Growing nitrogen fixing trees in acid soils

C. Buford Briscoe, Sergio M. de Faria, Veronique Lambert, Gregory Minnick, Ricardo Russo

Seed procurement

Once a grower has selected one or more nitrogen fixing tree species for a particular site and production goal, the first priority is to obtain high-quality seed. The chances of establishing and growing nitrogen fixing trees successfully can be greatly enhanced by using seed from good sources. Often, however, this key aspect of the production process does not receive sufficient attention.

Seed should contain genetic material that will produce trees that grow well in the proposed site and provide the desired services and products. In addition to the quality and suitability of the genetic material they contain, the seeds themselves should also be of high quality. Whenever possible, obtain seed from a reliable source, since inappropriate collection, storage, or treatment of seed can seriously reduce its viability (germination rate). Local seed banks, managed by government agencies, projects, or commercial companies, are often reliable sources of seed for nitrogen fixing trees. Preferably, the seed should be certified as to its provenance (original geographic area of genetic material), origin (exact location of source trees), purity, and viability. International seed banks are another good source. They sell certified seed in commercial quantities and may also sell or give away small amounts of seed for testing purposes. Appendix B provides a list of seed sources.

Seed collection

If seed is not available from a seed bank, it will have to be collected locally. Although seed collection methods vary depending on the flowering and pollination characteristics of each tree species, some simple "rules of thumb" should be followed.

· Seed should be collected from healthy mother trees that have no visible signs of weakness, disease, or insect attack.

· Because many characteristics are inherited, mother trees should be selected for the characteristics desired. For example, if nitrogen fixing trees are being planted for timber production, then mother trees should be selected that exhibit good form and self-pruning.

· Seed should be collected from many different trees, preferably 30 to 40, that are widely separated. At a minimum, seed should be collected from 10 mother trees spaced at least 100 meters apart. If a continuing genetic improvement program is planned, each parent tree should be selected from a different stand or uniform plantation to ensure genetic diversity.

· Only mature seed should be collected.

Seed storage

All tree seed falls into two categories. Most nitrogen fixing trees have orthodox seeds "mall or medium-size seeds with hard seed coats. Normally, their viability can be maintained for at least 12 months. The acacias are an example. Recalcitrant seeds generally have soft seed coats and may be larger than average. Their viability cannot easily be maintained in storage: in some case, germination rates fall drastically within a few days or weeks after the seeds are harvested. The Inga species are an example.

It is always best to plant seed as soon after collection as possible. In virtually all cases, germination rates will decline as storage is extended. If germination rates are expected to be low because of extended storage or any other reason, it is important to collect extra seed to ensure production of the required number of plants.

To store orthodox seed:

· Remove plant debris and cull seeds that are deformed, damaged, or show signs of pest attack

· Before seeds are stored, they should be dried in open shade or indoors at a temperature of not more than 30ø C until they reach a moisture content of 7-8 percent.

· If the seeds will be stored for a considerable time, they should be treated with a fungicide, but one that will not damage rhizobial or mychorrhizal symbionts.

Store seeds at a low temperature (3-5øC) in containers that will keep out moisture, such as powdered milk cans or double plastic bags.

Recalcitrant seed cannot be stored for long periods. It is best to plant such seeds as soon as possible after collection - within two weeks, for example, for Inga species. If seeds must be shipped over long distances, it may be possible to prolong viability somewhat by packing them in a moist, well-aerated medium, such as vermiculite or sawdust, in a perforated plastic bag inside an air-porous bag. They should be kept under refrigeration (3-10øC) and shipped by the fastest means available. It is occasionally possible to make prior arrangements with customs and plant-quarantine officials to give special priority to such perishable material. This almost invariably requires communication of shipping dates and waybill identification, plus close follow-up at the receiving customs office.

Propagation from seed

Seed pretreatment. Pretreatment is only necessary for orthodox seeds that have been dried. Table 1 lists recommended pretreatments for several tree species. In the past, seed was often treated with sulfuric acid, but this method is expensive and dangerous, and the results are seldom, if ever, superior to more conservative methods.

