Plants get most of their nutrients from the soil that they are planted in. Factors that play a part in determining how many nutrients are available to your plants include pH, structure and texture, and nutrients. Measures can be taken to improve all of these factors, but first, you must determine which are lacking in the soil, which you can do right in the garden.
Do a visual inspection of the soil to determine soil structure. It should clump together loosely when compacted. Nutrients leach out quickly and are not available for plant use in soils that are granular and do not have adequate cohesion. Soils that make large clumps make it difficult for plant roots to get through, making nutrients unavailable, as well. Amend soil with organic matter to improve soil structure.
Take several soil samples from different areas in your garden and squeeze them between your thumb and forefinger. Soil texture affects air and water circulation for plant roots, water and nutrient retention, and plant fertility. Desired soil texture is a sandy loam with a balanced sand and clay content, with nutrient-rich organic matter that retains adequate moisture and provides drainage and easy root growth. Soil with a high sand content feels gritty in your hand, and soil with high clay content feels slippery, while high amounts of silt makes soil feel greasy. Amending the soil with organic matter also improves texture.
Test soil pH by digging about 20 small holes in different areas around the garden and placing samples from them in a bucket. Follow the instructions on a home testing kit to test soil pH. Most plants do well in soil with a pH balanced in the 6.0 to 7.0 range. Add compost to help balance soil acidity and bring it into a preferable range.
Observe your plants to determine if your soil contains the proper nutrients. All plants need some degree of the three main nutrients found in soil--nitrogen, potassium and phosphorous, as well as secondary and micro-nutrients. A deficiency in any one of these needed nutrients will result in symptoms that are visibly recognizable in your plants. Plants that are weak and have poor growth indicate that soil amendment with organic matter is required. Organic matter can be added directly to the soil and allowed to decompose, or it can be added in the form of compost, which is a versatile organic fertilizer that contains many of the nutrients plants need and adds humus to the soil, as well as improving structure, texture and pH. For specific nutrient deficiencies, amend with soil additives that address the individual problem. Bat guano, blood meal, soybean meal and fish meal add nitrogen. Colloidal phosphorus, rock phosphate and bone meal provide phosphorous, and ground kelp, greensand and granite dust add potassium.