate row or plant zone of the crop. Adjust the rates accordingly.
• Don't mix lime and fertilizer together since it will tie up P or release ammonia gas from N fertilizers.
• Thoroughly wash all lime from any metal application equipment to prevent corrosion.
• How often to lime Where high rates of manure and other acid-forming fertilizers are used, liming may be needed as often as once every 2-5 years. Sandy soils and others of low buffering capacity will need reliming the most often, but they'll also require lower rates per application. Refer to Table 9-1 in Chapter 9 for more information on acid-forming fertilizers.
Avoid raising the pH by more than one full unit at a time, and don't raise it much above 6.5 . It may only be necessary to raise the pH up to 5.5 to 6.0 for best yields of most crops. Overliming can be worse than not liming at all because:
• Raising the pH above 6.5 increases the likelihood of micronutrient deficiencies (except for molybdenum), especially in the case of iron and manganese.
• Phosphorus availability starts declining above a pH of 6.5, due to the formation of relatively insoluble compounds with calcium and magnesium.