Seed pretreatments should always be tested on small samples, in increasing order of severity, before they are applied to the seedlot as a whole. For example, test samples should be soaked in boiling water for very short periods, and these should be extended in small incremeets. Germination tests should be used to determine which pretreatment works best.

Scrape a small hole in the seed coat by rubbing on a rough surface

Or cut a small hole in the seed coat with nail clippers or a knife (below).

Source: NFTA.

Area where the seed coat can safely be scarified. Source: Briscoe (1989).

Table 1. Suggested pregermination treatments for key nitrogen fixing tree and shrub species that tolerate acid soils.

Inoculation with rhizobia and mycorrhizae

To improve mineral nutrition of nitrogen fixing trees on acid soils it is important to ensure that the trees are infected with effective rhizobial and mycorrhizal symbionts. The rhizobial bacteria form nodules on the roots of leguminous trees and other plants: nitrogen fixation occurs inside these nodules through a symbiotic process that involves both bacteria and plant. The bacteria provide nitrogen to the plants, and the plants provide carbohydrates to the bacteria.

Like rhizobia, mycorrhizal fungi live in the soil and form symbiotic associations with the roots of nitrogen fixing trees. A tree provides essential carbohydrates to the fungus, and the fungus's network of hyphae functions like extra feeder roots, improving the tree's access to soil moisture and nutrients. This symbiotic relationship is particularly important for water uptake in arid environments or for uptake of scarce nutrients, especially phosphorus, in acid, infertile soils.

In most areas where nitrogen fixing trees are native or naturalized, the native rhizobia and mycorrhizae in the soil will form symbiotic associations with the trees' roots to enhance growth. However, the soil is not likely to contain effective symbionts in areas where trees have never grown or have not grown for a long time or where the top soil has been severely disturbed (for example, by mining). In such areas, it may be necessary to add rhizobia and mycorrhizae to the soil, a technique called "inoculation," to ensure good tree growth. Follow the general rule - if in doubt, inoculate!

Many nitrogen fixing tree species have been shown to require particular species of rhizobia in order to fix nitrogen from the atmosphere. Within a given species of rhizobia, particular strains may also be more or less tolerant of acid soils.

Tree species also need to be matched with particular types of mycorrhizae. These fungi can be classified into two broad groups: ectomycorrhizae and endomycorrhizae. The majority of tropical nitrogen fixing trees form symbiotic associations with vesicular-arbuscular mycorrhizae (YAM) in the endomycorrhizal group. However, Alnus and Casuarina species and some species in the Caesalpinoidaea subfamily associate with fungi in the ectomycorrhizal group as well as VAM fungi. It is important to inoculate nitrogen fixing trees with the appropriate mycorrhizal fungi to achieve maximum benefits in terms of tree growth and survival.

Figure

Until recently, it was difficult to procure just the right rhizobium or mycorrhiza for a particular tree species and site. Now, however, several international laboratories and private companies produce rhizobial and mycorrhizal inoculants for experimental and commercial purposes. These are listed in Appendix B.

Seeds should be inoculated after pretreatment immediately before planting. The Nitrogen Fixation by Tropical Agricultural Legumes (NifTAL) Center recommends placing seeds in a bag or bucket and covering them with a sticking agent such as gum arable, sugar, vegetable oil, or water (see illustration). Dissolve 40 g of gum arabic in 100 ml of hot water. After cooling, add 2 ml of this mixture to the bag or bucket for each 100 g of seed. Alternatively, dissolve 1 part sugar in 9 parts water and add 2 ml of this solution for each 100 g of seed. After either treatment, add inoculant and shake or mix the seeds until they are evenly coated. A 50 g bag of inoculant from NifTAL is sufficient to inoculate 1 kg of seeds. To eliminate any stickiness, allow inoculated seeds to air dry briefly before planting.

To inoculate seedlings in a nursery, mix inoculant in cool water and irrigate the seedlings with the suspension. Keep the suspension well agitated and irrigate until the inoculant is washed into the root zone. A 5 g bag of NifTAL inoculant can inoculate 10,000 seedlings of any species.

