As we know, NPK is nitrogen, phosphorus and potassium. Depending on what you are growing, is how you want to use the numbers. Basically, nitrogen is the first major element responsible for vegetative growth of plants above ground. With a good supply, plants grow sturdy and mature rapidly, with good, dark green foliage. Phosphorus is the second major element, essential for healthy growth, strong roots, fruit and flower developement and greater resistance to diseases. Potassium is the third element needed for the development of strong plants. It helps plants resist diseases, protects from the cold and during dry weather by preventing excessive water loss. If the soil is in good shape, compost and the like, you may not need much. For the lawn, you want more nitrogen, as you want more 'foliage' that's dark green. If too much nitrogen goes to the flowers, you will get more foliage but less blooms. Therefore, the need for the other two elements.

1) Every ingredient that you listed for the Fox Farm soil is extremely fine. Since water retention is dependant primarily on particle size, you should expect a medium made of all fine particulates to be extremely water retentive, something I try very hard to avoid in containers. 2) You can read about soils and how durable soils that remain well-aerated long after peat/compost-based soils have collapsed can offer a greater margin for grower error and make things much simpler for the grower if you follow this link. 3) Not necessarily, but if you think about it, what will you do if your pH falls outside the favorable range? Start adding acids and bases to change it? In commercial situations, pH is usually tested frequently and adjusted by injecting a wide variety of chemicals that have elements plants need into the irrigation water. Since pH is affected by temperature, fertility, plant material, moisture level, even time of day (Plants help to control the pH in the rhizosphere to make it easier for them to uptake nutrients. This effect is reduced when plants are undergoing periods of reduced transpiration, which in turn has an effect on pH.) I'm not saying that because I ignore trying to control pH that you should, but I've never found it necessary to fret about it in any container planting. I might consider the plant prefers acidic conditions and forgo the use of lime and maybe use a urea-based fertilizer to take advantage of the acid reaction, but other than that, I don't worry. Remember that Media pH is far less important in container media than in mineral soils, and if you're supplying nutrients in soluble form, unless the pH is WAY outside favorable limits, your plant will be able to assimilate the nutrients. You can find considerable discussion about this in Dr C Whitcomb's work, "Plant Production in Containers II". 4) On average, plants use about 6-10x more N than P. Since the P in NPK is not a true measure of P supplied (P is reportted as the % of P2O5 [phosphorous pentaoxide]) you need to use a multiplier of .43 to tell the actual amount of P. In 3:1:2 ratio fertilizers, there is 7x as much N as P, so you actually have a considerable cush in 3:1:2 ratio fertilizers. Plants use roughly the same amount of nutrients throughout the growth cycle in relation to N, so I'm not sure where the idea they need larger doses of P during flowering/fruiting comes from. Tissue analysis of fruits and blooms show roughly the same levels of P as other plant organs, which meshes nicely with the idea that consistent P levels at approximately 1/6 that of N are entirely adequate throughout the growth cycle. I actually don't boost K by adding the ProTeKt, I reduce the amount of N, which also must reduce the levels of P and K. Since I can see by my plants that what I said above about P is true (also confirmed in the text cited above, which is essentially the Bible on container production) and P levels remain adequate at reduced application rates of 3:1:2 ratio fertilizers, I only need to be sure plants are getting adequate K. I can do this by incorporating some potash in the soil when I make it in anticipation of the reduced N applications for tomatoes, or simply forgo the increase in TDC by supplying it later as needed by using the ProTeKt. Al

Fertilizers have reported NPK %s that are listed on the container, and identify the % Nitrogen, Phosphorous pentoxide (P2O5) , and Potassium oxide (K2O) in the product. When discussing fertilizer %s, separate the numbers representing the NPK %s with a dash (24-8-16, 12-12-12, etc). To determine the ACTUAL % of P and K in fertilizers, you multiply the amount of P2O5 reported by .43, and the amount of K2) reported by .83. Fertilizer RATIOS are the best reference point for determining what fertilizer is appropriate. If we consider only the NPK RATIO, 9-3-6, 12-4-8, 18-6-12, 24-8-16 would be equally appropriate (or inappropriate, if you prefer) because they are all 3:1:2 RATIOS (based on the amount of NPK REPORTED). Essentially they will yield approximately the same strength solution if mixed to the manufacturers specifications. Specifically, 24-8-16 diluted in twice the amount of water as the same measure of 12-4-8 yields the same concentration of elements and the same amount of each element per any given volume of solution. Note that when discussing fertilizer RATIOS and to avoid confusion, we should use a colon (3:1:2, 1:1:1) between the numbers to indicate you're discussing the fertilizers nutritional RATIO, rather than its NPK %s Usually, when we suggest or discuss ratios, we use them as a function of the REPORTED NPK %s, rather than the ACTUAL NPK %s - it's just much easier than having to do the .43 and .83 factoring and plugging the results into whatever it is you're factoring. Maca - I didn't realize I might have answered your questions here on 'Containers'. I looked all over the 'Citrus' forum yesterday or day before, and only stumbled on this thread today. I hope that between all the good folks that offered help, you got all the answers you needed. Best luck. Al

I agree with gardengal 48 and Jolj. And so do most university agriculture experts, extension offices, and documented and verifiable scientific studies. I am taking a course on this now. Oh, and to the comments about you can't have too much compost...well, that is inaccurate. One contributor said it is balance. I agree, but to know what the balance is or what is needed to maintain that balance requires testing. Most of us can't afford a complete soil test (it isn't just about NPK), so we guess as best we can. Sure, there are some NPK and other nutritional elements to compost, but compost is still considered an amendment, not a fertilizer. Part of it is WHEN or HOW do those nutrients become available to the plants. Anyway, this is several years after the original post, so I hope the poster is still gardening and is successful. :)