Agriculture Reference
In-Depth Information
Calcium, magnesium, and sulfur are often in sufficient supply in soil. However,
micronutrients may be lacking. Calcium, as ground limestone, is added to soil to
increase pH, while sulfur is added to decrease pH. Magnesium accompanies calcium
as it is a common constituent of limestone. Micronutrients are very different in that
they are needed in very small amounts and are thus added with other fertilizer elements.
They are of particular concern because in high concentration they can be toxic to plants
and in some cases animals and humans if excessive amounts are applied. Thus, great
care must be exercised in applying micronutrients to soil.
It is rare to find a soil that is equally deficient in all plant nutrients. It is more
common to find a soil particularly deficient in one nutrient and somewhat lacking in
other nutrients.
With continued cultivation and crop production on the same piece of land or field
without fertilizer, manure, or organic matter application, all nutrients become deficient
and crop production is dependent on the nutrients released from parent material during
the growing season. This leads to very low crop yields and a degradation of the soil and
its ability to support crop production. However, crop yield will never go to zero, as
there is always some decomposition of soil parent material that releases crop nutrients
into the soil, and nutrients from decomposing plant materials.
An exception is rice production. In South East Asia rice is often irrigated by water
from surrounding mountains. This irrigation water contains plant nutrients dissolved as
the water moves through soil and down rivers. Plants irrigated using this water can
make use of these nutrients. Therefore, these rice paddies have more natural plant nutri-
ents than might be expected. The result is that these rice paddies can sustain higher
yields without fertilization.
This type of irrigated rice production takes place in areas that have rainfall as high
as 2000 mm per year. This high rainfall means that there is no problem with salt buildup
in these areas.
In desert areas there is insufficient rainfall and so crops must be irrigated. Desert
areas generally have low humidities and high evaporation and evapotranspiration
†
rates. Therefore, salts in irrigation water are deposited in and on the soil when evapor-
ation of irrigation water occurs. These salts will have two effects on crop production.
First, the pH of the soil will be above 7.5. Second, high salt levels will prevent plant
growth when they exceed a certain level depending on the crop. In all cases irrigation
can result in increased salt in soil. Therefore, irrigation needs to be carried out with
caution and requires special management to keep these soils productive.
Because of the importance of plant nutrients in crop production, it is common to
take soil samples from a field and have them analyzed; see Figure 9.10. A representa-
tive soil fertility report is given in Table 9.5. The farmer generally gives the sample
number, and the report may also have a laboratory number assigned to each sample
that may also be on the report. Generally, it is desirable to have soil pH in the range
of 6.5 to 7.0 for most crops, although some specialized crops such as blueberries
may require lower pHs. For acid soils a buffer pH is given and can be related to
tables that give the appropriate levels of lime needed to bring acid soils to the
†
Evapotranspiration is the combined loss of water due to evaporation and transpiration.