If commercial inoculant is not available, look for vigorously growing trees of the same species you are planting or of a closely related species. Dig up some soil containing well-nodulated roots from under several of these trees and place this fresh material in the pots where seeds are planted. If a problem arises with parasitic fungi or bacteria, wait to apply the inoculant after seedlings have developed a dry bark and woody stem.

Germination

It is best to start tree seedlings in flats if you expect medium or low germination or directly in pots if you expect high germination. Use a potting mix of 1 part sand, 1 part organic matter, and 3 parts soil or any mix that will allow good water drainage and adequate water retention.

Seeds should be sown just below the soil surface with the hilum facing downwards and covered to a depth about equal to their diameter. Spread very tiny seeds on the surface. Keep soil moist but not wet until after the seedlings emerge. If germinating seeds in flats, transplant to pots or beds as soon as there is evidence of germination.

Field establishment

Site preparation. First, remove all unwanted plants and other material to reduce excessive competition for light, soil nutrients, and moisture. The most common method of site preparation in tropical countries is slashing all existing growth, at/owing it to dry, and then burning the entire area. The ash enriches the soil, and the lack of soil disturbance reduces the danger of erosion. On windy sites, such as along coast lines or exposed ridges, young trees may benefit from the protection of existing vegetation this may more than compensate for any disadvantages from competition.

Fertilizer can be applied in planting holes to correct the nutrient deficiencies normally associated with acid soils. Phosphorus is probably the most critical nutrient: if no phosphorus fertilizer is available, apply ash, manure, or compost. Other elements that may be needed are calcium, magnesium, and minor elements (micronutrients). Dolomitic limestone is recommended as a source of both calcium and magnesium. Additions of molybdenum or a complete micronutrient supplement may also be beneficial. Whether such applications are advisable in economic terms depends on the particular plant and soil situation.

Post planting care includes weeding (above) and mulching (below). Organic mulch should not touch the tree stem, particularly where rodents and insects are a problem. Where termites attack the young trees, only a non-organic mulch of stones or plastic may be acceptable. Source: Briscoe (1989).

For tree planting projects, adding lime in an attempt to correct acid soils is generally not a good strategy. Lime can be expensive and only affects the soil in the spot where it is mixed. Tree roots normally grow deep into the soil and are not affected by lime added to the top layers.

When preparing a field site for tree planting, follow these general guidelines:

· For any type of seedling, always prepare a large, deep planting hole. Make the hole deeper and wider than the seedling root width and length.

· If the soil is waterlogged for part of the year, plant trees in mounds of topsoil.

· In arid areas, plant trees in microcatchments, small triangular or semicircular bunds built downslope from the planting hole to catch rainfall.

Spacing. Closely spaced trees dominate a site quickly, reducing weed growth. They also provide greater woody biomass per hectare than widely spaced trees unless the close spacing inhibits growth or causes substantial mortality. Close spacing provides early high yields of small products such as foliage, and fuelwood.

Although closely spaced trees may grow faster initially, widely spaced trees tend to grow faster in the long term, both in terms of diameter and often in terms of height. This increased timber production with wide spacing may bring a substantial financial advantage. To produce trees of a given size, spacing on poor sites should be wider than on good sites. For most species, coppice regrowth after trees are harvested tends to be more reliable and vigorous at a wider spacing.

Post planting care includes weeding

Post planting care includes mulching

Forms of planting stock.

Source: Briscoe (1989).

Planting. Trees can be established in the field by direct seeding or by planting various types of seedlings or cuttings. The choice of planting method will vary according to tree species, site, weather conditions, and the availability of labor and other inputs. Even experienced farmers or field workers can benefit from a short training session before planting begins: emphasize the fragility of the root tips rather than the mecanics of planting. Two general rules apply:

· Always remember to keep roots moist and protect seedlings from any type of damage when lifting, transporting, and planting them in the field.

· When planting seedlings on a slope or at a site where surface erosion is expected, plant trees deeper than they were in the nursery. Never plant less deep.

Planting stock. Plants raised from seed or seedlings are the products of sexual reproduction, possessing a mixture of genes. Hence performance will be variable to some extent, either better or worse than the performance of the plant's parent tree or trees. By contrast, plants produced by vegetative propagation are genetically identical to a single parent tree. These are useful for identifying the genetic characteristics of the parent tree and for multiplying planting stock for tree improvement programs. In themselves, apart from environmental conditions, they are never inferior-but also never superior - to the parent. Planting stock based on sexual reproduction will be discussed first.

