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close this bookSoils, Crops and Fertilizer Use: A Field Manual for Development Workers (Peace Corps, 1986, 338 p.)
close this folderChapter 6: Soil fertility and plant nutrition simplified
View the document(introduction...)
View the documentLet's Make a Deal
View the documentHow plants grow
View the documentAvailable vs. unavailable forms of mineral nutrients
View the documentSoil negative charge and nutrient holding ability
View the documentSoil pH and how it affects crops growth
View the documentImportant facts on the plant nutrients

How plants grow

Plants grow by enlarging their cells and by developing new ones at their shoot and root tips.

Photosynthesis: How Plants Make Food and Tissue

Plants produce food for energy, tissue building, and storage by a process called photosynthesis which takes place in the green, chlorophyll-containing cells found mainly in the leaves. These cells take carbon dioxide from the air and combine it with water (taken in by the roots) to make simple sugars, using chlorophyll and sunlight as catalysts. As shown below, oxygen is also a byproduct:


Carbon dioxide + water -> Sugar + Oxygen Chlorophyll

This sugar is the real "food" of plants, and here's what they do with it:

· It's used for energy in a process called respiration in which the plant digests it much like we do and releases carbon dioxide.

· Sugar is also storied "as is" in varying amounts (i.e. maize contains a small amount, but sugarcane has a lot).

· It's used to make cellulose and other types of fiber that hold cells and plants together. Plants are the only source of fiber in our diets.

· Sugar can be converted to starch, the main component of most seeds and other starchy crops such as bananas, potatoes, cassava (manioc), and other root crops. It is also converted into fat, a principal constituent of some crops such as coconut, soybeans, peanuts, and avocados.

· Sugar, when combined with nitrogen, forms protein.

Photosynthesis governs the rate of plant growth and is the biggest factor affecting crop yields. It's encouraged by:

· Adequate sunlight. Cloudy weather lowers the rate of photosynthesis.
· Adequate moisture.
· Favorable temperatures, which vary with the type of plant.
· Adequate mineral nutrients like nitrogen, phosphorus, etc.
· Good insect and disease control which prevents the destruction of green tissue.
· Adequate carbon dioxide. Normal air contains enough. Some greenhouse growers try to raise the level.

A Note on "C4" Plants: Some crops such as maize, sorghum, amaranth, and sugarcane have an unusually efficient type of photosynthesis that functions best under high temperatures, full sun, and low-humidity conditions. They're called C4 plants.

So Where Do the Plant Nutrients Fit In?

The plant mineral nutrients like nitrogen are supplied by the soil and supplements of fertilizer. They are absorbed by the root hairs (tiny, delicate protrusions on the roots) in the form of ions (molecules with a + or - charge) from the soil water and perform many functions. Some, like potassium, are used in sugar and starch formation, while nitrogen is used for making protein and chlorophyll.