New gardeners and even some seasoned gardeners add far more vegetable garden fertilizer than their plants need. In many cases where plants have problems, fertilizer is not the problem. but sometimes garden fertilizers is the problem – too much of it.
How much fertilizer to add to your vegetable garden depends on the natural fertility of the soil, the amount of organic matter in the soil, the type of fertilizer added, and the crop that is grown. The only definitive way to know how much to add is to have your floor tested by a reputable laboratory. Garden floor test kits are also sold commercially, but sometimes do less than give good results.
Which nutrients influence the growth of a plant?
Light, water and nutrients are the main factors influencing the growth and health of plants. Outdoors, this is sunlight, rain or irrigation, and the nutrients available in the soil (and the pH of the soil that affects the availability of nutrients). If plants have problems or are not working properly, the problem may be with one of these problems. Adding garden fertilizer only helps if a lack of nutrients is the problem. For example, if your soil is soaked and not properly drained, or if your neighbor’s tree is shading your tomato patch, no amount of fertilizer on earth can help. Only a soil test will finally inform you if there is a nutrient problem with your garden soil.
To complicate matters, the soil structure also plays a role in plant health. If the soil of your garden has been / is heavily compacted by construction machinery or if your house was built on “filled” soil (as is common with new buildings), you must first build a healthy soil structure by adding a lot of organic content. Compost, untreated grass waste and shredded leaves are the best way to quickly build a soil structure and add slow-release nutrients. This improves the tendency (processability), promotes earthworm activity and supports biological activity, which in turn supports healthy plant growth.
Your location also determines what you may need to add to your garden floor. Clay is the norm in the Middle Atlantic, the Northeastern United States, and many other areas. Add organic changes to separate the clay and lots of nutrients become available. Coastal soils are usually sandy and very porous – nutrients are quickly lost to air and water. Here garden soil requires the regular addition of organic matter and the precise consideration of the nutrient content and the soluble salt formation through frequent fertilization. High desert areas with harsh weather conditions are a completely different matter. Each region has unique native nutrient profiles and a unique pH. Only a soil test can give you an accurate picture of what is missing and what is not.
The basics of vegetable garden fertilizer
Garden fertilizers are either organic or inorganic (a.k.a. synthetic). Organic fertilizers can be obtained from bat guano, chicken manure, cow manure, cottonseed meal, bone meal or any number of other sources. Inorganic fertilizers are man-made. The N-P-K numbers on the bag of garden fertilizer are very important – they indicate the percentage of nitrogen, potassium and phosphorus (potash) that is present in the fertilizer.
Nitrogen, potassium and phosphorus are the core elements that are required for plant growth. Plants need N-P-K in sufficient quantities (relative to each other) for optimal growth. Nitrogen is needed for plant growth – leaves, stems, roots, fruits; Phosphorus is needed for cell division to support this growth. and potassium is needed for the chemical processes a plant needs, disease resistance and starch formation. Just because a bag of garden fertilizer contains many of these elements does not mean that your garden plants use them. In fact, plants only take what they need and bypass the rest. Adding fertilizer when it is not required is not only a waste of money, it also helps these chemicals drain into local waterways.
Signs of nutrient deficiency in plants:
- Leaves are yellow, the plant is light green, leaves are small: lack of nitrogen
- Poor flowering or fruit growth, slow growth, dark blue-green leaves: lack of phosphorus
- Stunted growth and yellowish lower leaves: lack of potassium
However, these signs could also indicate other deficiencies or disease or pH problems that can inhibit the absorption of certain nutrients.
Adding too much vegetable garden fertilizer is not a good thing
- An excess of nitrogen can affect the quality of fruits and vegetables and increase pest and disease problems
- An excess of phosphorus can lead to chlorosis – yellowing of the leaf tissue due to a lack of chlorophyll
- Adding nutrients that plants do not need can lead to imbalance and subsequent deficiencies in other nutrients
How to choose a garden fertilizer
Fertilizers are marked with 3 letters: NPK, based on the weight percentage of nitrogen, phosphorus and potassium (potash). In many cases, N is the largest percentage. But how much matters. High nitrogen concentrations can “burn” certain plant roots. If too much nitrogen is added to plants like tomatoes, the plants are only grapevines and not fruits. too much for carrots and you have a lot of leaves and small roots. Add too little nitrogen to the corn, a heavy feeder, and the ears may be underdeveloped. Too much phosphorus kills symbiotic mycorrhizal fungi in the soil and reduces the plant’s ability to absorb iron and other micronutrients.
The only way to really know if your garden soil is low in nutrients is to have it tested by a local laboratory. However, the fertilizer recommendations returned by the laboratory can be formulated for commercial agriculture and recommend excessively high fertilizer levels to the home gardener. Use the soil test as a guide to find out where the deficiency is and how much organic content is in your soil. Adjust this accordingly, but only as needed.
What is the best garden fertilizer?
In my experience (clay soil, MidAtlantic USA) your garden needs little more than regular compost feeding, once in spring, summer and autumn. This can also be changed with mulched grass clippings (without turf treatment) and shredded tree leaves that are worked into the top 2 inches of the ground.
Sacked or liquid garden fertilizers only need to be added in spring as the growth begins and monthly until the plants stop growing – never in autumn because the nutrients are not absorbed and wash off with rain and snow. This does not apply to compost, which should be added regularly, as soil bacteria and fungi must act on compost to convert it into the nutrient form that plants can use.
Never apply lawn fertilizer to your garden. The high nitrogen content in fertilizers for grass “burns” vegetables and leads to an imbalance in the nutrient ratio.
Conclusion: Protect the health of your soil with biological methods and changes, and plants can make better use of the nutrients in the soil. Test for pH annually and add compost regularly to maintain and build soil structure and support microbial activity.
About the pH of the soil
The pH of your garden soil – the measure of acidity or alkalinity – has a direct impact on a plant’s ability to absorb and use nutrients. The pH of the soil varies from region to region and should be set to 6.0 to 7.0 for most vegetables. The exception are acid-loving plants such as blueberries, which thrive in soils of only 4.5. Only a soil test can tell you what your soil’s pH is, and the test results recommend what changes need to be added to make your garden soil neutral. Extreme pH values block the availability of certain nutrients and make them unavailable to plants. Conversely, the pH can make plants available to plants so easily that they actually become toxic (too much of a good thing).