Soil and nutrient management

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Soils also allow plants like rice to grow upright and turn towards the sun. They also provide needed nutrients to ensure enough yields, and store and supply water to plants. It is estimated that 99% of the food and fiber we produce grows on soils and only 10-12% of the earth's surface is covered by soils available for agriculture. Soils also provide many more essential services for humans. They help filter water; they immobilize many toxic substances, they mineralize crop residues and store carbon, as well as exchange gases with the atmosphere.

Soil is made up of air, water, mineral particles, organic matter, and organisms. About half of it is pore space that can be filled equally with water and air while most of the solid portion is made of mineral particles. Even though organic matter usually makes up about 2% of a top soil’s weight, it binds soil particles together, stores nutrients, and feeds soil organisms. Organic matter is created by tiny, living organisms that decompose dead plants and animals, creating nutrients that plants can use.

Although these nutrients already come from the soil, some plants like rice may still need supplemental nutrients (those added to the soil with fertilizers) especially when higher yields are required for a growing population. IRRI works on four areas that encompass soil and rice, including managing of nutrients.

Plant nutrition is only one of more than fifty factors which directly affect both crop yield and quality. The availability of required nutrients, together with the degree of interaction between these nutrients and the soil, play a vital role in crop development. A deficiency in any one required nutrient or, a soil condition that limits or prevents a metabolic function from occurring can limit plant growth. 

A soil nutrient management plan should include analyzing soil deficiencies to determine the type, application rate, application interval, and the placement of any nutrients required to optimize short and long term productivity.

There is a significant difference between an induced deficiency and a real soil deficiency. For example, certain crops require the addition of molybdenum at a specific rate for optimum growth. This is a real deficiency. In other crops zinc or iron deficiencies, caused by high levels of phosphorus and active calcium, can result in reduced yield. This is an induced deficiency. 

Typically, when deficiencies occur, the tendency is to foliar or soil apply copious amounts of product and hope for a favorable result. This ad hoc approach seldom achieves the expected result and is certainly not cost effective. 

The simple fact is, diagnosis is the first step in determining an appropriate corrective action which many include (1) a combination of treatments or (2) a program that incorporates several applications of different products at different application rates and intervals. 

When soil is depleted there are two methods for restoring its fertility (1) it can be left idle for several years allowing it to rebuild naturally or  (2) organic matter, in the form of crop residue, together with a microbial based inoculant can be applied from an external source. In the latter case, the rebuilding process is accelerated and optimal conditions for soil biological activity and long term soil fertility are maintained.

Soil organic matter is vital in rebuilding depleted soil as it ensures a continuous energy source for soil biomass. Soil biomass, consisting of microbes, fungi, algae, protozoa etc. (1) transform organic molecules into mineral elements that are readily available to plants and (2) help maintain good soil structure by transforming organic matter into humus and producing compounds that cement small soil particles together, promoting both increased drainage and moisture retention. 

Soil nutrient management involves not only the physical properties and mineral structure of the soil, but also the balance between soil pathogens and beneficial microbes. Beneficial microbes increase nutrient availability, reduce disease, reduce nutrient losses, and help degrade toxic compounds. Plants thrive or suffer, depending on the type of microbes in the rhizoshere (the area around the roots.) In a healthy rhizoshere, dominated by beneficial microbes, plant life and soil life work together to produce healthy plants. Conversely, in unhealthy soil, dominated by pathogenic microbes, optimum plant growth is  unattainable.