Subsoils nutrients

Sodium is an indispensable element for some crops (notably sugar beet), can partially substitute for potassium in several crops, contributes to neutralising soil and subsoil acidity, and has a positive effect on soil phosphoms solubiUty. Sodium is an essential nutrient for cattle, and sodium appHcation to soil increases its content in pastures. Sodium nitrate is particularly effective as a nitrogen source for sugar beet, vegetable crops, tobacco, and cotton (qv), and for any crop in acid soils. 

Grass roots can grow and absorb water down to 1 m, or deeper if soil structure is good, although subsoil water is usually poorer in nutrients than topsoil water (Frame, 1992). Transpiration is highest... 

Tarkalson DD, Jolly VD, Robbins CW, Terry RE (1998) Mycorrhizal colonization and nutrient uptake of dry bean in manure and compost manure treated subsoil and untreated top and subsoil. J Plant Nutr 21 1867-1878... 

Eichornia crassipes), water lettuce (Pistia stratiotes), and Eurasian milfoil (Myriophyllum spicatum). When planting in baskets, use a special, low-nutrient aquatic planting mix or garden subsoil. 

Soil often occurs as a series of horizontal layers called soil horizons, shown in Figure 15.7. The deepest horizon, which lies just above solid rock, is the substratum, which is rock just beginning to disintegrate into soil by the action of water that has seeped down to this level. No growing plant material is found in the substratum. Above the substratum is the subsoil, which consists mostly of clay. Only the deepest roots penetrate into the subsoil, which may be up to 1 meter thick. Above the subsoil is the topsoil, which lies on the surface and varies in thickness from a few centimeters to up to 2 meters. The topsoil usually contains sand, silt, and clay in about equal amounts. This is the horizon where the roots of plants absorb most of their nutrients. 

Many factors affect the development of roots in soils but these need not be discussed here (see Brouwer, 1965 and Barley and Greacen, 1967). Pearson (1966) has given an excellent discussion of the conditions in the subsoil that tend to inhibit growth and which often cannot be improved readily or quickly. These include soil acidity, deficient available nutrients, mechanical impedance, unfavorable moisture, and poor aeration. As he points out, the properties of surface soils can usually be modified readily but after this is done the root systems frequently have to contend with a subsoil environment that severely limits their development. 

In the face of all of these handicaps it is not surprising that the shifting system of cultivation has prevailed over the centuries. Under this system, trees with deep roots can assimilate nutrients in the subsoil and being carried down in the leachates. When the leaves and twigs fall, some of these nutrients are returned to the soil surface. When the... 

The term subsoil will not be used here in any very exact sense, but will refer to all of the layers below the A-horizon where plant roots commonly penetrate. The emphasis will be on the B-horizon, but not exclusively, for in actual agricultural practice it is seldom possible to study this horizon isolated from the others. A badly-eroded soil usually contains some A-horizon. Even where the B-horizon is exposed and crops are being grown on it, many of the plant roots may be in the C- and D-horizons and dependent upon these lower horizons for much of their nutrient supply and moisture. Furthermore, where extensive excavations or mining operations have been conducted the whole soil profile, including much parent material, is commonly mixed. The discussion here will, therefore, be concerned primarily with the growing of crops on soil or soil material that contains little or no A-horizon. 

Subsoils vary widely in their ability to support plant growth. In some cases the addition of adequate nutrients will result in good crop growth. However, many other subsoils are problem soils and may support only a limited vegetative cover. Some of the reasons for the wide differences in plant response, mentioned in part above, are discussed in the following pages. 

The possibility that limited root development is attributable to insufficient available major and minor nutrients has received considerable attention. Sometimes the addition of available nitrogen and phosphorus has increased the depth and extension of root systems, but in most cases the increase has not been marked, and there was no certainty as to how much of the increases obtained was due to the nutrients per se, and how much to the mechanical operations involved in the addition of the nutrients. Where the A-horizon is present, most of the nutrients needed for growth in the subsoil are readily obtained from the topsoil, but in exposed subsoils added available nitrogen and phosphorus may be very essential for extensive root proliferation. If the physical conditions are satisfactory, plants produce a surprisingly large amount of roots on a minimum of nutrients they appear to send out roots in search of nutrients, particularly nitrogen. 

Much emphasis has been placed on the plowed layer and its contribution to crop production, and comparatively little research has been done on the subsoil. In fact, in general discussions of the depletion of soil fertility the subsoil is often ignored, even though we know that it is often higher in essential nutrients, other than nitrogen, than is the A-horizon. These mineral nutrients may be less available to plants than those in the surface layer that is constantly subject to weathering and oxidation, but nevertheless subsoils do contribute in a major way to plant feeding. The plowed layer is only 6—12 inches deep whereas the subsoil tapped by the root system may have a depth of several feet. 

Peat in mixture with soil, and adequately supplied with nutrients, serves as an excellent medium for root growth if it is not too acid. In very compact soils growth is often curtailed because of the inability of roots to penetrate them readily. Peat corrects this and in addition releases nutrients that favor root development. Root penetration into the subsoil is often curtailed because of either the presence of a hardpan, lack of calcium, or too much soluble aluminum. The addition of peat is not likely to correct these conditions unless it is mixed with the subsoil. Where this can be done, a comparatively small amount of peat, reinforced with the needed nutrients, may have a long-term effect in correcting the unfavorable condition. 

Aluminum toxicity is particularly important on acidic tropical soils that may have low pH in their subsoils, which limits root development into these layers. Aluminum toxicities decrease the permeability of root cells and reduce root growth thus, the plant s ability to take jupjwateiLandjautrientsJs-decreased. This effect can be much more detrimental to the plant than P dcficiendes because of low pH d high A1 concentrations per se [41. In additim to nutrient availability due to solubility effects, mineralization of N from organic matter is decreased, arxl BNF by the rhizobia of legumes is decreased at low pHs. 

---