Incisions near the base of cuttings stimulate superficial root growth.

Source: NFTA.

Direct seeding is the least expensive planting method, but risks are high Try direct seeding if you want a dense planting, if you have a favorable site with good weed control, if you are planting a tree species with large seeds, and if you have an abundant supply of cheap seeds. Treat seed as for sowing in the nursery, and time planting for a period of good rainfall.

Bareroot seedlings are grown to a height of about 50 cm in the nursery and dug up and planted directly in the field without a rootball or container. This technique is widely known and fairly inexpensive: it is usually successful if properly done. In the tropics, it is particularly important to transport bareroot seedlings to the field and plant them as quickly as possible. If the planting hole cannot be deeper than the root length, prune the roots cleanly to fit the hole.

Striplings are bareroot seedlings that are stripped of all, or almost all, their leaves before removing from nursery beds. They are often more successful than bareroot seedlings on sites where rainfall is erratic during the planting season. Begin with large seedlings - 50-100 cm tall and 10-30 mm in diameter - and prune off all non-stiff roots along with the leaves. Plant as bareroot seedlings.

Potted seedlings are grown in individual containers. They tend to be expensive, but field planting is almost always successful if properly done. Keeping seedlings in pots until they are planted in the field reduces the pressure to transport and plant them quickly, but they are heavier and more difficult to transport than bareroot seedlings. Remove the seedling carefully from its container and insert it carefully into the planting hole without disturbing the ball of earth or breaking the fine roots.

Wildlings are seedlings that sprout naturally under existing trees. They are usually cheaper and may be more widely available than nursery stock, but they are generally less successful. Handle and plant them the same as striplings or bareroot seedlings.

Stumps Prepare them as bareroot seedlings with the stem trimmed back to 2- 5 cm, the primary root pruned to 12-18 cm, and the lateral roots pruned to very short stubs. They should be planted with the top of the stem no more than 1 cm above the ground to avoid multiple sprouting. This planting method is particularly successful in sites with low or erratic rainfall.

Pseudocuttings are cuttings with a short segment of stump attached. Prepare them as bareroot seedlings with the stem trimmed back to 15-30 cm, the primary root pruned to 10-20 cm, and all branches and lateral roots pruned to short stubs. This method is reportedly more successful in humid climates than stumps. Pseudocuttings can often maintain their viability if planting is delayed or they are subjected to rough handling. However, they tend to be susceptible to borer attack and may produce multiple sprouts.

Vegetative propagation from cuttings is inexpensive, especially if natural stock is available. Some nitrogen fixing trees, such as Erythrina species and Gliricidia septum, grow well from large cuttings (more than 1.5 m long and 3 cm in diameter), although initial problems in growth form may occur. For example, when Erythrina species are propagated from cuttings, initial rooting may be shallow. The following general guidelines are useful for propagating nitrogen fixing trees from cuttings:

· Make cuttings at the end of the dry season or the beginning of the rains.

· Plant cuttings immediately after cutting or store them temporarily, top end up, in a cool, shady place.

· Trim the top ends of cuttings with a sharp tool at a 45ø angle to avoid water accumulation that could lead to rotting. You may also cover the top cut with paraffin, plastic, or mud.

· With some species, such as Erythrina and Gliricidia, incising the bark to the cambium layer near the base of the cuffing may increase rooting..

Airlayering may improve establishment of species that are difficult to graft or to propagate from root cuttings. Remove a strip of bark 1-15 cm wide from a branch, preferably one that is more than 1.5 cm in diameter. Cover the stripped area with a double handful of moist sphagnum moss or similar absorbent material, enclose it in dear plastic film, and secure the film on both sides of the stripped area. Check weekly for visible roots. When roots are seen, dip off the branch above and below the plastic cover. Remove the plastic and plant.

It is also possible to propagate trees from selected genetic material by laboratory methods using tissue culture. Although this approach is still quite expensive, it is likely to become more widespread in the future. Trees produced from tissue culture are called emlings.